High Pore Water Pressure Research Articles

Landslides on Fluidlike Zones in the Deposits of Glacial Lake Hitchcock, Windsor County, Vermont

Landslides, in fine-grained lake deposits of glacial Lake Hitchcock, Vermont, consist of blocks of material that moved downslope on a layer of fluidlike material. The main cause of these slope landslides was high pore-water pressures. The 6 months preceding the landslides were marked by above-average precipitation leading to saturated soils and elevated pore-water pressures. About the time of the landslides in late May 1984, there were 3 days of rain with an intensity of 1 to 3 in. per day. Many of the 1984 landslides occurred in the vicinity of old landslides. The landslides have well-developed head scarps and are bounded by either lateral boundary shear planes or en echelon tension fractures. Tension fractures are common throughout the landslides, and in many places they divide the lower portions of the landslide into separate blocks. In many cases, the fluidlike soil from beneath the blocks squeezed into the fissures that developed between the blocks. Lobes of fine-grained, saturated material came out from beneath the slide blocks in the scarp area. These lobes have snouts similar to those found on debris-flow deposits.

Engineering geology of Quaternary soils: II Methods of treatment

Abstract The various soils of Quaternary age present a wide range of engineering problems such as low density, low strength, high compressibility, high permeability, collapsibility and high swell/shrink potential, as well as difficulties caused by the presence of high water tables and zones of high pore water pressures. This introductory paper describes a number of methods of ground treatment that can be used to alleviate these problems including exclusion techniques, grouting, ground freezing, drainage and groundwater lowering techniques, electro-osmosis and electro-chemical stabilization techniques, compaction and reinforcement, and methods of chemical and thermal stabilization.

Surface profiles and basal shear stresses of outlet glaciers from a Late-glacial mountain ice field in western Scotland

AbstractSurface and basal long profiles are reconstructed for 13 outlet glaciers that drained ice from a large ice field (80 km by 120 km) that formed in the western Grampians of Scotland during part of the Late-glacial period (c, 14000–10000years BP). Basal shear stresses are calculated at 5 km intervals along the central flowlines of the reconstructed outlet glaciers. Individual basal shear stresses for the outlet glaciers range from 10 to 204 kPa. Variations in calculated basal shear stresses within and between the glaciers are mainly explained by differences in bedrock topography, extending and compressional flow, and by differences in basal boundary conditions. Low basal shear stresses (<53kPa) calculated for the terminal zones of Creran, Menteith and Lomond glaciers are partly explained by the overriding of glaciomarine clays with inferred high pore-water pressures and a low yield strength that may have led to rapid basal sliding and thinning of the ice lobes.

Surface profiles and basal shear stresses of outlet glaciers from a Late-glacial mountain ice field in western Scotland

AbstractSurface and basal long profiles are reconstructed for 13 outlet glaciers that drained ice from a large ice field (80 km by 120 km) that formed in the western Grampians of Scotland during part of the Late-glacial period (c, 14000–10000years BP). Basal shear stresses are calculated at 5 km intervals along the central flowlines of the reconstructed outlet glaciers. Individual basal shear stresses for the outlet glaciers range from 10 to 204 kPa. Variations in calculated basal shear stresses within and between the glaciers are mainly explained by differences in bedrock topography, extending and compressional flow, and by differences in basal boundary conditions. Low basal shear stresses (<53kPa) calculated for the terminal zones of Creran, Menteith and Lomond glaciers are partly explained by the overriding of glaciomarine clays with inferred high pore-water pressures and a low yield strength that may have led to rapid basal sliding and thinning of the ice lobes.

