Showing papers in "Journal of Geotechnical and Geoenvironmental Engineering in 1977"

Probabilistic Modeling of Soil Profiles

Abstract: New concepts and methods for modeling the natural variability of soil properties are presented and illustrated. The proposed technique of modeling the statistical character of soil profiles serves a dual function: (1)It provides a format for quantifying the information gathered during site investigation and testing, about the subsurface conditions at a site; and (2)it provides the basis for predicting performance and for quantifying the reliability of performance predictions. Probabilistic soil profiles are characterized, first, by best estimates of layer depths and of pertinent engineering properties; and secondly, by the coefficient of variation and the correlation scales for the profile parameters of interest. Methodology is developed for dealing with problems that can be formulated in terms of extremes of averages of soil properties. The problems of limit equilibrium slope stability and differential settlement prediction fall into this category.

Stress state variables for unsaturated soils

Abstract: An unsaturated soil is visualized as a four-phase system, the fourth phase being the air-water interface commonly referred to as the contractile skin. Suitable stress state variables for an unsaturated soil are proposed on the basis of writing force equilibrium equations for each phase, within the context of multiphase continuum mechanics. The analysis indicates that any two of three possible normal stress variables can be used to define the stress state. Experimental null-type tests verified the proposed stress state variables for the soil structure and contractile skin of an unsaturated soil.

Time and Stress-Compressibility Interrelationship

Abstract: For any natural soil a unique relationship exists between secondary compression index C sub a and compression index C sub c. It is postulated that the relation between time compressibility and effective stress compressibility holds true at any time effective stress, and void ratio during consolidation. The observed values of C sub a/C sub c for a variety of natural soils, including highly sensitive clays, organic silts, and peats, are in the range of 0.025 to 0.10. The shape of the settlement curve in the secondary range can be predicted from the shape of the void ratio-logarithm of effective stress curve and the relationship between C sub a and C sub c. The predicted results are completely consistent with laboratory observations and with the existing field case records. The magnitude of C sub a/C sub c can be used to predict the shape of the volume change-logarithm of time curve in the transition from primary to secondary. The procedure provides a quantitative explanation for the observed shapes of the settlement curve. /Author/

Factors Affecting Liquefaction and Cyclic Mobility

Abstract: Liquefaction is a phenomenon wherein a saturated mass of sand loses a large percentage of its shear resistance and flows in a manner resembling a liquid until the shear stresses acting on the mass are as low as its reduced shear resistance. Cyclic mobility is the progressive softening of a saturated sand specimen when subjected to cyclic loading at constant water content. Cyclic mobility has been observed in the laboratory. The writers believe that most observed cyclic mobility deformations in dilative clean sands are due to a test error, redistribution of void ratio, which is not representative of in-situ behavior. The manner in which soil type, confining stress, and initial consolidation stress ratio affect both liquefaction and cyclic mobility are shown by means of laboratory test results, for the purpose of permitting the reader to develop rational procedures for designing against the effects of earthquake loadings on soils.

Bearing capacity and settlement of pile foundations

Abstract: The discussers point out further aspects of limit point bearing capacity of piles in sand as well as the overlayered stratum and critical depth. It is noted that if phi (angle of friction) is the parameter that can describe the part in p sub o N sub q, it cannot alone explain the part in q sub 1, which essentially depends upon the compressibility phenomena. Experiments on various sands have shown that the relationship between q sub 1 and phi is valid only for sands, the physical and mechanical properties of which are not very different. For homogeneous medium, A relationship is proposed between the critical depth D sub c and p sub c, or 1 sub 1. For overlayered stratum, experimental results have shown that the value of D sub c decreases rapidly with the value of the overload applied to the bearing stratum. It is shown that for long poles, the current rule of 3-5 diameters of penetration in the bearing stratum allows a mobilization of the full point resistance of the pile.

Effects of Sample Preparation on Sand Liquefaction

Abstract: The results of undrained stress controlled cyclic triaxial tests indicated that the dynamic strength of saturated sands, remolded to the same density by different compaction procedures, was significantly different. Variations in the dynamic strength of Monterey No.0 sand were found to be in the order of —100%; however, tests on other sands indicated that the magnitude of the effect of the method of preparation used may be a function of the type of sand. Fabric studies and electrical conductivity measurements indicated that the orientation and arrangement of the contacts between sand grains were probably the primary reasons for the observed differences in the dynamic strength of Monterey No.0 sand. Comparisons indicated that specimens prepared by moist tamping or moist vibratory compaction would exhibit dynamic strengths most similar to those of undisturbed samples; however, in most cases, the strength of undisturbed samples ranged from 0% to 45% higher than that of samples prepared by moist tamping.

