Influence of compaction procedure on the mechanical behaviour of an unsaturated compacted clay. Part 2: shearing and constitutive modelling

The influence of compaction pressure, compaction water content and type of compaction (static or dynamic) on subsequent soil behaviour was investigated by conducting controlled-suction triaxial tests on samples of unsaturated compacted speswhite kaolin. Compaction pressure influences initial state, by determining the initial position of the yield surface, thus affecting, among other things, the shape of stress–strain curves during shearing. Compaction pressure also influences, to a limited degree, the positions of the normal compression lines for different values of suction, but it has no effect on critical state relationships. The effect of compaction pressure can probably be modelled solely in terms of initial state if an anisotropic elastoplastic model incorporating rotational hardening is employed, whereas the parameters defining the slopes and intercepts of the normal compression lines for different values of suction require adjustment with variation of compaction pressure if a conventional isotropic hardening elastoplastic model is employed. Compaction water content influences the initial suction, but also has a substantial influence on normal compression lines and a noticeable effect on the volumetric behaviour at critical states. It is likely that soil samples compacted at different water contents will have to be modelled as different materials, irrespective of whether an isotropic or anisotropic hardening elastoplastic model is employed. A change from static to dynamic compaction has no significant effect on subsequent behaviour.

A Simple Triaxial System for Evaluating the Performance of Unsaturated Soils Under Repeated Loading

Unsaturated soils constitute a large proportion of the foundation materials supporting infrastructure throughout the world and they are subject to various loading conditions. This paper describes the development of a simple system for testing unsaturated soils under repeated loading. The equipment was comprised of a modified triaxial cell with hydraulic loading system, hall-effect transducers for on-sample strain measurements, and thermocouple psychrometer for suction measurements. A number of undrained monotonic and repeated loading triaxial tests were performed on compacted samples of kaolin clay in order to attest the newly developed system. The results yielded some useful information on the resilient modulus and permanent deformation of a soil when subjected to repeated loading. There is some difference between the failure deviator stress of samples subjected to repeated and monotonic loading, though repeated loading continued to result in a significant permanent deformation. This paper is aimed at demonstrating the key features of the equipment using preliminary data generated as part of the on-going research.

Performance of Unsaturated Compacted Clay Under Repeated Loading Effects of Confining Pressure, Initial Compaction Effort and Water Content

While a significant number of geotechnical structures are subjected to static loading, many, such as avement subgrade, also are subjected to cyclic or dynamic loading. While the performance of saturated soils under repeated, cyclic or dynamic loading conditions is still a topic of research, similar interests are growing when the soilcondition is unsaturated. This paper examines the performance of unsaturated soils under repeated loading. As part of the research, a triaxial system was developed which incorporates small strain measurements using Hall-effect transducers, in addition to suction measurements taken using a psychrometer. Tests were conducted on samples of kaolin under constant water mass conditions. The results address the effects of compaction effort and water content at the time of compaction on the overall performance of unsaturated soils, under different amplitudes of loading and different confining pressures. The results show that suction in the sample reduced with increasing number of loading cycles of the same magnitude. The resilient modulus initially increased with increasing water content up to approximately optimum water content, and substantially reduced with further increase in water content. Key Words: suction, resilient modulus, subgrade, repeated loading, small strain measurements, compaction.

Effects of confining pressure and water content on performance of unsaturated compacted clay under repeated loading

This paper examines the performance of unsaturated soils under repeated loading. As part of the research, a triaxial system was developed that incorporates small-strain measurements using Hall effect transducers, in addition to suction measurements taken using a psychrometer. Tests were conducted on samples of kaolin under constant water mass conditions. The results address the effects of compaction effort and water content at the time of compaction on the overall performance of unsaturated soils, under different amplitudes of loading and different confining pressures. The results show that suction in the sample reduced with increasing number of loading cycles of the same magnitude. The resilient modulus initially increased with increasing water content up to approximately optimum water content, and then reduced substantially with further increase in water content.

A laboratory based model study. Experimental observations of footings supported on soft clay reinforced with granular columns.

