Julie Lovisa

Consolidation Behavior of Soils Subjected to Asymmetric Initial Excess Pore Pressure Distributions

Although the consolidation settlements beneath foundations and embankments are rarely one dimensional, Terzaghis one-dimensional consolidation theory is often applied to these situations to approximate consolidation behavior. This paper investigates the consolidation behavior of a soil stratum subjected to various initial excess pore pressure distributions which occur under one-dimensional loading. The results show that analysis in terms of average degree of consolidation provides an incomplete representation of the consolidation behavior. While the average degree of consolidation curves for all uniform and linearly varying initial distributions are identical, the degree of consolidation isochrones for each distribution are unique. Furthermore, the application of a bottom-skewed initial excess pore pressure distribution results in a redistribution of pore pressures toward the skewed region so that an increase in excess pore pressure occurs at some depth after consolidation has already commenced. As a result, conventional consolidation relationships are considered inappropriate for these cases, and an alternative method of consolidation analysis in terms of normalized pore pressures is proposed.

Determination of c and φ of rocks from indirect tensile strength and uniaxial compression tests

Abstract Cohesion (c) and friction (φ) angle are the two key parameters required in numerical simulations and designs of underground openings, excavations, and foundations in rocks. The paper discusses a simple method, based on a theoretical framework, to determine the two parameters from uniaxial compression test and indirect tensile strength test. Laboratory tests were carried out on 35 rock specimens and the values of c and φ were computed. The predicted values of c appear to be more realistic than those of φ, and the scatter is attributed to the anisotropy and heterogeneity generally observed in the rock cores. The computed value of c is 1·82 times the indirect tensile strength (σt).

An Overview of Electrokinetic Consolidation of Soils

Electrokinetic stabilization is one of the techniques that improve the geotechnical properties of the soils. It was pioneered by Casagrande in late 1940s and has not seen much development since then, especially in large-scale field applications. Some bench scale studies have been carried out during the past two decades and there have been some small scale field studies and limited field applications, mostly on soft soils. Due to lack of understanding of the physiochemical and electrochemical changes in the soil during electrokinetic stabilization, uncertain energy costs, loss of efficiency with time and the corrosion of electrodes, this method is usually considered as a last resort for large-scale practical applications. The objective of this paper is to highlight the critical parameters affecting electrokinetic consolidation, and to discuss their effects on the efficiency of the process. A better understanding of these critical parameters and their effects will enable geotechnical engineers to design the electrokinetic consolidation operation more effectively and make it an economically viable process for many situations.

Tall Oedometer Testing: Method to Account for Wall Friction

AbstractThe oedometers used for one-dimensional consolidation tests are proportioned such that their height-to-diameter ratio lies in the range 0.17–0.40. Sometimes, it is desirable to test a soil specimen that has a significantly larger height-to-diameter ratio, where the wall friction must be considered in the analysis. Such tall oedometers can become useful tools in consolidation tests, if the wall friction can be accounted for rationally. The objective of this paper is to develop some theoretical basis for analyzing consolidation test data from tests carried out in tall oedometers. It is shown that the same average degree of consolidation versus time factor charts can be used for height-to-diameter ratios as much as 3, provided the specimen is doubly drained. Consolidation tests carried out in the standard oedometer and tall oedometer in the laboratory gave very similar values of coefficients of consolidation.

Simple Approach to Consolidation Due to Constant Rate Loading in Clays

AbstractTerzaghi’s one-dimensional consolidation theory is extended to constant rate of loading, where a simple expression relating average degree of consolidation and time factor is derived. This expression is used to assess Terzaghi’s approximation that the ramp load can be assumed as an instantaneous load, applied at t0/2, where t0 is the duration of loading. It is shown that this assumption overestimates the degree of consolidation by approximately 10%. Terzaghi’s approximation can be improved in two simple ways: (1) assume the load acts at t0/2, estimate the degree of consolidation, and reduce it by 10% or (2) assume the load to act at 2t0/5 rather than t0/2 in estimating the degree of consolidation, with no further adjustment. Nevertheless, the single U-T expression developed herein maintains a level of simplicity comparable to Terzaghi’s approximation and is adequate for all practical purposes for computing the degree of consolidation during the loading, as well as beyond completion of loading. A n...

