Values Of Pore Water Pressure Research Articles

Behaviour of Cohesive Soil Subjected to Low-Frequency Cyclic Loading in Strain-Controlled Tests

Abstract The subject of the paper comprises tests of cohesive soil subjected to low-frequency cyclic loading with constant strain amplitude. The main aim of the research is to define a failure criteria for cohesive soils subjected to this type of load. Tests of undrained cyclic shear were carried out in a triaxial apparatus on normally consolidated reworked soil samples made of kaolinite clay from Tułowice. Analysis of the results includes the influence of number of load cycles on the course of effective stress paths, development of excess pore water pressure and stress deviator value. Observed regularities may seem surprising. The effective stress path initially moves away from the boundary surface and only after a certain number of load-unload cycles change of its direction occurs and it starts to move consequently towards the surface. At the same time, it has been observed that pore water pressure value decreases at the beginning and after few hundred cycles increases again. It is a typical behaviour for overconsolidated soil, while test samples are normally consolidated. Additionally, a similar change in deviator stress value has been observed - at first it decreases and later, with subsequent cycles, re-increases.

Dynamic simulation of landslide based on thermo-poro-elastic approach

Catastrophic landslides often extend for surprisingly long distances, a trend consolidated by a decrease in the apparent friction coefficient with an increasing landslide volume. However, there is not yet a physical model able to predict the long distances traveled by landslides in the condition of deformation. A two-dimensional model for simulating catastrophic landslide motion is presented in this paper and is based on a lump model for thermo-poro-elastic mediums. The model can concurrently simulate landslide dynamic processes, and pore pressure and frictional heating evolution in the shear zone at the bottom of a landslide. These are based upon the shallow-seated groundwater table assumption and depth-averaged integration, as well as the thermo-poro-elastic approach. A combined computational method based on the finite volume method and Crank–Nicolson method is proposed to solve the coupled equations. Four computational experiments have been run and the numerical results indicate that frictional heating enhances pore water pressure and reduces the friction, resulting in higher mobility and longer distances traveled. The value of pore water pressure can also influence the deformation process of a landslide.

Three-dimensional consolidation deformation analysis of porous layered soft soils considering asymmetric effects

Long-term settlements for underground structures, such as tunnels and pipelines, are generally observed after the completion of construction in soft clay. The soil consolidation characteristic has great influences on the long-term deformation for underground structures. A three-dimensional consolidation analysis method under the asymmetric loads is developed for porous layered soil based on Biot’s classical theory. Time-displacement effects can be fully considered in this work and the analytical solutions are obtained by the state space approach in the Cartesian coordinate. The Laplace and double Fourier integral transform are applied to the state variables in order to reduce the partial differential equations into algebraic differential equations and easily obtain the state space solution. Starting from the governing equations of saturated porous soil, the basic relationship of state space variables is established between the ground surface and the arbitrary depth in the integral transform domain. Based on the continuity conditions and boundary conditions of the multi-layered pore soil model, the multi-layered pore half-space solutions are obtained by means of the transfer matrix method and the inverse integral transforms. The accuracy of proposed method is demonstrated with existing classical solutions. The results indicate that the porous homogenous soils as well as the porous non-homogenous layered soils can be considered in this proposed method. When the consolidation time factor is 0.01, the value of immediate consolidation settlement coefficient calculated by the weighted homogenous solution is 27.4% bigger than the one calculated by the non-homogeneity solution. When the consolidation time factor is 0.05, the value of excess pore water pressure for the weighted homogenous solution is 27.2% bigger than the one for the non-homogeneity solution. It is shown that the material non-homogeneity has a great influence on the long-term settlements and the dissipation process of excess pore water pressure.

