Variation Of Pore Water Pressure Research Articles

Ground Response during Pile Driving

The paper presents a synthesis of the field measurements obtained in a series of large-scale pile-driving tests. The field test was performed at the Chiayi-Taipo area of the high speed rail system currently under construction in Taiwan. The test project included a complete instrumentation of three precast concrete piles with a network of piezometers, inclinometers, level posts, and velocity sensors. The instruments were used to monitor the variations of the dynamic soil pore water pressure, lateral ground deformations, vertical surface displacements, and ground vibrations induced by the pile on the surrounding soil medium during the driving process. Based on the test results, a correlation between the pile-driving resistance and the geological condition of the test site is identified.

Three-dimensional numerical investigations of groundwater responses in an unsaturated slope subjected to various rainfall patterns

Three-dimensional (3D) numerical analyses were conducted to investigate groundwater responses in an initially unsaturated cut slope at Lai Ping Road in Hong Kong subjected to rainfalls with various patterns, durations, and return periods. Initial and boundary conditions were established from field monitoring data. The computed results show that rainfall pattern has a significant influence on pore-water pressures in soil layers near the ground surface but its influence gradually diminishes with depth. Rainfall with an advanced storm pattern of 24 h duration was found to be the most critical because it results in the highest pore-water pressure in the slope. At a given depth, the influence of rainfall pattern on pore-water pressures depends on the initial groundwater conditions: the higher the initial water table, the smaller the influence of rainfall pattern on pore-water pressures. Under a given rainfall duration, the rise of pore-water pressure at the study site is significant only when the return period increases from 10 years to 100 years, but not from 100 years to 1000 years. Short-duration, intense rainfall causes larger variations in pore-water pressure at shallow depths, whereas long-duration rainfall has a greater influence on groundwater in deep soils because of the generally greater amount of rainfall. For prolonged rainfalls, the difference in pore-water pressure distribution resulting from different rainfall patterns is less significant than that from short-duration, intense rainfalls.Key words: Lai Ping Road, rainfall patterns, pore-water pressure distributions, suction, unsaturated slope.

Hydrological response of hillslope soils above a debris-slide headscarp

The Jamieson Creek debris slide initiated in thin soils over a competent bedrock surface, on a planar section of hillslope, during a heavy rainstorm in November 1990. An array of automated piezometers and tensiometers was placed along a 22 m wide section of the headscarp in 1997 to monitor the temporal variation of pore-water pressures. Interpretation of the data addresses the hydrologic response to the storms in October and November 1997. The piezometers, which were designed for installation by driving, reveal very localized responses in what otherwise appears to be a uniform soil matrix. Peak positive pressures occur at the time of maximum rainfall intensity. The tensiometers indicate the hydrological response at the ground surface appears uncoupled from that at the bedrock interface. Implications of the extreme spatial variability in pore-water pressure are evaluated for conceptual models of hillslope hydrology. The assumption of parallel seepage flow is widely adopted in translational slope stability analyses, imposing a linear distribution of pore-water pressure with depth. None of the reported field data are consistent with such a linear distribution with depth or a uniform response across the slope.

Hydrological response of hillslope soils above a debris-slide headscarp

The Jamieson Creek debris slide initiated in thin soils over a competent bedrock surface, on a planar section of hillslope, during a heavy rainstorm in November 1990. An array of automated piezometers and tensiometers was placed along a 22 m wide section of the headscarp in 1997 to monitor the temporal variation of pore-water pressures. Interpretation of the data addresses the hydrologic response to the storms in October and November 1997. The piezometers, which were designed for installation by driving, reveal very localized responses in what otherwise appears to be a uniform soil matrix. Peak positive pressures occur at the time of maximum rainfall intensity. The tensiometers indicate the hydrological response at the ground surface appears uncoupled from that at the bedrock interface. Implications of the extreme spatial variability in pore-water pressure are evaluated for conceptual models of hillslope hydrology. The assumption of parallel seepage flow is widely adopted in translational slope stability analyses, imposing a linear distribution of pore-water pressure with depth. None of the reported field data are consistent with such a linear distribution with depth or a uniform response across the slope.