Nonlinear response of dams founded on alluvial deposits in narrow canyons

This paper presents a simplified model for the response of earth dams founded on alluvial deposits at the base of canyons. The goal of the model is to capture the effect of the most important factors influencing the response and to provide a realistic and efficient tool for preliminary safety evaluations. The developed theoretical closed-form solution combines the effects on the response of: (a) the foundation layer, (b) the canyon narrowness, (c) the variation of soil stiffness within the dam versus depth, and (d) soil nonlinearity. The latter is incorporated in a simple piece-wise linear scheme, in which the soil shear moduli and damping ratios within the dam and the layer are continuously updated at small time intervals during the analysis. The presence of a soft foundation layer is found to have a dramatic effect on the natural periods of the dam. A series of parametric studies is undertaken to investigate the effect on the response of the fundamental geometric and material characteristics of the dam-layer-canyon system subjected to transient and harmonic motion. The results show that a foundation layer with an average S -wave velocity similar or less than that of the dam under moderate or strong base excitation may experience significant shear strains. Such strains are capable of inducing high excess pore water pressures and perhaps liquefaction within contractive saturated cohesionless soils in the foundation. The cost of the simplified analysis is about three orders of magnitude less than that of an elastoplastic FE analysis.

Ice-marginal thrusting of drift and bedrock: thermal regime, subglacial aquifers, and glacial surges

Glacial thrust systems composed of blocks of drift and bedrock, associated with hummocky stagnation moraine along the margin of the Rainy lobe of the Laurentide Ice Sheet in Minnesota, are used in conjunction with paleoecological studies to constrain a numerical model of the ice-marginal thermal regime. Subglacial meltwater production in the thawed-bed zone was at least two orders of magnitude greater than the amount that could refreeze to the base of the glacier near the margin. The excess water recharged a thick subglacial aquifer, and thrust-system development was enhanced by the presence of a frozen toe and high pore-water pressures beneath the outer 2 km the glacier. The pore-water pressure required for thrusting is calculated from overburden pressures and basal shear stresses determined by numerical modeling. The heat generated by flow of water through the subglacial aquifer substantially affects the ice-marginal thermal regime, making a steady-state frozen toe 1.0–2.0 km in width unstable. Thrusting apparently occurred during multiple oscillations, or surges, when the ice was advancing over permafrost.

Postfailure analysis: Tramping Lake causeway, Saskatchewan, Canada

A traffic causeway placed on the sediments of saline Tramping Lake failed during construction in the summer of 1982. Vertical subsidence has continued until present (1988). The failure mechanism was controlled by sedimentary structure and artesian groundwater conditions. The shear zone is in a soft, near normally consolidated lacustrine sandy silt unit 22 m thick. The lake basin contains lacustrine, deltaic, and fluvial deposits of postglacial origin. Artesian conditions in the Upper Cretaceous Judith River Formation and postglacial fluvial sand and gravel dominate the hydrogeology at the site. The failure took place along a composite slip surface when excess pore-water pressures developed during loading [Formula: see text]. The estimated effective friction angle from triaxial tests and back calculation was 27° assuming c′ = 0. However, a parametric analysis showed that at very high pore-water pressures the effective friction angle required for equilibrium is very sensitive to small variations in ru. The calculated cohesion at [Formula: see text] required for equilibrium was 3.9 kPa, whereas the remolded vane strength measured in the field was 5.0 kPa. Key words: Foundation failure, artesian, saline environment, groundwater discharge, silty clays, postglacial fluvial and lacustrine deposits.

Dewatering and extensional deformation of the Shikoku Basin hemipelagic sediments in the Nankai Trough

The Shikoku Basin hemipelagic sequence, which underlies the Nankai Trough wedge, S.W. Japan, thins by 50% between the outer edge of the trench wedge and DSDP Site 582, 14 km arcward. A sedimentation model, which incorporates changes in sedimentation rates with time and with distance from the trench wedge toe, indicates that 57% of the total thinning occurs as a result of temporally varying sedimentation rates and a time transgressive facies boundary between the trench wedge turbidites and the underlying hemipelagites. Burial-induced consolidation beneath the wedgeshaped turbiditic overburden, accounts for the remaining 43% of arcward thinning within the hemipelagic unit. Rapid dewatering, modeled as one-dimensional consolidation suggests that the excess pore water pressures are quite low during this progressive dewatering. Thus, high pore water pressures should not be assumed to occur universally in convergent margin settings. Normal faults and vein structures in the hemipelagites suggest near-horizontal extension in addition to vertical consolidation. The estimates of excess pore water pressures together with fault geometries and horizontal extensional strains, determined from the geometry of the subducting oceanic plate, can be used to constrain the stress conditions at failure. The expulsion of hot water from the rapidly dewatering sediments in the Nankai Trough may help to explain anomalously high heat flow in the central part of the trough.

