Andrzej Sawicki

LIMIT ANALYSIS OF COHESIVE SLOPES REINFORCED WITH GEOTEXTILES

Abstract A method of plastic limit analysis of cohesive slopes reinforced with geotextiles is presented. First, the theoretical model of reinforced cohesive soil is presented, then the formulation of boundary value problems by a method of characteristics described. The bearing capacity of reinforced slopes and the failure surfaces are obtained as the solution to the system of governing equations. The method proposed allows for studying the influence of design parameters, like the unit weight of the soil, the angle of internal friction, cohesion, and the amount of reinforcement, on the stability of reinforced soil structures.

Apparent Creep of Saturated Sand Caused by Intrinsic Cyclic Loading

AbstractAn interesting phenomenon, designated as an apparent creep of saturated sand, is described on the basis of experimental data obtained from triaxial quasi-drained-controlled tests. In such tests, external stresses were kept constant while the pore pressure was cyclically changed (intrinsic cyclic loading). It is a kind of cyclic loading that takes place in seabeds, where the pore pressure changes cyclically because of the action of water waves. During the experiment, both vertical and horizontal strains were recorded. It is shown that these strains gradually increase as functions of the number of loading cycles in such a way that the volume of the specimen is constant, and only the deviatoric strain increases. The experimental curve resembles the creep curve for solid materials, in which real time is replaced by the number of loading cycles. The experiments are described and discussed, and a simple model of the phenomenon observed is proposed.

Estimation of Stresses in a Dry Sand Layer Tested on Shaking Table

Abstract Theoretical analysis of shaking table experiments, simulating earthquake response of a dry sand layer, is presented. The aim of such experiments is to study seismic-induced compaction of soil and resulting settlements. In order to determine the soil compaction, the cyclic stresses and strains should be calculated first. These stresses are caused by the cyclic horizontal acceleration at the base of soil layer, so it is important to determine the stress field as function of the base acceleration. It is particularly important for a proper interpretation of shaking table tests, where the base acceleration is controlled but the stresses are hard to measure, and they can only be deduced. Preliminary experiments have shown that small accelerations do not lead to essential settlements, whilst large accelerations cause some phenomena typical for limit states, including a visible appearance of slip lines. All these problems should be well understood for rational planning of experiments. The analysis of these problems is presented in this paper. First, some heuristic considerations about the dynamics of experimental system are presented. Then, the analysis of boundary conditions, expressed as resultants of respective stresses is shown. A particular form of boundary conditions has been chosen, which satisfies the macroscopic boundary conditions and the equilibrium equations. Then, some considerations are presented in order to obtain statically admissible stress field, which does not exceed the Coulomb-Mohr yield conditions. Such an approach leads to determination of the limit base accelerations, which do not cause the plastic state in soil. It was shown that larger accelerations lead to increase of the lateral stresses, and the respective method, which may replace complex plasticity analyses, is proposed. It is shown that it is the lateral stress coefficient K0 that controls the statically admissible stress field during the shaking table experiments.

COMPACTION DUE TO A CYCLIC FORCE ACTING IN THE INSIDE OF A SANDY SUBSOIL

Abstract The paper deals with some applications of the compaction theory to the analysis of densification and settlements of a sandy subsoil due to a cyclic force acting inside of it. First, fundamentals of the compaction theory are presented, then a numerical algorithm of dealing with boundary value problems is described. The method of analysis proposed is illustrated for both vertical and horizontal cyclic forces, as well as for some complex loading programmes. Some results of small scale experiments are presented and discussed against theoretical predictions. The analysis presented are of some importance to practical geotechnical problems as, for example, the behaviour of a subsoil loaded by anchors or the compaction of sandy subsoil by vibroflotation. One of the advantages of the method proposed is that the analysis of compaction can be performed using small microcomputers like IBM-PCs.

Dilation and Modelling of Sands in the Light of Experimental Data

Abstract The problem of dilation is discussed in the context of classical Cam-Clay model, which was developed on the basis of a specific assumption regarding the plastic work. This assumption leads to a special form of the dilation function, from which a shape of yield function is derived. The above mentioned assumption is compared with the results of the triaxial tests, performed on the model “Skarpa” sand. It is shown that the Cam-Clay approach is not realistic, as it is based on the assumption which is not consistent with experimental data. Some general considerations and discussion of this problem are also presented.

Structure and Calibration of Constitutive Equations for Granular Soils

Abstract The form of incremental constitutive equations for granular soils is discussed for the triaxial configuration. The classical elasto-plastic approach and the semi-empirical model are discussed on the basis of constitutive relations determined directly from experimental data. First, the general structure of elasto-plastic constitutive equations is presented. Then, the structure of semiempirical constitutive equations is described, and a method of calibrating the model is presented. This calibration method is based on a single experiment, performed in the triaxial apparatus, which also involves a partial verification of the model, on an atypical stress path. The model is shown to give reasonable predictions. An important feature of the semi-empirical incremental model is the definition of loading and unloading, which is different from that assumed in elasto-plasticity. This definition distinguishes between spherical and deviatoric loading/unloading. The definition of deviatoric loading/unloading has been subject to some criticism. It was therefore discussed and clarified in this paper on the basis of the experiment presented.

Mechanical Properties of Granular Soils: Triaxial versus Plane Strain Investigations

Abstract The paper compares the pre-failure behavior of granular soils investigated in the classical triaxial apparatus and in the true triaxial apparatus, under plane strain conditions. Both experiments are described within the framework of an incremental model of the pre-failure behavior of sands. The methods of tensor algebra are used to compare theoretical predictions with experimental results. The analysis presented deals with the pre-failure deformations of fully drained sand, as well as with its undrained behavior, including static liquefaction and the specific behavior of an initially dilative soil. Some key questions of soil mechanics are discussed, for instance, whether soil parameters determined from one configuration, such as triaxial conditions, can be applied in other cases.

The Pull-out Capacity of Suction Caissons in Model Investigations

Abstract A small-scale model experiment on the pull-out resistance of suction caissons is described. The pull-out force and suction developed within the caisson in the extraction process were recorded during the experiment. A simple breakout model, together with an elementary static formulae, is applied to predict the results obtained experimentally. There is a reasonably good agreement between the experimental results and predictions. An extensive discussion of the approach applied is included. The analysis presented in this paper is original, as it differs from other approaches mentioned in this paper, and leads to acceptable predictions. At the end, the results are also compared with another approach for predicting the capacity of suction caissons.

Theoretical analysis of model seabed behavior under water wave excitation

Abstract Theoretical analysis of the behavior of a model seabed subjected to water wave excitation is presented. The experiments were performed in the wave flume at the Danish Technological University in Lyngby. Such experiments are unique in engineering sciences and therefore provide unique empirical data for testing various models of the seabed. A controversial explanation of the experiments is presented in the literature. The goal of this research was to study pore pressure changes caused by water waves and the subsequent liquefaction of the seabed. The authors of the present contribution offer their own theoretical explanation of the wave flume experiments and discuss errors found in the literature cited. The analysis is based on the classical soil mechanics, including the Biot type approach

Liquefaction of Saturated Soil and the Diffusion Equation

Abstract The paper deals with the diffusion equation for pore water pressures with the source term, which is widely promoted in the marine engineering literature. It is shown that such an equation cannot be derived in a consistent way from the mass balance and the Darcy law. The shortcomings of the artificial source term are pointed out, including inconsistencies with experimental data. It is concluded that liquefaction and the preceding process of pore pressure generation and the weakening of the soil skeleton should be described by constitutive equations within the well-known framework of applied mechanics. Relevant references are provided