Reliability of Earth Slopes

Abstract: A probabilistic approach to three-dimensional limit equilibrium slope stability is presented. Failure events involving long or very short failure zones are shown to be highly improbable. The existence of a fairly distinct most probable failure length is revealed and a method to determine it is proposed. It is further found that the mean margin of safety is twice as large as that predicted by plane strain analysis. The paper also calculates the risk that a failure will occur anywhere along an embankment of given total length, and identifies as the dominant contribution to the risk a term that is linear with embankment length. This result has major implications with respect to the safe design of long linear geotechnical facilities such as dams, levees, and highways. The proposed analysis accounts for other sources of uncertainty in slope stability calculations. It is an aid in design and exploration in the pursuit of the benefits of risk reduction.

Three-dimensional slope stability analysis method

Abstract: Existing limit-equilibrium stability analysis methods are two-dimensional; a few methods for correcting or extending a two-dimensional analysis have been proposed. Presented in this paper is a general method for three-dimensional limit-equilibrium stability analysis. Limiting cases are considered in order to study preliminary implications of three-dimensional stability analyses. The method lends itself to both hand calculation and computer application. Method can also be used together with finite element analysis which should make it possible to set up a fairly realistic model for soil and slope behavior.

An Effective Stress Model for Liquefaction

Abstract: The important factors affecting the dynamic response of saturated sand layers to earthquake motions are: (1)The initial shear modulus in situ; (2)the variation of shear modulus with shear strain; (3)contemporaneous generation and dissipation of pore-water pressures; (4)changes in effective mean normal stress; (5)damping; and (6)hardening. Constitutive relations are formulated that take all these factors into account and these are incorporated into a nonlinear method for the dynamic effective stress analysis of saturated sands. The method predicts the phenomenological features of the dynamic response of saturated sand layers that commonly occur as the pore-water pressure rises in the sand during earthquake shaking. It allows the distribution of pore-water pressure and the effects that drainage and internal flow have on the location and time of liquefaction to be determined quantitatively.

Stabilization of potentially liquefiable sand deposits using gravel drains

Abstract: The installation of a drainage system offers an attractive and economical procedure for stabilizing an otherwise potentially liquefiable sand deposit. Such a procedure has already been used in one case involving the construction of stone columns in a relatively loose sand deposit, and it is being proposed for stabilization of a medium dense sand layer. The paper presents a simplified theory that provides a convenient basis for evaluating the possible effectiveness of a grain drain system in such cases. Where appropriate, additional analyses may readily be made using a computer program (LARF) based on the theory presented, but for most practical cases, it is believed that the charts presented in the paper will provide for effective stabilization of potentially liquefiable sand deposits.

Time-Dependent Behavior of Undisturbed Clay

Abstract: Time dependence of undrained stress-strain and strength behavior of a natural sensitive marine clay has been investigated with particular reference to the effects of strain rate, strain rate history, aging, and thixotropy. Triaxial samples, identically consolidated, were sheared under a variety of time-loading histories in order to simulate various deformation rate histories. The stress-strain and strength results from these tests were correlated using the hypothesis that stress is a function of current strain and strain rate only. It is illustrated that a stiffer stress-strain response and an increase in undrained strength result from an increase in rate of strain, length of aging, and thixotropic hardening. However, the failure in terms of effective stresses is found to be independent of time effects of any kind whatsoever.

Subsidence above shallow tunnels in soft ground

Abstract: The paper investigates the influence of the depth of burial and crown settlement on the surface subsidence above shallow tunnels driven in soft ground Model tests were conducted on tunnels in sand and in clay, and these are compared with one another and with observations of settlements above some real tunnels There is good agreement between the model behavior and the field records The error, or normal probability curve, may be used as an approximation for the trough of surface subsidence, and the standard deviation is an appropriate measure of the width Empirical relationships exist between the depth of burial, the trough width and the crown and surface settlements for tunnels in sands and in clays

Influence of Seismic History on Liquefaction of Sands

Abstract: It is shown both analytically and experimentally that deposits of sand subjected to low magnitude earthquakes that are not sufficiently strong to cause liquefaction will develop an increased resistance to liquefaction in subsequent earthquakes even though they may undergo no significant change in density. Accordingly, in order to determine the liquefaction characteristics of a sand it is necessary to perform tests on samples having the same density and structure as the in-situ material and conduct the tests, whenever possible, with the correct in-situ value of K sub 0. The study also indicates that the standard penetration resistance (or any in-situ measure of penetration resistance) is likely to provide a reasonable index of the liquefaction characteristics of a saturated sand deposit. Available data on field performance have been summarized to develop a correlation between penetration resistance and the cyclic stress ratio at which liquefaction has been found to occur in the field. /ASCE/

Consolidation Under Time-Dependent Loading

Abstract: In analyzing consolidation settlements caused by time dependent loading, the engineer typically replaces the actual load-time curve with a series of step loads and applies the theory for instantaneous loading. For the case of one-dimensional vertical flow and simple ramp loading, the time-settlement curve may be approximated using a graphical construction orginiated by Terzaghi or curves presented by Schiffman. However, the method is reviewed and suitable design curves are presented. For one-dimensional vertical flow all of the assumptions of Terzaghi.s basis theory are retained except the assumption of instantaneous loading. For the sand drain case, solutions are obtained for purely radial flow and for combined vertical and radial flow.

