Soil Behavior and Soft Ground Construction

In his first published paper in 1960 Professor Charles C. Ladd stated that several soil composition, state, and environmental factors govern the swelling behavior of a soil. He summarized the theoretical basis for analysis of interparticle forces and swelling in terms of diffuse double layer theory and osmotic pressure. He contributed data on the influence of compaction conditions and pore water chemistry on the swelling behavior of compacted fine-grained soils. When examined from the perspective of today's understanding of soil behavior, the principles he stated 40 years ago still apply. Theoretical and experimental studies carried out in the intervening years, and summarized in this paper, have provided further clarification of the most important factors controlling swelling and the qualitative and quantitative applicability of double layer and water adsorption theories of swelling. The most important single factor influencing the swell potential of a soil is the effective specific surface. The smectite clays (montmorillonite) have the greatest potential for expansion owing to weak interlayer bonding and, therefore, the large specific surface available for water adsorption. In the presence of high concentration salt solutions and/or potassium or divalent adsorbed cations interlayer swelling is suppressed, and the amount of swell is much less than when sodium is the dominant adsorbed cation. Current research on the fundamentals of water adsorption and clay swelling employs Monte Carlo and Molecular Dynamics modeling to obtain knowledge of the distribution and energy states of water and cations adjacent to clay particle surfaces. There may be applications of the results of these current studies to practical problems in geotechnical engineering in the future.

Soils are particulate materials. Therefore, the behavior of soils is determined by the forces particles experience. These include forces due to boundary loads (transmitted through the skeleton), particle-level forces (gravitational, buoyant, and hydrodynamic), and contact level forces (capillary, electrical and cementation-reactive). The relative balance between these forces permits identifying various domains of soil behavior. Furthermore, the evolution of particle forces helps explain phenomena related to unsaturation, differences between drained and undrained strength under various loading modes (including the effect of plasticity), sampling disturbance, and fines migration during seepage. Generally accepted concepts gain new clarity when re-interpreted at the level of particle forces.

The potential importance of coupled chemical and liquid fluxes in earthen materials has been recognized and investigated in the geologic and soil science disciplines for several decades. The mechanisms involved cause a family of coupled transport phenomena that are commonly referred to as osmotic or membrane phenomena. Recently these phenomena have attracted the interest of geotechnical researchers because they appear to provide mechanisms for enhancing the capability of clay barriers to impede the migration of contaminants. To facilitate geotechnical interest in these phenomena, this paper presents a brief review of the pioneering field studies of membrane phenomena in geologic systems, a summary of the pertinent coupled flow theory developed to date, and a synthesis of the experimental evidence obtained in laboratory studies during the last five decades.

The paper presents a review of the effects of sampling on the subsequent behaviour of soft clay. It takes for its starting point the classic paper by Ladd and Lambe in 1963 on this subject. The effects of perfect sampling, stress relief and of tube sampling are considered. Particular emphasis is placed on the effects of desaturation and gas exsolution, and of destructuring as a result of tube sampling strains. The benefits of sampling soft clays with a sampling tube of large diameter, thin wall, sharp cutting edge and no inside clearance, and of immediate extrusion and trimming of the sample, are emphasised. The dangers of improving only sample quality in design methods which rely on empiricism are highlighted.

One-dimensional compression of a saturated soil, subjected to an increment of total vertical stress, consists of a primary compression that takes place during the increase in effective vertical stress and a secondary compression that follows at constant effective vertical stress. A soil structure change commences as soon as the increase in effective vertical stress begins, and it continues with time to establish internal equilibrium under the new external condition.

This paper describes laboratory methods for measuring soft clay stress-strain-strength behavior and interpretation of test results for selection of design parameters. The paper traces the major practical contributions of experimental research to our understanding of soft clay mechanical behavior. Recommendations are given for implementation of research results in practice with the goal of improving the engineering state-of-the-practice, which often lags well behind the research state-of-the-art. Particular emphasis is given to the results of work produced by researchers working at the Massachusetts Institute of Technology under the leadership of Professor Charles C. Ladd. Significant advances in the understanding of clay behavior have been realized in the past 40 years including such issues as the influence of anisotropy, stress history, rate effects, and sample disturbance. Of these issues, sample disturbance remains by far the most difficult one to deal with in practice and in fact is often ignored. Field sampling and laboratory techniques that can reduce sample disturbance are described together with methods for quantifying sample disturbance. The merits of laboratory reconsolidation methods are discussed and are demonstrated through the use of three case studies ranging in sophistication in sampling and laboratory testing from the highest level down to the level more commonly encountered in practice.

