kChapter 11

P. ERIK MIKKELSEN FIELD INSTRUMENTATIOM

1. INTRODUCTION Transportation Research Record 482 (Transportation Research Board 1974) includes several docu- andslides often present the ultimate measure- mented case histories of geotechnical instrumenta- L ment challenge, in part because of their ini- tion applications. Considerable interest continued tial lack of definition and the sheer scale of during the 1970s concerning the most appropriate the problems. Intuitively, there is a desire to quan- methods for using instrumentation (Mikkelsen and tify the extent of a potential or real disaster, but Bestwick 1976; Wilson and Mikkelsen 1977). what should be measured? The answer can be com- Chapter 5 in Landslides: Analysis and Control plicated and should be addressed by drawing on the (Wilson and Mikkelsen 1978) was devoted to best available information concerning topography, landslide instrumentation. geology, groundwater, and material properties. The By the late 1980s, geotechnical instrumentation measurement problem usually requires information had evolved to the point that a major textbook was ranging from a coarse scale down to a fine scale and devoted to the subject (Dunnicliff 1988), and the involving a number of instrumentation tech- Transportation Research Board published a second niques. The ultimate goal is to select the most sen- Transportation Research Record containing reports sitive measurement parameters, the ones that will of new applications and methods (Green and change significantly at the onset of the landslide Mikkelsen 1988; Hinckley et al. 1988). Other case event. However, because of physical limitations histories were published during the same period and economic constraints, all parameters cannot (Patton 1983, 1984). In recent years the full im- be measured with equal ease and success. Geotechnical instruments have evolved tre- pact of new instruments and computer automation mendously during the past quarter-century. Early in data collection and data reduction has been re- versions of current instruments were developed ported in numerous papers (e.g., Mikkelsen 1989, and used on various projects beginning in the 1992; Dunnicliff and Davidson 1991; Tatchell 1960s. Early reports concerning instrumentation 1991; Gillon et al. 1992; Proffitt et al. 1992; Kane that have retained their value include those by and Beck 1994; O'Connor and Wade 1994). Shannon et al. (1962), Dunnicliff (1971), Gould Experiences with large-scale instrumentation and and Dunnicliff (1971), and Peck (1972). The com- automation of data collection for monitoring slope prehensive report on geotechnical field instrumen- stability at the huge open-pit mines of Syncrude, tation prepared by the British Geotechnical Canada, Ltd., in northern Alberta were described Society (1974) contains several important papers by McKenna et at. (1994) and McKenna (1995). (Burland and Moore 1974; Dibiagio 1974; Green The focus of this chapter is primarily field instru- 1974; Penman and Charles 1974; Vaughn 1974). mentation and measurement techniques associated 278 Field Instrumentation 279 with permanent borehole installations that serve to In the last situation, savings are often realized measure subsurface behavior. Typical instruments in remedial treatment by a planned and moni- are piezometers for measuring groundwater pres- tored sequence of construction. For example, sures and inclinometers and extensometers for drainage might be initially installed and its effect ground-displacement measurements. Excluded from monitored to determine whether planned me- this discussion are borehole testing and logging chanical stabilization is actually necessary. techniques that help define structure, lithology, and material properties; these are described in Chapter 2. PLANNING AND DESIGN 10. Surface surveying and other surface measure- Adequate planning is required before a specific ment technologies, such as seismic monitoring, re- landslide is instrumented. The plan should pro- mote sensing, and aerial photointerpretation, are ceed through the following stages: discussed in Chapters 8, 9, and 10, and so are also not treated in detail in this chapter. Determination of types of measurements re- Field instrumentation is most often used on quired; landslides that have already exhibited some move- Selection of the specific types of instruments best ment. In such cases, the overall characteristics of suited to make the required measurements; the landslide frequently may be readily observed Definition of the location and depth of instru- and noted. However, small movements of a soil or mentation and number of instruments; rock mass before or even during incipient failure are usually not visually evident. Thus, the value of Development of the data acquisition techniques; the information that can be obtained at the ground and surface is limited. Instrumentation can provide Decision as to the management and presenta- valuable information on incipient as well as fully tion of the data acquired. developed landslides. In this respect, use of instru- mentation is not intended to replace field observa- Initially the planning process requires develop- tions and other investigative procedures. Instead, ment of ideas on the causes of the landslide and the it augments other data by providing supplementary probable limits of the depth and outer boundaries information and by warning of impending major of the movements. Reconnaissance of the area, movements. Typical situations for which various study of the geology and groundwater seepage, re- instruments have been used are the following: view of rainfall records, and observation of topo- graphic features, especially recent topographic Determination of the depth and shape of the changes, will often provide clues. Unfortunately, no sliding mass in a developed landslide so that two landslides are alike in all details, and experi- calculations can be made to define the appro- ence alone without the application of judgment priate strength parameters at failure and design may lead to erroneous concepts. remedial treatments, An instrumentation system is a waste of time Determination of absolute lateral and vertical and money if the instruments do not extend below movements within a sliding mass, the zone of movement, were installed at the wrong Determination of the rate of sliding (velocity) locations, or are unsuitable. Instruments must col- and thus provision of a warning of impending lect data accurately, reliably, efficiently, and in a danger, timely manner; otherwise, corrective treatment Monitoring of the activity of marginally stable may be started too late, resulting in even greater natural or cut slopes and identification of effects economic losses than would have been experi- of construction activity or precipitation, enced without it. Monitoring of groundwater levels or pore pres- sures normally associated with landslide activity 2.1 Types of Measurements Required so that effective stress analyses may be performed, Provision of remote digital readout or informa- Landslides by definition involve movement. The tion to a remote alarm system that would warn magnitude, rate, and distribution of this movement of possible danger, and are generally the most important measurements re- Monitoring and evaluation of the effectiveness quired. Measurements of pore-water pressures of various control measures. within the landslide are important in resolving 280 Landslides: Investigation and Mitigation

many landslide problems. Such measurements are maintained at all times. High sensitivity is usually a especially important when landslides occur in lay- prerequisite when hazard warning is required or ered systems in which excess hydrostatic pressures landslide performance is monitored during con- may exist between layers. struction, since it is often the rate of change rather As discussed in Chapter 9, if the depth of slid- than the absolute value that provides the key to ing is readily apparent from visual observations, proper interpretation. surface measurements may be sufficient for obtain- The location of instruments requires a thor- ing the rate of movement. Surface measurements ough understanding of geologic and subsurface should extend beyond the uppermost limit of vi- conditions if meaningful data are to be obtained. sual movement so that any ground movements Location is particularly crucial for, pore-pressure that might occur before visible surface cracks de- recording devices that are intended to measure velop can be monitored. Movement of the ground pressures in specific zones of weakness or potential surface should be measured vertically and horizon- instability. Since most measurements are relative, tally at various locations within the landslide area. a stable base or datum must be provided so that Vertical offsets, widening of cracks, and heaving of absolute movements can be determined. the toe should be monitored. The direction of If the movement is known to be along a well- movement can often be inferred from the pattern defined shear surface, such as a bedding surface or of cracking, particularly by matching the irregular fracture, simple probe pipes will suffice to deter- edges of the cracks. mine the depth. If the movement is large and If the depth and thickness of the zone of move- rapid, accuracy is not an essential requirement and ment are not apparent, inclinometers or similar de- even relatively crude inclinometers may suffice. vices that can detect the movement with depth When the rate of movement is small and the depth must be used. Pore pressures at or near the sliding and distribution are not known, more precise in- surface must be measured to support effective stress strumentation is required. Carefully installed pre- analysis or to assess the adequacy of drainage mea- cision inclinometers are best in such instances, sures. Rapid-response piezometers are advanta- although there may be cases in which extensome- geous, particularly in impervious soils. ters or strain meters can be used to advantage. As discussed in Chapter 10, the Casagrande 2.2 Selection of Instrument Types type of piezometer is the most useful general pur- pose pore-pressure measuring device, but it may Many types and models of instruments are avail- give too slow an initial response in fine-grained able for theasuring the changing conditions in a soils. Therefore, pneumatic or vibrating-wire landslide; they vary in degree of sophistication, pressure-sensor types (transducers) may be prefer- particularly with regard to readout capabilities. able. High-air-entry, low-flow piezometers may Instruments have been developed to measure ver- warrant consideration in clays or clay-shales in tical and horizontal earth movements, pore-water which permeabilities are low or suction may be pressures, in situ stresses and strains, dynamic re- present because of unloading. sponses, and other parameters. However, in most landslides measurements of horizontal movement 3. SURFACE MEASUREMENT of the soil or rock and of pore-water pressures are of primary concern. Instruments commonly used Instrumentation that may be used for surface mea- for these purposes are described in this chapter. surement includes conventional surveying equip- The types of instruments, layout, and monitor- ment, portable tiltmeters, and recently developed ing schedules are usually determined by the specific differential Global Positioning System (DOPS) needs of a project. Several basic requirements, how- location methods. ever, should be thoroughly evaluated for any sys- tem. Instruments should be reliable, rugged, and 3.1 Conventional Surveying Equipment capable of functioning for long periods of time without repair or replacement. They must also be The application of conventional surveying meth- capable of responding rapidly and precisely to ods to landslide movement measurement and mon- changes so that a true picture of events can be itoring is discussed at length in Chapter 9. In an Field Instrumentation 281 active landslide area, surface movements are nor- trolytic sensors or servo-accelerometers. Electro- mally monitored to determine the extent of land- lytic sensors provide greater sensitivity, but servo- slide activity and the rate of movement. Both accelerometers have greater range. The prime optical instrument surveys and electronic distance advantages of these instruments are their light measurement (EDM) devices are used to determine weight, simple operation, compactness, and rela- lateral and vertical movements. Bench marks lo- tively [ow cost. Stationary electrolytic sensors with cated on stable ground provide the basis for deter- automatic data acquisition are gaining wider usage mination of subsequent movements of established in landslide monitoring. survey points, or hubs, by optical surveys, EDM equipment, or tape measurement. Recent advances 3.3 Differential Global Positioning in surveying equipment, including the so-called System total stations (see Chapter 9, Section 4.2.2.4), in- crease the automation and productivity of these The DGPS is emerging as a candidate surveying measurement procedures, especially on the often- method for monitoring surface movement at land- rough ground surface found on landslides. Survey slide sites. A base station at a known location is lines can be established so that vertical and hori- used to provide corrections and refinements to the zontal displacements at the center and toe of the computed locations of one or several roving sta- landslide can be observed. Lateral motions can he tions (Gilbert 1995). All stations use the same detected by theodolite and distance measurements global positioning satellite network. The DGPS from each huh. When a tension crack has opened relates observations at unknown roving station lo- above a landslide, simple daily measurements across cations to simultaneous observations at the base the crack can be made between two hubs driven stations placed at a known and fixed location. As into the ground. In many cases, the outer limit of the satellite signals are monitored, errors may sug- the ground movement is unknown, and establish- gest that the base station is moving. Since it is ing instrument setups on stable ground may be a known that the base station is not moving, these problem. fluctuations can be used as corrections to the ob- servations of the unknown roving stations to 3.2 Tiltmeters achieve their accurate location. All measurements are relative to the base station position. As long as Portable tiltmeters (Figure 11-1) can be used to de- that position is defined reasonably precisely, the tect tilt (rotation) of a surface point, but such de- other measurements will be internally consistent. vices have had relatively limited use. Their most If its location is very poorly defined, additional er- common application is to monitor slope move- rors may arise. An assumed value of the true loca- ments in open-pit mines and highway and railway tion (proper latitude and longitude) can be used cuts, but they may be used in any area where the without affecting the accuracy of the distances failure mode of a mass of soil or rock can be ex- pected to include a rotational component, such as within the survey. a landslide area. Tiltmeters utilize either elec- The accuracy of the DGPS is now approaching that of conventional surveys. Under favorable conditions, an accuracy of better than 1 cm is pos- sible (Kleusberg 1992; Gilbert 1995). The histori- cally large capital investments for such systems are now dropping (Gilbert 1994), and it appears that these systems already may be economical for mon- itoring larger landslide areas. However, the accu- racy of the DGPS may be degraded in areas where the ground surface is obscured by tree cover or in times of poor weather conditions. Nevertheless, FIGURE 11-1 the enhanced productivity of the DGPS approach Portable tiltmeter suggests that it will soon be economically practical (Wilson and for many landslide monitoring applications. Mikkelsen 1978). 282 Landslides: Investigation and Mitigation

