DOCSLIB.ORG

Borehole Instrument Package1

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Borehole Instrument Package1 Sacks, I.S., Suyehiro, K., Acton, G.D., et al., 2000 Proceedings of the Ocean Drilling Program, Initial Reports Volume 186 3. BOREHOLE INSTRUMENT PACKAGE1 Shipboard Scientific Party2 SYSTEM OVERVIEW Purpose The lack of in situ long-term observations in the oceans, which cover areas of major tectonic activity, limits our understanding of presently active geological processes. Since the beginning of the Deep Sea Drilling Project, there have been many attempts to utilize the drilled holes for observatory purposes. Recent circulation obviation retrofit kit deploy- ments (CORKS) to measure pressure and temperature changes in sealed boreholes are beginning to produce interesting results. The Ocean Drill- ing Program (ODP) continues to recognize the importance of observa- tory objectives (ODP, 1996). Many ODP sites have been drilled at plate subduction zones to un- derstand the accretion processes, to tackle the mass balance problems, or to construct the history of the making of island arcs, among other objectives. A major unknown in the plate subduction process is its epi- sodic nature. Our main goal for Leg 186 was to install two permanent observatories on the landward side of the Japan Trench directly above the seismogenic zone of the subduction plate boundary. The two sites, Sites 1150 and 1151, are located in presently seismically active and in- active zones, respectively. At a plate subduction zone, there is coupling between the two plates, but the definition of coupling can be confusing. One measure is the seismic efficiency, which is the percentage of plate motion released by interplate thrust earthquakes. Plate motion is determined from global plate velocity models. For the Japan Trench zone, the seismic efficiency 1Examples of how to reference the is probably <~30%. Another measure of coupling is how locked a por- whole or part of this volume. tion of a plate boundary is relative to adjacent portions. A 100%-locked 2Shipboard Scientific Party condition means that there is normally no differential motion across a addresses. part of the plate boundary. Eventually slip must occur, the presumption Ms 186IR-103 SHIPBOARD SCIENTIFIC PARTY CHAPTER 3, BOREHOLE INSTRUMENT PACKAGE 2 being that it will be manifest as a sizable earthquake. Since 1994, Global Positioning System (GPS) observations have not revealed any slow earthquakes comparable to or larger than the one that accompanied the 1994 Far-off Sanriku earthquake. The stress transfer across the plate boundary (shear coupling) is a third measure of coupling. In the Japan Trench, the plate motion direction largely governs the tectonic stress field in a wide area across the island arc to the Sea of Japan. The Japan Trench Borehole Geophysical Observatories provide seis- mic and deformation data to help quantify these characteristics. The observatory system is named ‘Neath Seafloor Equipment for Recording Earth’s Internal Deformation (NEREID). NEREID System Outline This section will outline the borehole geophysical observatory (Fig. F1). Each component is detailed in a separate section. The observatory F1. Schematic view of the NEREID is designed to last for many years, first as a stand-alone system, then as borehole geophysical observatory, a system cabled to the shore. Comprised of a strainmeter, a tiltmeter, p. 23. BOB temporary Back from and two broadband seismometers, the sensors reside near the bottom of storage BOB Ocean Bottom status check module the hole with each housed in a separate pressure vessel. Four cables con- infrared com. link PAT (Power Access Terminal) revivable data Storage nect the sensors uphole. Except for one seismometer, Guralp Systems seawater storage Acquisition battery 72-GB Module ROV landing pad data/status command Ltd.’s CMG-1T, the signals are sent in analog form. The relative orienta- power MEG 24-bit digitizers tion of the sensors is fixed and known. The absolute orientation of the data combiner Multiple-access 36-42 V power control Expandable reentry power real-time clock Gateway installed systems was not known at installation time but will be deter- cone unarmored cables mined to sufficient accuracy from the recordings of earthquake waves attached to 41/2-in casing pipes by the seismometers. hanging from uplink reentry cone downlink commands power The multiple-access expandable gateway (MEG) seafloor package dig- cased section analog/digital sensor signals sensor status change temperature multiple sensors itizes and combines the analog-to-digital (A/D) data from downhole to borehole strain (DTM) tilt (AGI) seismic (CMG/PMD) one serial data stream. It also distributes the power to individual mod- coupled to borehole ules. The data are stored in digital format in the storage acquisition sensor string with cement module (SAM) via an RS232 link using Guralp compressed format (GCF) uncased section protocol. The SAM