Ebook: Proceedings of the 16th International Conference on Soil Mechanics and Geotechnical Engineering

Site characterisation and in situ test interpretation have been evolving from basic empirical recommendations to a sophisticated area demanding a thorough knowledge of material behaviour and numerical modelling. With the advent of modern testing techniques and more rigorous methods of analysis, site characterisation in natural soils is gaining momentum. This Report presents a critical appraisal in the understanding and assessment of the stress-strain-time and strength characteristics of natural soil conditions and explores new interpretation methods capable of measuring soil properties shaped by effects of microstructure, stiffness non-linearity, small and large strain anisotropy, weathering and destructuration, consolidation characteristics and rate dependency. Interpretation methods in different soil formations such as clay, sand, silt and bonded geomaterials are explored using different testing techniques. Since the in situ behaviour of natural soils is complex, a single general recommendation is to cross-correlate measurements from different tests. When data are combined there is more scope for rational interpretation and, for this reason, emphasis has been placed on correlations with mechanical properties that are based on the combination of independent measurements.

Studies on constitutive modeling and numerical analysis methods have been well developed. Nowadays, numerical and analytical methods play a very important role in Geotechnical Engineering and in a related activity called Computational Geotechnics. Due to large deformations and failure, however, several issues are encountered in Computational Geotechnics. They include constitutive modeling and its calibration, mechanical instabilities: strain localization and progressive failure, the modeling of chemo-thermo-hydro-mechanical coupled behavior, and the verification and the validation of the mathematical modeling. In the present paper, we deal with the recent progress that has been made to solve these problems, in particular, the constitutive modeling of soil for rate-dependent models with structural degradation, strain localization, liquefaction of soil and their applications in the context of large deformations and the failure of geomaterials.

Although the exact date of when geotechnical engineers began to address environmental issues is uncertain, such issues became a primary concern in the field of Geotechnical Engineering around 1980. Thus, geotechnical engineers have been dealing with environmental issues on a formal basis for at least a quarter of a century. During the interim period, the significance of environmental issues in Geotechnical Engineering has grown to the extent such that a new sub-discipline in Geotechnical Engineering, commonly referred to as Environmental Geotechnics, has been established. As a consequence of these events, an assessment of some environmental issues that are currently of interest in Geotechnical Engineering, as well as some issues that are likely to be important in the future, is appropriate at this time. Accordingly, after a brief historical perspective, seven current and/or future environmental issues in Geotechnical Engineering are described, including the (1) long-term performance of waste containment systems, (2) acceptance of alternative barriers and barrier materials, (3) need for innovative barriers and innovative barrier materials, (4) emergence of new waste forms, (5) increasing significance of biological processes, (6) role of modeling, and (7) importance of professional identity. The significance of each issue is illustrated through one or more examples. The overall outcome of the assessment is that environmental issues will continue to represent a major sub-category within Geotechnical Engineering in the foreseeable future.

Design practice in offshore geotechnical engineering grew out of onshore practice, but the two application areas have tended to diverge over the last 30 years, driven partly by the scale of the foundation elements used offshore, and partly by fundamental differences in construction (or installation) techniques. Groups of many moderate-sized piles are replaced by a few very large diameter piles; excavation of shallow soft sediments is replaced by the use of deep skirts, transferring the effective foundation depth to the level of the skirt tips, or by forcing footings to penetrate several diameters into the seabed; underwater installation has allowed the use of ‘suction’ (or under-pressure) to aid installation of skirted foundations and caissons. Emphasis in design is focused more on capacity, paying particular attention to the effects of cyclic loading but generally with less concern on deformations compared with onshore design. These differences have led to the development of separate design codes for offshore structures, which are in most cases more prescriptive than onshore codes but are also more sophisticated in key areas. The paper describes design principles for foundation and anchoring systems ranging from shallow footings to piles and caissons, highlighting differences between onshore and offshore practice and also the link (or gap) between research and practice.