The Birth and Growth of Porsild Pingo, Tuktoyaktuk Peninsula, District of Mackenzie

The birth and growth of Porsild Pingo (ice-cored hill) can be taken as fairly representative of the birth and growth of the more than 2000 closed-system pingos of the western arctic coast of Canada and adjacent Alaska. Porsild Pingo, named after a distinguished arctic botanist, has grown in the bottom of a large lake that drained catastrophically about 1900. Porsild Pingo has grown up at the site of a former shallow residual pond. The "birth" probably took place between 1920 and 1930. The high pore water pressure that caused updoming of the bottom of the residual pond to give birth to Porsild Pingo came from pore water expulsion by downward and upward permafrost growth in saturated sands in a closed system. In the freeze-back period of October-November 1934, permafrost ruptured and the intrusion of water into the unfrozen part of the active layer grew a 3.7 m high frost mound photographed by Porsild in May 1935. Porsild Pingo has grown up at, or very close to, the site of the former frost mound. The growth of Porsild Pingo appears to have been fairly steady from 1935 to 1976, after which there has been a decline to 1987. The growth rate has been nearly linear with height, from zero at the periphery to a maximum at the top. The present addition of water to the pingo is about 630 cu m/y. Providing there is no major climatic change. Porsild Pingo may continue to grow for a few centuries.Key words: frost mound, intrusive ice, permafrost, pingo, Porsild

Magnetic and pebble fabrics and origin of the Sunnybrook Till, Scarborough, Ontario, Canada

The early Wisconsinan Sunnybrook Till is a mud-rich, pebble-poor, dark gray to gray–brown massive diamicton that is exposed in the Scarborough Bluffs, Ontario. It is interpreted here to be till.Previous separate, magnetic, and pebble fabric studies made on the till suggest a westward ice movement ranging from west of south to northwest. In this study, two dominant directions are found, northwest by west and northeast by north. Of these two directions, the northwest by west direction is the most common and this combined with the direction of shear in the underlying sediments and lithology of the till suggests that the glacier moved in a westerly direction.However, the fabrics are not typical of a lodgement till and to some extent resemble fabrics found in subaquatic debris-flow deposits. It is suggested that the glacier that deposited the Sunnybrook Till was buoyed up by high pore water pressure in the accumulating sediment at the sole of the glacier. Under these conditions, the till either flowed away from the zone of release at the base of the glacier and (or) was being lightly smeared by a partially grounded glacier.

Waipoapoa Landslide: A deep-seated complex block slide in Tertiary weak-rock flysch, Southern Hawke's Bay, New Zealand

Abstract The Waipoapoa Landslide (February 1976) in Southern Hawke's Bay is a complex failure in Upper Miocene weak alternating sandstone and mudstone (flysch). The landslide extends over approximately 18 ha and forms a reactivated portion of a larger ancient landslide complex. The Waipoapoa Landslide has a calculated volume of 8.35 × 106 m3. Morphologically the landslide is comprised of a head zone, front face, and debris-flow complex. Rock-mass defects have controlled slide block geometry. The crown escarpment of the head zone has propagated on nearly vertical intersecting fractures (joints and faults). The intersecting subhorizontal basal shear surface is coincident with bedding contact(s) in an alternating succession of friable porous sandstone and weakly consolidated slake-prone mudstone. Bedding attitude is inclined into the slope, with dips ranging up to 13°. Block movement has occurred against dip direction and has imparted a back-tilted geometry to the head zone depression. A multiple basal shear surface, and basal shear-gouge zone are inferred. The front face is postulated to have bulged outward during the main movement phase and successively collapsed, generating large, rapidly moving debris flows with an estimated volume of 1.75 × 106 m3. These flows stretch is excess of 900 m downvalley from the landslide toe. Failure mechanisms included both sliding and flowage. The failure involves several large blocks and is of multiple block geometry. Principal factors contributing to failure could have included: the presence of fractures (joints and faults) and other defects (e.g., bedding) along which block movement was facilitated; rapid ground-water infiltration along open fractures within the rock mass; high pore-water pressures in one or more porous sandstone beds; and the local steep relief.