Sabkha층 탄산질 모래의 입자파쇄에 따른 간극수압 변화

Abstract Carbonate sand of Sabkha layer in the middle east was made of deposition of shell fragments and it consisted of porous particles containing inner void. Generally, at yield stress the soil structure begins to break down, so the porewater pressure and the settlement are increased rapidly. In carbonate sand, unlike quartz sand if particle crushing happens, the inner voids are exposed and porewater pressure can be decreased under yield stress. Porewater pressure can be determined as the sum of excess porewater pressure due to increase of relative density, inner void expose of particle under particle crushing stress and rearrangement of crushed particle fragments. The porewater pressure can be negative value in case of greater amount of inner void expose, so if particle crushing is bigger, the porewater pressure value is smaller. The negative value zone of porewater pressure from triaxial test result means particle crushing effect is bigger than outer void decrease effect and the particle crushing effect dominant zone size was 1.50∼3.46% from triaxial test result of Sabkha layer.

Research on Liquefaction Characteristics of Saturated Undisturbed Loess under Different Level of Liquefaction

On the basis of the dynamic triaxial liquefaction test of the loess samples which were taken from the Shibei tableland, Guyuan city, and evaluating the liquefaction potential of the loess through laboratory experiment and response analysis. Then the liquefaction characteristics of saturated undisturbed loess under different level of liquefaction are analyzed based on the evaluation results. The results are shown that: when the saturated undisturbed loess liquefaction is light, dynamic stress is decayed barely, weak dynamic residual deformation is generated, pore water pressure begins to grow, the dynamic stress dynamic strain stagnant curves have not marked change, and the deformation of the soil is at viscoelastic stage. When the loess liquefaction is middling, attenuation of dynamic stress is not obviously, dynamic strain is increasing, amount of pore water pressure much higher than stage one, the long axis slope of hysteresis curves is reduced significantly, the sizes and shapes difference of adjacent hysteresis loop is obvious, and the deformation of the soil is at viscoelastic-plastic stage. When the loess is liquefied completely, dynamic stress is decayed evidently, dynamic strain increased dramatically, and its crest value could exceed 10%, the peak value of pore water pressure is sustainable increasing, and emerge the apparent fluctuation under the action of a drastic dynamic strain, the form of the stagnant curves tend to be irregular, the loess is in elastic-plastic stage.

Laboratory tests for measuring the time-lag of fully grouted piezometers

Summary Time-lag is the most important parameter describing the conformance of a piezometer because it represents the time that it takes for the instrument to reach equilibrium when there is a pore water pressure change in the soil. The use of fully grouted piezometers would make the installation easier and faster than the conventional one, since it prevents the failure risks during the placement of the sand pack and the overlying bentonite seal. This paper describes a number of laboratory tests performed to evaluate the time-lag of fully grouted piezometers. A pore water pressure change was applied at the bottom of cylindrical samples of grout and the pressure equalization was measured at the top. The samples were made of a cement–bentonite mixture and were 2 cm, 4 cm or 8 cm in height. Time-lags resulted in no longer than 6 min, that is a time generally acceptable in geotechnical projects. It was also found that the time-lag relies on the pore water pressure value. For a given draining path length, the higher the pore water pressure the shorter the response time. This behavior has been explained by assuming that the time-lag depends on both the hydraulic and compressible characteristics of the grout mixture and that these characteristics vary with the pore water pressure because the grout is in a quasi saturated state.

Multifunctional Piezocone Penetration Testing in Geotechnical Practice

Cone penetration test is a fast and efficient in-situ test technique. With the development of sensor technology and the use of new probes, such test is employed in more fields and reveals more soil parameters. Based on advanced CPTU equipment, soil types were classified. As CPTU has the function of porewater pressure test, the value of porewater pressure varies a lot when the soil changes, which is shown clearly in the CPTU feature map. So it can be easier to judge soil boundaries and its result is in good agreement with the borehole. Multi-function CPTU system is equip with SCPTU module, which enable to measure shear wave velocity of the soil easily. To meet the needs of conventional CPT equipment, the relationships between shear wave velocity measured by SCPTU and other CPT indexes were analyzed; then, two empirical formulas which suitable for kinds of soils are proved more consistent with the measured results, so it is a good method to estimate shear wave velocity without seismic wave test. With a view to get greater economic and technical benefits, more cone penetration testing experience in different regions should be accumulated for geotechnical engineering investigation and design.