4. GROUND-DISPLACEMENT widespread use as a monitoring instrument for MEASUREMENT dams, bulkheads, and other earth-retaining struc- tures and in various areas of research. Two broad 4.1 Inclinometers classes of inclinometers may be used: probe incli- nometers and in-place inclinometers. Both classes Before some of the sophisticated types of incli- use similar sensors. nometers are described, one of the simplest subsur- face measuring devices deserves mention: the 4.1.1 Inclinometer Sensors borehole probe pipe, also called the "slip indicator" or "poor boy." It typically consists of a 25-mm diame- One basic type of inclinometer sensor is currently ter semirigid plastic tube, which is inserted into a in use in the United States, the accelerometer borehole. Metal rods of increasing length are low- type, which uses a closed-loop, servo-accelerometer ered inside the tubing in turn, and the rod length circuit. Each sensor measures inclination in one that is just unable to pass a given depth gives a mea- plane, and the use of two sensors per unit (biaxial sure of the curvature of the tubing at that point. sensors) is common (Mikkelsen and Bestwick Use of the borehole probe pipe is an acceptable 1976). A few older, obsolete types are also still in technique for the location of slip surfaces in unsta- use but are gradually being replaced or abandoned ble slopes. This type of measurement can easily be because of lower accuracy and efficiency. Most in- performed in the riser pipe of an observation well or clinometers rely on commercially available, sensi- open piezometer. If there are several shear planes or tive servo-accelerometers. if the shear zone is thick, a section of rod hung on a The servo-accelerometer is composed of a pen- thin wire can be left at the bottom of the hole. dulous proof mass that is free to swing within a mag- Subsequently, the rod is pulled up to detect the netic field (Figure 11-2). The proof mass is provided lower limit of movement. with a coil or torque motor, which allows a linear FIGURE 11-2 The development of inclinometers has been the force to be applied to the proof mass in response to Servo-accelerometer for inclinometer most important contribution to the analysis and de- current passed through the coil. A special position measurements tection of landslide movements in the past two sensor detects movement of the proof mass from the (Wilson and decades. Although used most extensively to moni- vertical position. A signal is then generated and Mikkelsen 1978). tor landslides, the inclinometer has also gained converted by a restorer circuit, or servo, into a cur- rent through the coil, which balances the proof mass in its original position. In this manner, the current developed by the servo becomes an exact measure of the inertial force and thus of the trans- ducer's inclination. The proof mass usually consists of a flexure unit that operates on the cantilever principle. Temperature has only a negligible effect on readings taken with accelerometer transducers. Accelerometer systems have a resolution of ap- proximately 10 arc-sec in ranges from 30 to 50 de- grees. A typical example of such measurements is shown in Figure 11-3(b).

4.1.2 Probe Inclinometer

The probe inclinometer commonly used today was developed from a device built in 1952 by S.D. Wil- son at Harvard University (Green and Mikkelsen 1988). The same basic concepts have been incor- porated into instruments now manufactured by sev- eral firms. Such an inclinometer consists of a probe containing a pendulum-actuated transducer, which SLOPE BEBRIS INCLINOMETER 5-4 2100 a: /'PPROX LIMIT OF CE I OIU1VILE SWAK7 INCLINOMETER S-S ,—INCLINOMETER $-a I CRACKS BENIND TR 8 TRACKS / E LIMIT OF HUMMOCKY 285O z z AREA AT TOE OF SLOW GROUNDWAT ER SURFACE 0 0 BENI"BE_ I— \ > <8 w >w _j // —J -- \ Lu w 2800 :

7r HARD CLAY SHALE. KNOWN ZONE OF MOVEMENT KNOWN ZONE OF MOVEMENT DEPTH OF 26 m (85 2550

LATERAL MOVEMENT (mm) -20 -4U 0 127 25.4 38.1 77777777 77777 GROUND GROUND SURFACE SURFACE

O , 110

2( E

SURFACE OF HARO CLAY SHALE

2 41

I.1 51 '.1

NORTH SOUTH NORTH SOUTH 6 IL- -n -20 -4 00 0.5 1.0 1.5 CHANGE (INSTRUMENT DIAL UNITS) LATERAL MOVEMENT (in)

FIGURE 11-3 Inclinometer location and data, Fort Benton landslide, Montana (Wilson and Mikkelsen 1978): (a) section through Fort Benton landslide; (b) movement of inclinometer S-5 at toe of slope. 284 Landslides: Investigation and Mitigation

is fitted with wheels and lowered by an electrical lowing the oriented, longitudinal slots of the cas- cable down a plastic casing that is grooved to con- ing. Two sets of grooves in the casing allow the in- trol alignment. The cable is connected to a readout clinometer probe to be oriented in either of two unit, and data can be recorded manually or auto- planes set at 90 degrees to each other. At each in- matically (Figure 11-4). stallation one set of grooves is designated the A ori- A probe inclinometer system has four main entation plane, and the other set the B orientation components: plane. The response to slope changes in the casing is monitored and recorded at the surface. Readings A guide casing is permanently installed in a are taken at fixed increments, usually equal to the near-vertical borehole in the ground. The cas- length of the probe, throughout the entire depth. ing may be made of plastic, steel, or aluminum. The wheels of the inclinometer probe provide Circular sections generally have longitudinal points of measurement between which the incli- slots or grooves for orientation of the sensor nation of the instrument is measured. If the read- unit, but square sections are used in some types. ing interval is greater or less than the probe wheel A probe sensor unit is commonly mounted in base, correspondence between the position and a carriage designed for operation in the guide orientation of the instrument will be only approx- casing. imate in determining the total lateral displace- A control cable raises and lowers the sensor ment profile. Optimum accuracy is achieved only unit in the casing and transmits electrical sig- if the distance between each reading interval nals to the surface. For accurate depth control equals the distance between the upper and the of the sensor unit, the cable is usually graduated lower wheels of the probe. Gould and Dunnicliff or lowered on a separate surveyor's steel tape. (1971) reported that readings at depth intervals as A portable control and readout unit at the sur- large as 1.5 m result in poor accuracy. face supplies power, receives electric signals, Instruments differ primarily in the type of sen- displays readings, and can often store and pro- sor used, the accuracy with which the sensor de- cess data. tects the inclination, and the method of alignment and depth control within the borehole. Probe in- The probe inclinometer instrument sensor unit clinometers can measure changes in inclination on (Figures 11-4 and 11-5) is lowered and raised on an the order of 1.3 to 2.5 mm over a 33-rn length of accurately marked cable, its wheels or guides fol- casing (a precision of 1:10,000).

Pulley Assembly

Cable Total Displacement ELsin&e

Control & Sensor Displacement Readout Lsin38 Unit Drill ./ True Vertical Hole or initial Distance between "( 68 Profile Successive Readings'Z Coupling

FIGURE 11-4 Casing Casing Principle of Groove inclinometer operation 14 Backfill Guide (Wilson and Wheels Mikkelsen 1978; Green and Mikkelsen 1988). Field Instrumentation 285

one borehole. The maximum deflection range is FIGURE 11-5 (left) ±30 degrees; the precision is reported to be ±0.01 Probe inclinometer mm in 1 m, but this accuracy is not usually at- with manual readout, inclinometer casing, tamed because of long-term drift. A more realistic and couplings maximum accuracy is about 0.04 mm in 1 m, giv- (Green and ing a precision of 1:25,000, or about 2.5 times the Mikkelsen 1988). precision of a probe inclinometer. Monitor and alarm consoles and telemetry systems are available FIGURE 11-6 (below) for use with in-place inclinometers. Installation and detail of multiposition Since fixed units often consist of a number of in-place inclinometer sensors, they are more expensive and complex than (Wilson and Mikkelsen 1978).

8.4mm (3.34 in) O.D. ABS PLASTIC INCLINOMETER CASING JUNCTION ,BOX

,. FIXED SPRING WHEEL LOADED TOP WHEEL COR JDUMMY -d HOuSING),

WIRINGTO :'"' 4.1.3 In-Place Inclinometer ELECTRICAL TRUE HARNESS . The in-place inclinometer (Figure 11-6) employs 4 a series of servo-accelerometer or electrolytic sen- sors available for both one- and two-axis measure- ment (Cording et al. 1975). The sealed sensor SPRING z, ., SENSOR LOADED ; packages are spaced along a standard grooved in- HOUSING WHEEL clinometer casing by a series of rods. The rods and sensors are linked by universal joints so that they SENSOR - / GAUGE can deflect freely as the soil and the casing move. LENGTH The sensors are aligned and secured in the casing (ADJUSTABLE UPON - by spring-loaded wheels, which fit the casing / INSTALLATION) UNIVERSAL grooves. They are positioned at intervals along the JOINT SENSOR borehole axis and may he more closely spaced to ASSEMBLIES increase resolution of the displacement profile in TUBE zones of expected movement. Use of the grooved MIN 115-mm casing and guide wheels allows removal of the in- (4.5-in) ... strument for maintenance, adjustments, and sal- BORING PORTLAND CEMENT vage. Since the casing is a standard inclinometer GROUT BACKFILL-. casing, a probe inclinometer can be used in it after GROOVED PLASTIC CASING the fixed unit has been removed. CAST IN PLACE OR GROUTED I Readings are obtained by determining the IN PLACE IN BOREHOLE,,,, change in tilt of the sensor and multiplying by the gauge length or spacing between sensors. The re- sult is the relative displacement of each sensor; these relative displacements can be summed to de- terinine the total displacement at each sensor. Any number of sensors at any spacing can be used in 286 Landslides: Investigation and Mitigation

probe inclinometers. However, in-place inclinorn- tended to obtain a continuous profile of deforma- eters have major advantages over probe inclinome- tion. Rather, they measure critical movements of a ters, including enhanced accuracy and continuous few sections within the borehole. This instrument, recording capabilities. Fixed units can be moni- therefore, is not necessarily a replacement for the tored remotely by a datalogger or connected to probe inclinometer unit, but rather has its own alarm systems. Problems common to probe inch- purpose and is worthy of consideration for contin- nometers, such as tracking inaccuracies and repeat- uous monitoring of movements during critical able positioning (placement error), are eliminated. stages of the landslide. If the fixed unit is removed for any reason, the overall accuracy will, for the duration, be reduced 4.1.4 Principle of Operation to no more than that of a probe unit, which can be The primary application of the inclinometer to qsçd in its place. With probe inclinometer units, landslides is readily apparent: the definition of slip seiisor drift effects are canceled by taking readings surfaces or zones of movement relative to stable in opposite directions. Fixed accelerometer sensors zones by detection of any change in inclination of may also drift, and such effects cannot be so easily the casing from its original near-vertical position. compensated for. Better long-term prformance Readings taken at regular incremental depths in- and resolution are gained by using electrolytic sen- side the casing allow determination of the change sor technology. On some projects, an extra inèli- in slope at various points; integration of the slope nometer casing is used so that a probe inclinometer changes between any two points yields the relative can be used to check readings of the fixed unit. deflection between those points. The integration is Because of its potential accuracy (an achievable normally performed from the bottom of the boring precision of 1:25,000), an in-place inclinometer because the bottom is assumed to be fixed in posi- can be used to measure small movements in rock tion and inclination. Repeating such measure- and to measure velocity in the hourly time do- ments periodically provides data allowing the main. Its adaptability to various remote-monitor- distribution of lateral movements to be determined ing systems allows a continuous record of as a function of depth below the ground surface and displacement. By design, most systems are not in- as a function of time (Figure 1 17).

CHANGE IN INCLINATION (sin4) MOVEMENT(cm) 0.03 006 0.09 2.5 5 7.5 o I I I Ii 1.

2

3.

'5 - I-- CD ('4 FIGURE 11-7 Measurement of landslide 7 9/11 movements with 9/29 inclinometer: (a) B observational data, 9 Well A, and /27 (b) computed movements, Well A (a) 10 (b) (Wilson and Note: 1 m = 3.3 ft; 1 cm = 0.4 in. Mikkelsen 1978). Field Instrumentation 287

Inclinometers measure the slope of the casing in two mutually perpendicular planes, designated A 0.40 INCLINOMETER CASING GROUTED IN PLACE and B. Thus, horizontal components of movement, 0.35 • A DIRECTION B DIRECTION both transverse and parallel to any assumed direc- DISPLACEMENT BETWEEN 31 & 33 tion of sliding, can be computed from the inch- 0.30 FOOT DEPTH INTERVALS nomter measurements (Figure 11-8). Deviations 025 .... • 0.12 INCHES/YEAR from this ideal occur because of limitations and I 0z compromises in manufacturing and because of in- 0.20 SUBSTITUTE INSTRUMENT stallation circumstances. Instruments of different 0 DOWN • 0.15 SLOPE FUSED manufacture cannot generally be interchanged w II • 8 8 without causing discernible effects on .the results 0.10 0.03 INCHES/YEAR PP - 00 obtained [Figure 11-8(a)]. In the interest of accu- ww 0.05 8 racy, the interchange of instruments (probes and no _ 6_a cables) of the same manufacture should also be 0.0 avoided. INSTALLED 4-10-76 liii 11 liii liii 1111111 lilillililI ill II III Although the principle of operation has re- 1976 I 1977 I 1978 I 1979 mained unchanged, inclinometers have under- (a) TIME IN CALENDAR MONTHS gone rapid development to improve reliability, provide accuracy, reduce weight and bulk of the instrument, lessen data acquisition and reduction EICLINOMETER OHS-i LATERAL MOVEMENT time, and improve versatility of operations under 104 FT. 10 122 FT. DEPTH adverse conditions. Use of automatic data record- NORTHERLY 1 MEASUREMENTS OSCONTINUED ing, personal computers, and data-base software DEFORMATION DUE TO BLOCKAGE IN CASING

has become standard practice. /EASTERLYt- DEMAON INCLINOUE DHS-2 I LATERAL MOVEM it