has four 18-GB small computer system interface (SCSI) hard disks equivalent to more than seven channels of 1-yr-long continuous data of 24-bit dynamic range at 100-Hz sampling rate. In this case, there are three channels each for the seismometers (100 Hz for the CMG seismometer and 20 Hz for the PMD seismometer), two chan- nels for the tiltmeter (4 Hz), and nine channels for the three-compo- nent and three channels for the single-component strainmeter (50-Hz sampling rate). The borehole sensors are permanently cemented in as required for the strainmeter operation and to assure good coupling for the other sensors. The MEG, on the other hand, may be serviced by a submersible remotely operated vehicle (ROV). The MEG can be physi- cally replaced and accepts commands and software upgrades through the SAM. The SAM needs to be replaced by a submersible ROV before the disks become full, which takes ~1 yr in the present design. The back from ocean bottom module (BOB) provides a communica- tion link to the borehole system while the station is being serviced by a submersible ROV. The BOB can extract data through the SAM, calibrate the clock, and check the health of the system. Except for the BOB, all power is supplied by the seawater battery (SWB) system, which can provide ~18 W with more than 300-kWhr ca- pacity. Its source of energy is the magnesium anode, which needs to be replaced once it is consumed by electrolytic dissolution. Replacement of the magnesium anode is also handled by a submersible ROV. SHIPBOARD SCIENTIFIC PARTY CHAPTER 3, BOREHOLE INSTRUMENT PACKAGE 3 Site Characteristics The Japan Trench is probably one of the most suitable sites for test- ing various ideas associated with the episodic nature of plate subduc- tion and its relation to earthquakes and tectonics. Although the seismogenic zone itself cannot be reached by present drilling capability, the present target depth of ~1 km below seafloor provides stations within 10 km of the seismogenic zone of the plate boundary. Since the plate dip angle is very shallow (<10°), a wide area can be viewed by these stations. The nearest geodetic stations are coastal stations ~50 km above the plate boundary; that part of the plate interface may be below the lower-dip end of the seismogenic zone. Site 1150 and Site 1151 can be considered as two end-members in terms of seismicity. The background seismicity is high around Site 1150 but the area around Site 1151 is aseismic. The former has experienced M7-class events, but there is no historical record of large earthquakes rupturing the area of Site 1151. Although both the microseismicity and large events have contrasting characteristics, there is no compelling evi- dence of structural differences between the two sites. For high-sensitivity measurements, pressure fluctuations caused by ocean long waves or temperature changes can be sources of noise, as can water motion near the sensor. Thus the sites are located at key geo- physical coordinates and in boreholes for noise reduction. Because of its principle of operation, the strainmeter needs to be grouted inside the borehole to respond accurately to strain changes in the surrounding rock. INSTALLATION TECHNIQUES Requirements A major objective of the installation of the observatories is to moni- tor the changes in the deformation state of the overlying strata in an ac- tive subduction zone. To achieve this, a suitable instrument, described in “Borehole Instruments,” p. 5, has to be in intimate contact with the rock. This is accomplished by cementing a resilient instrument in the bottom of an open hole in competent, indurated rock. Although all in- struments benefit to some extent by being emplaced in competent rock, this is critical for successful strain monitoring. Depths at least 1 km below seafloor are generally required in areas with thick sediment cover. One complication of subseafloor installations results from having to cope with irreducible ship heave during hole entry. Since heave can be 1 m or more, cables linking the instruments with the seafloor data han- dling units have to be protected from stresses created by the relative motion between the cables and the hole wall and between the cables and any insertion tools. Even though the heave compensator is used during hole entry, the instrument string has to hang from the rig floor with no compensation while pipe is being added, and this is repeated every 10–30 m for more than 1 km of hole penetration. In addition, in order for the cement to set up properly, the instru- ment package has to be completely undisturbed for ~1 day after the ce- ment is introduced. SHIPBOARD SCIENTIFIC PARTY CHAPTER 3, BOREHOLE INSTRUMENT PACKAGE 4 Method The technique we developed to satisfy the installation requirements is illustrated in Figure F2. The instrument package is supported on 4½- F2. Instrument installation tech- in diameter casing pipe that hangs on the base of the reentry cone at nique, p. 24. J-type 103/4-in riser the seafloor.
