Pore pressure response during failure in soils EDWIN L. HARP U.S. Geological Survey, 345 Middlefield Road, M.S. 998, Menlo Park, California 94025 WADE G. WELLS II U.S. Forest Service, Forest Fire Laboratory, 4955 Canyon Crest Drive, Riverside, California 92507 JOHN G. SARMIENTO Wahler Associates, P.O. Box 10023, Palo Alto, California 94303 111 45' ABSTRACT Three experiments were performed on natural slopes to investi- gate variations of soil pore-water pressure during induced slope fail- ure. Two sites in the Wasatch Range, Utah, and one site in the San Dimas Experimental Forest of southern California were forced to fail by artificial subsurface irrigation. The sites were instrumented with electronic piezometers and displacement meters to record induced pore pressures and movements of the slopes during failure. Piezome- ter records show a consistent trend of increasing pressure during the early stages of infiltration and abrupt decreases in pressure from 5 to 50 minutes before failure. Displacement meters failed to register the amount of movement, due to location and ineffectual coupling of meter pins to soil. Observations during the experiments indicate that fractures and macropores controlled the flow of water through the slope and that both water-flow paths and permeability within the slopes were not constant in space or time but changed continually during the course of the experiments. INTRODUCTION The mechanism most generally ascribed (for example, Campbell, 1975, p. 18-20) to account for rainfall-induced failure in soils indicates that an increase in the pore-water pressure within the soil mantle results in a reduction of the normal effective stresses within the material. Failure Figure 1. Location of experiment sites 1 and 2, north of Salt Lake occurs when pore pressure increases to the point that the frictional resis- City, Utah. Ricks Creek' Figure 2. Oblique aerial photo- graph showing location of sites 1 and 2 and local features. View to east. Geological Society of America Bulletin, v. 102, p. 428-438, 12 figs., 1 table, April 1990. 428 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/4/428/3380785/i0016-7606-102-4-428.pdf by guest on 02 October 2021 PORE PRESSURE RESPONSE DURING FAILURE IN SOILS 429 tance to downslope movement drops below the downslope component of Accordingly, typical characteristics of pore-pressure behavior during fail- weight. ure could be delineated, and any premonitory responses could be discov- Iverson and Major (1986), however, favor the use of seepage forces ered and evaluated. The experiments were carried out at three sites, two in to describe slope stability. Their analysis concluded that minimum slope north-central Utah and one in southern California. Landslide nomencla- stability occurs when the angle between the seepage force vector and the ture used in this report is after Varnes (1978). normal to the slope, A, is 9QP-<f>, where 4> is the internal angle of friction of the slope material. This implies that on steep slopes, where the slope angle is nearly equal to cj>, horizontal seepage results in minimum slope stability. EXPLAN ATION Numerous studies have measured pore-water pressures in slopes and • Pz6 (66.0) Piezometer, location have recorded the response of the soil to both seasonal and storm rainfall number to side, (Wieczorek, 1987; Keefer and Johnson, 1983). Few data exist, however, depth in cm in parentheses which show the pore-pressure response of a slope during failure. One recent study from Alaska reported positive pore pressures in piezometers 10.6 Surface contours in adjacent to a failure and suggested that the magnitude of these pressures m, elevations relative to arbitrary causing failure can be quite small (Sidle and Swanston, 1982). control points The experiments described here were designed so that pore-water I DM-1 -, pressure could be monitored in slopes during the actual failure process. Displacement meter line "TTTT- Failure scarp, hachures on hanging wall Figure 3. Location of in- struments with respect to soil failure at site 1 near Ricks Creek, Utah. (68.6) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/4/428/3380785/i0016-7606-102-4-428.pdf by guest on 02 October 2021 430 HARP AND OTHERS UTAH SITE2 o s 111 49'W CENTER TRENCH — . _ OF • 40°53'N - Pz4 (30.5) Pz6 (35.5) Pz8 (35.5) Pz3 (61.0) Pz5 (61.0) Pz7 (61.0) 2.5m Pz9 (55.8) V 2.0m Figure 4. Location of in- struments with respect to soil SCARP failure at site 2 near Ricks Creek, Utah. Pz1 O (53.3) 1,5m Pz11 (50.8) 1.0m VERTICAL CUT 0.5m 0 1 meters Om EXPLANATION Pz4 (3 0.5) Piezometer, location number to side, depth in cm in parentheses 0.5m Surface contours in