4.1.5 Casing Installation IORTVERLY DEFORMATION _....,.:_:.o_: In landslide applications, inclinometer casings are ' EASTERLY DEFORMATION normally installed in exploration drill holes. They I9IIIl7mI1971I1072 I 19" I I74 I 1075 I 19m must extend through soil and rock suspected of TIME IN YEARS movement and well into materials that, in the best (b) judgment at the time, are assumed to be stable. Some installations extend to depths of more than 200 m (Imne and Boume 1981). The annular space tom must be extended about 6 m or more into ma- FIGURE 11-8 between the drill hole and the outer wall of the cas- terial that will not undergo lateral displacement. If Typical landslide ing is generally grouted or backfilled with sand. The any doubt arises as to the stability of the casing deformation data from inclinometers success of the casing installation depends on the ex- bottom, movement of the casing top should be (Green and perience and the skill of the personnel; soil, rock, checked by precise surveying methods. The casing Mikkelsen 1988): and groundwater conditions; depth of installation; for the inclinometer is usually installed in 1.5- and (a) creep and accessibility to the area. 3.0-rn lengths, which are joined with couplings movements on The accuracy of the inclinometer observations that are either riveted or cemented, or both, to en- thin shear zone in is generally limited not by the sensitivity of the in- sure a firm connection (Figure 11-9). Each cou- reactivated ancient clinometer sensor, but by the requirement that phing represents a possible source of entry for grout landslide; (b) successive readings be made with the same orien- or mud, which can seep into the casing and be de-- deformations measured on deep tation of the instrument at the same point in the posited along the internal tracking system. For this shear zones on a casing. Because the casing must provide a reliable reason, couplings should be sealed with tape or landslide in Canada. orientation and a continuous tracking for the sen- glue. The bottom section of the casing is closed sor unit, proper casing installation is of paramount with an aluminum, plastic, or wooden plug, which importance. also should be sealed. If the drill hole is filled with Since the displacements are referenced with re- water or drilling mud, the inclinometer casing spect to the bottom of the casing, the casing hot- must be filled with water to overcome buoyancy. 288 Landslides: Investigation and Mitigation

FIGURE 11-9 Details of OUTSIDE OF COUPLING inclinometer casing (Wilson and Mikkelsen 1978).

ALUMINUM ABS PLASTIC

GROOVES TO ALIGN INCLINOMETER SENSOR

152mm (6 in) "s— ALUMINUM CASING ALUMINUM COUPLING BUTTJOINTAT -. CASING END - I "POPRIVETS 305 mm(12 in) . AT END OF ALUMINUM COUPLING COUPLING P0 INT OF CO NTACT 1 --FOR SETTLEMENT STANDARD 152mm (6 in) J PROBE WHEN CASING I MAKING SETTLEMENT LENGTHS MEASUREMENTS 1.5m(5ft) ALUMINUM or3m(lOft) CASING END :.: CASING .11L ALUMINUM - rn . BOTTOM PLUG POP RIVETSAT :... END OF COUPLING ______-POP RIVETS AThPOINTS

Sometimes, extra weight, such as sand bags or a material. Grouting is generally preferred but may drill stem, may be needed. not always be possible, particularly in pervious ge- The annular space between the boring wall and ologic materials such as talus. Grouting may be fa- the inclinometer casing may be backfilled with cilitated by use of a plastic tube with a diameter of sand, pea gravel, or grout. The selection of backfill 20 to 25 mm firmly attached to the casing bottom depends mostly on soil, rock, and groundwater through which grout is pumped from the ground conditions (i.e., whether the borehole is dry, wet, surface until the entire hole is filled. In a small- caving, or stable without mud). The type of clearance drill hole, grout can be placed (tremied) drilling technique (e.g., rotary, hollow-stem, cased through drill rods via a one-way valve at the inside holes) is also important. Poorly backfilled casings bottom of the casing. can introduce scatter into initial measurements. If In the as-manufactured casing, the grooves may maximum precision is required, great care should have some spiral. During installation, the casing be taken in the selection of the proper backfihling can become even more twisted or spiraled so that, Field Instrumentation 289 at some depth, the casing grooves may not have servations is reliable. Measurements of the origi- the same orientation as at the ground surface. nal profile should be established by a minimum of Spiraling as great as 18 degrees in a 24-m section a double set of data. If any set of readings deviates of plastic casing has been reported (Green 1974); 1 from the previous or anticipated pattern, the in- degree per 3-rn section is not uncommon. Because clinometer should be checked and the readings re- significant error in the assumed direction of move- peated. A successful record of measurements ment may result, deep inclinometer casings should generally requires at least two trained technicians, be checked after installation by using spiral indica- who should be allowed the proper amount of time tors available from the inclinometer manufacturer. for setting up, recording observations, and under- For a particular installation, groove spiraling is taking maintenance of the equipment. Use of the generally a systematic error occurring for each sec- same technicians and instrument for all measure- tion in the same amount and the same direction. ments on a particular project is highly desirable. In any event, spiraling, whether it is due to manu- When an inclinometer installation is surveyed facture or to torque of the casing during installa- for the first time with a probe inclinometer to pro- tion, can be measured and is thus not really an vide the initial set of data, a fixed reference for the error in inclinometer measurement. Spiraling is probe should be selected so that each time a survey important only when a determination of the true is repeated the same set of grooves is used for align- direction of the movement is required; in such ment orientation and the probe will have the same cases, the amount of spiraling should be measured orientation in the casing. For the typical biaxial in- and the results adjusted accordingly. clinometer (Figure 11-5), it is generally recom- Small irregularities in the tracking surface of mended that the A component (or sensor) be the casing can lead to errors in observation, espe-. oriented so that it will register the principal di- cially if careful repetition of the depth to each pre- rection of anticipated deformation as a positive vious reading is not exercised. If plastic casing has change. For example, in an area suspected of land- been stored in the sun before installation, each slide activity, the first set of readings is taken by ori- section may be locally warped (so that opposing enting the fixed wheels of the probe in the casing groove closest to the downhill position. grooves are not parallel), and large measurement Probe inclinometer measurements generally are errors can occur with minor variations at the recorded as the algebraic sums or differences of pairs depths at which the readings are taken (Gould of readings. At selected measurement depths, a se- and Dunnicliff 1971). ries of readings in one vertical plane are taken and Aluminum casings should be used with some then repeated with the instrument turned through caution because severe corrosion may occur if the 180 degrees. Computing the differences of these casing is exposed to alkaline soil, corrosive ground- readings minimizes errors contributed by irregulari- water, or grout. Epoxy coatings help to minimize ties in the casing and instrument calibration. One this problem. Burland and Moore (1974) recom- excellent check on the reliability of each measure- mended cleaning the casing with a stiff brush before ment is to compute the algebraic sum of each pair taking readings. Frequent flushing with water is also of readings. These algebraic sums should be ap- helpful. The top of the casing is generally capped proximately constant except for those readings with a tight-fitting plug to prevent intrusion of de- made while one set of wheels is influenced by a cas- bris. In addition, the inclinometer casing should be ing joint. Differences between any sum of two read- protected and padlocked at the ground surface. ings and the observed constant usually indicate that When a protective casing or monument covers are an error has occurred or that opposite sides of the selected for use around the top, the need to place a casing wall are not parallel. This check allows er- casing-mounted pulley or similar device during in- rors resulting from mistaken transcription, faulty clinometer measurements should be kept in mind. equipment, or improper technique to be identified. Thus, the algebraic sum of pairs of readings should 4.1.6 Observations be computed on site in the field, so that corrective actions may be taken to resolve transcription errors. Because all inclinometer readings are referenced However, if the algebraic sum does not remain to an original set of measurements, extreme care nearly constant, the sensor unit, readout, and cas- must be taken to ensure that the initial set of ob- ing should be rechecked before further use. 290 Landslides: Investigation and Mitigation

4.1.7 Data Reduction the accumulation of field readings, the data reduc- tion for a particular casing must be performed with Manual computation and reduction of inclinome- the knowledge of both former movement and ex- ter data is a tedious and time-consuming operation. pected ground behavior. A single movement profile for a 33-rn casing can Manual data recording is adequate for many involve more than 200 separate computations. small projects. A special data sheet is recom- Because of the amount of effort involved in taking mended, which can be used to transcribe the data readings and subsequently computing displace- into a computer. Once this is done, the computer ments, a successful measurement program depends can handle the data with any desired degree of so- primarily on organization and discipline. Field phistication. It must be recognized, however, that readings must be transposed to discernible mea- in addition to the time delay, this method is subject surement, preferably in the form of summary plots to two potential sources of error. The first is in FIGURE 11-10 indicating successive movement profiles, as soon as recording the data manually, and the second is in Simple field sheet possible after the field observation. Successive transcribing the data. Figure 11-10 shows a special formerly used to movement profiles also provide perhaps the best field data sheet formerly used for recording incli- transcribe check of instrument reliability. If a record of suc- inclinometer data nometer data that were subsequently transcribed for cessive movement profiles is established, the con- before tabulation on sistency of new measurements can be referenced to computer processing. Major improvements have punched cards been made in inclinometer data acquisition and (Green and previous readings and judged in light of anticipated Mikkelsen 1988). materials behavior. Because of occasional error in processing during the past few years. Personal computers may be used as data collec- tion systems that eliminate hand recording of data and prompt the observer to fill in the right data. SHEE1 Ciflithea,,. a,..., IIWI.a U8A Most inclinometer manufacturers now provide units that can support automated data collection. (VJO8o u_)JOBTrTLE (IMYtXTtNQT*uI.4O) M0 These units can record, store, file, and reduce in- l.-.t. . 17. . Iii .o.C.T. ,.7.4II.3. .0r . .2.0..I clinometer data as collection proceeds in the field, ®HcLE No @SET No()DATE ®TIME ®STAT INT including the correction of systematic errors and Ig.3.i .3.8 I.'Z.o.o. .1 . .3.1 I ®INS C NoREAD6REV 1R-SCALE ©D-SCALE 6ERRDI1 A ®ERROR B incorporation of information defining the spiral- ing of the casing. One commonly used product is lg.o.o,o,o,. • .4. I . . .A.2.• . . • B.I . . IIS'L ClST. ®DIR. A+ ©DIR. A- ®DIR. 8+ (?)DIR. B- the Digitilt recorder, processor, and printer (RPP) I II I I I unit, shown in Figure 11-11. This unit can tabu- A-LAD.L ® late results, graph them on a built-in electrostatic DEPTH DIR. A+ DIR. A- DIR. 8+ DIR. 8- printer, and store the data on magnetic tape. The 2.0 , . ,/,2. RPP can also operate as a computer terminal and 4.0 2.7 -.2.5 . . .2.0. send data via telephone modem to a remote com- . Z4 , -.26 . , .2.3.5 .2.6 . -'2.5 . .1.9.5 . puter for additional processing and plotting. /2. . , .4 . -.2.4 . .1.5.5 . .-,I '.4.0 . . 2,3 A . -.2.3 .1.3.0 . Software for reducing and plotting the results has also greatly improved and now provides rapid, .2,0.0 , . 2,4, , , ...,2,5. .4 _3? simple operations (Figure 11-12). Typically such DIDITLT SE .2,2. , . ,2,(, , -.2,6 ,...,5 ,...-4 MDCCI. 50325 . . .2,4.0 . . .3,1. 3,2.0 . , ,j,4 . , software includes the following features: 24.0 ,_. .3,4. , ,-,3,1. .. .—.I .9 , • .6 .2.5." . , .3, . ,-.3.5 . -.4.4' 3.0.0 . , 2,9. , ,-,2,S5 , , .6.5 UPPEAW 2 , . .3.2.0 . • ,q,, , ..,,,.6 Utilization of simple puildown menus so that -a .o.7 there is very little, if any, need to refer to the .4.o. ._,.2.6 instruction manual; flU . . .4.2.0 . . .1.9- .-.t.9- . lAS , .-.I .5.3 ,.8. .-.1.5. . . • a.Q.0 Allowance for all data sets for each inclinome- .4.6.0 . . /9,0 . ,-,1.9,0 . ,1.7. . ter, representing readings collected at different POI.AITY .4,5.0 . . ,j,i,t' , ,.-,i,7,/ . 4,1 , ILl ANGLJ times, to be stored in a unique data file; and Generation of multiple plots by many types of printers, with color plots possible if appropriate equipment is available. Field Instrumentation 291

These problems are common to all inclinometers; FIGURE 11-11 therefore, they are discussed in some detail so that, Digitilt inclinometer even without a detailed understanding of the used with recorder, processor, and inner workings of the instruments, these anom- printer (RPP) unit alies, the limits of accuracy, and the meaning of (Green and the results can be recognized. In simple terms, the Mikkelsen 1988). person closest to the job should be able to answer the following basic questions:

Is the landslide active? How fast is the landslide moving? How deep is the landslide?