Stability of streambanks formed in partially saturated soils and effects of negative pore water pressures: the Sieve River (Italy)

Streambanks of alluvial channels are usually composed of loose materials, which are unsaturated in ambient conditions. Unsaturated soils are subject to negative pore water pressures, which cause an apparent cohesion. The latter is the main factor in allowing the stability of near-vertical banks. Even during moderate in-bank flow events, the apparent cohesion can be strongly reduced as the material approaches full saturation; therefore, during the drawdown phase, as the confining pressure of the water in the channel disappears, a bank failure is likely to occur. Channel bed-level lowering along the Sieve River, Central Italy, has caused widespread bank instability. A geomorphological reconnaissance of forms and processes was followed by in situ tests to determine the shear strength of the banks. Interpretation of the tests and a streambank stability analysis were based on concepts of soil mechanics for unsaturated soils, in order to obtain relations between bank angle and height in limit equilibrium conditions. A stability chart was obtained with curves for different apparent cohesion values, and a stability analysis was performed taking into account the effects of flow events. In order to investigate the pore pressure effects, a series of piezo-tensiometers were installed in a streambank of the Sieve River. Data from a 1 year monitoring period show variations in pore water pressure and matric suction as a consequence of rainfall, evapotranspiration, and water stage variations. A planar failure with a tension crack occurred in the upper cohesive part of the bank during December 1996. The safety factor has been expressed as a function of the geometry of the bank and of the shear strength of the material. Safety factor variations through time are therefore shown as a function of seasonal variations in matric suction.

Performance of Diaphragm Wall Constructed Using Top-Down Method

This paper presents the performance of an excavation using the top-down construction method. Strut loads, wall displacement, wall bending moment, ground surface settlement, pore-water pressure and bottom heave were measured. Results obtained from those observations are correlated with the construction activities. Field observations indicate that strut loads, wall displacement, and ground surface settlement correspond to those reported in the literature. Bending moments of the wall are studied based on the results of the rebar strain gauge and inclinometer measurements. The supported wall and the soil near the wall have a deep inward movement, which accounts for the magnitude of the lateral earth pressure acting on the wall. The behavior of the supported wall and soil over time is consistent with the variation of pore-water pressure during excavation. Analysis of excavations in soft clay should therefore consider the creep factors and/or pore-water pressure dissipation.

Consolidation of Clay by Sand Drain under Time-Dependent Loading

The mathematical theory and physical laws of one-dimensional consolidation of clayey soils are well-known. Sand drains accelerate the consolidation process. Several analytical and experimental studies have already reported on sand drain consolidation of clayey soils. However, the literature does not contain a nonlinear theory of sand drain consolidation under time-dependent loading that can take into account any effective stress! void ratio/permeability variations. A generalized governing equation, capable of yielding a large class of ana­ lytical solutions for these variations is derived in this paper. Closed-form solutions are presented for the variation of pore water pressure with a time factor and load increment ratio under time-dependent loading. The analytical formulation is validated by comparing the solution with the standard results available in the literature for in­ stantaneous loading, constant permeability, and constant compressibility. Following the equal strain approach, this paper presents a particular form of generalized governing equation, capable of yielding a large class of analytical solutions for time-depen­ dent loading, which can handle any effective stress!void ratio/ permeability variations. The use of this governing equation is illustrated by presenting the closed-form solution for the case where the effective stress/void ratio/permeability response takes the form U) a~~r) =1 :leo (~;) In the equal strain approach, e is a function of t only. Therefore

EVALUATION OF DRAINAGE WELL USING NUMERICAL SIMULATIONS OF SATURATED-UNSATURATED SEEPAGE IN SOIL

In order to investigate the influence of some important factors at rainfalls on the behaviour of pore water pressure in slope, the finite element analyses using saturated-unsaturated seepage were performed on the model slopes of sand and clay layer with one condition of rainfalls and groundwater inflow. The obtained results are discussed by a detailed look at the different types of soil profile, the variation of vector due to rainfalls and inflow to the reference section in the slope, and the variation of pore water pressure with elapsed time. In addition, the effects of the drainage wells are also investigated.

An experimental design for laboratory simulation of periglacial solifluction processes

An experimental slope of gradient 12° was constructed, comprising two 5 m × 2 m × 0·3 m contiguous strips of natural soils. Soil freezing and thawing took place from the surface downwards in an open hydraulic system, water being supplied at the base of each soil. Thermal conditions, porewater pressures and soil displacements were monitored using a PC-based logging system, with readings taken at half-hourly intervals. Soil surface displacements due to frost heave and solifluction were measured using linear voltage displacement transducers. Soil temperatures were determined using thermistors and semiconductor temperature sensors. Antifreeze-filled miniature ceramic-tipped pressure transducers were used to determine porewater pressure variations. The potential of this experimental approach for precise monitoring of mass movement processes associated with thawing of ice-rich soils is demonstrated.

