D. Gallipoli

On a 2D hydro-mechanical lattice approach for modelling hydraulic fracture

A 2D lattice approach to describe hydraulic fracturing is presented. The interaction of fluid pressure and mechanical response is described by Biots theory. The lattice model is applied to the analysis of a thick-walled cylinder, for which an analytical solution for the elastic response is derived. The numerical results obtained with the lattice model agree well with the analytical solution. Furthermore, the coupled lattice approach is applied to the fracture analysis of the thick-walled cylinder. It is shown that the proposed lattice approach provides results that are independent of the mesh size. Moreover, a strong geometrical size effect on nominal strength is observed which lies between analytically derived lower and upper bounds. This size effect decreases with increasing Biots coefficient.

Soil suction monitoring for landslides and slopes

Abstract Rainfall is the most frequent triggering factor for landslides and the development of early warning systems has to take account of this. It is suggested that direct measurement of pore pressure gives the most reliable prediction of failure of a slope. The amount of rainfall can be only a crude indicator of failure as the processes that occur between rain falling on a slope and the resulting pore water pressure change are complex, highly non-linear and hysteretic. The paper describes high-capacity tensiometers developed within the EU-funded MUSE Research Training Network that have been used for measuring suctions in slopes. High-capacity tensiometers are capable of direct measurement of pore water pressure down to −2 MPa and are also able to record positive pore water pressures. Two methods of field installation are discussed; one developed by ENPC in France uses a single tensiometer per hole, and the second technique, developed by Durham University in the UK, allows multiple tensiometers to be used at different depths within a single borehole. Continuous monitoring of pore water pressure has been carried out over several months and shows the responses to climatic events.

A new procedure for the determination of soil-water retention curves by continuous drying using high-suction tensiometers

Soil-water retention curves (SWRCs) can be determined using high-suction tensiometers (HSTs) following two different procedures that involve either continuous or discrete measurement of suction. In the former case, suction measurements are taken while the sample is permanently exposed to the atmosphere and the soil is continuously drying. In the latter case, the drying or wetting process is halted at different stages to ensure equalization within the sample before measuring suction. Continuous drying has the advantage of being faster; however, it has the disadvantage that the accuracy of mass measurements (necessary for the determination of water content) is affected by the weight and stiffness of the cable connecting the HST to the logger. To overcome this problem, an alternative continuous drying procedure is presented in this paper in which two separate but nominally identical samples are used to obtain a single SWRC; one sample is used for the mass measurements, while a second sample is used for suction measurements. It is demonstrated that the new continuous drying procedure gives SWRCs that are similar to those obtained by discrete drying.

A sequential method for selecting parameter values in the Barcelona basic model

The popularity of the Barcelona basic model (BBM) has grown steadily since its publication in 1990, due to its ability to capture key aspects of unsaturated soil behaviour. Nevertheless, the BBM is still rarely employed by practitioners, partly because of the absence of simple and objective methods for selecting parameter values from laboratory tests. One difficulty is that, in the BBM, individual aspects of the isotropic virgin behaviour are controlled by multiple parameters, while at the same time a single parameter controls more than one aspect of soil behaviour. This has led to iterative procedures where parameter values are adjusted in turn to match experiments, which requires significant experience and can lead to the selection of widely varying parameter values depending on the user. The proposed method streamlines parameter selection with a view to increasing the appeal of the BBM for practitioners. The method adopts a “sequential” procedure where the five parameters governing isotropic virgin beh...

Parallel Hybrid Particle Swarm Optimization and Applications in Geotechnical Engineering

A novel parallel hybrid particle swarm optimization algorithm named hmPSO is presented. The new algorithm combines particle swarm optimization (PSO) with a local search method which aims to accelerate the rate of convergence. The PSO provides initial guesses to the local search method and the local search accelerates PSO with its solutions. The hybrid global optimization algorithm adjusts its searching space through the local search results. Parallelization is based on the client-server model, which is ideal for asynchronous distributed computations. The server, the center of data exchange, manages requests and coordinates the time-consuming objective function computations undertaken by individual clients which locate in separate processors. A case study in geotechnical engineering demonstrates the effectiveness and efficiency of the proposed algorithm.

Determination of the soil water retention curve with tensiometers

An alternative technique for the determination of the soil water retention curve has recently been proposed whereby a tensiometer is used to measure soil suction and a balance to record the water content variations. The soil water retention curve is obtained by drying the soil either continuously or by stages (i.e. each drying stage is followed by an equalization period). Initial results from tests on compacted soil suggest that the relatively fast evaporation rate during continuous drying affects the water retention curve whereas the stage drying procedure provides more accurate results. Factors such as sample geometry and tensiometer position (relative to the sample) are also likely to affect the response obtained during continuous drying. These are the object of future investigation.