The inclusion of granular columns in soft clay deposits leads to improvements in bearing capacity and overall stiffness along with a reduction in consolidation settlement. Many laboratory investigations have focused on aspects of bearing capacity, but published data on settlement performance is limited. This paper reports on some interesting findings obtained from a laboratory model study in respect of these issues. In this investigation, 300 mm diameter by 400 mm long samples of soft kaolin clay were reinforced with single or multiple granular columns of various lengths using the displacement and replacement installation methods. The experimental findings revealed that, for the same area replacement ratio, limited settlement reduction was achieved for single long floating columns and end-bearing column groups. Marginal improvements in settlement performance were also achieved for columns installed by the displacement method. No settlement reduction was achieved for short single floating columns while short floating granular column groups produced increased settlements. These observations were verified using contact pressure measurements between the footing and column/surrounding clay.

Wetting of compacted clays under laterally restrained conditions: Initial state, overburden pressure and mineralogy

Compacted clay fills are generally placed at the optimum value of water content and, immediately after placement, they are unsaturated. Wetting might subsequently occur due, for example, to rainfall infiltration, which can cause volumetric deformation of the fill (either swell or collapse) with associated loss of shear strength and structural integrity. If swelling takes place under partially restrained deformation, due for example to the presence of a buried rigid structure or a retaining wall, additional stresses will develop in the soil and these can be detrimental to the stability of walling elements and other building assets. Factors such as dry density, overburden pressure, compaction water content and type of clay are known to influence the development of stresses. This paper investigates these factors by means of an advanced stress path testing programme performed on four different clays with different mineralogy, index properties and geological histories. Specimens of kaolin clay, London Clay, Belfast Clay and Ampthill Clay were prepared at different initial states and subjected to ‘controlled’ wetting, whereby the suction was reduced gradually to zero under laterally restrainedconditions (i.e. K0 conditions). The results showed that the magnitude of the increase in horizontal stresses (and therefore the increase of K0) is influenced by the overburden pressure, compaction water content, dry density at the time of compaction and mineralogy.

The influence of high air entry filter on the testing of unsaturated soils using the axis translation technique

Suction is an important stress variable that is required for reliable predictions of the likely performance of unsaturated soils. The axis translation technique is the best established method of measuring or controlling suction; however, the success of this application is heavily dependent on the rating of the high air entry filter (HAF) and how it is incorporated into the testing system. This paper reports some basic experiments in which samples of unsaturated kaolin were brought to saturation in stages using 5 bar and 15 bar HAFs. The results have shown that the water equilibrium in unsaturated soils is greatly affected by the rating of filters. The findings also suggest that the flow through unsaturated soils is not necessarily governed by the one-dimensional consolidation theory that was developed for saturated soils, and this may be attributed to the bimodal pore size distribution of unsaturated soils.

Selection and characterisation of geological materials for use as geopolymer precursors

Geopolymer binders are generally formed by reacting powdered aluminosilicate precursors with alkali silicate activators. Most research to date has concentrated on using either pulverised fuel ash or high purity dehydroxylated kaolin (metakaolin) in association with ground granulated blast furnace slag as the main precursor material. However, recently, attention has turned to alternative calcined clays that are abundant throughout the globe and have lower kaolinite contents than commercially available metakaolins. Due to the lack of clear and simple screening protocols enabling assessment of such geological resources for use as precursors in geopolymer systems, the present paper presents results from experimental work that was carried out to develop a functional binder using materials containing kaolinite taken from the Interbasaltic Formation of Northern Ireland. The influence of mineralogy has been examined, and a screening process, using three Interbasaltic materials as examples, that will assist in the rapid selection of suitable geopolymeric precursors from such materials is outlined.