By means of repeated surveys, inclinometers measure changes in the orientation of a borehole over certain depth intervals during a period of time. A probe inclinometer records this change in orientation at any depth within the limitations of the cable on which the probe is lowered (i.e., weight, strength, and elongation characteristics). Once the active zone has been detected from suc- cessive sets of data, the rate of deformation can be Some recently developed software systems determined by plotting the change versus time. incorporate data-base building and management Usually, the major zone of movement is only a few capabilities. Some software allows linkage of incli- meters thick; hence, the sum of the changes over a nometer data with other data automatically col- few consecutive intervals will often be representa- lected from other instruments and telemetered tive of the magnitude and rate of movement of the from the landslide site. Such software allows the entire landslide. investigator to readily make a coherent assessment The most commonly encountered errors that of multiple data sources. Routines are included in affect the quality of inclinometer data are the so- many systems for performing time-series analyses, called "zero-shift" error and rotation errors due correcting systematic errors, and producing illus- to spiraling of the casing (Figure 11-13). When trations suitable for inclusion in reports. readings are obtained in a slightly inclined When combined with the new data acquisition casing, a correction for the apparent drift in capabilities, such computer programs can in just a the servoaccelerometer readings is necessary. few minutes complete the data analysis, a process Rotation produces a more irregular pattern in the that previously required hours. Thus, more time is displacement-versus-depth profile. Both of these made available to analyze and make systematic effects may mask shear movements occurring at error corrections to the data. Since the mid-1970s, discrete zones and should therefore be evaluated it has been possible to make systematic error correc- and compensated for during data processing. tions to inclinometer data. Because of the speed and To obtain the greatest observation accuracy, the convenience of current software, corrections for original installation should be as close to vertical as sensor drift and sensor azimuth alignment changes practicable. The error in inclinometer surveys is caused by rotation of the casing can be easily made proportional to the product of the casing inclina- to improve the results of inclinometer surveys. tion and angular changes in the sensor alignment. As shown in Figure 11-14, angular changes in sen- sor alignment on the order of 1 degree within in- 4.1.8 Evaluation and Interpretation clined casings, corresponding to a sensor rotation of Data value (sin a) of about 0.02, may produce errors of All too often, confusing or unexplainable results several centimeters per 30 in of casing. Sensor- appear because of instrumentation problems. alignment change occurs from time to time because 292 Landslides: Investigation and Mitigation

GEO-8.QPE I P C - S L I N ISEO-8_OPE P C - S L I N 80526 ----- 00526-- DEICIBIARK EXAI5t.E NO. I seQlAMc EXAMA_E NO I DIBITILT DATA DIGITILT 0414 HOLEIASIBER: 0-1 HOLE lENDER. 0-1 PASTFILE NAME:A:SII.SN P451 FILE N€:A:51l.ER PRESENT FILE NAIE:A:SIi1.SN PRESENT FILE NPNEA:SIII.SN PAST SET NUMBER: 1 PRESENT SET NUMBER: I PAST SET 11.91585, 1 PRESENT SET 14211585, 1 PAST DATE: PAST DATE:T 20/73. PRESENT DATE,OCT. 11/76 T 20/73 PRESENT DATE: . T. * 1/76 a- a. a- A. DSA043E IN READIN8 DEFLECTION - INDIES -900. -600. -300. 0. 300. 600. 900. -2.250 -1.500 -.750 .000 750 1.500 2.250 I * I S I 0 I S II 12.0 11.0 $ *1 I S I S I S I 0 II 22.0 21.0 5 I 0 I S

32.0 31.0

1-1

II *1 41.0 SI II SI 01 DI 92.0 II 51.0 SI 0! SI 'I *1 SI II 62.0 SI 61.0 01 *1 SI SI SI SI 12.0 51 71.0 SI (a) SI

SEQ-SLOPE PC-SLIN I of one or several factors, such as wheel play in the SERIAL NO. 85019 HOLE MD1DER: S-I groove, wear of the sensor carriage (particularly PAST FILE NAME: A:BNI.SI PAST DATE: NOV. 16/82 wheel assemblies), internal changes in the sensor FILE NAME: A:8N2.SI DATE: PlAY. 18/83 0 FILE NAME: A:8N3.5I DATE: JUNE. 14/83 x itself, and changesin the alignment between sen- FILE NAME: A:0N4.9I DATE: JULY 11/83 * sor and carriage. FILE NAME A:0N5.SI DATE: AUG.22/93 - FILE NAME: A:EN6.S1 DATE: SEPT 27/83 Accuracy of deflection-versus-depth plots is

A- 4* usually discussed in terms of the repeatability of an DEFLECTION - INCHES integrated curve of deformation for a depth increment of 33 m. However, on deeper installa- tions, this may be misleading to the interpreter. Although the instrument may be operating within its range of accuracy, over a period of time it may suggest tilting of several centimeters back and forth and perhaps the beginning of a small kink some- where in the curve. This situation is somewhat similar to that with an open-ended traverse. The primary concern should lie with the developing kink in the curve rather than with the overall tilt, which is often related to instrument error at the time. of measurement. Again, it must be remem- bered that the greatest accuracy of the inclinome- ter lies in its ability to measure change in inclination at a speciflcdepth rather than to survey an exact displacement profile. '- F 4.1.9 Maintenance FIGURE 11-12 Graphical computer output of inclinometer data: (a) single data set, (b) Inclinometers are special instruments; conse- multiple data set (Green and Mikkelsen 1988). quently, the number of suppliers is limited. Repair Field Instrumentation 293 or replacement of an inclinometer can be expen- sive and time-consuming and can result in loss of NO CORRECTION WITH ZERO SHIFT CORRECTION CHANGE DEFLECTION CHANGE DEFLECTION important data. The best insurance against dam- age is careful use and systematic maintenance. The sensor unit should be checked frequently dur- ing operation and its wheel fixtures and bearings tightened and replaced as necessary. After each casing has been read, the guide wheels should be cleaned and oiled. Most important, the electric readout should be protected against water at all FF' F times. The introduction of a few drops of water into the readout circuitry can cause the gal- NO CORRECTION CASING PROFILE, B-AXIS CORRECTED vanometer in a pendulum inclinometer to drift or DEFLECTION, A-AXIS (CROSS-AXIS INCLINATION) DEFLECTION. A-AXIS can induce variations in the numbers on the digi- cc tal voltage display of an accelerometer instrument. cc cr If readings are made too rapidly or if an automatic recorder is used, these variations or drift may not be detected and erroneous measurements may re- sult. When the data are subsequently analyzed, the cause of such results cannot be identified.

4.2 Extensometers 4.2.1 Strain Meters FIGURE 11-13 Extensometers and strain meters measure the in- Typical zero-shift crease or decrease in the length of a wire or rod con- When the above devices are used as strain meters, and rotation errors necting two points that are anchored in the the repeatability and accuracy are essentially the (Green and borehole and whose distance apart is approximately same as the sensitivity. Thus, for a grouted-in- Mikkelsen 1988). known. One commercially available device is place 3-rn-long extensometer assembly composed shown in Figure 11-15. When gauge lengths are on of an invar rod with a range of 25 mm, unit strains the order of a meter or less, these devices are often as low as 0.0001 may be detected with relatively referred to as strain meters rather than as exten- inexpensive instrumentation. Horizontal stretch- someters. When they are used as extensometers, ing of embankments has been observed by in- measurement accuracy and repeatability depend on stalling anchors at various positions at a given the type of sensing element and its range of travel elevation and attaching horizontal wires to dead- and also on the type of connecting wire or rod and weights on the downstream face, as was done at the methods used to control the tension. Dead- Oroville Dam (Wilson 1967). Care is required to weights are best for maintaining constant tension in ensure that the wires (or rods) do not get pinched wires; if deadweights cannot be used, constant ten- if localized vertical shear movements occur. Other sion springs are acceptable, although there may be applications are described by Dutro and Dickinson some hysteresis. The simplest measurements involve (1974), Heinz (1975), and Gillon et al. (1992). manual readings using a scale or micrometer. Alter- natively, linear displacement transducers (potentio- 4.2.2 Rod and Wire Extensometers meters or vibrating wire) are often used as sensors. These transducers can be monitored by connecting Wire extensometers, especially those with long dis- and reading measurements obtained with relatively tances between the anchor and the sensor, are par- simple, battery-operated voltmeters. Sensitivity is ticularly responsive to changes in wire tension on the order of 0.1 percent of the range of travel, but caused by friction along the wire and to hysteresis repeatability and accuracy may be no better than effects in the sensor or constant tension spring. 0.55 mm, depending on the type of anchor and con- The sensitivity of these instruments is controlled necting member. Nevertheless, these parameters are by these factors. For example, a typical installation usually sufficient for landslide applications. might be constructed with a 16-gauge stainless- 294 Landslides: Investigation and Mitigation

FIGURE 11-14 CASING DRIFT (m) should be given to the rate of change in length, be- Measurement error cause any increase in this rate may be an indication 0.0• 40 as a result of casing of impending failure. inclination and Extensometer readings are also especially sensi- sensor rotation tive to temperature changes. If the connecting rods (Wilson and or wires are made of steel, any increase in temper- Mikkelsen 1978). 0.0 II...uI ature will result in an increase in their length and thus a reduction in the actual extensometer read- ing. For example, when the extensometer de- 3° scribed in the previous paragraph is subjected to an 0.0 average change in temperature of 12°C, the same 11111 wire will change in length by 3.0 mm. This change in length is about 20 times greater than that de- tected by the basic sensitivity of the instrument. Daily and seasonal temperature variations are likely to show similar variations in the gauge read- ings. Not only does the length of the wire change 20 with temperature, but the ground itself expands and contracts as its temperature varies. At a hill- cr 00.0 side stability project in Montana, rock outcrops Itil were found to expand and contract seasonally by as much as 1.5 mm. Electric lead wires also change their electric resistance with temperature and will cause erroneous readings unless such a change is 0. RV properly taken into account.

4.2.3 Slope Extensometers

0.0 When landslide movements are concentrated along relatively thin shear zones, inclinometers are only capable of measuring movements that are less than 2 to 5 cm. Larger movements exceed the diameter of the inclinometer casing, and the inclinometer 0.0 00 0 becomes useless owing to the inability to access the CASING DRIFT (in) probe. In such cases, extensometers involving ca- bles and anchors provide a means for continuing NOTE: CASING DRIFT OVER ANY DEPTH the monitoring of the movement. Sometimes ex- INCREMENT IS MEASURED AS HORIZONTAL tensometer cables and anchors can be installed DEVIATION FROM TRUE VERTICAL AXIS IN within the damaged inclinometer casing. TWO MUTUALLY PERPENDICULAR PLANES. DEFLECTION ERROR IN ONE PLANE IS RESULT Extensometer cables do not provide accurate OF DRIFT IN OTHER PERPENDICULAR PLANE. measurements until the displacements are greater than the thickness of the shear zone. Before the inclinometer casing becomes crushed, a special steel wire 33 m long that is subjected to a pull of detailed survey of the shear zone with the incli- 67 N. If the sensor requires 1.11 N to actuate it, the nometer probe may be able to determine the minimum change in length of the wire that can be thickness of the shear zone, and future slope ex- detected by the sensor is 0.15 mm, which is the ef- tensometer results can be corrected accordingly. fective sensitivity of the extensometer. Up-to-date Alternatively, a completely new subhorizontal ex- plots should be kept of changes in length from tensometer can be installed across the shear zone each anchor to the sensor and of computed to provide maximum sensitivity and minimize changes between anchors. Particular attention measurement error. Field Instrumentation 295

FIGURE 11-15 Extensometer (Wilson and Mikkelsen 1978).

