Numerical Modelling of Seepage in the Presence of Phreatic Surfaces.

A numerical model has been made for the solution of phreatic surface seepage (single-phase fluid flow), assuming continuity of flow between unsaturated and saturated soil zones. It allows for continuous variation of the hydraulic properties as a function of the pressure head using real soil data taken from relevant literature. Soil anisotropy and layering, one-, two-, or three- dimensional problems, seepage faces and hysteresis can also be handled. The computer code based on this model has been validated in comparison with analytical, experimental, field and numerical results found in the literature. The impact of several factors on the accuracy of the results has been investigated. The various methods for the mathematical simulation of the hydraulic properties are shown not to influence significantly the numerical results. Conversely, not all hysteresis models performed equally well in all situations. Slight improvement was achieved using lumping of the time-dependent matrix. Extensive comparisons have been made between the writer’s model results and those derived by other methods, used for phreatic surface seepage solutions. Such methods usually ignore flow in the unsaturated zone, or make oversimplifying assumptions about the hydraulic property variation in this zone. The comparisons were based on dam, trench drainage and local or regional pumping problems. Case studies of pumping have also been made, and the writer’s model has been applied to the simulation of groundwater level changes in inner cities (with particular reference to London). It has been shown that the results derived using the various methods may differ significantly, especially in transient seepage analyses. Ignoring hysteresis may also lead to significant discrepancies with observations. Realistic modelling of the hydraulic property variation in the unsaturated zone was demonstrated to be of major importance. It is claimed that, overall, the writer’s model has been able to produce improved results compared with other methods investigated herein

Effects of testing techniques on the SWRC of a partially saturated soil

The paper studies the laboratory measurement of the Soil Water Retention Curve (SWRC) of statically compacted London Clay focusing on the effects of different specimen preparation methods and testing techniques to acquire a better understanding of how measurements can be affected by these factors. Testing methods include filter paper, two modified pressure plate systems, and a ceramic pressure membrane extractor. A discussion on the repeatability of the measurements is also made

A comparative assessment of chemical stabilisers including waste materials, for the treatment of swelling-shrinking soils

This paper assesses comparatively the performance of a number of innovative soil stabilisers for the treatment of a highly swelling-shrinking soil, against that of commercial calcium lime. The production of lime, a most common soil stabiliser, involves high energy consumption, carbon dioxide emissions and the depletion of natural raw materials. Alternatives are actively sought, in particular industrial wastes and by-product materials or lower energy demand cements e.g. reactive magnesia (MgO) cements. In this paper calcium lime, reactive magnesia, industrial wastes and mixes of these with lime are comparatively assessed, based on a number of conventional measures of the propensity of a soil to swell, i.e. plasticity characteristics and swelling characteristics (swelling strains, swelling pressures, swelling indices). Furthermore, as expansive soils are typically in an unsaturated state hence sensitive to both changes in water content and suction, filter paper testing was performed to provide additional evidence of the effect of the treatments on the swelling/shrinking soil. According to the main findings, for the treatment of swelling shrinking soils, binders coming from the paper recycling industry show most promise as alternatives to lime; reactive magnesia cement had a smaller effect than calcium based stabilisers in improving the swelling-shrinking of the soil, yet it also suppressed swelling and shrinkage considerably; it thus shows potential for use as an alternative to common soil stabilisers (Portland cement and calcium lime) to alleviate the environmental impact of the latter

Effects of testing techniques on the SWRC of a partially saturated soil

The paper studies the laboratory measurement of the Soil Water Retention Curve (SWRC) of statically compacted London Clay focusing on the effects of different specimen preparation methods and testing techniques to acquire a better understanding of how measurements can be affected by these factors. Testing methods include filter paper, two modified pressure plate systems, and a ceramic pressure membrane extractor. A discussion on the repeatability of the measurements is also made

Implementation of biocementation for a partially saturated problematic soil of the UK railway network

This paper refers to biocementation of a problematic soil of the UK railway network as a potential stabilisation technique of this soil using indigenous ureolytic bacteria. The soil is peat, a soft foundation soil also subject to oxidation wastage. As the peat is under existing embankments, electrokinetics (EK) is proposed as a promising technique to implement treatments. In the context of unsaturated soils the paper thus focuses on two particular aspects relevant for the implementation of treatments and the stability of this soil, namely: a) the effect of degree of saturation of the peat on the bio-electrokinetic treatment ; b) the soil water retention curve of the soil affecting flow and transport; these are relevant as we focus on understanding and modelling the implementation of treatments through electrokinetics; moreover for the peat it is of importance to understand moisture exchange in the vadose zone and control groundwater table levels (e.g. during electrokinetics) in order to prevent further oxidation. After isolation and screening of indigenous microorgansisms Bacillus licheniformis was selected for further testing. The results in terms of unconfined compressive strength, CaCO3 content, swelling and compression behaviour and water retention proved the feasibility of biocementation using this indigenous microorganism. Ongoing work is assessing the required treated soil characteristics and related required biocementation degree to solve UK rail's peat foundation problems. Upscaling of the techniques towards in situ implementation is also planned in the next stage of the research