The paper summarises the experience gained by the writers in the interpretation of the cone penetration test (CPT) and flat dilatometer test (DMT) for the assessment of the geotechnical properties of sands. In the first part of the paper, the problem of determining the relative density (D_R) as function of the penetration test results and ambient stress (σ'), for silica sands, is dealt with. In the second part of the paper, the assessment of the peak angle of shearing resistance (φ'_p) is dealt with. The attention is given to the use of the Bolton's strength-dilatancy theory in order to estimate φ'_p. Engineering correlations, based on Bolton's work, are proposed allowing estimation of φ'_p as function of penetration resistance and σ', taking into account the compressibility and the curvilinear shear strength envelope.

Increasing demand for sustainable transportation systems and for modernized infrastructure poses new challenges to the geotechnical engineer. More than before, the profession should aim at engineering more economical solutions, developing improved and safer transportation solutions and encouraging innovation while preserving the environment. To meet these challenges, an alliance of good practice and research is required. Such is illustrated with examples where improved in situ characterization of the foundation soil considerably reduced costs, and improved performance. The extent of site investigations should be a decision taken considering hazard and consequence, including cost, and therefore it is a risk-based decision.

There are many problems in geotechnical engineering where the inadvertent or controlled ground loss or displacement of soil have important implications in geotechnical design. This paper shows that simplified analytical methods can provide reliable and practical solutions for estimating ground movements in two applications: Analyses of pile installation using the Shallow Strain Path Method simulates consistently the downward movement of soil beneath the advancing pile tip and the more general heave towards the stress-free ground surface. Far field movements can be estimated using simple closed-form solutions. Field monitoring data from a large piling project in East Boston provide a clear demonstration of the predictive capabilities of these analyses. Tunneling-induced ground movements can be obtained by similar approximate techniques, considering deformations around the tunnel cavity as the sum of three component mode shapes. Analytic solutions have been obtained for 2D deformations around a cylindrical tunnel cavity using assumptions of linear elastic and constant plastic flow behavior. Examples presented in the paper show that the analytical solutions are not only consistent with well established empirical methods, but also provide a more comprehensive framework for understanding the distribution of ground movements.

The paper presents a case history involving a soft bay mud site that had been reclaimed for commercial development by filling over old marshlands along the margins of San Francisco Bay. The paper reviews the design of the preload program and the settlements recorded during the preload period. It reviews the post-construction investigations, summarizes the geotechnical characteristics of the bay mud, and evaluates the performance of the preload. It summarizes the results of the monitoring program and reviews the lessons learned regarding the use of vertical drains in combination with preloading to accelerate consolidation of soft clay soils.

Prefabricated vertical (PV) drains are commonly used to decrease the drainage paths within soft soils in order to accelerate the time for primary consolidation. PV drains are full displacement drains of small volume that are thought to exhibit considerably less disturbance to the soil mass than displacement sand drains. Experience at three sites where excessive disturbance due to installation of PV drains is thought to have occurred, combined with available published data regarding the performance of displacement sand drains and PV drains, provides the basis for an empirical approach to assess the excessive installation disturbance effects imposed by both PV drains and displacement sand drains. The size of both the installation mandrel and the anchor are important factors in the disturbance effects. The disturbance caused by PV drains is shown to be similar to displacement sand drains when the drain spacing ratio is based on the effective diameter of the mandrel/anchor combination calculated using the mandrel/anchor combination perimeter, not the end area. With this approach, a modified drain spacing ratio (effective drain spacing / effective mandrel diameter) greater than 7 to 10 is considered necessary to reduce the excessive disturbance effects of PV drain installation.

This paper describes in general terms the unique and innovative embankment design approaches developed and implemented by Dr. Charles C. Ladd in the late 1960s and early 1970s. Some of these approaches included 12.7 cm (5 in) diameter undisturbed tube sampling, in situ strength testing using the Geonor field vane shear device, innovative laboratory strength testing (Geonor Direct Simple Shear Tests), development and application of the SHANSEP approach to fall scale test and production embankments, use of test embankments during design not only to verify soil properties but also to assess impacts of sand drain installation techniques on in situ soil properties, and first time use in the United States of the Dutch jet bailer (non-displacement) method of sand drain installation. Many of these approaches, particularly SHANSEP, have withstood the test of time and have been used on many other projects over the past 30 years.