Two types of special extensometers that de- displacements of pipe sections and kept the inside serve some mention are designed to measure large of the pipe free of debris. A 30-m extensometer was movements. The first was invented in Europe in established in the first section of the line by an- the 1970s for monitoring gross landslide and glac- choring the upper end of the surveying tape to an ier movements. It was adopted for use in the eyebolt installed in the cliff face. This extensome- United States to monitor a natural gas pipeline ter was tensioned by attaching a weight suspended that moved several feet where it crossed a land- in a vertical pipe to the lower end of the surveying slide at Douglas Pass, northwestern Colorado. tape by means of a pulley system installed at the top The second type, a gravity-tensioned slope of the vertical pipe. Two other extensometers were extensometer, was developed by the Bureau of installed in the lower two sections of the line. They Reclamation for use on the Big Bull Elk landslide were constructed in a similar fashion to the upper- in Montana (L.R. Carpenter, personal communi- most extensometer. The upper ends of each of cation, 1995, Bureau of Reclamation, U.S. Depart- these extensometers were anchored by attaching ment of the Interior). The site is located on the their surveying tape to the axle of the pulley system Bighorn Reservoir 16 km upstream from Yellowtail of the weight-tensioning system of the preceding Dam and 72 km southeast of Billings. During ini- extensometer. slumped tial filling of the reservoir, the landslide The displacement of each of these extensome- 3 to 6 m at its contact with a limestone cliff. Mea- ters is determined by monitoring the distance be- surement points were installed along two lines es- tween the lower end of the tape and the position of tablished on the landslide. These lines were the lower end of the protecting pipe. This distance labeled A and B. Manual readings are taken annu- is measured by attaching the body of a linear po- ally between a point scribed on the cliff and rein- tentiometer to the lower end of the pipe and the forcing bars placed at three points on the landslide. Determination of the Line A displacements was sliding potentiometer rod to the tape. The poten- automated in 1985 by establishing a line parallel to tiometers exhibit a 5-kQ electrical change for each and 2 m uphill from the manually read points. The 305-mm change in length. The three potentiome- line was divided into sections 30, 50, and 98 m ter values are monitored by a solar-powered auto- long. A 10.2-cm diameter polyvinyl chloride mated data analysis system (ADAS) that transmits (PVC) pipe was buried below the ground surface to the values via a radio link to a personal computer protect a surveying tape, which was supported by located in the control room at the Yellowtail Dam. pulleys placed in the pipe at 1 7-m intervals or at These values and other data collected at the dam points where the pipe bent to follow a major slope are relayed through the Geostationary Operational change. Bellows constructed of rubberized fabric Environmental Satellite (GOES) to a computer at connected the individual sections of pipe at such the Bureau of Reclamation office in Denver, bends. The bellows allowed both linear and angular Colorado. 296 Landslides: Investigation and Mitigation

5. GROUNDWATER MONITORING so that the recorded water level may be of little significance for further analysis. Therefore, at all Groundwater levels and pore pressures in a land- installations, boreholes between different water- slide area can be measured by a variety of com- bearing strata should be properly sealed, and each mercially available piezometers. The selection of water-bearing zone should be separately moni- the best type for a particular installation involves tored. Not only will better measurements result, several considerations, which are discussed in but unnecessary cross-contamination of ground- Chapter 10 (Section 9 and Tables 10-6 and 10-7). water supplies will be avoided and most local reg- ulations concerning well construction will be 5.1 Open Standpipe Piezometers satisfied. Sealed, open standpipe piezometers vary The most common water-level recording tech- mainly in diameter of standpipe and type and vol- nique, despite the availability of more sophisti- ume of collecting chamber. The simplest type cated methods, is observation of the water level in (Figure 11-16) is merely a cased well sealed above an uncased borehole or observation well. A par- the monitoring portion. The depth to water is ticular disadvantage of this system is that perched measured directly by means of a small probe. In water tables or artesian pressures occurring in spe- this case, the static head represents the average cific strata may be interconnected by the borehole conditions over the entire zone being monitored. This measured head may be higher or lower than the free water table, but in the case of moderately

.-SCREW CAP OR impervious soils, it may be subject to a large time WOODEN PLUG lag before a stable water level is observed (Hvor- slev 1951). The simplicity, ruggedness, and overall reliability of the open standpipe piezometer dic- tate its use in many installations. However, it is CEMENT/SAND ORBENTONITE often a poor choice of instrument for measuring BACKFILL water levels in. impervious soils because of the in- herent time lag, and in partially saturated soils the significance of the measured head may be difficult to evaluate. The time lag may be reduced by decreasing the diameter of the riser pipe and increasing the diam- eter of the inflow screen or porous filter (Hvorslev -5-cm DIAMETER STAN DPIPE 1951). The Casagrande type of standpipe piezome- ter (Figure 11-17) consists of a porous stone tip em- bedded in sand in a sealed-off portion of a boring and connected with a 1-cm diameter plastic riser tube (Shannon et al. 1962). When properly in- stalled, this type of piezometer has proven success- SAND OR SAND AND GRAVEL ful for many materials, particularly in the long BACKFILL term, because it is self-deairing and its nonmetallic construction resists corrosion. The reliability of unproven piezometers is usually evaluated on the basis of how well their results agree with those of adjacent Casagrande piezometers. However, so PERFORATED SECTION OF PIPE that commercial water-level probes can be in- APP ROX. serted, larger 1- to 2-cm diameter risers, which in- FIGURE 11-16 1mLONG crease the time lag, are commonly used with the Open standpipe -PLUG Casagrande-type piezometers. If rapid piezometer piezometer 0.3 m •::; Note: 1 m = 3.3 ft; (Wilson and V :.:.. lcmO.4in. responses are a major concern in a landslide, pres- Mikkelsen 1978). sure sensor piezometers should be used. Field Instrumentation 297

slope face, at any convenient location and eleva- FIGURE 11-17 tion. In the pneumatic piezometer shown in Figure Casagrande 11-18, flow of air through the outlet tube is estab- borehole piezometer (Wilson and lished as soon as the inlet-tube pressure equals the Mikkelsen 1978). pore-water pressure. Pneumatic piezometers have the following advantages: (a) negligible time lag be- cause of the small volume change required to oper- ate the valve, (b) simplicity of operation, (c) 10mm capability of purging the lines, and (d) long-term -BACKF ILL stability. Their main disadvantage is the absence of a deairing facility. Electrical piezometers have a diaphragm that is deflected by the pore pressure acting against one face. The deflection of the diaphragm is propor- tional to the pressure and is measured by means of various types of electric transducers, the most com- mon being either vibrating-wire or strain-gauge pressure transducers. Such devices have a negligi- -BENTON lIE SEAL 1 m / ble time lag and are extremely sensitive. However, they cannot be deaired nor can the sensitive elec- tric transducers normally be recalibrated in situ (Dibiágio 1974). For short-term observations at in- stallations where data transmission is over limited 30 cm distances, standard resistance strain-gauge pressure transducers may be used. Because they are affected 30 cm POROUS TUBE by the environment and have poor long-term sta- 38-mm 0.0. - - :-.-.- bility, resistance strain-gauge pressure transducers 30 cm SAND FILTER are generally not recommended for installations in Note: 1m=33ft; which reliable readings are required over an ex- 1 cm = 0.4 in; 15 cm lmmO.04in. tended period of time. A typical vibrating-wire electrical piezometer is shown in Figure 11-19. These generally exhibit better long-term stability characteristics. However, not all vibrating-wire types of piezometers have provided consistent 5.2 Pressure Sensor Piezometers long-term stability records. Electrical vibrating-wire and pneumatic sensors Although somewhat expensive, the multipoint dominate the bulk of the geotechnical piezometer piezometer offers major advantages for monitoring instrumentation market, with the old standby, the groundwater in deep landslides. Invented in the Casagrande-style standpipe, as the backup. Con- mid- 1970s and initially used on the Downie land- siderable improvement in these instruments has slide located between Mica and Reveistoke on the occurred during the last decade, and several new Columbia River in British Columbia (Patton commercial products have become available. Each 1979, 1983, 1984), this system has since become a type has distinctly favorable and unfavorable attri- seasoned and reliable commercial product. butes relative to short- and long-term monitoring. Integrated data gathering and monitoring sys- Both the electrical vibrating-wire and pneumatic tems that take full advantage of the latest in sensors (or transducers) have become less expen- microtechnology and single-wire transmission of sive and easier to use. digital data were launched about a decade ago. The pneumatic piezometer consists of a sealed These systems, such as the MP System with modu- tip containing a pressure-sensitive valve (Penman lar subsurface data acquisition (MOSDAX) de- 1961). The valve opens or closes the connection veloped by Westbay (Patton et al. 1991), are between two tubes that lead to the surface, or the revolutionizing the methods of obtaining and eval-

298 Landslides: Investigation and Mitigation

SUPPLY PRESSURE SUPPLY PRESSURE FILTER COUPLING

TRANSDUCER INPUT COUPLING0- TRANSDUCER OUTPUT SUPPLY PRESSURE COUG GAUGE

OUTPUT PRESSURE GAUGE

300mm(l2in) PVC SLEEVE GASKET-7 r— DIAPHRAGM POROUS [FILOTNEER INSULATING RESIN /

3.8 mm 0.5 in)

T 3 rnrn (0.125 in)/ ALL LSTAINLESS POIYETHYIENE CHECK STEEL PLUG TUBING VALVE BODY

FIGURE 11-18 uating groundwater data in complex hydrogeologi- selected on the basis of the drill log and identifica- Pneumatic cal regimes at reservoirs, dams, and landslide areas. tion of materials encountered in the borehole, the piezometer The MP System facilitates the measurement of estimated position of the water table, and the esti- (Wilson and mated location of the sliding surface. Mikkelsen 1978). groundwater or gas pressure at any number of points spaced as close as 1.5 m apart in a single The simplest open standpipe piezometer is con- borehole. Sections of 48- x 38-mm PVC pipe are structed by placing a tube within the drilled hole combined with special valved couplings and that extends to the ground surface. The monitor- packer units to isolate multiple pressure-measuring ing portion of the piezometer installation should ports. If desired, ports can serve as monitoring sites be pervious to allow observation of the ground- or be opened to pump and sample groundwater, or water pressure. This monitoring section may be a permeability test may be run from these loca- constructed of a porous material, or the appropri- tions. Installations have extended to depths of ate portion of the tube can be slotted or con- more than 300 m. MOSDAX is a pressure-sensing structed from a material with a sufficiently fine module that can be used to probe any individual screen so that free access of groundwater is per- port in turn, provided manual data collection is ac- mitted but movement of soil into the piezometer ceptable. Automated data collection is possible, is prevented. The piezometer tube assembly is even at unattended sites, when dataloggers are centered in the drilled hole, and a known volume combined with multiple MOSDAX units attached of clean sand is placed around the piezometer to selected ports. monitoring section to create a sand filter between the soil and the piezometer sensor. 5.3 Importance of Piezometer Sealing Construction of an impervious barrier above the piezometer tip and sand pocket is essential. Casa- Piezometers in landslides are commonly installed grande, pneumatic, and electric sensors placed in boreholes advanced into soil or rock. The depth within open standpipe piezometer assemblies in of the monitoring location for each piezometer is boreholes can be sealed in a similar manner. One Field Instrumentation 299

cement-bentonite grout. Even if the permeability of FIGURE 11-19 the grout is significantly higher than that of the sur- Electrical rounding soil, little error will rèsult because of the piezometer (Wilson and relatively large grouted length. In many cases, a sand Mikkelsen 1978). filter need not be included because the piezometer tip can be grouted directly with little error. —ELECTROUAGNETS Installing only one, or at most two, piezometers in a single borehole has generally been found pref- erable because bentonite seals are sometimes diffi- VIBRATING lIRE cult to construct and may leak slightly despite precautions. However, special-purpose, multiple- point piezometers (with up to four tips) have been used successfully by Vaughan (1969) and merit SENSING TUBE - C consideration. o c In soft and medium-stiff soils, it may be prefer- able to push the piezometer directly into the E E ground. Flush-coupled heavy steel pipe is required I OIAPHRAGU C., to allow for the driving of the probe, and the piezometer tip must be robust enough to resist damage and should be designed to minimize dis- turbance around the tip (Mikkelsen and Bestwick 1976). Casagrande, pneumatic, or, less commonly, FILTER POROUS STONE electric sensors may be used. These driven piezo- meters are self-sealing and rapidly installed. If pneumatic sensors are used, these piezometers have a rapid response. Since the piezometer tip cannot 7-----STEEL CONICAL TIP be deaired after installation, it should be soaked in deaired water beforehand and kept in the water until it is driven into place. A low-air-entry filter - - tip can be placed in a saturated sand pocket, but a Note: 1 mm 0.04 in. (APPROX.) = high-air-entry tip should be pushed into the soil 4-40mm - beyond the base of the hole or surrounded by a porous grout such as plaster of paris. This proce- dure will ensure more rapid pressure equalization, well-established procedure is to drop balls of soft which may be of considerable value in obtaining bentonite into the space between the piezometer reliable pore-pressure measurements soon after in- tube and the borehole wall and then to tamp those stallation in low-permeability clay. Both electrical balls around the piezometer tube by using an annu- and pneumatic piezometers should be checked for lar hammer. Prepared bentonite in chip or pellet malfunction before and during installation, and form is available commercially; it has a specific particular care is needed in pushing electrical gravity that is sufficiently great to allow it to sink piezometers to prevent overpressuring them. through the water in the piezometer hole so that hammering is not necessary. Alternatively, a 5.4 Presentation of Data cement-bentonite grout can be tremied into the hole above the sand filter. Such a seal can be Piezometric heads should be plotted on time pumped through a 1.3- to 2.0-cm diameter pipe ad- graphs showing amounts of rainfall and other data jacent to the piezometer tube. An alternative that may influence the pore pressure. If drainage method of sealing piezometers in boreholes within has been installed, the quantity of seepage should fine-grained materials was described by Vaughan also be recorded and plotted. If possible, the re- (1969). Instead of bentonite being placed above a sponse of each piezometer should be checked peri- sand filter, the borehole is completely grouted with odically to determine its recovery rate. 300 Landslides: Investigation and Mitigation