Load more

Recommended publications
  • Sloping and Benching Systems
    Trenching and Excavation Operations SLOPING AND BENCHING SYSTEMS OBJECTIVES Upon the completion of this section, the participant should be able to: 1. Describe the difference between maximum allowable slope and actual slope. 2. Observe how the angle of various sloped systems varies with soil type. 3. Evaluate layered systems to determine the proper trench slope. 4. Illustrate how shield systems and sloping systems interface in combination systems. ©HMTRI 2000 Page 42 Trenching REV­1 Trenching and Excavation Operations SLOPING SYSTEMS If enough surface room is available, sloping or benching the trench walls will offer excellent protection without any additional equipment. Cutting the slope of the excavation back to its prescribed angle will allow the forces of cohesion (if present) and internal friction to hold the soil together and keep it from flowing downs the face of the trench. The soil type primarily determines the excavation angle. Sloping a method of protecting employees from cave­ins by excavating to form sides of an excavation that are inclined away from the excavations so as to prevent cave­ins. In practice, it may be difficult to accurately determine these sloping angles. Most of the time, the depth of the trench is known or can easily be determined. Based on the vertical depth, the amount of cutback on each side of the trench can be calculated. A formula to calculate these cutback distances will be included with each slope diagram. NOTE: Remember, the beginning of the cutback distance begins at the toe of the slope, not the center of the trench. Accordingly, the cutback distance will be the same regardless of how wide the trench is at the bottom.
    [Show full text]
  • SECTION A-A NTS 2" Grade Board Notches
    Figure 4.1 – Alternative Flow Dispersal Trench NOTES: 1. This trench shall be constructed to prevent point discharge 1' - 6" min galvanized bolts and /or erosion. *20% max 2. Trenches may be placed no closer than 50 feet to one *20% max 2" x 12" another (100 feet along flowline). pressure 3. Trench and grade board must be treated grade level. Align to follow contours of board site. 4" x 4" support 4. Support post spacing as post required by soil conditions to 2" x 2" notches ensure grade board remains 18" O.C. 36" max 12" min. clean (< 5% fines) level. 3 1 4" - 1 2" washed rock filter fabric *15% max for flow control/water quality 18" O.C. treatment in rural areas 2" SECTION A-A NTS 2" grade board notches Figure 4.2 – Pipe Compaction Design and Backfill O.D. O.D. limits of pipe W (see note 4) 3' max. 3' max. compaction limit of pipe zone 1' - 0" bedding material for flexible 1' - 0" pipe (see 0.15 O.D. min. note 6) B O.D. limits of pipe compaction 0.65 O.D. min. foundation level A* gravel backfill gravel backfill for for pipe bedding foundations when specified * A = 4" min., 27" I.D. and under 6" min., over 27" I.D. A. Metal and Concrete Pipe Bedding for Flexible Pipe span span span 3' max Flexible Pipe NOTES: 3' max 3' max 1. Provide uniform support under barrels. 1'-0" 2. Hand tamp under haunches. 3. Compact bedding material to 95% max.
    [Show full text]
  • Frequency and Magnitude of Selected Historical Landslide Events in The
    Chapter 9 Frequency and Magnitude of Selected Historical Landslide Events in the Southern Appalachian Highlands of North Carolina and Virginia: Relationships to Rainfall, Geological and Ecohydrological Controls, and Effects Richard M. Wooten , Anne C. Witt , Chelcy F. Miniat , Tristram C. Hales , and Jennifer L. Aldred Abstract Landsliding is a recurring process in the southern Appalachian Highlands (SAH) region of the Central Hardwood Region. Debris fl ows, dominant among landslide processes in the SAH, are triggered when rainfall increases pore-water pressures in steep, soil-mantled slopes. Storms that trigger hundreds of debris fl ows occur about every 9 years and those that generate thousands occur about every 25 years. Rainfall from cyclonic storms triggered hundreds to thousands of debris R. M. Wooten (*) Geohazards and Engineering Geology , North Carolina Geological Survey , 2090 US Highway 70 , Swannanoa , NC 28778 , USA e-mail: [email protected] A. C. Witt Virginia Department of Mines, Minerals and Energy , Division of Geology and Mineral Resources , 900 Natural Resources Drive, Suite 500 , Charlottesville , VA 22903 , USA e-mail: [email protected] C. F. Miniat Coweeta Hydrologic Lab , Center for Forest Watershed Research, USDA Forest Service, Southern Research Station , 3160 Coweeta Lab Road , Otto , NC 28763 , USA e-mail: [email protected] T. C. Hales Hillslope Geomorphology , School of Earth and Ocean Sciences, Cardiff University , Main Building, Park Place , Cardiff CF10 3AT , UK e-mail: [email protected] J. L. Aldred Department of Geography and Earth Sciences , University of North Carolina at Charlotte , 9201 University City Blvd. , Charlotte , NC 28223 , USA e-mail: [email protected] © Springer International Publishing Switzerland 2016 203 C.H.