Investigating the impacts of climate change on slopes: field measurements

Abstract Climate change has the potential to have significant effects on the stability and serviceability of earthworks slopes, impacting on the performance of transport infrastructures. This paper describes how a unique facility for engineering and biological research was established in NE England through the BIONICS Project (Biological and Engineering Impacts of Climate Change on Slopes). It describes the building and monitoring of a full-scale embankment representative of road/rail embankments in the UK. The paper presents the results of monitoring of pore water pressure carried out between 2007 and 2009. The pore water pressures within the upper 3 m of the embankment have been largely positive, in some case approaching hydrostatic conditions after wetter periods. This is largely due to the wet nature of the summers in both 2007 and 2008 since monitoring began, as well as the application of artificial inundation using a climate control system. Negative pore water pressures (suctions) of the order of −30 kPa were recorded at greater depths below 3 m. The experimental facility provides essential field measurements that can be used to calibrate numerical models of soil responses to climatic changes and gain a better understanding of the response of engineered UK fills to climate events.

Image Analysis of Strains in Soils Subjected to Wetting and Drying

A novel image based technique has been developed to measure strains of soil specimens subjected to wetting-drying by using vapor equilibrium with closed-loop ventilation. A digital SLR camera, fitted with a macro telephoto lens, and an advanced photo system type-C (APS-C) sensor have been used to take images of specimens during wetting-drying. By using the simple radial model of optical distortion, it has been shown that macro fixed focal length lenses introduce extremely small strain errors, which can be neglected. It has also been shown that APS-C sensors are preferable to large Full Frame sensors when the specimen image does not fit the entire sensor area and pixel density, rather than pixel resolution, becomes the key parameter controlling measurement precision. Images of specimens have been acquired at different times during wetting-drying of the soil and processed by using the Adobe Photoshop software to assess evolution of strains. In particular, the planar strain field has been measured by means of a virtual “optical” strain gage rosette tracking the same three points on the specimen image throughout equalization. Multiple optical strain gage rosettes at different scales and positions have been employed to assess the uniformity of the strain field. The method was validated by imposing six different values of suction on six distinct specimens of the same isotropically compacted bentonite, with five values producing soil shrinkage (drying) and one value producing soil swelling (wetting). An excellent repeatability of results was observed in terms of both water content and strains. In addition, for each suction level, almost identical strain equalization curves are obtained from the analysis of multiple optical strain gage rosettes of different sizes placed at different locations over the specimen image. This confirms uniformity of the strain field and excludes the presence of friction at the specimen base.

Towards a Tensiometer Based Suction Control System for Laboratory Testing of Unsaturated Soils

This paper presents the development of an automated tensiometer based suction control system for testing unsaturated soil samples. The system is able to dry and wet soil, while measuring suction (pore water pressure – air pressure) and water content. The system uses air circulation within a closed loop to dry the soil, or water injection to wet the soil, to achieve the required pore water pressure while the air pressure is kept atmospheric. Pore water pressure is controlled by using a feedback computer system that dries or wets soil samples according to measurements obtained from sample-mounted high suction tensiometers. Excess moisture in the air circulation loop is captured by an in-line moisture trap consisting of a sealed cell containing a desiccant (silica gel), which is placed on an electronic balance to give continuous measurements of retained water. Changes of the sample water content are measured as the difference between the amount of water injected and that retained by the moisture trap. The system is fully automated and runs controlled by software with minimum assistance. The proposed suction control system presents advantages over the conventional axis translation technique as it avoids the need for elevated pore air pressures and hence better replicates the natural processes of wetting and drying of soils. The system was developed for use in a triaxial cell, but could also be used with other instruments and for the determination of water retention curves.

On the Measurement of Water Pressure in Soils with High Suction Tensiometers

Past studies on the use of high suction tensiometers for measuring negative pore water pressure have focused on three different aspects, namely, initial saturation of the probe, calibration over both positive and negative pressure ranges, and measurement procedures. Among these three areas, the one focusing on measurement procedures has undeservedly received less attention. Aspects related to measurement conditions during laboratory or field testing are as important as the initial pre-conditioning or calibration of the probe. According to the particular type of measurement, different aspects of the testing procedure have to be considered in order to obtain accurate readings of pore water pressure. This note presents preliminary data to highlight the importance of factors, such as measurement time, soil-probe contact, and material type, when measuring suction by means of high suction tensiometer.