Three Dimensional Seepage Analyses in Mollasadra Dam after Its Impoundments

Mollasadra dam is an earth fill dam with a clayey core and a height of 72 m from river bed, constructed on Kor River. pore water pressure in the dam was investigated following its construction and first and second impoundments. The dam was modeled by a finite element mesh. After the first and second dam impoundments, the overall trend in monitored pore water pressure was well modeled by the transient analysis. The result showed the six month time period between impoundments was long enough for the pore water pressure to reach equilibrium everywhere throughout the core, except where considerable initial constructioninduced pore water pressure was observed. High values of construction-induced pore water pressure at elevation2050 m did not dissipate completely during the 6 month period of almost constant reservoir level (el. 2098.3 m) and the pore pressures were still at the transient state throughout the core. Therefore, it was concluded that porepressures in the core of earth fill dams may not achieve steady state conditions even several months after the dam construction and impoundments.

تقويم مقاومة القص للتربة غير المتماسكة نتيجة زيادة ضغط الماء المسامي

The shear strength of cohesionless soil is reduced as the water pressure inside the pores of the soil mass increases. The mathematical relationship between the shear strength and the pore water pressure was derived using Mohr–Coulomb failure criteria as a function of the confining pressure and the effective angle of friction. Experimentally, a series of consolidated drained triaxial tests with back pore water pressure was run on samples of saturated uniform dense sand. The tests were conducted at different confining pressures in the range of 100–400 kPa with an increment of 100 kPa. At each level of confining pressure, the tests were repeated at different values of back pore water pressure in the range of 0–100 kPa with an increment of 25 kPa. For each test, the initial applied back pore water pressure was kept constant during the test for comparing the results at the same effective confining pressure. This study concludes that the mathematical relationship gives accurate results at any level of confining pressure and/or pore water pressure as a function of the effective angle of friction that can be evaluated using single consolidated drained triaxial test at zero back pore water pressure.

Pore Water Pressure Contribution to Debris Flow Mobility

Problem statement: Debris flows are very to extremely rapid flows of saturated granular soils. Two main types of debris flow are generally recognized: Open slope debris flows and channelized debris flows. The former is the results of some form of slope failures, the latter can develop along preexisting stream courses by the mobilization of previously deposited debris blanket. The problem to be addressed is the influence of the mode of initiation on the subsequent mechanism of propagation. In particular the role of pore water pressure on debris flow mobility in both types was debated. Approach: Laboratory flume experiments were set up in order to analyze the behavior of debris flows generated by model sand slope failures. Failures were induced in sand slopes by raising the water level by seepage from a drain located at the top end of the flume, and by rainfall supplied by a set of pierced plastic pipes placed above the flume. Video recordings of the tests were performed to analyze debris flow characteristics. Results: In all the tests the sand water mixture flows were unsteady and non uniform and sand deposition along the channel bed was a relevant phenomenon. The flows were characterized by a behavioral stratification of the sand water mixture along the flow depth. Back analyzed pore water pressure were just in excess to the hydrostatic condition. The reliability of the experimental results was checked by comparison with other flume experiment data. Conclusion: Debris flow behavior was influenced by the mode of initiation, the inclination of the channel and grain size of the soils. These factors affected the attained velocities and the pore water pressure values. The mobility of debris flows was not always enhanced by high excess pore water pressure values.

