Measuring horizontal stresses during jacked pile installation

Jacking is an installation technique for displacement piles which is commonly used onshore in urban environments owing to its low noise and vibration. During pile jacking, stress changes occur in the soil which are substantial close to the pile, but also extend a significant radial distance. These stresses are difficult to measure accurately owing to arching around stress sensors. In the field, stress measurements are commonly made by means of an adjacent pile whose stiffness changes the stress field within the soil. Accurate measurements of stresses due to the installation of a single pile under laboratory conditions are needed in order to quantify this error. In this paper, null gauges that do not suffer from membrane deflection are used to measure horizontal stress changes during the jacked installation of a cylindrical pile in dry sand. Stresses are measured by means of both an adjacent pre-installed square pile and in-soil sensors. The paper also presents a comparison between the centrifuge results and the radial stress distribution estimated using conventional methods, such as Boussinesq's elastic analysis and elasto-plastic spherical cavity expansion. The research was funded by Giken Ltd. and the Coleman-Cohen Exchange Programme to whom a great acknowledgement is addressedThis is the author accepted manuscript. The final version is available from ICE via http://dx.doi.org/10.1680/geng.14.0006

Continuous and discontinuous stability analysis of the bell-shaped caverns at Bet Guvrin, Israel

The stability of two systems of bell-shaped caverns excavated some 1000 years ago at Bet Guvrin National Park is investigated. The caverns were excavated in a weak, anisotropic, and moderately discontinuous chalk. The cavern stability is considered based on two separate and independent methods: a continuum model framework—FLAC, used for stress analysis, and a discontinuous approach—block theory, used for critical key block analysis. The numerical stress analysis reveals that in the case of very large span openings, tensile fracture of intact rock may be responsible for instabilities, which may lead to global failure. Evidence of tensile rupture at margins of failed caverns is abundant at the Park. The discontinuous block theory analysis reveals that the moderate joint set spacing at Bet Guvrin, up to 45% of the roof area may be comprised of removable blocks. The removable keyblocks in the roof remain in place due to arching stresses, which develop through the roof material. The chalk at the roof can sustain the maximum loads in existing caverns, as predicted by the numerical stress analysis. However, local failures due to exceedingly high compressive stresses at the abutments or by tensile fracture at the roof, may lead to relaxation of arching stresses followed by keyblock displacement. Such a “mixed failure mode” process could eventually lead, over time, to global collapse. Indications that “mixed failure mode” processes are presently active in the studied caverns are substantiated by in-situ measurement of keyblock displacements. It is suggested that in weak and discontinuous rock environments where “mixed failure mode” processes may be active, long term stability evaluation should be based on both continuous and discontinuous stability analyses

Measurement of soil strains under earthquake loading

A new device has been developed for the direct measurement of soil strains under earthquake loading in centrifuge testing. The main body of the device is a potentiometer used to measure soil deformation directly. Two MEMS accelerometers are attached to either end of the device to obtain soil deformation as a semi-direct measurement method. Two different centrifuge models, a level sand bed and a slope, were prepared with dry sands to achieve small and large soil deformation, respectively. A series of model earthquake motions were applied to the centrifuge models. The strains obtained with the new devices are compared with the indirect measurement of Piezo-electric accelerometers. It is found that this new direct method to measure soil strain could be applicable for medium to large soil deformation, that is, in the case of relatively strong earthquakes

Effect of width of geosynthetic reinforcement within the granular cover on the load distribution over the tunnel lining

A realistic estimation of load distribution over the buried structures is necessary for proper analysis of tunnels, culverts and pipes/conduits. Tunnels with linings are often constructed in transportation and hydraulic engineering. For the design of tunnel lining, it is essential to know the load over the lining. Load distribution over the buried structures has been investigated scientifically during the past several decades. The method of investigation includes experimental, numerical and analytical methods. The finite-element models based on some commercial software have been developed for load analyses for design of the tunnel linings and buried structures. The geosynthetic is an effective reinforcement layer to reduce the load over the buried structure. Although some studies have indicated that the geosynthetic layer can reduce the load over the buried structure, but no attempt has been made to determine the optimal width of the geosynthetic reinforcement within the granular cover. Therefore, in this paper, an attempt is made to present effect of width of geosynthetic layer on the load distribution over the tunnel lining. The study has been carried out by developing a numerical model of the problem. The commercial software PLAXIS 2D has been used for numerical modelling. The results have been presented in the form of design charts, mentioning the optimum width of geosynthetic layer, so that they can be used by practising engineers