Design Problems in Soil Liquefaction

An attempt is made to clarify some aspects of the problems encountered in evaluating the stability of embankments under conditions where a potential for soil liquefaction exists. It is suggested that at the present time, the most prudent method of minimizing the hazards associated with liquefaction‐induced sliding and deformations is to plan new construction or devise remedial measures in such a way that either high pore water pressures cannot build up in the potentially liquefiable soil, and thus liquefaction cannot be triggered, or, alternatively, to confine the liquefiable soils by means of stable zones, so that no significant deformations can occur; by this means, the difficult problems associated with evaluating the consequences of liquefaction (sliding or deformations) are avoided. However, when large deformations can possibly be tolerated, it may be adequate and economically advantageous to simply ensure the stability of the embankment against major sliding after liquefaction has occurred. Evaluati...

Stress release, undrained storage, and reconsolidation in simulated underwater clay

Triaxial testing has examined the influence of storage time and reconsolidation procedures on normally consolidated and overconsolidated samples of reconstituted clay prepared with high pore-water pressures to simulate underwater deposits. "Samples" of illite were consolidated one dimensionally, off-loaded without drainage, stored undrained, reconsolidated, and sheared undrained. The storage times were 15 min, 1 day, and 7 days. Reconsolidation pressures were (1) isotropic to [Formula: see text], (2) isotropic to [Formula: see text], and (3) anisotropic to [Formula: see text], [Formula: see text]. The results were compared with "specimens" that had not been off-loaded.Different undrained storage times had no effect on the strengths after reconsolidation, but the strengths were affected by the reconsolidation pressures. Anisotropic reconsolidation produced the best agreement between the control specimens and samples in terms of their strengths and Af-values. The duration of storage time did affect stress–strain parameters such as the undrained compression modulus E50 and the pore-water pressure parameter Af. Key words: underwater, offshore, clay, illite, sampling disturbance, triaxial, storage, reconsolidation, undrained, strength, stiffness.

Alleghanian episode of K-bentonite illitization in the southern Appalachian Basin

Mixed-layer illite-smectite (I/S) from Middle Ordovician potassium (K)-bentonites was found to be uniformly illitic throughout the southern Appalachian Basin regardless of variable depths of burial of the K-bentonites. The K/Ar ages of illitization are narrowly confined between 272 and 303 Ma (Late Pennsylvanian to Early Permian); this suggests that illitization was a short-lived episode coincident with, and prompted by, the Alleghanian orogeny. The illitization is explainable by the fluid expulsion hypothesis recently proposed by others (e.g., Oliver). Hot saline fluids were flushed to the basin edges from the deeply buried part of the foreland basin during the orogeny; these fluids are thought to be effective agents of illitization. Some of our K-bentonite samples are stratigraphically and geographically close to Mississippi Valley-type lead-zinc deposits, suggesting a similar mode of origin. Rapid illitization of shales, in a fashion similar to that observed for the bentonites, should have led to high pore-water pressures that enabled them to act as ideal decollements during Alleghanian thin-skinned deformation.