    [Show full text]
  • Division 2 Earthwork
    Division 2 Earthwork 2-01 Clearing, Grubbing, and Roadside Cleanup 2-01.1 Description The Contractor shall clear, grub, and clean up those areas staked or described in the Special Provisions. This Work includes protecting from harm all trees, bushes, shrubs, or other objects selected to remain. “Clearing” means removing and disposing of all unwanted material from the surface, such as trees, brush, down timber, or other natural material. “Grubbing” means removing and disposing of all unwanted vegetative matter from underground, such as sod, stumps, roots, buried logs, or other debris. “Roadside cleanup”, whether inside or outside the staked area, means Work done to give the roadside an attractive, finished appearance. “Debris” means all unusable natural material produced by clearing, grubbing, or roadside cleanup. 2-01.2 Disposal of Usable Material and Debris The Contractor shall meet all requirements of state, county, and municipal regulations regarding health, safety, and public welfare in the disposal of all usable material and debris. The Contractor shall dispose of all debris by one or more of the disposal methods described below. 2-01.2(1) Disposal Method No. 1 – Open Burning The open burning of residue resulting from land clearing is restricted by Chapter 173-425 of the Washington Administrative Code (WAC). No commercial open burning shall be conducted without authorization from the Washington State Department of Ecology or the appropriate local air pollution control authority. All burning operations shall be strictly in accordance with these authorizations. 2-01.2(2) Disposal Method No. 2 – Waste Site Debris shall be hauled to a waste site obtained and provided by the Contractor in accordance with Section 2-03.3(7)C.
    [Show full text]
  • 3 Groundwork and Foundations
    P1: JZW/JZZ BY032-03 BY032-Emmitt BY032-v1.cls September 13, 2004 9:6 3 Groundwork and Foundations The foundation of a building is that part of walls, piers and columns in direct contact with, and transmitting loads to, the ground. The building founda- tion is sometimes referred to as the artificial foundation, and the ground on which it bears as the natural foundation. Early buildings were founded on rock or firm ground; it was not until the beginning of the twentieth century that concrete was increasingly used as a foundation base for walls. With the introduction of local and then national building regulations, standard forms of concrete foundations have become accepted practice in the UK, along with more rigorous investigation of the nature and bearing capacity of soils and bedrock. 3.1 Functional requirements The primary functional requirement of a foundation is strength and stability. Strength and stability The combined, dead, imposed and wind loads on a building must be transmit- ted to the ground safely, without causing deflection or deformation of the build- ing or movement of the ground that would impair the stability of the building and/or neighbouring structures. Foundations should also be designed and constructed to resist any movements of the subsoil. Foundations should be designed so that any settlement is both limited and uniform under the whole of the building. Some settlement of a building on a soil foundation is inevitable. As the building is erected the loads placed on the foundation increase and the soil is compressed. This settlement should be limited to avoid damage to service pipes and drains connected to the building.
    [Show full text]
  • Trench Blasting with DYNAMITE a TRADITION of INNOVATION
    Trench Blasting with DYNAMITE A TRADITION OF INNOVATION Dyno Nobel’s roots reach back to every significant in- novation in explosives safety and technology. Today, Dyno Nobel supplies a full line of explosives products and blasting services to mines, quarries and contractors in nearly every part of the world. DYNAMITE PRODUCT OF CHOICE FOR TRENCH BLASTING One explosive product has survived the test of time to become a true classic in the industry. DYNAMITE! The dynamite products manufactured today by Dyno Nobel are similar to Alfred Nobel’s original 1860s invention yet, in selected applications, they outperform any other commercial explosives on the market. The high energy, reliability and easy loading characteristics of dynamite make it the product of choice for difficult and demand- ing trench blasting jobs. Look to Unigel®, Dynomax Pro® and Unimax® to make trench blasting as effective and efficient as it can be. DISCLAIMER The information set forth herein is provided for informational purposes only. No representation or warranty is made or intended by DYNO NOBEL INC. or its affiliates as to the applicability of any procedures to any par- ticular situation or circumstance or as to the completeness or accuracy of any information contained herein. User assumes sole responsibility for all results and consequences. ® Cover photo depicts a trench blast using Primacord detonating cord, MS ® Connectors and Unimax dynamite. SAFE BLASTING REMINDERS Blasting safety is our first priority. Review these remind- ers frequently and make safety your first priority, too. • Dynamite products will provide higher energy value than alternate products used for trenching due to their superior energy, velocity and weight strength.