6. DATA RECORDING AND TRANSMISSION 6.1 Readout Boxes with Memory

Clearly, the evolution of automated data recording A variety of dedicated readout boxes containing in the field has been the greatest innovation in geo- large volumes of digital memory are appearing on technical instrumentation during the past decade. the market from several manufacturers. These read- The accuracy and speed of data transfer from field out boxes are lightweight and weather- and splash- to office are increased drastically by electronic proof, have power for at least one work day, and can recording (Mikkelsen 1992). Methods range in communicate with a personal computer. Currently sophistication from data keyed into general-purpose available readout boxes are able to process signals electronic notebooks to fully automated data analy- from inclinometers and vibrating-wire electrical sis systems (ADAS). At the heart of this technol- and pneumatic piezometer sensors. Typically, these ogy are the continuously evolving microchips, readout boxes are carried into the field and manu- which seem to get smaller and faster and have more ally connected to a series of instruments in a se- memory every few months. The versatility, speed, quential fashion. The data received from each power, portability, and convenience of perso- instrument are temporarily stored in the memory of nal computers provide capabilities that were only the readout box. At the end of a suitable period, dreamed of 10 years ago. Today, some data evalua- usually no more than a day, the stored data are off- tions can even be conducted in the field with de- loaded from the readout box into a personal com- vices that are very simple to use. The day of the puter, with which an appropriate data base is slide rule is long gone, and so is the manual record- maintained. Expertise in programming is not nec- ing of numerical readings, with date, time, and essary for this method of electronic recording. An comments, for many applications. infrequent user is usually guided through the proper Successful instrumentation and data-gathering recording procedure by menu displays. Some data- automation require a blending of expertise from di- entry mistakes are even allowed without the neces- verse areas, including civil engineering, electronics, sity of restarting the reading session. communications, instrumentation, and computers For example, preparations for a typical field ses- (Dunnicliff and Davidson 1991). Much of the nec- sion of reading a series of inclinometers at a land- essary synergism was encouraged by those involved slide would start in the office by connecting the in dam instrumentation, in which increased sensi- readout box to a personal computer with a serial tivity to dam safety issues coupled with dramatically interface cable and bringing up the data-base man- improved computer hardware and software offered agement program. A single data-base file would potential solutions to reductions in monitoring staff contain information concerning all the inclinome- and other budgetary constraints. Some of the in- ters and all the readings for one project, including struments and much of the experience in the special notes and installation parameters. From automation of field instrumentation networks de- this file, any of the inclinometer installation pa- veloped by those involved in dam monitoring can rameters and sets of previous readings would be ob- be applied to other geotechnical applications, in- tained. In an on-going data acquisition program, cluding landslide investigations. On the basis of ex- the installation parameters for each inclinometer perience with dam instrumentation, Dunnicliff and would be loaded into the readout-unit memory, but Davidson (1991) reviewed methods for automating modifications in the parameters would perhaps be data collection from various classes of geotechnical required. Deletion of previously transferred data instruments and the ease with which such automa- sets is usually necessary to free up memory in the tion can be achieved (Table 11-1). readout box before initiation of a new field reading Automated data recording and transmission can session. Only a few minutes would be required to be undertaken in several ways, each reflecting dif- accomplish these tasks before the readout box was ferent levels of automation, investment, and sophis- ready for the field. tication. Three broad approaches can be defined: In the field, the reading of the individual incli- nometers would proceed as usual; the readout box Readout boxes with memory, would be connected to the inclinometer probe and Multichannel dataloggers, and activated by selecting the menu option for collec- Integrated computer-based ADAS tion of readings. Then, by scrolling through a list Table 11-1 Automated Data Collection from Various Geotechnical Instruments (Dunnicliff and Davidson 1991)

INSTRUMENT CATEOORYb AUTOMATION METhOD MANUAL BACKUP

Piezometers Open standpipe 2 Pressure transducer down standpipe Usually straightforward, by measuring water surface Twin-tube hydraulic 2 Pressure transducers connected Straightforward, by existing pressure to lines gauges Pneumatic 3 Pressure actuating and measurement Straightforward, with manually system operated readout Vibrating-wire 2 Frequency counter Straightforward, with manually operated readout Electrical resistance 2 Strain gauge completion circuitry Straightforward, with manually strain gauge operated readout 4- to 20-mA pressure 1,2 Electrical current readout Straightforward, with manually transmitter operated readout Deformation gauges Tiltmetei' 1,2 _C Straightforward, with manually operated readout Probe extensometer 4 N/A N/A Embankment and 1,2 _C Straightforward, with manually borehole extensometet operated readout Inclinometer, 4 N/A N/A probe type Plumb line and inverted 3,4 Infrared or light sensor line and Straightforward, with manually pendulum' position transducers operated readout Liquid level gaugec 1,2 Pressure transducer Straightforward, with manually operated readout

Earth pressure cells Pneumatic 3 Pressure actuating and measurement Straightforward, with manually system operated readout Electrical resistance 2 Strain gauge completion circuity Straightforward, with manually strain gauge operated readout Load cells and strain gauges Electrical resistance 2 Strain gauge completion circuitry Straightforward, with manually strain gauge operated readout Vibrating-wire 2 Frequency counter Straightforward, with manually operated readout Hydraulic load cell 2 Pressure transducer Straightforward, with manually operated readout

Temperature -. Thermistor 1,2 Resistance readout Straightforward, with manually operated readout Thermocouple 2 Low-level voltage readout Straightforward, with manually operated readout

Miscellaneous Surface water 1,2 _C Straightforward, with manually elevations, rain gauges operated readout Weir for measuring 1,2 -c Straightforward, with manually water flow operated readout Seismograph 4 N/A Seismograph chart record For a definition of instument terms, see work by Dunnicliff(1988). There are four categories in order of increasing difficulty, specialization, and cost of automation: Category 1, Straightforward: Automation can be done by numerous manufacturers of laboratory and industrial ADAS equipment. Category 2, More Specialized Requirements: Problems exist in thermally unconditioned environments or with inaccessible power; requires non- standard signal conditioning, low-level voltage measurement, and wide common mode voltage range. Category 3, Most Specialized Requirements: Problems exist in hostile outdoor environment; radio network communication required if hard-wire communication unavailable; sensors are difficult to automate (e.g., plumb lines or pneumatic piezometers); involves sensors that communicate via serial or parallel interface. Category 4, Usually Not Practical To Automate: Technical complexity or cost outweighs benefits; significant reliability problems exist in hostile environment; customized automation hardware required or impractical to automate. - Automation method and difficulty depend on type of transducer used. 302 Landslides: Investigation and Mitigation

on the display, the user would select the desired in- days, weeks, and months. Sufficient battery power clinometer. The time and date would be logged and a great deal of memory are necessary require- automatically, and the stored inclinometer param- ments. Continuing improvement in low-power eters would be retrieved from memory. These pa- electronics permits dataloggers to be operated un- rameters would be quickly confirmed by the opera- attended from a month to up to a year. Use of solar tor, and the reading session would begin. The power for battery recharging can extend unat- readings would be recorded at each depth by tended use for several years. Dataloggers can mon- pressing a remote hand switch. After a series of itor from one to a few dozen sensors at a remote field readings was completed, the data would be site. The sensors can be polled according to a pre- checked for errors in the field and simple calcu- programmed sequence or activated by a triggering lations performed. Subsequently, in the office, event or by manual command. the new readings would be uploaded into the per- sonal computer, the data base updated, and new 6.3 Integrated Computer-Based ADAS displacement plots produced. A datalogger may form the heart of an ADAS, but 6.2 Multichannel Dataloggers much more powerful and flexible systems are built around personal computers. A computer-based Manufacturers of geotechnical instruments have ADAS is generally capable of dealing with a vari- produced a number of sensor-dedicated dataloggers ety of transducer types and instrument- monitoring in the past, but it is only recently that more intel- needs. The use of an ADAS leads to some cost re- ligent field-hardened units that can handle several ductions and easier system operation because a signal types have emerged. In the early 1980s, single computer can replace several dataloggers. Campbell Scientific, a manufacturer of weather The resulting integrated system is more robust. and environmental data collection units, produced Computer-based systems generally provide power- their initial CR10 instrument, the first significant ful communications capabilities that allow the new data collection unit to be widely utilized by ADAS to be accessed by several remote users, in- the geotechnical industry in North America cluding the system instrumentation manufacturer (Figure 11-20). The CR10 still forms the heart of for monitoring and evaluation of operational diag- most of the small unattended datalogger systems nostic tests. produced by several suppliers. The heart of an ADAS is the central network Because they must remain in the field continu- monitor, which houses the host computer and ously for extended periods, dataloggers have to be communications hardware allowing receipt of even more rugged than readout boxes, which are data from one or several sites. Data are acquired at only carried into the field during actual reading sites by local monitoring units (LMUs), which are sessions. Dataloggers have to withstand whatever directly connected to the central network moni- field and weather conditions are experienced for tor, or by remote monitoring units (RMUs), which transmit data to the central network moni- tor by telemetry (usually radio) links. The indi- vidual RMU or LMU systems contain modules for the multiplexing (switching), signal conditioning, power excitation, control, and communications functions required to acquire data from each trans- ducer. RMU and LMU systems can be designed to convert raw data into desired engineering units, to collect data readings according to a prepro- grammed sequence, or to be polled from the cen- tral network module. All of the above ADAS components can be lo- FIGURE 11-20 cated at a remote site, for example, a single land- Campbell Scientific slide or group of nearby landslides. A variety of CR10 datalogger. telephone, microwave, radio, or even satellite data Field Instrumentation 303 transmission links have been used to connect re- case histories of landslide instrumentation that had mote ADAS installations to a central office com- been reported before 1978. Although now some- puter system. Dunnicliff and Davidson (1991) what dated, these case histories include sev- provided many important recommendations con- eral classic examples. These earlier landslide cerning the specification, design, and maintenance instrumentation applications included those at a of ADAS installations for dam projects, but some rock cut along Interstate-94 in Minneapolis, further comments are warranted concerning their Minnesota (Wilson, 1970, 1974); in the Potrero application to landslide investigations. Economic Tunnel of the Southern Pacific Railroad near San considerations are most important. These systems Francisco, California (Wilson, 1970; Smith and were developed mainly for large dam projects and Forsyth, 1971); in the stabilization of glacial till remain relatively costly for many landslide investi- slopes along Interstate-S in downtown Seattle, gation and monitoring activities. Nevertheless, as Washington (Wilson, 1970; Palladino and Peck, noted in the case histories presented later in this 1972); and at the Fort Benton landslide along the chapter, they are already in use at several landslide Missouri River in western Montana (Wilson and locations, usually those that exhibit considerable Hilts 1971). Other early landslide instrumentation potential for having significant economic impacts case studies were reported by Dun (1974), Munoz or pose large safety hazards. The cost of such sys- and Gano (1974), and Tice and Sams (1974). tems continues to fall, in line with the rapidly de- Five recent landslide monitoring projects that clining costs of most electronic components. As feature a variety of instrumentation issues have the novelty of these automated systems passes and been selected for inclusion in this chapter. The in- their cost continues to diminish, their increased strument programs developed, the purposes of the use at landslide sites can be expected. investigations, the measurements achieved, and the Dunnicliff and Davidson (1991) noted that the lessons learned are described for each case history. initial expectation that automation would cut labor costs does not appear realistic. Compared 7.1 Downie Slide near Revelstoke Dam with manual retrieval of readings, automation does and Reservoir, British Columbia, Canada not eliminate the need for personnel. Introduction of a new and more sophisticated technology, the Investigations in 1956 of alternative dam sites on need for maintenance of additional components, the Columbia River north of Revelstoke, British and processing and evaluating an increased volume Columbia, led to discovery of a large prehistoric of data generally require the same or a greater num- rock slide on the west side of the river (Piteau et ber of people. However, better and more timely at. 1978). The volume of this landslide is approx- data, and even real-time continuous data, are ad- imately 1.5 km3; today its toe extends into Revel- vantageous because a much better understanding stoke Reservoir 66 km upstream from the of the landslide problem is provided. 160-rn-high concrete gravity Revelstoke Dam. Automated data collection technology is new The Downie landslide was first studied in to most geotechnical engineers; only a few spe- 1965-1966 as part of the design for Mica Dam, lo- cialists and instrument suppliers have the ex- cated 70 km upstream from the landslide (Imrie pertise to carry out successful programs. The 1983). The investigation was undertaken to deter- technology is better known in other application mine how a blockage of the Columbia River caused sectors, such as hydrological and meteorological by reactivation of the landslide might affect the sta- monitoring, and by the telecommunications in- bility of the downstream toe of the dam. The early dustry. Geotechnical practitioners should seek as- investigation consisted of geologic mapping, bor- sistance from those in other fields experienced ings to study the rock layers, and installation of with this technology before attempting to use it instruments to measure slide movement and for the first time in geotechnical applications. groundwater conditions. Five drill holes (three on and two off the landslide) were involved in which core was recovered using double-tube core barrels. 7. EXAMPLE PROJECTS Inclinometer casings were placed in the drill holes Chapter 5 in Landslides: Analysis and Control (Wil- on the landslide, and measurements were taken. son and Mikkelsen 1978) contained several brief Other early instrumentation included two surface 304 Landslides: Investigation and Mitigation

surveys with 21 monuments, strong-motion seis- 1989). Additional instrumentation included 45 mometer measurements, rock noise surveys, and surface survey monuments, 2 meteorological sta- water-level measurements in the three leaky incli- tions and 3 snow-course stations, 7 streamfiow and nometer casings. The results of the study, described adit-flow gauging stations, and 1 seismometer. The by Piteau et al. (1978), were helpful in answering combination of surface surveys measured by elec- questions concerning the stability of Mica Dam but tronic distance measurement (EDM) and inch- were inconclusive with regard to the nature and nometers cemented vertically into holes that depth of the slide (Patton 1983). extended as much as 250 m into the landslide Beginning in 1973, a new study was undertaken monitored movements as small as 2 mm/year to evaluate the stability of the landslide and to de- (Imrie 1983). The locations of the drainage adits termine the effect on the landslide of the future and many of the instrument stations are shown in reservoir created by the proposed Revelstoke Dam, Figure 11-21. which would raise the water level 70 m into the During reservoir filling in 1983, a continuous toe of the landslide (Imrie and Bourne 1981). This surveillance system monitored a microseismic unit, new study proceeded in several stages using instru- three in-place inclinometer sensors, three exten- mentation, stability analysis, and geologic observa- someters, two automatic flume recorders, and four tions. The instrumentation program eventually fluid-pressure transducers (Patton 1983). Some of involved 41 drill holes instrumented with 113 the instrument readings were telemetered to piezometers (both single- and multiple-position) Revelstoke and Vancouver, British Columbia, to and 10 inclinometer casings (Patton 1983; Moore provide constant information on landslide move-

