Development Of Excess Pore Pressure Research Articles

Finite-Element Parametric Study of the Consolidation Behavior of a Trial Embankment on Soft Clay

The paper presents a case study for numerical analysis of the consolidation behavior of an instrumented trial embankment constructed on a soft soil foundation. Details are given to the geological profile, field instrumentation, laboratory test results, and determination of soil parameters for numerical modeling. Embankment settlement is estimated based on one-dimensional consolidation analysis and nonlinear finite-element analysis following Biot's consolidation theory. Finite-element results are calibrated against the measured field data for a period of more than 3 years. Development and dissipation of excess pore pressure, long-term settlement, and horizontal displacement are predicted and discussed in light of sensitivity of embankment performance to some critical factors through a parametric study.

Fluid-Soil-Structure Interaction in Liquefaction around a Cyclically Moving Cylinder

Coastal or offshore structures such as pipelines installed on the seabed are submitted to cyclic horizontal loads either by the direct hydrodynamic action of waves or through the cyclic movement of risers or flow lines transmitted by floating structures. In fine sandy or silty soils these cyclic loads can induce a liquefaction of the surrounding bed, which can play an important part in the processes of erosion, trenching, or self-burial of the pipes. As part of the LIMAS program, a full-scale physical model was built to study the fluid-soil-structure interaction with special emphasis on the conditions leading to liquefaction around a pipe instrumented with pore pressure sensors. The experiments indicate a development of excess pore pressure at the pipe-soil interface much higher than the effective overburden stress, and a lateral visualization provided evidence of the liquefaction of a soil band in the vicinity of the pipe. The penetration of the structure can be related to the phenomenon of liquefaction.

Working mechanism of cutoff walls in reducing uplift of large underground structures induced by soil liquefaction

The uplift of large underground structures in saturated liquefiable soils under strong earthquake loadings may induce severe damages to the structures. Various mitigation procedures have been proposed to alleviate such damage, among which installation of cutoff walls next to underground structures was found to be effective. However, the working mechanism of cutoff walls in alleviating uplift of underground structures and the corresponding design parameters are still not clear. The liquefaction induced uplift behaviour of a subway tunnel in saturated sandy deposit over a layer of non-liquefiable soils and the working mechanism of cutoff walls for uplift mitigation purpose were investigated using the fully coupled dynamic finite element code DIANA Swandyne-II. A generalized plasticity model capable of simulating both cyclic liquefaction and pressure dependency of soils was used to model the sandy deposit. It is found that the small effective unit weight of underground structures, the development of excess pore pressure and the flow of liquefied soils were the sufficient and necessary conditions for underground structures to uplift during earthquakes. Cutoff walls could restrain the flow or deformation of liquefied soils and inhibit the uplift of underground structures but they could not necessarily prevent the liquefaction of the enclosed soils. After earthquake loadings, underground structures might settle due to the consolidation of soils and cutoff walls could also reduce the magnitude of settlement. The design parameters of cutoff walls, including the acting lateral pressure, the position, the stiffness and the permeability of cutoff walls, were also analyzed, the findings of which, together with the unveiled working mechanism, would be relevant for the design of cutoff walls for uplift mitigation purpose.

Factors affecting seismic response of submarine slopes

Abstract. The response of submerged slopes on the continental shelf to seismic or storm loading has become an important element in the risk assessment for offshore structures and "local" tsunami hazards worldwide. The geological profile of these slopes typically includes normally consolidated to lightly overconsolidated soft cohesive soils with layer thickness ranging from a few meters to hundreds of meters. The factor of safety obtained from pseudo-static analyses is not always a useful measure for evaluating the slope response, since values less than one do not necessarily imply slope failure with large movements of the soil mass. This paper addresses the relative importance of different factors affecting the response of submerged slopes during seismic loading. The analyses use a dynamic finite element code which includes a constitutive law describing the anisotropic stress-strain-strength behavior of normally consolidated to lightly overconsolidated clays. The model also incorporates anisotropic hardening to describe the effect of different shear strain and stress histories as well as bounding surface principles to provide realistic descriptions of the accumulation of the plastic strains and excess pore pressure during successive loading cycles. The paper presents results from parametric site response analyses on slope geometry and layering, soil material parameters, and input ground motion characteristics. The predicted maximum shear strains, permanent deformations, displacement time histories and maximum excess pore pressure development provide insight of slope performance during a seismic event.