    [Show full text]
  • Fact Sheet: Trenching and Excavation Safety
    Fact sheet Trenching and excavation safety Between 2016 and 2019, three Minnesota workers died in a trench, excavation or ground collapse. Cave-ins pose the greatest risk and are much more likely than other excavation-related accidents to result in worker fatalities. Minnesota law requires employers to provide a workplace free of recognized hazards that may cause serious injury or death. This includes the trenching and excavation requirements of 29 CFR 1926.651 and 1926.652. An excavation is any man-made cut, cavity, trench or depression in an earth surface, formed by earth removal. A trench – or a trench excavation – is a narrow excavation (in relation to its length) made below the surface of the ground; in general, the depth is greater than the width, but the width of a trench (measured at the bottom) is not greater than 15 feet. Dangers of trenching and excavation In addition to cave-ins, other potential hazards include falls, falling loads, hazardous atmospheres and incidents involving mobile equipment. Two cubic yards of soil can weigh about 6,000 pounds or as much as a small car. An unprotected trench is an early grave. Do not enter an unprotected trench. Trench safety measures Trenches five feet deep or greater require a protective system unless the excavation is made entirely in stable rock. If the trench is fewer than five feet deep, a competent person may determine a protective system is not required. Trenches 20 feet deep or greater require that the protective system be designed by a registered professional engineer or be based on tabulated data prepared and/or approved by a registered professional engineer in accordance with 1926.652(b) and (c).
    [Show full text]
  • Slope Stability
    Slope stability Causes of instability Mechanics of slopes Analysis of translational slip Analysis of rotational slip Site investigation Remedial measures Soil or rock masses with sloping surfaces, either natural or constructed, are subject to forces associated with gravity and seepage which cause instability. Resistance to failure is derived mainly from a combination of slope geometry and the shear strength of the soil or rock itself. The different types of instability can be characterised by spatial considerations, particle size and speed of movement. One of the simplest methods of classification is that proposed by Varnes in 1978: I. Falls II. Topples III. Slides rotational and translational IV. Lateral spreads V. Flows in Bedrock and in Soils VI. Complex Falls In which the mass in motion travels most of the distance through the air. Falls include: free fall, movement by leaps and bounds, and rolling of fragments of bedrock or soil. Topples Toppling occurs as movement due to forces that cause an over-turning moment about a pivot point below the centre of gravity of the unit. If unchecked it will result in a fall or slide. The potential for toppling can be identified using the graphical construction on a stereonet. The stereonet allows the spatial distribution of discontinuities to be presented alongside the slope surface. On a stereoplot toppling is indicated by a concentration of poles "in front" of the slope's great circle and within ± 30º of the direction of true dip. Lateral Spreads Lateral spreads are disturbed lateral extension movements in a fractured mass. Two subgroups are identified: A.
    [Show full text]
  • Our Broad (TUNNEL) Vision
    5 OUR BROAD Royal Boskalis Westminster N.V. Rosmolenweg 20 PO Box 43 (TUNNEL) VISION 3350 AA Papendrecht The Netherlands [email protected] Immersed tunnel execution brings Boskalis’ fields of expertise Our in-house experts are from a broad range of disciplines T +31 78 6969000 together. that span the capabilities necessary for immersed tunnel F +31 78 6969555 activities, including: Construction is technically complex and involves an extreme www.boskalis.com degree of accuracy. Design of the elements is governed by Geotechnics temporary phases, meaning that the early involvement of Hydraulic Processes an experienced contractor is crucial to producing optimal Maritime Engineering solutions. Environmental Management IMMERSED Boskalis combines both operational and engineering They work in close cooperation with structural design capabilities and is able to handle the entire installation partners and combine insight about the design process with process, from feasibility studies to completion. a thorough understanding of the practicalities of works, which often leads to opportunities for clients. TUNNELS Immersed tunnel execution involves our core activities of dredging, rock placing, inland infrastructure and towage. Boskalis has the skills, resources and experience to engineer Our fleet of specially developed marine equipment enables and execute all marine activities involved in any immersed us to execute them safely and with the required precision. tunnel project, anywhere. OUR SOLUTIONS 1. Construction of building dock and water management 6. Outfitting 2. Dredging the tunnel trench 7. Immersion management and survey (including closure 3. Laying the gravel or sand-flow foundation joint) 4. Towage 8. Locking fill, trench backfill and rock protection 5. Transportation overseas 9.