The flood plain deposits of the Lower Thames

Summary The flood plain of the tidal Thames is underlain with soft clays, peats, silts and sands which in turn rest on gravel. These deposits vary in thickness from a few to tens of metres. As with all soft deposits considerable care is required in order to obtain good quality samples for geotechnical testing. The wide range of soils which often occur down single soil profiles and from location to location can make it very difficult to assign soil parameters for design purposes and careful categorization is required. The presence of varying percentages of organic matter as well as silt and sand lenses in the clays can lead to difficulties in the interpretation of in situ tests such as the field vane. While the foundations for large structures are taken down to the underlying gravel and chalk, problems arise from services and smaller foundations. Stability problems arise during the design and construction of road and flood embankments. Since the gravel aquifer is exposed in the river channel along much of the length, pore water pressures transmitted through gravel during high tides can affect the stability of flood banks. High pore water pressures in the gravel can also lead to instability of excavations unless the pressures are relieved by groundwater lowering. When it is necessary to lower the water pressures for construction purposes consideration needs to be given to the additional settlement of adjacent ground and small structures founded on the alluvium. The paper looks at the distribution of the various soil types, gives typical geotechnical properties and discusses some engineering implications.

Rock slope movements in the Canadian Cordillera

Some recent work on natural rock slope movements in the Cordillera is reviewed. Climate, geology, and seismicity combine to produce rockslides in the Mackenzies and Rockies; slower, more complex bedrock flows in the Omineca Belt; spreads and flows within the volcanics of the Intermontane Belt; and rockslides, falls, debris flows and earthflows in the Coastal and Insular Belts.Major rockslides occur on overdip slopes in sedimentary, metamorphic, and volcanic successions. Natural causes of the slides include erosion of the toes and lateral margins of the rock slopes, destruction of cohesion along discontinuities by physical weathering and solution, high pore-water pressures, and seismicity. Some of the rockslides from the Mackenzies and from the young volcanoes in the Coast Plutonic Complex have shown travel distances that are extremely high for their volumes.While no regional mapping of landslide risk has been carried out in the Cordillera, some regional mapping of landslides has been undertaken, usually as part of a surficial geology mapping program. Detailed studies of hazards have sometimes followed settlement rather than preceded it, though some useful maps of particular hazards have been produced. In two cases, landslide hazard has caused the relocation of communities. Key words: landslides, rockslides, spreads, flows, rockfalls, natural hazards, Cordillera, Rockies.

Frost heaving of bedrock in permafrost regions : Dyke, L D Bull Assoc Engng GeolV21, N4, Nov 1984, P389–405

Frost heaving of bedrock in permafrost areas is a widespread process that produces a variety of landforms and exhibits movements of possible concern to engineering endeavors. While the study of ice growth in soils is an actively researched subdiscipline of soil mechanics, the same process in rock escaped notice until major engineering projects were considered in areas outside the dominantly soil covered parts of the North American arctic. Geology and hydrology exert a fundamental control on the style of the rock heave process. Displacement of single blocks, panels, and domes has taken place where saturated zones in the active layer become confined by the downward-advancing freezing front. The attempted expulsion of water by continued freezing supplies the heaving force, much in the manner of the closed-system pingo. This mechanism is favoured where the water table lies close to the surface in the massive granites, gneisses, and quartzites of the Canadian Shield. The mechanism requires that ice-filled cracks be able to seal in high pore water pressures. Field measurements show that water pressures up to 400 kPa are attained within the confined water bodies, a pressure considerably in excess of that needed to offset the overburden pressure. Where drier conditions prevail and rocks less resistant to weathering are present, the accumulation of weathering products in cracks can provide a site for heaving. In this case, the forces are produced by the growth of segregated ice in soil-filled cracks. This mechanism is common in sandstones and limestones and contributes to the mechanical breakdown of outcrops. Laboratory experiments confirm that horizontal movements can be produced by vertically-downward freezing in vertical soil-filled cracks. This mechanism is widespread because saturated conditions are not necessary. Resurveying of markers at several locations from Churchill, Manitoba to the central Arctic Islands reveals yearly movements of up to 5 cm, horizontally and vertically. Recorded movements are most common at sites subject to the ice segregation mechanism. Vertical movements tend to be cyclical due to settlement during thaw. Features formed by the water expulsion mechanism presumably reach an equilibrium height and show little further movement.