An experimental design for laboratory simulation of periglacial solifluction processes

An experimental slope of gradient 12° was constructed, comprising two 5 m × 2 m × 0·3 m contiguous strips of natural soils. Soil freezing and thawing took place from the surface downwards in an open hydraulic system, water being supplied at the base of each soil. Thermal conditions, porewater pressures and soil displacements were monitored using a PC-based logging system, with readings taken at half-hourly intervals. Soil surface displacements due to frost heave and solifluction were measured using linear voltage displacement transducers. Soil temperatures were determined using thermistors and semiconductor temperature sensors. Antifreeze-filled miniature ceramic-tipped pressure transducers were used to determine porewater pressure variations. The potential of this experimental approach for precise monitoring of mass movement processes associated with thawing of ice-rich soils is demonstrated.

The performance of push-in spade cells installed in stiff clay

Keywords: clays ; diaphragm and in situ walls ; earth pressure ; field instrumentation ; in situ testing ; site investigation ... Show All

Documentation of earthquake-induced disturbance of lake sediments using subbottom acoustic profiling

Subbottom acoustic profiling in lakes on the Canadian Shield and in the Canadian Cordillera has revealed disruptions in deltaic and lacustrine sediments that can be attributed to large historical earthquakes. The Timiskaming earthquake of 1935 (Magnitude 6.25) and the Vancouver Island earthquake of 1946 (Magnitude 7.2) triggered extensive slumping and flow of postglacial sediments in lakes located within a few tens of kilometres of the epicentres of these events. There is some evidence in one of the surveyed lakes for in situ mobilization of sediment on relatively gentle slopes, with minimal horizontal displacement; this may be a product of liquefaction induced by coseismic shaking or displacements along faults beneath the basin. In another lake, widespread coseismic slumping resuspended large amounts of fine sediment, which then settled out onto the lake floor to produce a texturally anomalous layer. Features similar to those produced by the 1935 and 1946 earthquakes can also result from a variety of nontectonic processes, for example, the collapse of sediment over buried ice blocks, depositional oversteepening of deltas, groundwater sapping, and variations in pore-water pressures due to lake-level fluctuations. Records of sediment disruption in lakes therefore must be interpreted with caution. In general, many lakes in a region should be surveyed and a variety of different features identified before sediment disruptions are attributed to an earthquake.

Field measurement of anchor forces, ground temperatures, and pore-water pressures behind a retaining structure in northwestern Ontario

Anchor forces, ground temperatures, and piezometric pressures were measured at a retaining wall in northwestern Ontario over a period of 2 years. The anchor forces were measured with strain gauges attached in pairs directly to the anchor rods. This method appeared practical in the field for time periods of less than 2 years as long as the strain gauges were carefully protected against moisture. The anchor forces increased from an average of 5 kN initially up to values of 50 kN during the winter periods and dropped during the summer periods back to the same values measured initially. The anchor forces were largely independent of pore-water pressure variations behind the wall. Rapid drawdown conditions, however, which were experienced during the second summer, were reflected in a load increase that was equivalent to the associated unloading effect in front of the wall. The pore-water pressures behind the wall were not noticeably affected by rapid drawdown, possibly due to the restraining effect of the anchors and the high rigidity of the low sheet pile wall. Ground temperatures at or below the groundwater table never dropped below 0 °C thus restricting the depth of frost penetration. Key words : anchor loads, freezing pressure, retaining walls, pore-water pressures, ground temperatures, field measurements.

Frequency Response Characteristics for Incident P-SV Waves in Multi-Layered Irregular Strata Including Water-Saturated Layers Using Discrete Wave Number Method.

It is important to investigate wave propagation characteristics taking into account the effects of pore-water in, e.g., soft sand layers with topographical and/or subsurface irregularities. To obtain information on the spatial variation of displacements, stresses and pore-water pressures, the authors have extended the Discrete Wave Number Method proposed by Aki and Larner (1970) to the P-SV wave incident problems in multi-layered media with topographical and/or subsurface irregularities including water-saturated layers. This paper first explains the analytical method and then verifies it using numerical examples incorporating spatial variation characteristics of frequency response functions of displacements, stresses and pore-water pressures. The effects of subsurface irregularities on the frequency response are discussed. The response characteristics of an elastic model and a water-saturated model are compared. It is thus confirmed that the Discrete Wave Number Method can be efficiently used to calculate amplifications of ground motions taking into account wavetype coupling and wavenumber coupling in a multi-layered alluvial deposit with subsurface irregularities and a very permeable water-saturated layer.

Strain, pore pressure and fatigue characteristics of sandstone under various load conditions

This paper presents results of an experimental study of the strain, pore water pressure and fatigue characteristics of a saturated sandstone under quasi-static, repeated and cyxlic loadings. Based on the experimental results, a relation between the accumulation of axial strain and the fatigue life of the sandstone studied was established. It was found that under some testing conditions there exists a critical axial strain beyond which the failure of the specimens will occur almost immediately. Variation of the pore water pressure in the sandstone under various loadings was observed and interpreted. Three distinct modes of pore water pressure variation were observed in the cyclic loading process. For all loading conditions studied, the pore water pressure induced in the rock specimens drops rapidly right before the failure. The variation of the pore water pressure was interpreted in terms of internal structural changes of the rock specimens.