Selected topics in the field of pile foundations are addressed. The effects of the installation technique on the bearing capacity and the load-settlement response of a single pile are discussed. The latter effect is shown to be less significant; a settlement controlled design is thus less dependent on the technological factors. Monitoring of the installation parameters shows some potential for controlling the pile response. The available experimental evidence on the behaviour of pile foundations under vertical loads (settlement, load sharing, bearing capacity), by monitoring of full scale structures or by research experiments, is reviewed. Simple empirical methods for a preliminary evaluation of the settlement are suggested. The (more limited) evidence about horizontal loading is also reviewed and discussed. The methods for the analysis of pile foundations under vertical load are next reported. They may be considered satisfactory for engineering purposes, provided they are used paying due attention to the correspondence relations between theories and reality. The criteria for an optimum design, achieving maximum economy while keeping satisfactory performances, are different for different kinds of pile foundations (small groups, large rafts). Safety against a bearing capacity failure, average settlement, differential settlement, moment and shear in the raft and cost are the quantities to be controlled. It is claimed that the conventional capacity based approach, still prevailing in practice, is not suited to develop a proper design. Present codes and regulations, essentially based on this approach, at the time being act as a restraint rather than a stimulus and need some revision.

This Terzaghi Oration addresses advancing insight in geotechnics with a focus on the interaction of soil and water. It considers the societal position and the innovation potential of the geotechnical profession with special attention to aspects of knowledge and communication and makes suggestions as to how the profession could benefit from expertise and experience, using modern concepts.

Japan is one of the regions which are most prone to natural hazards. Among such disasters as indeced by heavy rainfalls, typhoons, and volcanic actions, earthquakes have been causing a variety of bad consequences in the history. Under this hard natural environment, technologies have developed historically and in modern times in a variety of directions in order to prevent or mitigate natural hazards. The earliest examples of those developments are the famous seismic coefficient method of seismic design and Mononobe-Okabe earth pressure theory. Repeated strong earthquakes in densely populated area have kept showing new kinds of problems. Under many disasters, it was often possible to demonstrate the effectiveness of new mitigative technologies. Problems related with soil liquefaction is one of the examples of this type. Starting with the assessment of liquefaction potential and prevention of the onset of liquefaciotn, geotechnical engineering in the recent decades is shifting to the performance-based approach in which the consequence of liquefaction is assessed and, if the consequence is not allowable, it is mitigated. This situation has provided significant research topics, which are extremely difficult, and important achievements have been accomplished. The latest problem to be solved is the introduction of broader viewpoints in which the seismic resistance of individual structurea is studied and determined so that the seismic vulnerability of the whole public may be minimized. This would be a combination of geotechnical engineering and regional economic planning.

This paper presents briefly some research works involved in the field of soil mechanics and rock mechanics and some part of actual monitoring results on problems of the high earth-rock cofferdam in deepwater, the stability analyses of rock foundation of the gravity dam and the high rock excavated slope of ship lock, the most challenging works in the civil engineering construction at TGP.

The high cofferdam, an earth rock dam with the height of about 90m, two concrete cut-off walls inserted at the body, is formed by dropping the weathered granite in deep water (60m depth), and its density is quite loose. The stress and strain in walls are very complicated and the construction work is also very difficult. Nevertheless, the practical result is very well, even beyond the expected.

The high rock slope of permanent ship lock has been excavated in rock mass, with the maximum height of 170m. The concern is focused on the displacement and stability of the rock slope, as the deformation of slope is very much related to the safety of ship lock operation. Different kinds of tests in field and numerical analysis are performed during the design, construction and even operation period. The analysis results are proved to be reliable by the actual deformation measurements during ship lock excavation.

The stability of rock foundation of the gravity dam has been comprehensively studied by means of the rock mechanical tests, numerical analysis and the large scale of geological mechanics model test. The actual situations of the dam are working well up to now.

All those results are very important to Three Gorges Project, and also will be significant to other dams as well.