Organo-mineral complexation alters carbon and nitrogen cycling in stream microbial assemblages

Inland waters are of global biogeochemical importance receiving carbon inputs of ~ 4.8 Pg C y^-1. Of this 12 % is buried, 18 % transported to the oceans, and 70 % supports aquatic secondary production. However, the mechanisms that determine the fate of organic matter (OM) in these systems are poorly defined. One important aspect is the formation of organo-mineral complexes in aquatic systems and their potential as a route for OM transport and burial vs. their use potential as organic carbon (C) and nitrogen (N) sources. Organo-mineral particles form by sorption of dissolved OM to freshly eroded mineral surfaces and may contribute to ecosystem-scale particulate OM fluxes. We tested the availability of mineral-sorbed OM as a C & N source for streamwater microbial assemblages and streambed biofilms. Organo-mineral particles were constructed in vitro by sorption of ¹³C:¹⁵N-labelled amino acids to hydrated kaolin particles, and microbial degradation of these particles compared with equivalent doses of ¹³C:¹⁵N-labelled free amino acids. Experiments were conducted in 120 ml mesocosms over 7 days using biofilms and streamwater sampled from the Oberer Seebach stream (Austria), tracing assimilation and mineralization of ¹³C and ¹⁵N labels from mineral-sorbed and dissolved amino acids.Here we present data on the effects of organo-mineral sorption upon amino acid mineralization and its C:N stoichiometry. Organo-mineral sorption had a significant effect upon microbial activity, restricting C and N mineralization by both the biofilm and streamwater treatments. Distinct differences in community response were observed, with both dissolved and mineral-stabilized amino acids playing an enhanced role in the metabolism of the streamwater microbial community. Mineral-sorption of amino acids differentially affected C & N mineralization and reduced the C:N ratio of the dissolved amino acid pool. The present study demonstrates that organo-mineral complexes restrict microbial degradation of OM and may, consequently, alter the carbon and nitrogen cycling dynamics within aquatic ecosystems.

Fate of organo-mineral particles in streams: Microbial degradation by streamwater & biofilm assemblages

Inland waters are of global biogeochemical importance. They receive carbon inputs of ~ 4.8 Pg C/ y of which, 12 % is buried, 18 % transported to the oceans, and 70 % supports aquatic secondary production. However, the mechanisms that determine the fate of organic matter (OM) in these systems are poorly defined. One aspect of this is the formation of organo-mineral complexes in aquatic systems and their potential as a route for OM transport and burial vs. their use as carbon (C) and nitrogen (N) sources within aquatic systems. Organo-mineral particles form by sorption of dissolved OM to freshly eroded mineral surfaces and may contribute to ecosystem-scale particulate OM fluxes. We experimentally tested the availability of mineral-sorbed OM as a C & N source for streamwater microbial assemblages and streambed biofilms. Organo-mineral particles were constructed in vitro by sorption of ¹³C:¹⁵N-labelled amino acids to hydrated kaolin particles, and microbial degradation of these particles compared with equivalent doses of ¹³C:¹⁵N-labelled free amino acids. Experiments were conducted in 120 ml mesocosms over 7 days using biofilms and water sampled from the Oberer Seebach stream (Austria). Each incubation experienced a 16:8 light:dark regime, with metabolism monitored via changes in oxygen concentrations between photoperiods. The relative fate of the organo-mineral particles was quantified by tracing the mineralization of the ¹³C and ¹⁵N labels and their incorporation into microbial biomass. Here we present the initial results of ¹³C-label mineralization, incorporation and retention within dissolved organic carbon pool. The results indicate that 514 (± 219) μmol/ mmol of the ¹³:¹⁵N labeled free amino acids were mineralized over the 7-day incubations. By contrast, 186 (± 97) μmol/ mmol of the mineral-sorbed amino acids were mineralized over a similar period. Thus, organo-mineral complexation reduced amino acid mineralization by ~ 60 %, with no differences observed between the streamwater and biofilm assemblages. Throughout the incubations, biofilms were observed to leach dissolved organic carbon (DOC). However, within the streamwater assemblage the presence of both organo-mineral particles and kaolin particles was associated with significant DOC removal (-1.7 % and -7.5 % respectively). Consequently, the study demonstrates that mineral and organo-mineral particles can limit the availability of DOC in aquatic systems, providing nucleation sites for flocculation and fresh mineral surfaces, which facilitate OM-sorption. The formation of these organo-mineral particles subsequently restricts microbial OM degradation, potentially altering the transport and facilitating the burial of OM within streams.