Pore pressure response during failure in soils

Three experiments were performed on natural slopes to investigate variations of soil pore-water pressure during induced slope failure. 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. Piezometer 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.

Spatial Variations in Subsurface Pore Fluid Properties in a Portion of Southeast Louisiana: Implications for Regional Fluid Flow and Solute Transport: ABSTRACT

ABSTRACT Fluvial-deltaic deposition, growth faulting, and salt diapirism in the Tertiary section of southeast Louisiana have combined to produce a structurally complex geologic framework which contains potential pathways for large-scale lateral and vertical fluid migration. A study has been made of spatial variations in pore water salinity, temperature, pressure, density, and viscosity to a depth of 10,000 ft (3 km) in a five-parish area situated across the northern margin of the South Louisiana salt basin to help identify driving forces and pathways for subsurface fluid migration and solute transport. Three regional hydrologic flow regimes can be defined in the area as a function of depth: 1) a shallow, hydropressured, meteoric flow regime which is driven by differences in topographic elevation; 2) an intermediate-depth, hydropressured, thermohaline regime apparently driven in part by variations in pore water density; and 3) a deep, geopressured regime driven by high pore fluid pressure gradients. Although the sediments in the area originally contained fresh to marine waters at the time of deposition, the interval between 2000 and 10,000 ft is now largely filled with hypersaline waters. These brines are derived from the dissolution of salt domes, which act as local sources for both saline waters and heat. Some domes have salinity plumes that extend thousands of feet above the top of salt and many miles laterally. Two areas in which overpressured saline brines are apparently migrating upward along radial faults into the overlying hydropressured zone appear to be related to deep-seated salt structures. Faults and lack of lateral continuity of interbedded shales have made the sand-dominated section to a depth of 10,000-ft hydrologically open on a sub-regional scale. Significant temperature, pressure, and salinity gradients have made it hydrologically dynamic. Of economic interest is the possible relation of hydrocarbon migration to the migration of pore water and dissolved salt.

Pore pressure loads on gravity structures

It is common practice to compute wave-induced loads on the immersed surface of gravity structures exposed to the wave motion and disregard the pore-water pressure variation on the foundation surface. However, when the soil is porous, wave-induced pressures propagate within the soil under the structure and result in a rather significant contribution to overall loads. This paper describes a practical method for numerical modeling of the pore pressure under a gravity platform foundation for compressible water and a rigid, but porous soil. The porous soil may be bounded by an impermeable horizontal layer at some arbitrary depth. The paper outlines the basic boundary element procedure for pore pressure analysis and presents numerical results for a typical gravity structure as well as results for comparison with an existing analytical solution for a vertical circular cylinder.

On pore water pressure variations causing slide velocities and accelerations observed in a seasonally active landslide

AbstractIndividual block displacements, continuously recorded in spring 1987 in a landslide in the French Alps, are strongly affected by rain and dry spells. Although a viscous component is present in the sliding mechanism, variation of the plastic strength characteristics is necessary to explain the controlled sliding. A stick‐slip movement behaviour and a diurnal variation in slide rate, attributed to the diurnal variation in evapotranspiration, were recorded. The observed numerous accelerations and decelerations of the blocks movements are attributed to small variations in pore pressure. These variations are explained and their magnitude is calculated: starting from limit equilibrium increases of pore water pressure smaller than 0‐1 k Pa are sufficient to provoke the maximum recorded acceleration and velocity for a block of 2000 tons in weight.

The Variation of Pore Water Pressure on the Landslide Slip Surface due to Seepage of Rain

Rainwater percolates slowly downward through the parts of high permeability in the layers and finally it is deposited near the landslide surface.According to the general consideration, a ladslide is caused by the pore water pressure due to the deposition of ground water on the landslide surface. Furthermore, it should be noted that when rainwater seeps downward in the layers, which are mostly unsaturated, air in the void of the unsaturated parte of the layers is compressed by the seepage pressure due to the descent of the wetting front; as a direct effect upon the pore pressure on the landslide surface.The results of model tests for the phenomena as described above, are as follows;(1) When the wetting front descends into the unsaturated layers, the pore water pressure equivalent to 17-83% of the seepage pressure occurs on the landslide surface.(2) When the artesian pressure is acting on the landslide surface prior to the descent of the wetting front, the water pressure on the landslide surface increases slowly, provided that the wetting front seeps down in the unsaturated layers, and as time proceede, the pore water pressure will be equal to the seepage pressure in value.