Laterally Loaded Piles: Program Documentation

Abstract: The documentation is presented for the computer program COM622, which solves for the deflection and bending moment of a pile under lateral loading as a function of depth. The calculations are performed on a finite difference model of the pile, and the soil is represented by a series of nonlinear curves of force per unit length versus deflection. This is the first program documented and distributed under the standards developed for the Geotechnical Engineering Division.

Specimen Preparation and Cyclic Stability of Sands

Abstract: The results of a study on the cyclic structural stability of saturated sands as determined by the cyclic triaxial strength test (liquefaction test) are presented. The results clearly demonstrate that the method of specimen preparation can significantly affect the cyclic behavior of sands, and this effect is most likely directly related to variances in the fabric of the sand. It is also concluded that a better understanding of the strain development characteristics of sands could play an important role in evaluating the results of cyclic triaxial strength tests.

Soil Restraint Against Horizontal Motion of Pipes

Abstract: The safe and economical design of buried pipes subjected to lateral motion requires an accurate knowledge of the subgrade reaction. Too often values of the coefficient of horizontal subgrade reaction, k\dh, reported in the literature are used without understanding the author’s intentions and without thought as to whether or not the conditions in the particular application are compatible to those in the literature. A survey of the available literature discloses that the numerical values and formulations proposed were such as to underestimate the soil restraint against flexible culverts and laterally loaded piles. These values are inadequate for the safe design of buried pipes laterally displaced by settlement, thermal expansion, earthquake shaking or wave action. The results of a laboratory testing program show the complex soil-conduit interaction and the associated failure mechanisms. An analytical method is presented to determine the load-displacement curve for any size pipe embedded at any given depth. A moderate size in-situ test was performed, which confirmed that the results of the laboratory model tests could be successfully applied to in-situ conditions.

Membrane Penetration Effects in Undrained Tests

Abstract: It was found that membrane flexibility can change the soil behavior and pore pressure characteristics far more than may be caused by, e.g., the flexibility of the pore pressure measuring system. Expressions were developed for the pore pressure parameters, B and A, which include the effects of flexibilities of membrane and pore pressure measuring system. Measured and calculated B-values agreed well for saturated specimens in which membrane penetration could occur. The effects of time and partial saturation on the measured B-values were also studied. The effect of membrane penetration in undrained tests was to reduce the rate of change in pore pressure; and the effective stress-path, stress-strain relations, and undrained strength were modified accordingly. The strengths obtained from cyclic loading tests performed on saturated specimens of granular materials for determination of liquefaction potential were also shown to be affected by membrane penetration.

Saturation Effects on Initial Soil Liquefaction

Abstract: Soil liquefaction has become an important consideration in the design of structures in seismically active regions. Many researchers have in the past attempted to determine the liquefaction potential of soils, but these studies concerned themselves with the liquefaction potential of fully saturated sands. In this paper, the writers establish a general relationship between the pore-pres-sure parameter, B and the initial degree of soil saturation, S o . By introducing the experimental values of B obtained from tests on loose Ottawa sand.

Equivalent Uniform Cycle Concept for Soil Dynamics

Abstract: While additional studies of this nature would be appropriate, especially using direction independent tests such as cyclic simple shear, torsion shear, or shaking table, the writers feel justified in concluding that the data presented herein generally confirm the validity of the equivalent cycle or the cumulative damage method of dealing with irregular loading effects on soil. Some other aspects include a clarification of the relations between damage potential and equivalent number of uniform cycles, certain peculiarities of the cyclic triaxial test on saturated sands that may not be significant for clays, and a perspective comparison of the range of data scatter for cyclic tests on soil as compared with other materials. Thus, while the damage potential or equivalent cyclic concept may not be exactly valid for all cases, its continued use in the field of seismic stability analyses of soils seems to be a valid pragmatic approach.

Modulus of Soil Reaction Values for Buried Flexible Pipe

Abstract: A table of modulus of soil reaction (E') values for use in the Iowa Formula has been empirically developed by the Bureau of Reclamation. Use of the methods and values suggested can reasonably predict the initial (no time effect) deflection of buried flexible pipe under fill up to 50 ft (15 m). The E' values vary according to the type of soil placed beside the pipe and the degree of compaction. The accuracy of predicted deflections varies according to the degree of compaction. Laboratory soil container tests and data from over 100 field installations were used in the investigation.