PLAN LEGEND Slide boundary Survey monuments .00 Driliholes I 1400 = Inclinometer p Piezometer Drainage adit

p s 31 O

I A AI P

0 1000 meters

FIGURE 11-21 Plan view and section for Downie slide indicating —1200 locations of E drainage adits, boreholes, z - 800 0 instrumentation, Lower and survey targets Zone > - T11I 0 1000 w (modified from - 400 —j lmrie and Bourne LIJ meters 1981). Field Instrumentation 305 ment, seismic activity, and groundwater flow. The 7.2 Cromwell Gorge Landslides, Clyde telemetry allowed the instruments to serve as a Dam Project, New Zealand warning system that would allow suitable action, such as lowering the reservoir, to be taken quickly An extensive network of instruments has been in- (Imrie 1983). stalled to monitor 13 large landslides in Cromwell The Downie slide has been stabilized by means Gorge of the Clutha River, central Otago, New of an extensive drainage system (Imrie et al. 1992). Zealand (Gillon et al. 1992; Proffitt et al. 1992). Some 2430 m of exploratory and drainage adits was The landslides, shown on the map in Figure 11-22, excavated in and adjacent to the landslide mass in border Lake Dunstan, the reservoir formed by the three separate stages between 1974 and 1981. Clyde Dam, a 100-m-high concrete gravity dam Observations made during initial stages were used owned by the Electricity Corporation of New to lay Out subsequent plans. About 12 200 m of Zealand (Gillon and Hancox 1992). Approxi- drainholes was drilled from the adits, and a wide- mately 25 percent of the shoreline of Lake Dunstan spread lowering of water pressures at the basal is bordered by extensive, deep-seated landslides shear zone was achieved (Patton 1983; Moore formed mainly in schist bedrock, schist debris, and 1989). colluvium. The total area of landsliding along the Largely on the basis of the Downie slide stud- shores of the lake is about 20 km2, and the land- ies, Patton (1984) arrived at the following con- slides are composed of about 1.6 km3 of material. clusions regarding the collection and use of By mid-1991, more than 2,000 individual piezometric data: points were monitored on or adjacent to these landslides, with the expectation that this moni- Dormant slides can be affected by maximum toring would increase to more than 3,500 points long-term piezometric levels and therefore are by the completion of lake filling in mid-1992 affected by long-term climatic changes; (Table 11-2). The instruments are distributed over Slope stability analyses should generally consider all the landslides along an 18-km stretch of the three classes of piezometric observations: (a) Clutha River and in more than 16 km of tunnels short-term measurements, (b) seasonal or storm- (Gillon et al. 1992). The elements being measured related maxima over a several-year period of are deformation, water pressure, water flow rate, record, and (c) estimated long-term maximum and weather. In general, the monitoring tech- piezometric levels; niques are conventional. The challenge is in ob- Long-term piezometric records are almost al- taining data from the great number of monitoring ways unavailable for slope stability analyses, but points spread over a large area in steep terrain. they can be estimated for a period to coincide with the expected life of the project using avail- 7.2.1 Deformation Measurement able piezometric records as a basis; and Piezometric levels used for slope stability analy- The main deformation-detection instruments ses should, as a minimum, be based on the max- being used at Cromwell Gorge are (Gillon et al. imum levels recorded over a period of several 1992) years. Inclinometers for detecting the location and After 10 to 15 years of use, the monitoring sys- magnitude of small lateral movements; tem has begun to decay. Several instruments have Geodetic deformation survey networks for de- become blocked or otherwise inoperable and will termining the areal extent and magnitude of have to be replaced (Imrie et al. 1992). The Origi- larger, longer-term movements; and nal ADAS, first implemented in 1983 to gather Multipoint borehole extensometers installed real-time data from key piezometers, inclinome- from tunnels across specific geologic features, ters, extensometers, seismometers, and flow meters such as landslide failure surfaces. (Tatchell 1991), was no longer state of the art and was replaced, beginning in 1991, by advanced Lower-accuracy techniques include installation telemetry, including hardware and software com- of rod extensometers along tunnel walls and across ponents (Imrie et al. 1992). surface scarps and surveying between points in 306 Landslides: Investigation and

_____ TWO- , I BRIDGc S HINTONS DUNI.AYS SLIDE SLIDE CILYOE SLIDE SH CLYDE DAM No5CREEK River _..' \ SLIDE \, /z 1 BYFORO CREEK c. SLIDE LE NINE MILE CREEK SLIDE AREA JACKSON CREEK FLYING FOX SLIDE 1. SLIDE CLYDE SLIDE

DOWNSTREAM ,A / SEGMENT /

UPSTREAM SEGMENT

BREWERY CflEEK'" -'--- SLIDE 2 i KEY 0' SUDE CROMWELL CAIRNMUIR Landslide aseas SLIDE C) SMINERS I 2 3 km Oraunage drIves rIOCKFALL I I I --- Slab HQI?way No 8 (\ SCAIE O COANISH POINT SLIDE

FIGURE 11-22 tunnels with EDM equipment. The satellite-based packer, casing components, and pumping port at Map of Cromwell GPS was used on a trial basis on one landslide the bottom of a 70-mm casing to provide an effi- Gorge landslide with relative accuracies similar to those of con- cient and accurate inclinometer-piezometer. The area, New Zealand, ventional survey methods. showing locations packer provides a good base bond, and the hole was deliberately drilled deeper to ensure that the and extent of 7.2.1.1 Inclinometers drainage adits piezometer filter avoids any zone of deformation. (modified from By mid-1991 there were 180 inclinometer instal- Gillon and Hancox lations. About 150 of these, with an average depth 7.2.1.2 Multipoint Borehole Extensometers 1992). of 90 m and a maximum depth of 300 m, were Before the filling of the Lake Dunstan reservoir, REPRINTED FROM D.H. BELL being regularly monitored (Gillon et al. 1992). (ED.). LANDSLIDES! approximately 35 multipoint borehole exten- GLISSEMENTS DE TERRAIN, During filling of the Lake Dunstan reservoir, incli- PROCEEDINGS OF THE someters were installed from tunnels to intersect SIXTH INTERNATIONAL nometers were monitored twice weekly or weekly basal slide surfaces. Designed and manufactured in SYMPOSIUM, CHRISTCHURCH, 10-14 at sites showing unusual activity or where con- house, the extensometers use four untensioned, FEBRUARY, 1992, VOLUME I, struction work could cause instability. 1992-1994, 1800 PP., 3 VOLS., fully grouted fiberglass rods up to 50 m long. A.A. BALKEMA, OLD POST Early in the investigation phase, drilling prob- ROAD, BROOKFIELD, Readings are obtained either manually with a dial VERMONT 05036 lems and the practice of combining piezometers gauge or electronically using linear potentiome- with inclinometers caused several poor installa- ters linked to a datalogger. These extensometers tions. In general, combining standpipe piezometers are accurate to 0.1 mm. with inclinometers tends to create a zone of poor bond at each piezometer sand filter, which may re- 7.2.1.3 Surface Extensometers duce data quality for the inclinometer. However, a Two types of extensometers for measuring surface recent development has been the use of a Westbay deformation have been installed across head scarps Field Instrumentation 307

Table 11-2 Instruments and Monitoring Points on Cromwell Gorge Landslides (modified from Gillon et al. 1992)

INSTRUMENTS AND POINTS JULY 1991 FUTURE Standpipe piezometers 760 800 Westbay multipoint piezometer ports 260 400 Inclinometers 180 200 Multipoint borehole extensometers 20 140 Drive leveling points 30 300 Surface monuments 250 280 Surface leveling points 250 350 Drainage flow meters 350 1,000 Weather and miscellaneous monitoring devices 120 120 Total 2,220 3,590

and other surface expressions of slide movement critical locations to provide data where complex (Gillon et al. 1992). For small movements, rod groundwater conditions exist on high-hazard land- extensorneters were buried in special casings at a slides and to confirm the response of landslides to depth of 1 m. The rod material is a carbon-fiber drainage works. composite with a near-zero coefficient of thermal expansion. 7.2.3 Drainage Flows For measurement of larger surface movements, a As of mid-1991, drainage flows were measured at wire extensometer was developed. The 6-mm wire, about 350 locations, principally at individual mounted on 2-rn pylons to prevent animal damage, drain holes. Ultrasonic pipe-flow meters linked to consists of a kevlar core protected against ultravio- dataloggers are used when a continuous record is let light by a nylon sleeve. The wire runs across an required from individual drain holes. instrumente4 pulley and is tensioned by weights. 7.2.4 Data Processing 7.2.2 Groundwater-Level Instrumentation The use of several software packages, including re- gional data bases, has facilitated the storage, selec- 7.2.2.1 Standpipe Piezometers tive retrieval, filtering, manipulation, quantitative Standpipe piezometers are the most common in- assessment, and analysis of a large and rapidly ac- strument on the Clyde Project (Gillon et al. cruing data base from the Clyde Project landslide 1992). As of mid-1991 there were 760 standpipe instrumentation. The following software packages piezometers in about 360 boreholes. Typically, are being utilized (Proffitt et al. 1992): they were installed in cored 96-mm (HQ) diame- ter boreholes at depths of 100 to 300 in. Multiple TECHBASE, a relational data base and model- standpipe piezometers, normally two or three but ing software package for storage, management, sometimes up to five, were installed in each bore- manipulation, statistical evaluation, and output hole. Most standpipe piezometers are monitored of engineering, scientific, and topographic data; manually at frequencies ranging from daily to TIDEDA, a data base for storing, manipulating, quarterly, with weekly being typical. and plotting time-dependent data, including water levels, flows, weather, and surveying in- 7.2.2.2 Westbay Multipoint Piezometers formation; and By mid-1991 Westbay multipoint piezometers had GTILT, a data-base system used for analysis and been installed in 13 holes, with more installations plotting of inclinometer surveys. Several types planned (Gillon et al. 1992). Placement of these of deflection plots are available in GTILT; how- instruments began about midway through con- ever, on the Clyde project graphical output has strsiction as part of the move to improve data qual- generally been restricted to cumulative deflec- ity and quantity. They have been restricted to tion and time-series plots (Figure 11-23). 308 Landslides: Investigation and Mitigation

per year. Episodes of major movement correlate Deflection (mm) with critical rainfall events, each of which gener- -25 25 ally includes a period of prolonged and high- intensity rainfall of at least 24 hr duration. Catastrophic failure of the 215-rn-wide by 27-Nov-'87 580-rn-long mass occurred most recently on March 23, 1993, and resulted in closure of US-101 for ap- 5. N -5 proximately 2 weeks (Squier Associates, Inc. 1994). Evaluation of the geologic setting, depth to the - Depth failure surface, and high groundwater pressures led to the conclusion that only drainage alternatives would provide cost-effective, long-term stabiliza- 10 - tion of the landslide. Geotechnical exploration focused on the following key issues (Squier Assoc- 10-Jul-'90 iates, Inc. 1994):

(a) Further definition of the artesian zone; 15 1 15 Measurement of piezometric pressures in the -25 0 25 landslide mass, the artesian zone, and the un- derlying bedrock; Relationship of the unconfined water levels to the confined aquifer; Relationship of piezometric pressures to precip- Calendar Dates itation; 27-Nov 14-Jun31 -Dec 19-Jul 04-Feb 23-Aug 11-Mar 27-Sep Permeability and variability in the landslide 1987 1988 1988 1989 1990 1990 1991 1991 I I I I I I I U I I I I I I mass and in bedrock; Displacements in X-direction Source of artesian pressures; and Quantity of groundwater. 120 Skew = 2ldeg. E

