M. R. Coop

The Development of a New Micro-Mechanical Inter-Particle Loading Apparatus

The inter-particle coefficient of friction comprises an essential input parameter in computer codes that utilize the discrete element method. This paper describes the main features of a custom-built apparatus of a new generation, capable of performing inter-particle shearing tests at very small displacements on the order of tens to hundreds of microns and measuring the frictional forces developed at the contacts of coarse-grained particles of sand to gravel size. Linear, micro-stepping motors are used for the inter-particle shearing tests of a displacement-controlled type and the application of the vertical confinement of a force-controlled type at the particle contacts. The apparatus is designed to work at very small confining forces, in general between 1 and 20 N, and utilizes a system of bearings of small friction, which can be calibrated following simple procedures. The experiments are controlled and monitored through a computer code developed for the apparatus. The signal conditioning and data-logging systems were optimized to give the minimum environmental and electrical noise in the experimental data. The particles tested must have a relatively convex shape and be fairly symmetrical about the axis of shearing to avoid significant lateral forces in the out-of-plane horizontal direction during sliding, and, in general, the size of particles is limited from about 0.50 to 5.0 mm. Tests on reference particles composed of chrome steel balls and quartz particles demonstrated high repeatability of the results and agreement with the literature data. The experimentally derived horizontal force–displacement data showed that the stiffness of the apparatus is sufficiently high to prevent significant stick-slip phenomena, allowing a stable sliding.

The Development of a Suction Control System for a Triaxial Apparatus

A new method of controlling suction in the triaxial apparatus is presented that consists of two subsystems: one each for drying and wetting that is used in combination with an independent measurement of suction. The drying system is based on a development by Cunningham et al _2003_, with improvements in the measurement of the current water content. The wetting system involves the staged circulation of air and water. This new technique enables continuous monitoring of all state variables during tests, including water content, degree of saturation, net stresses, suction, and the different components of strain. Some typical results from tests on a compacted silty clay are presented in order to demonstrate the capabilities of the system. The main advantage of the apparatus is that there is no need to raise the ambient air pressure as is required with the axis translation technique and tests may therefore be conducted at negative pore water pressure. The system may also be easily incorporated into other standard apparatus.

Experimental Accuracy of the Initial Specific Volume

Many advanced soil models rely on the current state relative to normal and critical state lines to describe soil behavior. The position of these lines, therefore, requires an accurate estimation of the specific volume or void ratio. A series of one-dimensional compression tests was performed both on a coarse and a fine grained soil to investigate the experimental accuracy of the initial specific volume. This was obtained comparing independent calculations of the initial specific volume that were based on redundant measurements of height and weight of the specimen, both at the beginning and at the end of the test. The redundancy in the measurements was a key factor to obtain independent calculations. It was found that the excess water, such as may be stored in the filter papers, was the main cause of inaccuracy, when gross errors did not occur. Two novel confining rings having a closed-base were designed to reduce this effect. Although this was possible for the coarse grained soils tested, the fine grained soils retained more water due to the higher suction at the end of the test and water adsorption could not be avoided. The assumption of saturation is shown to be far from accurate, meaning that both the bulk unit weight and the water content should be measured independently to obtain a reliable measurement of the specific volume. The specific volume of the intact soil was found to be less accurate than when reconstituted. The experimental scatter was compared with the theoretical accuracy obtained from the error propagation theory. Good agreement was found between the theoretical and experimental accuracy.

Transitional Behaviors in Well-Graded Coarse Granular Soils

AbstractDrained triaxial compression tests were carried out for a well-graded coarse granular soil (CGS) to investigate the effect of the initial specific volume on the location of the critical state line (CSL). A family of parallel CSLs in the v∼logp′ plane was observed for the well-graded CGS, indicating that it exhibited transitional behavior. The degree of transitional behavior was quantified from the relationship between the intercepts of the CSLs and the initial specific volumes, giving a value of 0.59, which indicated a substantially transitional behavior. The observations of the CSL pattern in the CGS illustrated that transitional behavior could be extended to large-sized granular soils, beyond the usual transitional soils that have been observed so far, which generally have gradings between those of clean sand and plastic clay.

Mechanisms of compression in well-graded saprolitic soils

AbstractSoils originating from weathering processes present considerable heterogeneity in their composition, which can make it difficult to analyse their behaviour in a systematic way. For the granitic saprolites discussed in this paper, based on a trend between soil density and weathering degree, there appears to be two different domains of behaviour, a granular domain and a clay matrix one, according to the degree of weathering reached. Recognition of these domains can reduce the apparent scatter of data for the engineering behaviour of weathered soils. A number of one-dimensional compression tests are presented for saprolitic soils from Hong Kong having different weathering degrees. In addition, isotropic and one-dimensional compression tests from the literature on other saprolites from Hong Kong and around the world were reanalysed and used to identify possible trends in the mechanisms of compression for these two domains. From practical considerations, the trends considered were between compressibility and common engineering grading descriptors. An attempt was also made to provide the physical explanations behind the behaviour observed, and the particle breakage was investigated in detail, both from a quantitative and qualitative point of view. It was found that the values of relative breakage (Hardin in ASCE J Geotech Geoenviron Eng 111(10):1177–1192, 1985), for a same stress level, might be very similar for soils having different compressibility values and different initial gradings. When studying particle breakage in further detail, it can be observed that it is linked to the amount of large particles and their characteristics. The maximum particle size, rather than the amount of fines in a mixture, may be a better predictor for differences in compressibility and breakage.