Initial Pore Pressure Distributions Research Articles

A simple method to account for non-uniform initial excess pore water pressures in settlement computations

The variation in percentage consolidation with time within a clay layer subjected to a non-uniform initial excess pore water pressure distribution can be difficult to evaluate, and as a result, often a uniform initial distribution is assumed in most analyses. However, by utilizing some of the key features of consolidation in terms of excess pore water pressure dissipation, it is possible to simply adjust the uniform case to account for any number of non-uniform initial excess pore pressure distributions. By observing the decay of excess pore water pressure with time resulting from various non-uniform initial distributions, it is clear that any initial asymmetry or skewness is quickly dispersed, and the distribution of excess pore pressure with depth becomes sinusoidal (or half-sinusoidal if singly drained) shortly after consolidation has commenced. In other words, once the pore pressure decay due to a non-uniform initial distribution has become sinusoidal, it will actually decay at the same rate as the uniform case – however, it will be “ahead” or “behind” the uniform case by some constant factor. Once this factor has been determined, it is possible to simply adjust the rate of consolidation resulting from a uniform initial pore pressure distribution (the values of which are widely available in literature) to account for any number of realistic non-uniform initial excess pore pressure distributions.

A simple analytical solution to one‐dimensional consolidation for unsaturated soils

SUMMARYThis paper presents a simple analytical solution to Fredlund and Hasan's one‐dimensional (1‐D) consolidation theory for unsaturated soils. The coefficients of permeability and volume change for unsaturated soils are assumed to remain constant throughout the consolidation process. The mathematical expression of the present solution is much simpler compared with the previous available solutions in the literature. Two new variables are introduced to transform the two coupled governing equations of pore‐water and pore‐air pressures into an equivalent set of partial differential equations, which are easily solved with standard mathematical formulas. It is shown that the present analytical solution can be degenerated into that of Terzaghi consolidation for fully saturated condition. The analytical solutions to 1‐D consolidation of an unsaturated soil subjected to instantaneous loading, ramp loading, and exponential loading, for different drainage conditions and initial pore pressure conditions, are summarized in tables for ease of use by practical engineers. In the case studies, the analytical results show good agreement with the available analytical solution in the literature. The consolidation behaviors of unsaturated soils are investigated. The average degree of consolidation at different loading patterns and drainage conditions is presented. The pore‐water pressure isochrones for two different drainage conditions and three initial pore pressure distributions are presented and discussed. Copyright © 2013 John Wiley & Sons, Ltd.

One-Dimensional Consolidation of Unsaturated Soil Subjected to Time-Dependent Loading with Various Initial and Boundary Conditions

This study assesses the one-dimensional (1D) consolidation of unsaturated soil subjected to time-dependent loading based on the 1D consolidation theories of unsaturated soil. The differential quadrature method (DQM) is used to produce a general solution that considers various boundary conditions, various initial pore-water and pore-air distributions, and complex time-dependent loading. A special case in which the analytical solution is available in the literature is used for verification and accuracy analysis. It is found that the DQM solution can deliver more accurate results compared with the finite-difference method with a small number of sampling points. The general solution can avoid cumbersome computations in solving eigenequations encountered with the analytical solution. In addition, the proposed solution is more suitable for practical engineering because of its generality in complex initial, boundary, and loading conditions. Finally, the characteristics of the 1D consolidation of unsaturated soils under various initial pore pressure distributions, boundary conditions, and complex time-dependent loading conditions are investigated, and it is found that the initial and boundary conditions have a significant influence on the consolidation of unsaturated soils.

Calculating cv based on non-uniform initial excess pore pressure

The coefficient of consolidation (cv) is often determined by comparing the characteristics of the experimental and theoretical consolidation using empirical curve-fitting procedures which are based on the theoretical U−T curve generated by a layer subjected to a uniform initial excess pore water pressure distribution. However, in cases where settlement–time data are a result of a non-uniform initial pore pressure distribution, these curve-fitting procedures are no longer valid. In this technical note, a generalised procedure for Taylor and Casagrande's popular curve-fitting procedures is proposed, where the user is directed to select appropriate adjustment factors, depending on the type of non-uniform initial excess pore pressure distribution encountered. These factors were determined by approximating separate regions of the U−T curves using simple power and exponential functions. In non-uniform cases where the power approximation only captures a small portion of the U−T curve it may be difficult to use the corresponding modified curve-fitting procedure objectively.