Frost Heaving of Bedrock in Permafrost Regions

Frost heaving of bedrock in permafrost areas is a widespread process that produces a variety of landforms and exhibits movements of possible concern to engineering endeavors. While the study of ice growth in soils is an actively researched subdiscipline of soil mechanics, the same process in rock escaped notice until major engineering projects were considered in areas outside the dominantly soil covered parts of the North American arctic. Geology and hydrology exert a fundamental control on the style of the rock heave process. Displacement of single blocks, panels, and domes has taken place where saturated zones in the active layer become confined by the downward-advancing freezing front. The attempted expulsion of water by continued freezing supplies the heaving force, much in the manner of the closed-system pingo. This mechanism is favoured where the water table lies close to the surface in the massive granites, gneisses, and quartzites of the Canadian Shield. The mechanism requires that ice-filled cracks be able to seal in high pore water pressures. Field measurements show that water pressures up to 400 kPa are attained within the confined water bodies, a pressure considerably in excess of that needed to offset the overburden pressure. Where drier conditions prevail and rocks less resistant to weathering are present, the accumulation of weathering products in cracks can provide a site for heaving. In this case, the forces are produced by the growth of segregated ice in soil-filled cracks. This mechanism is common in sandstones and limestones and contributes to the mechanical breakdown of outcrops. Laboratory experiments confirm that horizontal movements can be produced by vertically-downward freezing in vertical soil-filled cracks. This mechanism is widespread because saturated conditions are not necessary. Resurveying of markers at several locations from Churchill, Manitoba to the central Arctic Islands reveals yearly movements of up to 5 cm, horizontally and vertically. Recorded movements are most common at sites subject to the ice segregation mechanism. Vertical movements tend to be cyclical due to settlement during thaw. Features formed by the water expulsion mechanism presumably reach an equilibrium height and show little further movement.

Hydrogeologic Control and Statistical Prediction of Active Mass Movement

Based on an extensive research study of selected natural slopes on three continents, a special type of wide-spread mass movement is noted, explained and quantitatively analyzed. The paper focuses on the long-observed intensity of landslides on slopes consisting of permeable materials overlying cohesive soils. In non-arid areas, the overlying aquifer, in acting as a reservoir for infiltrated precipitation, furnishes a steady supply of water to the slope. This initiates and reactivates mass movement mainly by sustaining high pore water pressures in the slope-forming materials. Pore-pressure induced failure is not exclusive to the mass-movement scenario addressed. However, the accelerated rate of failure is. This is because the rate of failure is a direct function of the sustained supply of water from the overlying aquifer. Hydrogeologic variables are discussed in relation to other controls contributing to patterns and activity of mass movement. New knowledge is contributed not so much by the further explanation of a relatively well-known concept, but by the presentation of a systematic methodology involving the collection of quantitative data from aerial photographs, and the processing of these data by means of computer-assisted techniques. The results obtained are consistent both with geotechnical theory and field data and have predictive value in terms of estimating the extent of mass-movement activity on slopes. As such, the methodology outlined offers an approach for researching similar problems.

Geomorphological observations as aids to the design of coast protection works on a part of the Dee estuary

Summary A section of the southeast shore of the Dee estuary is occupied by unstable cliffs formed of a complex succession of glacial deposits. Design of remedial works has necessitated an investigation of the nature and balance of geomorphological processes operating. Between 1973 and the present, a balance over a year has generally been achieved between failure of a clayey flow till in the upper cliff and erosion of the resulting colluvium from the toe. During exceptional storms, colluvium and in situ cohesive and non-cohesive materials are eroded, producing a rapid and larger-scale response at the crown of the slope. During winter, high pore water pressures are generated in perched water tables within the succession. Recent stabilization works at the toe in one downstream section appear to have caused severe toe erosion and slope instability over a distance of approximately 73 m of cliff line immediately upstream of the works. Geomorphological studies have also indicated appropriate angles for naturally stable slopes within this mixed succession. Grading or backfilling of the toe to achieve these specified slope angles, and toe protection with or without drainage, have been undertaken or are planned.