Measurement and Analysis of Pore Water Pressure in Large Caverns and Shafts in a Hydro-electric Project

With an objective to understand the behaviour of pore water pressure around large underground openings, piezometers were installed in the rock around the desanding chambers, surge shaft, its drainage galleries, and the transformer cavern of a large hydro-electric project. The monitored data indicated that the pore water pressure was insignificant in the rock surrounding the desanding chambers prior to lining. On completion of the concrete lining of the chambers, the maximum pore water pressure was about 50 kPa as the flow of water was restricted. When one of the desanding chambers was charged with water, the piezometers around it indicated an excessive increase in the pore water pressure due to seepage of water from the chamber. Thus the piezometer readings were useful in taking appropriate remedial measures. In the surge shaft, higher values of pore water pressure were recorded on the hill side of the shaft as compared to the valley side. In one of the drainage galleries, the pore water pressure gradually increased to 450 kPa, which was released by drilling relief holes in the area. In the other drainage gallery, no significant change in the pore water was recorded. In the transformer cavern, the recorded pore water pressures were within 20 kPa.

The effect of plastic fines on the pore pressure generation characteristics of saturated sands

Pore water pressure generation during earthquake shaking initiates liquefaction and affects the shear strength, shear stiffness, deformation, and settlement characteristics of soil deposits. The effect of plastic fines (kaolinite) on pore pressure generation in saturated sands was studied through strain-controlled cyclic triaxial tests. In addition to pore pressure generation, this experimental study also focused on evaluating the threshold shear strain for pore pressure generation and the volumetric compressibility of specimens during pore pressure dissipation. The results reveal that specimens having up to 20% plastic fines content generated larger values of pore water pressure than clean sand specimens. At 30% fines content, the excess pore water pressure decreased below that of clean sand. The threshold shear strain, which indicates the strain level above which pore pressures begin to generate, was assessed for different kaolinite–sand mixtures. The threshold shear strain was similar for 0–20% fines ( γ t∼0.006–0.008%), but increased to about 0.025% for 30% fines. The volumetric compressibility, measured after pore pressure generation, was similar for all specimens. The transition of behavior at fines contents between 20% and 30% can be attributed to a change in the soil structure from one dominated by sand grains to one dominated by fines.

Mechanical behavior of a volcanic ash soil (Typic Hapludand) under static and dynamic loading

Andisols are often subject to landsliding and coinciding liquifaction of the existing soil structure because they dominate at steeper slopes and show often an insufficient hydraulic conductivity. Thus, soil erosion at those sites is considered as one of the major risk. In order to quantify the proportion of the mechanically and of the pore water dependent soil strength, we evaluated their dependency during compression (divergent process) and shear tests as a function of the pore water pressure in a volcanic ash soil (Typic Hapludand). Undisturbed samples collected at two depths (0–15, 40–55 cm) were equilibrated at two pore water pressure values (−60 and −300 hPa). Thereafter they were compressed for either 10 or 30 min, respectively at each total stress applied. Shear tests with a shear speed of 0.2 mm min −1 were performed at the same two initial pore water pressure values, after the samples had been statically pre-stressed with identical loads all smaller than 400 kPa. The pre-compression stress was similar between time intervals when the samples were dried at −60 hPa, but on average for the two depths the pre-compression stress was 32% lower when 30 min steps were performed because of a more complete settlement due to the time dependency of water flow during soil deformation. There was an increase in cohesion of 30% in topsoil and 900% in subsoil when the pore water pressure changed from pF 1.8 to pF 2.5. Irrespective of the loading time interval, pore water pressure increased to even positive pore water pressure values when mechanical stresses of >300 kPa were applied, being more important in samples after pre-drying at −60 hPa than at −300 hPa. In the shear test the behavior was similar, but positive values of pore water pressure already occurred when the vertical stress exceeded 200 kPa. Thus, we can conclude, that hydraulic properties are specially important in soils showing thixotropic behavior and/or when shear stresses are applied resulting in a more pronounced weakening by increasing the destabilizing pore water pressure as a reason for an enhanced particle rearrangement and soil homogenization compared with the original soil structure.

