Influence of heave reducing piles on ground movements around excavations

The research concerns the influence of piles, installed beneath deep excavations, as a means of reducing movements in the surrounding ground. The work focussed on the use of piles installed as a part of top down basement construction, a technique used in conjunction with deep excavations in urban areas. The investigations sought to explore the effectiveness of bored piles as a means of enhancing the stiffness of the soil beneath the excavation and so reducing the spread of movements to the surrounding ground. Experimental data were obtained from a series of 19 centrifuge model tests undertaken at 100g. The plane strain models consisted of a pre-formed excavation temporarily supported by fluid pressures acting at formation level and against the retaining wall. The fluid support was removed as the test proceeded and successive levels of props were advanced against the retaining wall using pressurised hydraulic cylinders as jacks. Ground movements were measured using a combination of transducers and analysis of digital images from a camera viewing the front of the model seen through the Perspex side of the model container. These systems gave ground surface, formation level and wall displacement profiles as well as overall patterns of movement. The general model behaviour was characterised in a series of datum tests. These established the magnitude of displacements generated with ground support provided by the retaining wall alone in key positions throughout the model. Following this the overall stiffness of the soil below excavation formation level was enhanced by the introduction of either one or two rows of cast in situ piles installed at distances of 3 and 6 pile diameters from the retaining wall during model making. Direct comparison was then made between the various test results. These procedures were repeated in a small number of additional tests in which the retaining wall embedment depth was reduced. The use of piles was found to reduce both horizontal movements and settlement behind the retaining wall. Maximum reductions in settlement behind the retaining wall were found to be about 55%. The influence of piles on settlement was limited to a distance of about two times the excavation depth behind the retaining wall. Maximum reductions in horizontal displacement, near to the retaining wall, were about 70%. The effectiveness of the piles in reducing ground movement diminished with increasing prop stiffness such that when lateral displacement of the retaining wall was effectively prevented maximum movements were reduced by 40% (settlement) and 50% (horizontal). The piles were found to create a general stiffening effect that reduced horizontal movement at the toe of the retaining wall and led to reductions in overall prop load. Additionally the piles provided restraint against heave movements at the excavation formation and therefore also acted in tension. As a result the soil mass around the piles tended to behave as a block. This behaviour was observed for excavations in which both one and two rows of piles were used despite the relatively discrete nature of the elements. With increasing time after completion of the excavation the block behaviour became less well defined although the effect was better maintained when the greater number of piles were used. Finite element analyses of the centrifuge models also predicted reductions in displacement when piles were modelled at excavation formation level although the magnitude of reduction was less than that observed in the centrifuge tests.