Performance of embankments with and without reinforcement on soft subsoil

A case history of both reinforced and unreinforced embankments on soft subsoil built-to-failure is described and analyzed. The effect of geotextile reinforcements on embankment behavior is discussed by comparing the field and numerical analysis results of cases with and without reinforcement. The results of a laboratory model test on the behavior of embankments on soft subsoil are discussed. Both field and laboratory tests, as well as analysis results, indicate that the reinforcement had a positive effect on embankment stability. However, at a working state (for a factor of safety of FS = 1.2~1.3) the reinforcement did not have an obvious effect on the subsoil response. The effect of reinforcement on subsoil deformation could be noticed only when the unreinforced embankment was close to failure. The laboratory model test results indicated that if the reinforcement is stiff and strong enough, the effect of reinforcement is considerable. It is suggested that although the geotextile has a beneficial effect on embankment over soft subsoil due to its relative lower stiffness, to achieve a substantial improvement on embankment behavior, the stiffer and stronger reinforcements should be used. This case history also demonstrated that the rate of lateral displacement and excess pore pressure development are sensitive indicators of the stability of embankment on soft subsoil.Key words: embankment, reinforcement, soft ground, field tests, laboratory tests, FEM analysis.

Consolidation phenomena in sheared granitic magma: effects of grain size and tortuosity

Granitic (and other) magmas with crystal contents between 50 and ca.70% are expected to show dilatant behavior during deformation. The grain size at which the magma has been crystallised is shown to be relevant to the development of excess pore pressure at continued shearing. The reigning pressure regime is compared to the stresses required for fracturing of the skeletal elements. At rates of loading in excess of average tectonic rates (≥ 10−14 s−1), shear-induced dilation in granitic magmas with high solidosities (crystal contents >50%), can lead to fracture. The available excess skeletal pressure at a given strain rate is a function of two coupled parameters, grain size and tortuosity, with higher skeletal pressures favoured by smaller mean particle size. Our analysis suggests that the common occurrence of brittle-like features thought to have formed in the magmatic state during pluton crystallisation can only be achieved where strain rates (emplacement loading) are at least of the order 10−13 s−1 or greater, consistent with similar estimates of strain rates during pluton emplacement based on field studies.

Large-scale drainage behaviour of composite geotextile and geogrid in residual soil

The reinforcing capability of high strength geotextile and geogrid in residual soil has been studied recently, using pullout tests and wide width tensile tests. However, the drainage behaviour of these geosynthetics products in reinforcement applications has not been well addressed so far. This paper presents experimental evaluation of the drainage performance of a high strength composite permeable geotextile and a HDPE geogrid in a poorly draining Singapore residual soil. The drainage behaviour of these geosynthetics products embedded in poorly draining Singapore residual soil was evaluated with large-scale drainage behaviour tests. Pore pressure transducers were installed at strategic locations in the soil mass to evaluate the development and dissipation of excess pore pressure. The test results showed that with the placement of permeable geosynthetics materials, the rate of pore pressure dissipation was faster than without any geosynthetics product or with a non-permeable geosynthetics product.