Monitoring and modelling of pore water pressure changes and riverbank stability during flow events

AbstractPore water pressures (positive and negative) were monitored for four years (1996–1999) using a series of tensiometer‐piezometers at increasing depths in a riverbank of the Sieve River, Tuscany (central Italy), with the overall objective of investigating pore pressure changes in response to flow events and their effects on bank stability.The saturated/unsaturated flow was modelled using a finite element seepage analysis, for the main flow events occurring during the four‐year monitoring period. Modelling results were validated by comparing measured with computed pore water pressure values for a series of representative events. Riverbank stability analysis was conducted by applying the limit equilibrium method (Morgenstern‐Price), using pore water pressure distributions obtained by the seepage analysis.The simulation of the 14 December 1996 event, during which a bank failure occurred, is reported in detail to illustrate the relations between the water table and river stage during the various phases of the hydrograph and their effects on bank stability. The simulation, according to monitored data, shows that the failure occurred three hours after the peak stage, during the inversion of flow (from the bank towards the river). A relatively limited development of positive pore pressures, reducing the effective stress and annulling the shear strength term due to the matric suction, and the sudden loss of the confining pressure of the river during the initial drawdown were responsible for triggering the mass failure.Results deriving from the seepage and stability analysis of nine selected flow events were then used to investigate the role of the flow event characteristics (in terms of peak stages and hydrograph characteristics) and of changes in bank geometry. Besides the peak river stage, which mainly controls the occurrence of conditions of instability, an important role is played by the hydrograph characteristics, in particular by the presence of one or more minor peaks in the river stage preceding the main one. Copyright © 2004 John Wiley & Sons, Ltd.

Mechanisms of riverbank failure along the Arno River, central Italy

AbstractRiverbanks along the Arno River have been investigated with the aims of defining the main mechanisms of failure and retreat, their spatial distribution, and their causes. Geomorphological aspects were investigated by a reconnaissance of riverbank processes, for a number (26) of representative sites. Laboratory and in situ tests were then performed on a selected number of riverbanks (15). Based on the material characteristics, six main typologies of riverbanks have been defined, with homogeneous fine‐grained and composite banks representing the most frequent types. Slab‐type failures are the most frequent mechanism observed on fine‐grained banks, while cantilever failures prevail on composite banks.The role of river stage and related pore water pressure distributions in triggering the main observed mechanisms of failure has been investigated using two different types of stability analysis. The first was conducted for 15 riverbanks, using the limit equilibrium method and considering simplified hypotheses for pore water pressure distribution (annulment of negative pore pressures in the portion of the bank between low water stage and peak stage). Stability conditions and predicted mechanisms of failure are shown to be in reasonably good agreement with field observations. Three riverbanks, representative of the main alluvial reaches of the river, were then selected for a more detailed bank stability analysis, consisting of: (a) definition of characteristic hydrographs of the reach with different return periods; (b) modelling of saturated and unsaturated flow using finite element seepage analysis; and (c) stability analysis with the limit equilibrium method, by adopting pore water pressure values derived from the seepage analysis. The results are compared to those obtained from the previous simplified analysis, and are used to investigate the different responses, in terms of stability, to different hydrological and riverbank conditions. Copyright © 2003 John Wiley & Sons, Ltd.

Predicted and Observed Performance of an Oil Tank Founded on Soil-Cement Columns in Clayey Soils

ABSTRACT In this Paper, a back-analysis of the behaviour of an oil tank during a loading test is presented, and the results of the analysis are compared with the actual performance of the tank resulting from field measurements. The tank is founded on stiff cohesive soils and on a clayey backfill improved with soil-cement columns, obtained with the deep-mixing technique. The constitutive model used in the analysis is capable to reproduce the soil non-linearity and has been calibrated using in-situ measurement of the stiffness at small strains. The backfill improved with stabilised columns is regarded as an equivalent homogenous material, characterised by a non-linear stress-strain behaviour. A coupled consolidation analysis is carried out, that provides a satisfactory simulation of the observed time-settlement curve and of the overall field of displacements monitored during the loading test. A less satisfactory agreement is obtained between measured and computed values of pore water pressure. The effectiveness of the soil improvement scheme adopted is finally evaluated, comparing the back-analysed performance with the results of additional analyses, in which no treatment or a different geometry of treatment are hypothesised.