m, elevations relative to arbitrary control points DM Displacement meter line Failure scarp, hachures on hanging wall, dashed where fracture only SITE DESCRIPTIONS AND INSTRUMENTATION had been active during the period of intense debris-flow formation in May-June, 1983. The head of the slide complex contained several incip- Ricks Creek, Wasatch Range, Utah ient soil slides and slumps that had begun to undergo failure in 1983. We decided to attempt to initiate further sliding of this material and also of Two sites were chosen near Bountiful, Utah, in the Wasatch Range adjacent unfailed material near the head of the slide complex (sites 1 and about 20 km north of Salt Lake City (Fig. 1). The sites are located in the 2; Fig. 2). headwaters of Ricks Creek at about 2,500 m elevation. They are located at The headwall scarp of the slump/debris-flow complex is located just the head of a slump/debris-flow complex developed in residual soils that downslope from a nearly horizontal topographic bench within a cirque- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/4/428/3380785/i0016-7606-102-4-428.pdf by guest on 02 October 2021 PORE PRESSURE RESPONSE DURING FAILURE IN SOILS 431 11 8° 00' of the slope and to observe the effects of the seepage of water from the upper trench through the slope. At site 2, a single trench 2 m long and about 0.6 m deep (Fig. 4) was dug along the slope strike at the head of a 43° planar slope. The flow of water from the trenches into the slope was intended to simulate the flow of water through the soil that occurs when the water table (perched or annual) is extremely high and near the surface. We had previously at- tempted experiments to trigger slope failures by the use of sprinkler systems. Even at sprinkling rates many times higher than normal rainfall and duration rates, all attempts were unsuccessful. Despite the fact that water infiltration from a trench may not exactly reproduce natural water- flow conditions in soils, it does introduce a high water table, elevate pore-water pressures, and initiate failures. The objective of the experiments was to study the pore-water pressure variation and slope deformation during failure rather than to exactly reproduce natural ground-water con- ditions. During the course of the experiments, the trenches were kept as full as possible to saturate the slopes. Data from the piezometers and displacement meters were recorded digitally and stored on cassette tapes. Monroe Canyon Site, California: San Dimas Experimental Forest A site on the west side of Monroe Canyon near the summit of a Dimas Experimental Forest, southern California. prominent peak in the San Dimas Experimental Forest near Glendora, California (Fig. 5), was chosen to initiate a soil failure because of the numerous debris flows that had occurred there in previous years. The slopes at this site are grass covered; they had been converted from chapar- shaped valley that contains several abandoned beaver ponds. The beaver ponds are fed by springs that produce water at a combined rate of -175 1/min, which is enough to infiltrate the slopes at the experiment sites and cause small soil failures. The ponds are about 100 m from the experiment sites. Water was pumped from the ponds into a 220-1 holding bag just SAN DIMAS SITE above the sites, to be dispensed into trenches dug within the slopes to be failed. The locations of the sites and ponds with respect to each other are 3340 shown in Figure 2. Bedrock at the sites consists of Archean and early Proterozoic gneiss and schist of the Farmington Canyon Complex. The soils vary considera- bly but are of three basic types. Soil at site 1 at the head of the slide complex is a gravelly, silty sand. Site 2 (Fig. 2) has a surficial soil consist- ing of permeable silty sand about 0.6 m thick underlain by a relatively impermeable bluish-gray silty sand (silt fraction 22%-25%). Site 1 was located at the head of the slide complex on a 6-m2 soil slide that had become detached from soil above the main slide scarp but had not mobilized into a debris flow (Fig. 2). Nine electronic piezometers were installed within the incipient slide mass at depths varying from 30.5 to 172.7 cm (Fig. 3). Each piezometer is constructed of a differential pressure transducer housed in a cylindrical nylon chamber having a coni- cal tip (see Harp and others, 1984). The pressure resolution of the pie- zometers is ~0.6 mm head, and the accuracy is ±1.3 mm head. Two displacement meters were also installed on the slide; these consisted of light-weight braided wire cable attached to 10-turn potentiometers. The EXPLANATION failure surface of the soil slide was —70 cm deep.