The first part of this paper describes the offshore oil and gas exploration activities in Southeast Asia. Typical subsurface conditions in the Sunda Shelf in Southeast Asia are highlighted and hazards in oil and gas exploration caused by the peculiar soil profile and parameters in the region are identified. These hazards include the punch through failure of spudcan supporting mobile jack-up rigs during installation and difficulties in removing the spudcans during extraction. The second part of this paper describes the results of centrifuge model study carried out at the National University of Singapore on punch through hazards during installation of spudcans in layered soils and performance of spudcans during extraction. The study on spudcan punch through reveals that a plug consisting of the overlying stiff soils develops when a spudcan punches through the underlying soft clay layer. Owing to such failure mechanism, conventional bearing capacity theories are not applicable to predict the spudcan punch through phenomenon. The study on spudcan extraction reveals that the suction developed at the spudcan base is the major component in the breakout resistance of spudcan with long operation periods.

Kansai International Airport was constructed as a marine airport 5km offshore in Osaka bay so as not to burden urban dwellers with noise pollution. The second phase construction works are being performed even farther offshore than the first phase. To reclaim land over vast area and complete the island in a short period of time, maximum use is being made of the experience accumulated during the first phase construction works. And by introducing the latest technology and careful planning to procure the large quantity of materials and equipment required, construction works are proceeding in a smooth and efficient manner. This report shows an overview of the construction works of the second phase construction project of Kansai International Airport.

Activity, as introduced in 1953 by Skempton, was defined as the ratio of plasticity index (PI) to the clay fraction (CF) of a soil and was used by Skempton to help distinguish mineralogic differences in fine-grained soils. While activity (A) provides an approximate method of delineating fine-grained soils by mineralogy, PI is not really a fundamental soil property. Conversely, Specific Surface Area (SSA) and Cation Exchange Capacity (CEC) are more fundamental soil properties that dominate engineering behavior of fine-grained soils. SSA and CEC are “inherent” soil parameters and may be combined with the clay fraction in order to help classify mineralogic composition of fine-grained soils. Two parameters are examined: 1. Specific Surface Area Activity (S_c), which is defined as the ratio of surface area to the clay content, and 2. Cation Exchange Capacity Activity (CECA), defined as the ratio of cation exchange capacity to the clay content. These two parameters show clear groupings of low, medium and high “activity” when used in relationship with other index parameters that may help explain fundamental clay behavior. The relationships between S_c and CECA and engineering properties are presented and described. The results indicate that Specific Surface Area Activity and Cation Exchange Capacity Activity may be a more fundamental and convenient basis for describing the mineralogic composition of fine-grained soils.

The influence of compaction water content on the structure has been well known for clayey soils, but has never been studied for granular materials. In this paper the effect of compaction moisture on the structure of a non-plastic till is investigated by means of scanning electron microscopy, water retention curve, permeability and mercury intrusion porosimetry tests. The results show that the structure of pulverulized materials, in some degrees similarly to cohesive materials, is affected by the compaction condition. When compacted on the dry side of the optimum water content, the porous system is characterized by a relatively uniform medium pores, while in case of compaction on the wet side, due to formation of coarse aggregates, pores size is very diversified. The few large pores are surrounded by numerous small pores, and the hydraulic behavior of the porous system of the soil is dominated by the small pores.

The purpose of the investigation reported in this paper was to study the influence of strain rate on stress-strain and strength relationships of Mexico City lacustrine soils from the Central Park site (Alameda). An isotropic consolidation test to define the yielding stress, σ'_y(=95kPa), was done by triaxial-cell method. The paper describes results from 16 consolidated-undrained triaxial compression tests on Mexico City lacustrine soils. Specimens were consolidated to four confinement pressures (σ'_c=40, 80, 160, and 300 kPa), and for each, σ'_c, undrained shear was performed using four axial strain rates (1%, 5%, 100%, and 800%/h). The results show that the peak shear resistance increased about 336% in passing from the slow to the fast strain rate for the structured domain. For destructured domain, the increase was about 229%.