    [Show full text]
  • Trenching Precautions to Take Before Trenching and Tips on Safety
    Safety Matters is intended to promote discussions of safety issues among underground construction professionals. You should always read and understand the operator’s manual before operating any equipment. For additional information, please e-mail [email protected]. INFORMATION/FACTS TOPIC: • Trench sides can cave in up to a distance of 3/4 x the depth of the trench. For example, if the trench is 4 feet (1.2 Trenching m) deep, the cave-in can start up to 3 feet (0.9 m) away from trench. POTENTIAL HAZARDS • Trenchers can react if the chain hits an underground • Caught in obstruction such as a tree branch or concrete footing. On • Struck by pedestrian machines, this can cause the handles to be • Laceration forced downward suddenly. • Drawn into • Machine will move when digging starts. • Buried hazards TALES FROM THE TRENCH • Falls onto • The injured was using a shovel on the right side of the digging chain to cave off the trench side and allow the PRECAUTIONS digging chain to pull soil from the trench. The accident • ALWAYS have all underground utilities located prior to occurred when the injured got too close and attempted to digging. Know and follow local regulations for excavating near step across the trench with his left leg. His foot slipped utilities. down the trench side and into the digging chain. His leg • Stay away and keep others at least 6 feet (1.8 m) away from was amputated below the knee and he had severe tissue machine and moving parts of machine. Stop machine if damage to his thigh.
    [Show full text]
  • Soil, Plant and Water Reference Methods for the Western Region1
    SOIL, PLANT AND WATER REFERENCE METHODS FOR THE WESTERN REGION 1 2005 3rd Edition Dr. Ray Gavlak Dr. Donald Horneck Dr. Robert O. Miller 1 From: Plant, Soil and Water Reference Methods for the Western Region. 1994. R. G. Gavlak, D. A. Horneck, and R. O. Miller. WREP 125. PREFACE This manual has been developed as a guide of standard analytical methods for agricultural laboratories for use in the Western Region, byt the Western Coordinating Committee on Nutrient Management. This publication an update of Plant, Soil and Water Reference Methods for the Western Region , 1994, (WREP 125) written by Dr. Ray Gavlak formerly of the University of Alaska, Dr. Donald Horneck of Oregon State University, and Dr. Robert O. Miller of Colorado State University. These represent accepted methods for the analysis of soil and plant samples and were selected for the express purpose of identifying common methods on which a group of agricultural laboratories analytical results can be statistically evaluated. We would like to thank all those individuals who have contributed to this manual. The specific soil, plant and water analytical methods listed represent those analytical procedures that are recommended for use in the current North American Proficiency Testing Program organized by the Soil Science Society of America. We encourage all suggestions and comments from participating laboratories for improving this manual for future publication. The authors would like to thank Dr. Byron Vaughan of MDS Harris Laboratory Services, Dr. Kelly Belden of the University of Wyoming for reviewing this publication. WREP-125, 3 rd Edition TABLE OF CONTENTS Page Quality Assurance in the Agricultural Laboratory .....................................
    [Show full text]
  • Gabion Basket Installation Construction Design Standards
    Gabion Basket Installation Construction Design Standards, suggest that a 500mm trench be dug to allow for future excavations in front of the wall, this also gets below the frost layer, and provides a toe in to stop the baskets slipping, (This toe in is not normally used within the structural calculations for the standard designs). Another reason for this is that the top soil contains organic matter that may rot or compress. The majority of installations for short walls are started at ground level, it is down to the customer to make a decision on site looking at the ground conditions. The worst material for gabion walls to sit on is soft clay, if the ground is likely to sink under baskets weight, a gabion retaining wall may not be possible. 100-200mm of hardcore (type 1) should be compacted with a vibrating plate as a footing for the gabion wall. If 200mm was put in the 500mm deep trench the baskets would start 300mm below ground. Baskets should be lent back at 6 degrees. The Face of a gabion wall can be flushed or stepped. On taller walls baskets are stepped back to equalize the pressure between the heel and toe of the wall. Gabions can be cut on site to achieve your required dimensions. The standard thickness for walls up to 3m in height is 3mm. 4mm is the architectural spec, 5mm is what we would refer to as the military spec or for use on higher walls. For inexperienced installers a 4mm face can be added for walls 3m and below to reduce the risk of bulging.
    [Show full text]