The Effect of Mechanical Remoulding on the compression and strength characteristics of a Mercia Mudstone

A series of laboratory tests have been undertaken as part of a larger study into the effect of continuous flight auger pile installation on the soil-pile interface in the Mercia Mudstone Group [1]. This paper reports the results of stress path triaxial tests on bulk samples of the mudstone which have been subjected to varying amounts of mechanical remoulding representing the remoulding that occurs due to the action of the auger. In addition, the water content at which remoulding takes place has been varied. The tests were undertaken to investigate whether the silt sized aggregates of clay particles that exist [2] within this soil could be broken up by the mechanical remoulding and whether this would affect the compression and strength properties of the mudstone. It was found that both mechanical remoulding and the water content at which this is carried out affect the subsequent response of the soil

Twin-tunnelling-induced changes to clay stiffnesses

Tunnels used for transportation in urban environments are often constructed in pairs. Projects in which tunnels are constructed sequentially and within close proximity are referred to as ‘twin tunnelling’. Case studies and recent research indicate that the prediction of settlements for such schemes cannot be determined using existing simple methods derived from consideration of a single tunnel. To establish the reasons for the observed variation in settlements, a series of centrifuge tests was undertaken on various twin-tunnel arrangements in overconsolidated clay. The tests consisted of preformed cavities from which a specific quantity of supporting fluid could be drained, with precision, creating a predetermined magnitude of tunnelling volume loss. Data were obtained for surface and subsurface displacements, changes in pore-water pressure near the tunnels and the support pressure within the tunnels. The systematic use of cavity contraction models was found to be an informative method of explaining the observations. Use of an elastic–perfectly plastic cavity contraction model coupled with observations from the experiments enabled the shear stiffness of the clay around the tunnel to be described. Further analysis demonstrated a reduction in shear stiffness of the soil prior to and during the second tunnel excavation, explaining the increase in volume loss observed in that event

Modelling the effect of recent stress history on the deformation of overconsolidated soils

The aim of the research was to study the behaviour of overconsolidated soils subjected to small changes of strain or stress appropriate to the investigation of ground movements around excavations, retaining walls or foundations, and to develop a constitutive soil model that can predict such behaviour. The principal feature of soil behaviour investigated was the effect of recent stress history, defined by 9 the angle of rotation between the previous and current stress path directions. Stress path triaxial tests were carried out on both reconstituted and undisturbed samples of speswhite kaolin and London clay. The tests, which followed on from previous work by Richardson (1988), examined details of the influence of recent stress history, which was found to have a significant influence on the stress-strain response of the soil for the current loading path. The data from the tests together with a re-evaluation of the existing experimental data and a limited investigation of the effect of recent stress history in 3D stress space, enabled the main features of the soil behaviour to be identified. The stress-strain response of the soil was found to be highly non-linear, inelastic and dependent on recent stress history; if the stress path rotation was 18O, i.e. a complete reversal, the soil stiffness was at a maximum and was at a minimum for no rotation. As the loading path continued the influence of the recent stress history gradually diminished until it was no longer evident. Recent stress history also affects strain paths and effective stress paths measured during drained and undrained loading respectively. The significance of mean effective pressure and overconsolidation ratio was also investigated. Soil models which can predict this behaviour need to include at least one kinematic yield surface which allows plastic straining inside the state boundary surface. A two-surface yield model of this type, formulated by Al Tabbaa (1987) was evaluated. The predictions of this model did not compare well with several important aspects of the experimental observations and it was shown that an additional kinematic surface is necessary to model non-linearity, inelasticity and the recent stress history effect. A new three-surface model based on the two-surface model was developed, within the framework of Critical State soil mechanics, which successfully predicts all the main features of the soil behaviour. It is described by eight parameters which are all soil properties and, with one exception, all have a clear physical meaning

Explanation for twin tunnelling-induced surface settlements by changes in soil stiffness on account of stress history

In this article, a group of representative centrifuge tests were selected for numerical modelling to explain the surface settlements induced by sequential twin tunnelling. Both Modified Cam Clay model (MCC) and Three-Surface Kinematic Hardening model (3-SKH) were adopted in the simulation, which indicated the use of 3-SKH model conduced to mimicking more closely centrifuge model response. Via performing more contrastive numerical analyses with 3-SKH model, the influence of the first tunnel event on the stiffness of the soil around the second tunnel was quantitatively investigated, whereby the mechanism behind the observed surface settlements was finally made clear

Test development for the investigation of soil disaggregation during slurry tunnelling

Slurry tunnelling uses a water based slurry to aid in tunnel face support and transportation of the excavated material. Unlike other tunnelling methods this requires expensive surface separation plant to remove the excavated ground from the slurry. Incorrect specification of this plant can lead to significant delays and added cost to a tunnel drive. Due to the tight budgets and space constraints this can cause contracts to become unprofitable, in particular where small diameter slurry tunnels are excavated by pipe jacking. For this reason accurate prediction of the size distribution of the soil particles and lumps in the disaggregating slurry is required. This research concerns the development of test procedures and methods of soil classification that will enable improved predictions of the degree to which soils/weak rocks will disaggregate during the slurry tunnelling process