A Critical Reappraisal of the Average Degree of Consolidation

Since its development in 1925, Terzaghi’s expression for the average degree of consolidation has been extensively used in teaching, research and consulting to determine the consolidation settlement that has taken place at a specific time in a geotechnical problem. Recent research by the authors has revealed that the term degree of consolidation is not very meaningful when the initial pore pressure distribution is highly skewed. In view of these findings, a review of all aspects of Terzaghi’s consolidation theory is required, particularly when considering asymmetric initial pore pressure distributions. An exact method for estimating the consolidation settlement of a clay stratum using the mass flux per unit area out of the drainage boundaries is proposed. Graphical representations of both exact and traditional methods reveal identical average degree of consolidation curves. In light of this, Terzaghi’s expression for the average degree of consolidation was re-examined. Upon application of the original governing one-dimensional consolidation equation, it was found that the traditional and exact methods for calculating the average degree of consolidation are identical. Thus, the widespread proclivity for geotechnical engineers to use Terzaghi’s average degree of consolidation equation to estimate the consolidation settlement of a clay stratum has been validated.

Transient deterministic shallow landslide modeling: Requirements for susceptibility and hazard assessments in a GIS framework

Application of transient deterministic shallow landslide models over broad regions for hazard and susceptibility assessments requires information on rainfall, topography and the distribution and properties of hillside materials. We survey techniques for generating the spatial and temporal input data for such models and present an example using a transient deterministic model that combines an analytic solution to assess the pore-pressure response to rainfall infiltration with an infinite-slope stability calculation. Pore-pressures and factors of safety are computed on a cell-by-cell basis and can be displayed or manipulated in a grid-based GIS. Input data are high-resolution (1.8 m) topographic information derived from LiDAR data and simple descriptions of initial pore-pressure distribution and boundary conditions for a study area north of Seattle, Washington. Rainfall information is taken from a previously defined empirical rainfall intensity–duration threshold and material strength and hydraulic properties were measured both in the field and laboratory. Results are tested by comparison with a shallow landslide inventory. Comparison of results with those from static infinite-slope stability analyses assuming fixed water-table heights shows that the spatial prediction of shallow landslide susceptibility is improved using the transient analyses; moreover, results can be depicted in terms of the rainfall intensity and duration known to trigger shallow landslides in the study area.

An approach to evaluation of field CPTU dissipation data in overconsolidated fine-grained soils

Dissipation of excess pore pressures during piezocone testing in firm to stiff overconsolidated fine-grained soils provides data curves that cannot be interpreted using published theoretical solutions. Available solutions are based on either cavity-expansion or strain-path methods, which have been developed for soft, normally consolidated soils and do not adequately model the response in overconsolidated soils. During penetration in overconsolidated soils, large pore-pressure gradients and unloading can exist as the soil moves past the singularity at the cone tip shoulder; these gradients modify the initial pore-pressure distribution around the tip and along the friction sleeve and give rise to nonstandard dissipation curves. The different types of response resulting from the modified pore-pressure distribution are discussed and classified and a correction technique is proposed. In this way, the application of available theoretical models can be used to evaluate in situ flow characteristics. Examples of the different types of pore-pressure response are presented from various sites worldwide.Key words: cone penetration test, in situ, clay, overconsolidation ratio, pore pressure, dissipation, consolidation.

Consolidation After Undrained Piezocone Penetration. II: Interpretation

A rational interpretation method for estimating the in situ consolidation and permeability characteristics of clays from pore pressure dissipation measurements after piezocone penetration is presented. The method is based on linear consolidation analyses and initial pore pressure distributions calculated by the strain path method for undrained penetration in Boston blue clay (BBC). Extensive dissipation measurements in BBC are consistent with predictions for two cone angles and various locations of the porous element. Backfigured values of the coefficients of consolidation and permeability are in reasonable agreement with laboratory and field performance data.

Consideration on Critical Rainfall for Prediction of Occuring Time of Slope Failure

In order to develop a prediciion method for the occurring time of a slope failure based on the critical rainfall, the following studies were carried out. Conditions which has influence on the critical rainfall were discussed. Then, the change of pressure head in natural condition were investigated. Finally, the pore pressure distribution in a slope and the stability of slope were analysed numerically.Consequently, the following results are obtained:(1) The critical rainfall curve which is the relation between rainfall per a week and rainfall per a day of the failed slopes can be obtained numerically.(2) The seepage characteristic and the thickness of permeable layer have a great influence on the total rainfall till the slope failure.(3) The quantity of successive rainfall for the slope failure can be accurately estimated by numerical analysis if the initial pore pressure distribution in the slope is known.