Bank and near-bank processes in an incised channel

Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion. The interaction of these forces control streambank mechanics. Hydraulic forces exerted by flowing water on in situ bank-toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower-bank profiles. Seepage forces exerted on in situ bank material by groundwater, downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength. Data from a study site on Goodwin Creek, MS, USA clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore-water pressures. Negative pore-water pressures (matric suction) have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear. A stable bank can be transformed into an unstable bank during periods of prolonged rainfall through: 1. increase in soil bulk unit (specific) weight, 2. decrease or complete loss of matric suction, and, therefore, apparent cohesion, 3. generation of positive pore-water pressures, and, therefore, reduction or loss of frictional strength, 4. entrainment of in situ and failed material at the bank toe, and 5. loss of confining pressure during recession of stormflow hydrographs. Relatively small frequent flows during the winter have the ability to erode failed bank materials, maintain oversteepened, unstable bank surfaces and promote prolonged periods of bank retreat, channel migration and high yields of fine-grained sediment. Confining pressures provided by stormflow are not as significant in maintaining bank stability as the counteracting force of fluid shear on the bank toe, which steepens the bank. For example, more than 2 m of bank retreat occurred during the study period at the research site on Goodwin Creek, northern Mississippi. The loss of matric suction (negative pore pressures) due to infiltrating precipitation has been found to be as significant as the development of excess pore pressures in contributing to mass bank instability. Apparent cohesion, friction angle, soil bulk unit weight and moisture content were measured in situ. Matric suction was measured continuously, in situ with a series of five pressure-transducer tensiometers. A bank-failure algorithm, which combines the Mohr–Coulomb approach, for saturated conditions and the Fredlund modification for unsaturated conditions was developed for layered cohesive streambanks. The resulting equation has been used successfully to investigate the role of matric suction, positive pore-water pressures and confining pressure for layered streambanks composed of cohesive materials.

Anisotropy of Undrained Shear Strength of an Over-Consolidated Soil by Triaxial and Plane Strain Tests

In order to find out the real cause leading to the anisotropy of undrained shear strength of clayey soils, two types of undrained shear tests were carried out; 1) undrained plane strain tests on lightly over-consolidated kaolin clay, and 2) undrained triaxial tests on heavily over-consolidated kaolin clay. Analysis of the data has shown that the anisotropy of undrained shear strength of over-consolidated kaolin clay is primarily caused by the direction-dependent (anisotropic) shear strength parameters c' and ϕ' in terms of effective stresses. The anisotropy of pore water pressure parameter Ay is artificially generated as a result of the anisotropic change of the undrained shear strength itself, rather than of the anisotropic excess pore-pressure development during shearing, and is of secondary, if any, importance to interpret why the undrained shear strength is anisotropic. Intensity of the anisotropy in the undrained shear strength is not altered much by either changing the boundary conditions, such as triaxial and plane strain or increasing the over-consolidation ratio up to 32. The minimum values of cohesion c' and friction angle ϕ' appear when a specimen is compressed with the major principal stress inclined at about 30° to the consolidation plane. When a specimen is compressed either normal or parallel to the consolidation plane, values of cohesion c' and friction angle ϕ' are high with no significant difference between them. The difference between the maximum and the minimum friction angle ϕ' is almost the same as that observed in the triaxial tests of sands. In the case of sands, however, a specimen compressed normal to the bedding plane yields the maximum friction angle ϕ', while a specimen compressed parallel to the bedding plane yields the minimum value in the triaxial tests and a value close to the minimum in the plane strain tests. These observations suggest that the micro-deformation mechanism leading to the strength anisotropy is quite similar in both clay and sand, but not exactly the same.

Experimental factors governing the internal stress state of the intervertebral disc

This paper presents an experimental study of the response of the intervertebral disc to short-term static-axial loading within the conceptual framework of mechanical consolidation as applied to swelling materials. An experimental technique developed previously for articular cartilage was used to load the disc in compression and measure simultaneously the matrix internal excess pore pressure under both radially constrained and radially unconstrained conditions. Specifically the short-term pattern of development of the hydrostatic excess pore pressure was investigated. The average ratio of the maximum excess pore pressure u i to the nominal applied stress for unconstrained loading was approximately 1.9 while for constrained loading it was about 1.4. This behaviour is interpreted in terms of the generation of a pore pressure in the nucleus which reflects a non-uniform partitioning of the applied load across the inhomogeneous structure of the disc. These results provide a quantitative insight into the role that physico-chemically generated swelling plays in determining the internal mechanical response of the disc to an externally applied load and suggest a quantitative means of investigating the important influence of degenerative changes on the internal stress state of the disc without needing to pierce the annular wall in order to insert a pressure sensing device.