Evaluation of these issues has been assisted by drilling of additional exploratory borings and in- stallation of multiple vibrating-wire piezometers in the boreholes, continuous monitoring and eval- uation of piezometric pressures and rainfall, and stressing the groundwater system using newly in- stalled wells and pump tests. The landslide is now being monitored by an in- 0 200 400 600 800 1000 1200 1400 strumentation system incorporating automated Days since 27-Nov-1987 monitoring and recording of a number of instru- ments. Continuous monitoring of one inclinome- ter, piezometric pressures, and rainfall is being 7.3 Arizona Inn Landslide, Oregon accomplished by an ADAS that allows monitor- ing of 26 piezometers simultaneously. The loca- The Arizona Inn landslide (Figure 11-24) on US- tions of borings, test wells, and instrument sites 101 in the Kiamath Mountains of southwestern are shown in Figure 11-24. Oregon has a history of persistent creep and recur- Six inclinometers were installed by the Oregon ring major movements since highway construction Department of Transportation (ODOT) during in the 1930s. The landslide mass consists primarily March through May 1993. Then during early April of fine-grained, intensely sheared, mylonitized 1994, ODOT installed (a) vibrating-wire transduc- sandstone and mudstone. The landslide area re- ers in eight open standpipe piezometers, (b) a con- ceives an average of 1900 to 2000 mm of rainfall tinuous-reading slope inclinometer in one hole, FIGURE 11-23 (opposite page) LEGEND Landslide in (SHALLOW) P1 0 BORI NGS/PI EZOMETERS Cromwell Gorge: (a) P7 • BORI NGS/PI EZOMETERS (DEEP) Cumulative deflection in Direction A for io x RECENT (1994) BORINGS/PIEZOMETERS Inclinometer RD-6 between November S11 INCLINOMETERS 27, 1987, and July W2 A TEST WELLS (1994) 10, 1990 (July 1990 data inClude SCARPS corrections for zero PRIMARY CRACKS shift and rotations) time-series DATA LINES and (b) plot (Proffitt et al. 0 DATA LOGGER/MUX BOARD 1992).

FIGURE 11-24 Locations of borings, test wells, and instrument sites IN on Arizona Inn landslide, Oregon (Squier Associates, Inc. 1994).

------I I PACIFIC OCEAN 310 Landslides: Investigation and Mitigation

and (c) an on-site tipping-bucket rain gauge. All of average rate of about 4 cm/year. Figure 11-25 is a these instruments were connected to the ADAS time plot of piezometric readings for the upper using two dataloggers. The continuous monitoring part of the landslide. system was programmed and placed in operation on April 10, 1994. 7.4 Last Chance Grade Landslide, Subsequently, 16 vibrating-wire piezometers California were connected to a datalogger after, the addition of two multiplexing boards. Another datalogger The Last Chance Grade landslide, located along records hburly data for four piezometers and US- 101 in far northwestern California, is approx- continudus-reading inclinometer. New software imately 520 in wide and 460 in long. The land- was added; it has three'primary routines that mon- slide, underlain by interbedded shale, sandstone, itor and record data from 20 piezometers and from and conglomerate, is very active; at the road ele- rain gauges as well as system information. The vation, it moves on the order of 1.5 to 3.0 rn/year routine includes (Squier Associates, Inc. 1994) and affects approximately 235 in of the 'roadway. daily maximum and minimum piezometric head Investigation of the Last Chance Grade landslide values, complete hourly data recording of all in- by the California Department of Transportation struments, and user-selectable reading frequencies (Caltrans) included drilling, borehole logging, ge- during special test periods. Summary plots have ologic mapping, and developing recommendations been produced for the inclinometers, piezometers, for stabilizing the highway (Kane and, Beck 1994). and rain gauges. Zones of shear showed rates of Three alternatives are being considered for movement of about 2.5 to 7.5 cm/year, with an highway stabilization. The first is to realign the

Upper Landslide Piezometers

FIGURE 11-25 Time plot of piezometric readings for upper part of Arizona Inn landslide, Oregon; symbols indicate individual piezometers (modified from w 1? Squier Associates, Inc. 1.994). TIME (calendar dates) Field Instrumentation 311 roadway in a tunnel excavated behind the sliding FIGURE 11-26 surface. The second alternative is to realign the Diagrammatic sketch of TDR cable roadway slightly and stabilize the landslide mater- installed in borehole ial below the roadway with a soldier-pile and (Dowding and tieback wall and the material above the roadway Huang 1994) with several rows of slope reinforcement. The REPRINTED WITH PERMIS- SION OF AMERICAN third choice is to realign the highway in a cut ex- SOCIETY OF CIVIL cavated behind the sliding surface. Preconstruct- ENGINEERS ion determination of the location of the sliding surface will be essential in selecting one of these measures. Caltrans commonly uses inclinometers to mea- sure landslide movement. The primary disadvan- tage of the ordinary inclinometer system is the necessity of visiting the site for each set of instru- ment readings. Because the Last Chance Grade landslide is located far from any Caltrans geotech- nical engineering facilities, an alternative method was sought that could be used for denoting zones of landslide movement without actually having to visit the site. For this purpose, as part of the field creases in length as the cable is deformed. At ten- investigation, while inclinometer casings were in- sion failure, a small necking trough is visible in the stalled in two boreholes on the landslide, a coaxial TDR signal, which is distinguishable from a shear- cable was fixed to the outside of one of the casings failure reflection (Dowding et al. 1989). The cable to allow comparison of time domain reflectometry tester can be connected to a datalogger, which (TDR) data with standard inclinometer results records and stores reflections. The datalogger con- (Kane and Beck 1994). trols the cable tester and supplies power during TDR is a technique that uses characteristics of measurements. The data can be collected automat- returned electrical pulses to determine the amount ically from a remote location using a computer with of strain, or the existence of a rupture, in a coaxial a telemetering system (Dowding and Huang 1994). cable. Although developed for cable testing, TDR At the Last Chance Grade landslide, an 82-rn- is finding use in geotechnical applications, espe- long coaxial cable (type RG 59, commonly used for cially mining (O'Connor and Wade 1994). Most videocassette recording) was attached to the outer, commonly, a cable tester connected to a coaxial upslope side of an inclinometer casing by means of cable installed in a borehole emits a stepped volt- nylon ties spaced every 1.5 m. Because the incli- age impulse (Figure 11-26). As rock-mass move- nometer hole was positioned on the actively trav- ments deform the cable, its capacitance is eled roadway of US-101, the cable was continued changed, and this causes changes in the reflected beyond the highway shoulder in a shallow groove waveform of the voltage pulse. The time delay be- in the bituminous pavement, thus allowing TDR tween a transmitted pulse and the reflection from readings to be obtained without interference by a cable deformity determines the damage location traffic. To further explore the possibility of remotely (Dowding et al. 1989). collecting the data and monitoring the landslide, Different wave reflections are received for differ- the TDR system was connected to a datalogger op- ent cable deformations. The length and amplitude erated by a telemetry system that could be activated of the reflection indicate the severity of damage. A using a cellular telephone connection. With this cable in shear reflects a voltage spike that increases arrangement, the TDR data could be directly in magnitude in direct proportion to the amount of loaded into a Caltrans computer located in shear deformation. A distinct spike occurs just be- Sacramento, more than 500 km away, and no one fore failure. After failure, a permanent reflection is would have to visit the landslide site. recorded. If the cable is in tension, the wave reflec- At 148 days after installation, the reflection sig- tion is a subtle, troughlike voltage signal that in- nature for the TDR cable indicated two notable 312 Landslides: Investigation and Mitigation

FIGURE 11-27 spikes (Figure 11-27). The upper spike, for a depth TDR output for of 8.6 m, indicated a crimp in the cable where it borehole on Last was squeezed against the lip of the casing. At a Chance Grade depth of approximately 40 m, roughly the same landslide, California; note spikes where depth as the inclinometer deflections, a trough cable enters was visible in the reflection signature, which was inclinometer hole interpreted as indicating distress of the cable at and in vicinity of the sliding surface (Kane and Beck 1994). surface of sliding The results of this study indicate that in certain (modified from Kane cases TDR may be used instead of, or in parallel and Beck 1994). with, inclinometers for monitoring landslide movement and that the technology exists to re- Top of Hole Om 30 ...... : motely access the TDR installation. The advan- . tages of a remotely accessed system are as follows:

There is no need to visit the site, 40 ...... Debris does not have to be cleaned from the in- clinometer casing before TDR readings are taken, 50 ...... Cable is inexpensive and readily available, and TDR Voltage Response The ability exists to monitor up to 512 holes 14.5 mp/div with one cable tester and multiplexer hookup. 0

Remote data collection results in cost savings. It 134 ...... also means that hazardous areas can be closely mon- itored by frequent sampling to determine incipient movements. Another benefit of this technology is 144...... that it can be used as part of an early warning sys- tem to alert transportation officials of a possible rm catastrophic highway failure (Kane and Beck 154 ...... ..L...... 1994). Depth of Shear Plane 40m(132ft) 7.5 Golden Bypass Landslide, Golden, 164 ...... Colorado In the spring of 1991, during construction of a high- 174 ...... way bypass around the city of Golden, Colorado, forming a part of Colorado State Highway 93, an ..... initial minor landslide failure began in one wall of an excavation in Pierre shale (Highland and Brown ducers were installed to measure groundwater ele- 1993). Slow movement and enlargement of this vations at the landslide. Automated monitoring of earth flow continued until 1994, when the land- the load cells and piezometers was desired, along slide was stabilized by means of a Brazilian-concept with two-way, remote telemetry between the land- tieback panel wall, installation of horizontal drains, slide and Colorado Department of Transportation and removal of 75 000 m3 of overburden near the (CDOT) headquarters in Denver, 30 km to the landslide headwall (Ed Belknap, personal commu- east. Neither power nor communication facilities nication, 1995, Colorado Department of Trans- were available at the site. portation, Denver, Colorado). As a solution to the problem, an integrated Seven of the tieback anchor cablçs were monitoring and telemetry system was installed to equipped with load cells. In addition, five 12.5-mm- remotely automate the load cell and piezometer diameter piezometers with vibrating-wire trans- measurements and provide direct, two-way radio Field Instrumentation 313 communication (Dennis E. Gunderson, personal standard whip antennas, communicate with the communication, 1995, Geomation, Inc., Golden, networked gateway unit through MCU1, which is Colorado). A network monitor station (NMS) was equipped with a high-gain directional antenna. established at CDOT headquarters. The NMS, an Each MCU has its own microprocessor and is IBM-compatible personal computer running capable of multitasking functions, including GEONET software, provides the operator inter- face with the system for programming, alarm Automated monitoring of any standard mea- announcement and acknowledgment, real-time surement signal (analog or digital); monitoring, supervisory control, and data logging. Time-and-date stamping of all measurements in An RS-232 cable from a serial port in the NMS the field at the point of capture; connects to a radio gateway unit, which is con- Conditioning and converting physical- nected to a high-gain directional antenna parameter signals to desired engineering units; mounted on a nearby signal tower. The gateway Peer-to-peer communication; provides a link between the radio communication Acting as a repeater and performing store-and- from the Golden landslide site and the NMS. forward functions for other network nodes; Three measurement and control units (MCUs), Enacting direct or remote-control functions; and which are battery-powered, solar-recharged, and Accomplishing data reduction in the field, if radio-linked, were installed at the landslide (Figure desired. 11-28). MCU1, located at the top of the landslide, automatically monitors three piezometers. MCU2 The user, at any time without system disruption, and MCU3 are located near the bottom of the can initiate system reconfiguration diagnostics and landslide and automatically monitor (a) two load measurement verification and can enact remote cells and (b) five load cells and two piezometers, re- control functions from the NMS. spectively. All monitoring and telemetry is user- programmable and is currently set at 66-hr 8. PROBLEMS PREVENTING FURTHER intervals. MCU2 and MCU3, both equipped with DEVELOPMENT OF NEW INSTRUMENTATION TECHNOLOGY

Instrumentation and associated monitoring tech- niques have made significant advances during the past decade. The general proliferation of minia- ture electronics has revolutionized many aspects of data acquisition, processing, and communications, and has profoundly benefited both industry and the public. Personal computers have entered of- fices and homes, and people have come to expect the speed and convenience of information with- out delay. However, such progress in the geotech- nical field, and in the transportation industry generally, has been less dramatic. Four main rea- sons govern the slow-paced and uneven gains in geotechnical monitoring relative to other data management technologies. First, the geotechnical instrumentation indus- FIGURE 11-28 try is relatively small and highly specialized. Its MCU and solar practice and progress are easily influenced by eco- power supply nomic conditions, levels of government spending, attached to and existing procurement practices. In the United monitoring well States, a highly competitive and shrinking heavy at Golden Bypass landslide...... -.-. - -. ... ._ construction market has also shrunk the market •'- -,. COURTESY OF --{ ; .' - for instrument suppliers. GEOMATION. INC. 314 Landslides: Investigation and Mitigation

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