The principal factor to which the observed engineering performance of the soils in the humid tropical zone of Southern Nigeria can be attributed has been determined to be the laterization phenomenon. During the process, oxides and hydroxides of iron and aluminum accumulate as the cementing agents binding the soil particles together. Clay-size particles aggregate into coarse grains giving the impression of a granular texture. The soil mass, in turn, correspondingly exhibits characteristics reflecting this texture when undisturbed in the field. When subjected to mechanical reworking or remolding however, such as in the process of pre-test sample preparation, the grains are disaggregated releasing a large quantity of fines. The laboratory test results therefore usually differ from predictions based on field observations.

This paper presents a broad framework, which can be used for a preliminary estimate of the engineering performance of the soils as well as formulating a generalised soil classification scheme for the area. The framework has been realised from a synthesis of selected previous works on the soils, a task, which included detailed evaluation of all the soil formations in the whole area and correlation of their engineering properties with various physical characteristics. The genetic and climatic factors and the post-formation alterations, which influence the engineering behaviour of the soils, were also investigated, with emphasis on their significance in engineering design and construction.

Attempts have been made by various researchers to suggest methods for identification and classification of expansive soils. This paper is concerned with the comparison of the predicted equations of swelling potential of expansive soils. A comparison between nine different predicted equations for swell percent was carried out. The results brought from each equation are later compared to experimental results for soil samples obtained from three different site locations chosen according to their swelling potential: moderate, high and very high. A conclusion for the use of the predicted equations based on these comparisons is outlined in this work.

An experimental study of the K₀ of a loose reconstituted sand is presented. The experimental technique consists of drained vertical compression of a triaxial specimen using the strain path control, in which a positive control of the strain increments rather than the conventional stress increments is exercised. The method does not require the use of any lateral strain sensors as used in previous studies and is free from any side friction affects, typical of oedometer testing. Possible influence of stress/strain history the soil inevitably experiences during sampling or reconstitution together with the initial state of stress prior to initiating zero lateral strain loading on the measured K₀ are critically reviewed and evaluated experimentally. It is also shown that the requirement of absolute zero lateral strain may not be essential for obtaining credible estimates of K₀ as long as the ratio of radial to axial strain during vertical compression does not exceed about 3 to 5%.

Metropolitan Tehran is located on the highly cemented granular deposits in upper central to northern parts of the city which is fairly close to the Alborz Mountains with altitudes around 4000m. Due to the specific geological conditions, the deposited alluviums are very non-homogeneous in different zones, but generally the cementation is high to very high contributing to cohesions of up to 160 kPa, mostly resulted from the existence of calcite. The main objective of this paper is to investigate experimentally the effect of calcite content on the cohesion and angle of internal friction. A special chemical analysis method consisting of solution in HCl was conducted to measure the calcite content at 10 different locations in northern Tehran. In order to simulate the effect of calcite content, soil samples with different lime percentages (1, 3, 5 & 7 %) were prepared and cured in the laboratory, and large scale direct shear tests were performed. The results showed considerable effect of cement content (calcite) on the soil behavior and shear strength, in particular substantial increase in cohesion.

Creep of granular clay is studied by one-step loading up to 1000 kPa in oedometers. Dry and loose specimens were used. Two phases of creep were found. The first one, up to 10⁴ min., represents a regular secondary compression of semilogarithmic nature and is ascribed to the transformation of inborn fabric (produced by the specimen preparation) into the compression fabric. The other one is formed by a periodic variation of the s-creep and g-creep. S-creep is produced by the diffusion of the oedometric compression from the loaded plate downwards, g-creep results from the nonhomogeneity of the specimen. Granular clay is supposed to model waste clayey (double-porosity) material of open coal mines. Complexity of the creep process even in uniaxial compression points to the difficulty of numerically modelling of problematic soils.

The fall cone test has been recommended in many test standards for determining the liquid limit of fine-grained soils. A number of the existing fall cones and test methods are reviewed. A new approach of using the fall cone test to determine the plastic limit has recently been recommended. Key factors in using the fall cone test to determine both the liquid limit and the plastic limit are examined. It is concluded that the fall cone test with a fall cone of one weight is suitable for determining both the liquid limit and the plastic limit.