Consolidation Behavior of Soils Subjected to Asymmetric Initial Excess Pore Pressure Distributions with One-Way Drainage

A consolidating clay layer can be singly or doubly drained. The degree of consolidation is a function of the initial excess pore pressure distribution and the drainage conditions. Traditional geotechnical engineering practice assumes a uniform initial excess pore pressure distribution, for which the singly drained and doubly drained solutions to Terzaghis one-dimensional consolidation theory are available in most textbooks. In this paper, several symmetric and asymmetric initial excess pore pressure distributions are studied using a series solution method. Excess pore pressure isochrones and average degree of consolidation plots are generated for all cases, assuming an impervious boundary at the base of the clay stratum. These plots enable users to determine the average degree of consolidation and pore pressure at a specific depth for a variety of loading scenarios. Pore pressure redistribution is also clearly more prevalent in singly drained clays, where pore-water pressures at certain depths increase at certain times, a phenomenon not generally expected during consolidation.

Time factor in consolidation: critical review

The magnitude of consolidation settlement is often calculated using Terzaghis expression for average degree of consolidation (U) with respect to time. Developed during a time of limited computing capabilities, Terzaghis series solution to the one-dimensional consolidation equation was generalized using dimensionless time factor (T), where a single U-T curve is used to describe the consolidation behaviour of both singly and doubly drained strata. As a result, any comparisons between one- and two-way drainage are indirect and confined to discrete values of time. By introducing a modified time factor T* in terms of layer thickness (D) instead of the maximum drainage pat length (Hdr), it is now possible to observe the effect of drainage conditions over a continuous range of time for a variety of asymmetric initial excess pore pressure distributions. although two separate U-T plots are required (for singly and doubly drained cases), the time factor at specific times remains the same for both cases, enabling a direct visual comparison. The importance of a revised time factor is evident when observing the endpoint of consolidation, which occurs as U approaches 100%. This occurs at T*≅0.5 for two-way drainage and at T*≅2 for one-way drainage, an observation not possible using the traditional expression for time factor.

A Method to Determine cv Under Sinusoidal Pore Pressure Distributions

The coefficient of consolidation (cv) is traditionally evaluated by fitting experimental settlement-time data to the theoretical percentage consolidation-time factor curve of a layer subjected to a uniform initial excess pore water pressure (ui) distribution. Over the years, numerous curve-fitting techniques have been developed for this experimental-theoretical correlation, the most popular of which are Casagrandes logarithm-of-time method and Taylors square-root time method. These classical curve-fitting techniques have recently been generalized to account for a variety of different ui distributions. In this paper, basic consolidation principles have been applied in a novel fashion to both standard and tall oedometer tests on two clays to simulate a sinusoidal ui distribution (operating under either singly or doubly drained conditions) within a laboratory setting. The settlement-time data obtained from these tests were analyzed using the modified curve-fitting procedures previously put forward by the authors to determine appropriate values for cv, which were found to closely align with values of cv obtained using the traditional Taylor and Casagrande methods, when the ui distribution is considered uniform. The most valuable feature of these modified curve-fitting techniques was found to be their ability to analyze traditionally obtained settlement-time data to supplement any conventionally obtained cv values. This is useful in terms of validation when settlement-time data do not exhibit the usual trends and traditionally calculated values of cv require further authentication.

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

Abstract 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.

Consolidation settlement due to ramp loading

Abstract In one-dimensional consolidation analysis, it is generally assumed that the load is applied instantaneously. In reality, the load is applied over certain duration which has an effect on computations. This paper presents some ramp loading tests using a standard oedometer to support some simple theoretical developments. It is shown that during ramp loading, the plot of settlement v. time, normalized with respect to the end of ramp loading values, is relatively independent of the clay type and loading duration. This normalized plot can be used to predict the settlement pattern during the construction period and the coefficient of consolidation, using a single measurement at any time during construction. Post-construction settlements are predicted separately by assuming a sinusoidal initial pore water pressure distribution at the end of the construction period that is applied instantaneously. Laboratory oedometer test data show very good agreement with the theory.