Remoulding of the Mercia Mudstone Group around CFA pile shafts

A field test has been undertaken to investigate changes to in situ Mercia Mudstone at the pile soil interface after installation of four 5.5 m long 350 mm diameter CFA piles. The test investigated whether a remoulded zone exists, the extent of the zone, changes caused by remoulding and the effect of the installation method. The piles and surrounding soil were excavated after installation to a depth of just under 2 m. The sections of pile and surrounding soil were returned to the laboratory where a variety of detailed observations at both micro and macroscopic scales were undertaken, together with chemical and mineralogical analysis. It was found that a remoulded zone existed in all piles but that this varied in thickness both laterally and vertically around a pile. Across all piles the maximum thickness observed was 55 mm. The average thickness, fabric and texture were all affected by installation method. A distinctive vertically orientated fabric was observed in which up to four vertically orientated layers could be distinguished. There was evidence of changes in texture/fabric, structure, colour, mineralogy and chemistry

Mechanisms for the disaggregation of soil cuttings in slurries

A series of tests has been undertaken on a variety of different soils to understand how soil cuttings disaggregate when pumped from the tunnel face to the separation plant during slurry tunnelling for pipe jacked tunnels. It is important to understand this process to ensure that the separation plant can be optimised to remove the maximum amount of soil from the slurry prior to the reuse of the liquid. In pipe-jacking operations this liquid is normally water, which is recirculated after the soil has been re-moved to minimise and if possible eliminate the production of liquid waste. The paper will present results from a series of “mixing tests”, devised by the authors to investigate disaggregation, and also from standard laboratory tests undertaken to establish basic soil properties such as soil strength. The “mixing tests” simulate the softening of the cuttings due to the presence of water and the shear forces applied to the slurry by the pumps and have al-ready allowed the effect of these factors to be quantified. This series of tests will demonstrate how the mechanism of disaggregation varies with strength resulting in different proportions of disaggregated soil particles in slurries subjected to the same mixing time and shear forces

Predictions of changes in pore-water pressure around tunnels in clay

Any underground construction causes changes to the stress state in the ground and this change generally causes the generation of excess pore-water pressures in saturated fine grained soils. Subsequent dissipation of these pressures can lead to settlements and potential damage and hence there is a need to understand and predict these changes in pore-water pressure. Simple plasticity and non-linear elastic solutions have been used to calculate pore-water pressure changes as a tunnel is constructed in clay. These are compared with previous centrifuge tests involving the simulation of tunnel excavation as well as new tests specifically designed to investigate the generation and subsequent dissipation behaviour of excess pore-water pressures. The paper reports on the new tests, presents the findings within the simple plasticity and non-linear elastic analysis framework

Development of a model pile for heat transfer experiments in the centrifuge

Conventional energy piles use embedded plastic pipes to circulate a fluid through solid concrete which enables transfer of heat into, or out of, the ground as required. Such piles are relatively low efficiency owing to the poor conductivity of the concrete in which the pipes are embedded. They are also known to be susceptible to damage during construction and their adoption as a sustainable energy source has, as a result, been limited. A novel method of heat transfer, which is much less susceptible to damage during construction, and has been found in field trials to be more energy efficient, exploits the significantly higher conductivity of water in a rotary augured hollow, cast in-situ, or precast pile. In such a pile the plastic pipes are placed in the water filled central void of the pile. Such an arrangement will lead to the ground around the pile experiencing a lower range of temperature variation compared with standard energy piles and the influence of this effect on pile capacity will be explored. In order to model multiple cycles of temperature variation to which the ground around a prototype pile may be subjected it is necessary design experimental apparatus that is capable of rapid heating and cooling and with high thermal conductivity materials. The paper will describe the design of a model pile which incorporates an immersed heating element capable of bringing the pile temperature to a specific maximum value and a means of quickly purging the heated water to return the pile temperature to the desired minimum value whilst the pile carries a constant axial load in the centrifuge

A low-cost miniature immersible pore water pressure transducer

The ability to measure pore water pressure accurately in geotechnical models is vital for allowing researchers to quantify effective stress and, frequently, its temporal variation. The earliest use of electrical devices to measure pore water pressure in specific locations within a model employed a standard laboratory pressure transducer located outside the boundaries of the model connected to tubing inserted into the model. However, this technique was superseded by the development (some 40 years ago) of submersible miniature pore water pressure measuring devices that could be located within a model to measure pore pressure directly at specified locations. In particular, miniaturisation allowed these transducers to be used in small scale centrifuge models. This facilitated, across the full spectrum of geotechnical engineering modelling, both enhanced understanding of mechanisms, by permitting quantitative analysis, and validation of numerical techniques. The earliest miniature transducers, the PDCR81 device manufactured by Druck, rapidly became universally adopted by the geotechnical modelling community. The production of this device was halted about 10 years ago, but other manufacturers have developed similar miniature pore water pressure transducers. Whilst the newer devices have been demonstrated to be effective their unit cost is relatively expensive; owing to the low volume of manufacture. This can result in forced limitations in the number that may be in a model, or a reluctance to use them in zones of models where they might be damaged. The requirement for a cheaper but equally reliable device prompted the work described in this paper. This is now achievable because of the development for the consumer market of a mass produced robust, immersible pressure device that has a very low unit cost. The paper will describe the development of a miniature transducer, employing this device, that can be located within models in a similar way to the PDCR81