Vibroflotation Compaction of Cohesionless Soils

Abstract: Vibroflotation uses a horizontally vibrating probe to densify in-situ cohesionless soils with simultaneous vibration and saturation The method is described in general terms and each factor affecting the density achieved is analyzed in detail An example test program is presented which was used to investigate the influence of in-situ soil type, backfill gradation, probe withdrawal rate, and probe spacing on the densities achieved The densities achieved decreased with increasing probe withdrawal rate and probe spacing Coarse granular soils were easier to compact and were more suitable backfill material than fine granular soils A rating system is presented for evaluating the suitability of alternate backfill materials Recommended inspection and quality control procedures are provided for verifying that adequate densities have been achieved

Stability Analysis for Eccentrically Loaded Footings

Abstract: Footing foundations are often subjected to eccentric loads. The problem has been studied in detail by several investigators. The effective width method of Meyerhof is widely used for calculating the bearing capacity of eccentrically loaded footings. While some of the experimental evidence supports Meyerhof.s hypothesis others report a large deviation. A stability analysis based on a slip surface derived from experimental results is presented herein for eccentrically loaded footings resting on sand.

Laboratory Standard Penetration Tests on Fine Sands

Abstract: The Standard Penetration Test (STP) is critically examined with respect to its ability to estimate relative density in situ. Twenty-six test specimens, 4-ft in diameter by 6-ft high, were constructed using Reid Bedford Model sand and Ottawa sand. Using field drilling equipment, the SPT was performed at three overburden pressures on test specimens built to various densities. The results are presented as a family of curves correlating relative density with the SPT N-values at the testing pressures. It is concluded that the SPT is fairly repeatable in homogeneous deposits; however, variations in density, structure, or lateral stress within the test medium will produce widely scattered N-values. Thus, estimates of in-situ relative densities from N-values should be considered gross values or trends and should not be interpreted as accurate determinations for any specific case.

SPT and Relative Density in Coarse Sands

Abstract: The Standard Penetration Test (SPT) is critically examined with respect to its ability to estimate relative density in situ. SPT's were performed using field drilling equipment at three overburden pressures on 4-ft (1.22-m) diam by 6-ft (1.83-m) high test specimens constructed using Platte River and Standard Concrete sands at three relative densities. The results are presented as a family of curves correlating relative density with SPT N values for the three testing pressures. This research was an extension of a previous SPT test series on Reid-Bedford Model and Ottawa sands. The results of testing the four sands were compared and a statistical analysis produced an empirical equation relating relative density to effective overburden pressure, N value, and coefficient of uniformity. Comparisons are also made between this work and the previous work of Gibbs and Holtz at the Bureau of Reclamation and Bazaraa at the University of Illinois.

Uplift Resistance of Cohesive Soils

Abstract: Effects of vertical uplift forces on circular anchors in purely cohesive soils are considered. Model tests indicate a general type of ultimate uplift failure in shallow anchors, occurring by yielding above and below the anchor due to shear and tensile stresses and cracking in the soil. Existing shallow anchor theories, although in substantial agreement with one another, overestimate actual ultimate uplift resistance since they do not take into account tensile stresses and cracking. Model tests indicate a local type of failure in deep anchors and confirm existing deep anchor theories that take account of soil compressibility. Dimensional analysis indicates that caution must be exercised when interpreting model uplift test results in terms of the prototype, particularly when the prototype soil has low shear strength. A finite element program has been developed to estimate, within limitations, the magnitude and direction of stresses occurring in each element of the mesh at any stage of uplift resistance.

Torsional Vibrations of Pile Foundations

Abstract: Figures are presented to facilitate this application and the response of various representative footings to torsional excitation is also shown. For a pile group the contribution of pile twisting to the total response in torsion is usually small. For a single pile or pier torsion can be important and the inclusion of material damping is essential. The following dimensionless parameters govern the soil-pile interaction in torsion: Shear wave velocity ratio, slenderness ratio, mass ratio, dimensionless frequency, and material damping ratio. With increasing shear wave velocity ratio, both pile stiffness and damping increase. For soft soils and pile slenderness ratio greater than 25, the response is independent of the slenderness ratio and the tip condition. Pile foundations can have higher resonant frequencies but smaller resonant amplitudes than shallow foundations. The analysis presented compares favourably with the static solution of Poulos.

Seismic testing of reinforced earth walls.

Abstract: This paper is a summary progress report of ongoing studies at UCLA toward developing a rational seismic design method for reinforced earth retaining walls. A 20-foot (6-m) high reinforced earth test wall was constructed and then subjected to low strain level forced vibration tests using mechanical vibrators and high strain level explosive tests. In addition, four existing reinforced earth walls were also subjected to forced vibration tests. Criteria are presented for determining the design modal frequencies of a reinforced earth wall needed for the design as a function of its effective height and level of seismic excitation. Data are also presented showing the static and dynamic tie force distribution.