Mathematical modelling of gravitational compaction and clay dehydration in thick sediment layers

SUMMARY Gravitational compaction is an important process in sedimentary basins which controls the reduction of porosity with burial depth and the development of high pore fluid pressures. Often, sediments contain hydrated, expandable clay (smectite) that can undergo a transition to a dehydrated, non-expandable clay (illite). During dehydration, structural water bound within the sheet layers of smectite is released into the pore space, which can increase the pore pressure and influence geological processes such as solute transport, hydrocarbon migration and hydrothermal fracturing. A multicomponent continuum mechanics model that accounts for Darcy's Law, Terzaghi's principle of effective stress and a thermally activated dehydration reaction is derived and solved numerically. A closed-form solution is available in the limiting case of hydrostatic pore pressure and no dehydration. The results show that excess pore-pressure development is controlled by the sedimentation parameter, the dimensionless ratio of the hydraulic conductivity to the sedimentation rate. For relatively impermeable sediments, chemically released water can increase the excess pore pressure by as much as 30 per cent, and the excess pressure persists over geological time-scales. The pressure contribution due to the excess pore water is important provided that the dehydration goes to completion at sufficient burial depth, which depends in part on the activation energy of the reaction. If sediments overlie a permeable basement, fluid can flow out of the sediments and relieve pore pressure throughout the sedimentary column.

Chemical Kinetic Model of Hydrocarbon Generation, Expulsion, and Destruction Applied to the Maracaibo Basin, Venezuela

This paper describes the development and application of a compositional chemical model of hydrocarbon generation, expulsion, and destruction for the Cretaceous La Luna Formation source rock of the Maracaibo basin, Venezuela. Applications include both laboratory and geological settings. Laboratory pyrolysis experiments were used to study bulk oil generation, expulsion, and associated changes in composition of the kerogen, extractable organic matter, and expelled and unexpelled hydrocarbons. The laboratory experiments were also used to determine kinetic parameters to quantitatively describe organic reactions, via a computer model that also includes simulation of pressure-driven primary expulsion, over widely varying conditions. We show that the chemical model accurately sim lates the experimental results. Thermal history models for wells in the Maracaibo basin were used to simulate hydrocarbon generation and pore pressure development in the La Luna Formation and expulsion into nearby Cretaceous reservoirs. Results of the modeling indicate that both compaction disequilibrium and organic maturation play important roles in the development of excess pore pressure in the La Luna Formation. The model simulation of the variation of indicators such as Rock-Eval parameters and extract and oil compositions shows generally good agreement with measurements from remaining kerogen, oils, and extracts recovered from the La Luna Formation and from nearby Cretaceous reservoirs.

Finite element consolidation analysis of embankments

Abstract The construction of earth embankments may lead to the development of excess pore pressures in foundation and embankment soils. The ability to predict the development and dissipation of these pore pressures is important in assessing the performance of such structures. This paper describes a finite element analysis method that allows the prediction of the behaviour of embankments including stress-deformation and consolidation behaviour. The application of the method is demonstrated through application to a reinforced embankment on peat and clay and to a high rockfill dam.

A New Technique for Reduction of Excess Pore Pressure during Pile Driving: Discussion

The authors are to be complimented on an interesting case history presenting an original method of solving the commonly occurring problem of development of excess pore pressures, when driving piles in soft clays. In Fig. 5, the authors present the measured pore pressures and number of piles driven on specific days. This figure indicates that the developed pore pressures were considerably smaller after the installation of the Geodrain on the piles, as compared with the measurements before the drain application. However, as the magnitude of the measured pore pressures is less dependent on the number of piles driven per day and more on the distance from the piles to the piezometers, a different plotting of the results gives a better basis for the conclusions drawn by the authors. In Fig. 1, the writer has plotted £or each driven pile the distance against time to two piezometers, numbers P-1 and P-3. A black dot shows the distance (and date) of a pile relative to piezometer number P-1 and an open dot that of the same pile relative to piezometer number P-3. The lower diagram in Fig. 1 shows the piezometric elevations measured in piezometer numbers P-1 and P-3. A study of the measurements taken before