Jamie R. Standing

Gas generation and accumulation by aquifer drawdown and recharge in the London Basin

Air at high pressure has recently been encountered during ground investigation borehole drilling for the Thames Tideway project in London. The air was released from within the Upnor Formation (at the base of the Lambeth Group beds), which is at the top of the Lower Aquifer. Cavitation within the granular beds of the Lower Aquifer would have occurred as a consequence of drawdown from the extensive water extraction that began at the start of the nineteenth century, resulting in the water table dropping by almost 70 m. Estimated historical pore pressure profiles are postulated and the cavitation process that occurred, along with the subsequent pressurization of the air during recharge that has occurred in recent years, are discussed using behavioural concepts from unsaturated soil mechanics. The hypotheses are corroborated by a series of simple column experiments where the processes are modelled using analogous soils. Engineering implications of the presence of the pressurized air are discussed: besides the potential for borehole blow-out, another serious consideration is the depletion of oxygen from the trapped air by chemical reaction with the soil, increasing the risk of confined space hypoxia.

Centrifuge Modelling of Diaphragm Wall Construction Adjacent to Piled Foundations

Diaphragm walls are often implemented to support the sides of deep excavations and in urban environments situations increasingly occur where it is necessary to install them close to existing piled structures. There is a lack of good quality field monitoring case studies to help understand this complex three-dimensional soil–structure interaction problem. It can be investigated using numerical analysis or an alternative approach is to perform small-scale model tests in a geotechnical centrifuge. This paper describes a sophisticated model construction system that was developed to simulate the construction sequence (excavation and casting of concrete) of a single or series of three diaphragm wall panels in sand as part of a study to investigate the influence of constructing diaphragm walls adjacent to piled foundations. Prior to wall construction an adjacent instrumented model pile was driven in-flight and a constant force maintained at the pile head to simulate axially loaded piles. The system response was captured through the use of miniature soil stress cells, LVDTs, and laser displacement sensors. A detailed description of the centrifuge model and test procedures developed is presented and the necessary simplifications and associated errors discussed. The effectiveness of the small-scale physical model is demonstrated by presenting some selected test results (soil stresses and deformations).

Geomaterials: aggregates, building stone and earthworks: papers from 50 years of QJEGH

Numerous papers included within the broad subject of geomaterials have been published in QJEGH over the past 50 years. These have been compiled and divided here into three main categories: aggregates, building stone and earthworks. The aim of this review is to provide a comprehensive summary of the relevant papers published in QJEGH with a view to identifying the main areas of interest historically, now and in the future. Some clear trends are evident from the survey and review; for example, there has been a steady interest in building stone and in particular its deterioration, a decline in papers on earthworks and an increase in those on ground improvement (also covered here). It is also noted that methods of characterizing geomaterials are becoming more sophisticated with advancing technology. The review makes relevant links with other Special Publications from the Geological Society, including also Engineering Geology Special Publications.

Experimental Set‐up for Determining Soil Water Retention Curves for Granular Soils During Drying

Soil water retention curves (SWRCs) provide an important means of describing the response of unsaturated soils during drying / wetting processes in terms of variations of degree of saturation, water content or void ratio with suction. A key consideration in generating these curves is how to measure the suction. Frequently the filter paper technique is adopted, especially when high suctions are developed, e.g. with plastic clays. As each measurement takes at least a week with this technique, it can take months or years to generate a full SWRC in drying and wetting. Developments in laboratory tensiometers now allow matrix suctions up to about 1.5 MPa to be measured. With such a device it is possible to develop SWRCs for granular soils such as silts and clays in hours or days by continuous measurement. This paper describes an experimental set-up that was developed to measure changes in volume, water content and matrix suction during drying of three granular soils. Limitations of the apparatus and usefulness of the curves are discussed.

A Miniature Soil Inclusion for Measuring Axial Force and Radial Stress

This paper describes the design, development, and manufacture of a miniature soil inclusion 175 mm long and 8 mm in diameter for measuring axial force and radial stress at three points along its length. The individual devices rely on strain gages for their operation and were found to be stable in a laboratory environment. The axial force and radial stress measurements can be resolved to less than 1 N and 1 kPa, respectively. The model inclusion has been used in a study to investigate the development of shear stress along a soil nail. The instrumentation allowed the influence of arching to be positively identified. The paper includes discussion of factors such as calibrating for cross-effects, converting the electrical outputs to engineering units, and cell-action effects. These factors along with the soil stiffness govern the accuracy of measurement.