Effect of repeated tractor wheeling on stress/strain properties and consequences on physical properties in structured arable soils

The discussion about the effect of repeated short time wheeling on long-term changes in soil structure and pore functioning reveals a great uncertainty. On the one hand it is told that soil structure elements are rigid and do not undergo intense changes in pore functions as a consequence of the short loading interval during each single wheeling. On the other hand, the complete deterioration of the structure elements and pore functions is assumed to occur, which also results in changes of the shrinkage pattern, soil strength including even strength regain. Consequently, the effect of wheeling on soil deformation and stress/strain distribution was investigated in a soil bin which contained Hiwassee clay at the NSDL, Auburn. If the soil is very strong due to aggregation, plow pan formation or dryness, soil stress applied by repeated wheeling results in an increased primarily vertical soil particle displacement in the Hiwassee clay soil while during repeated wheeling (up to 10×) a more pronounced displacement linked with a more intense movement of particles can be proofed. With increasing number of wheeling events, new platy or again coherent structure elements are formed, which create a very different pore system. The more intense is soil wheeling, the smaller is the saturated hydraulic conductivity and the higher is the unsaturated one at a given pore water pressure value. Such changes are the more pronounced the more completed is the rearrangement of the still existing aggregates into new units like plates. Due to shear because of the three-dimensional soil displacement even under dry conditions such aggregates can be redisturbed and a coherent but very compacted soil horizon can be formed. Under those conditions the values of bulk density are even higher than the Proctor density.

Stress–strain effects in structured unsaturated soils on coupled mechanical and hydraulic processes

The stress–strain processes in structured unsaturated arable and forest soils depend very much on the internal soil strength and on the existing hydraulic and mechanical boundary conditions. These variables affect soil deformation by compaction and shearing to a great extent. As soon as the internal soil strength, defined as the precompression stress value, is exceeded by external forces, an intense virgin compression process occurs in combination with shearing forces at high pore water pressure values. These processes result in a complete homogenisation of the soil profile down to a certain depth. Consequently, both the hydraulic and the gas fluxes are affected, which results in an intense alteration of ecological and mechanical properties of the site. The consequences of such compaction and shearing on soil strength and on sustainable land use must be considered with respect to the sustainability of the system.

Stability of Saturated Cohesionless Soil Layer Due to Water Flowing from a Broken Underground Pipeline

An experimental study was conducted to assess the effect of the increase in pore water pressure, caused by water flowing from an underground pipeline, on the stability of a saturated cohesionless soil layer. The value of the buildup pore water pressure depends on several factors such as the water pressure inside the pipeline, the size of the hole at the broken section of the pipeline, and the depth of embedment of the pipeline. In this study, three series of tests were run to evaluate the effect of the above mentioned factors on the buildup of pore water pressure and on the stability of the soil layer. The increase of the water pressure inside the pipeline and the increase of the size of the hole at the broken section of the pipeline were found to significantly increase the buildup pore water pressure and to reduce the stability of the soil layer. However, increasing the depth of embedment of the pipeline reduced the effect of the buildup water pressure on the stability of the soil layer due to the increase of the effective stress with depth.

Performance of Geosynthetic-Reinforced Soil Structures at Limit Equilibrium State

Potential failure mechanisms representing various limit equilibrium states of geosynthetic-reinforced soil walls/slopes are employed to study the behavior of reinforced soil walls/slopes. Parametric analyses were conducted to investigate the effects of varying the values of pore water pressure, soil strength, geosynthetic strength, soil-geosynthetic interaction coefficients, vertical spacing of geosynthetic, surcharge load, facing slope, and backslope. In the analyses, geosynthetic-reinforced soil walls/slopes are assumed to have competent foundations. Seepage forces and other dynamic forces such as earthquake forces have not been taken into account. Similarly, the stabilizing effects of facing rigidity and strength are neglected. The results are presented in a nondimensional graphical form. Methods of optimizing the design of geosynthetic-reinforced soil structures are suggested.