Compensation grouting is a multiple injection process | My Assignment Tutor

Laboratory investigation of multiple grout injections into clayK. SOGA*, S. K. A. AU†, M. R. JAFARI and M. D. BOLTON§Compensation grouting is a multiple injection process in which large numbers of grout injections are carried out in a designated grouting zone. Each injection alters the in-situ stress state at the site of neighbouring injections, influencing the grout behaviour as well as the deforma-tion of the surrounding soil. In this study, multiple grout injection tests were performed in the laboratory on clay specimens prepared at different overconsolidation ratios ranging from 1 to 10. Two types of grouting mode for compensation grouting were examined: (a) facture grout-ing by injecting epoxy resin, and (b) compaction grouting by expanding a latex balloon placed inside the soil speci-men. The sequence of multiple injections was also varied by injecting either simultaneously or sequentially with different waiting periods. For highly overconsolidated clay the grout efficiency, defined as the ratio of the volume of heave achieved to the injected grout volume, was close to 1 irrespective of grout spacing and injection sequence. For normally consolidated and lightly overcon-solidated clays the grout efficiency increased when the separation in space and time between the injections was reduced. A better grout efficiency was obtained in com-paction grouting than in fracture grouting. The results from finite element analyses of the laboratory tests show that the magnitude and extent of excess pore pressure reduce when many closely spaced simultaneous injections are performed. A few non-simultaneous injections create large stress concentrations around the injection points, leading to larger soil consolidation.KEYWORDS: grouting; tunnels; settlement; clays; consolidationLa cimentation de compensation est un processus ainjec-tions multiples dans lequel de grands nombres d’injec-tions de ciment sont effectue´s dans une zone de cimentation de´signe´e. Chaque injection change l’e´tat de contrainte in-situ a l’endroit des injections voisines, influ-ençant le comportement du ciment ainsi que la de´forma-tion du sol aux alentours. Dans cette e´tude, des tests d’injections multiples ont e´te´ re´alise´s en laboratoire sur des spe´cimens d’argile pre´pare´s adivers taux de surcon-solidation, allant de 1 a 10. Nous avons examine´ deux types de modes de cimentation pour la cimentation de compensation : (a) une cimentation de fracture en injec-tant de la re´sine e´poxyde et (b) une cimentation de compaction en gonflant un ballon de latex place´ al’inte´r-ieur du spe´cimen de sol. La se´quence des injections multiples a e´galement e´te´ varie´e en faisant les injections soit simultane´ment, soit les unes a la suite des autres, avec diverses pe´riodes d’attente. Pour l’argile très sur-consolide´e, l’efficacite´ de la cimentation, de´finie comme le rapport entre le volume de rejet obtenu et le volume de ciment injecte´, e´tait proche de 1, quels que soient l’espa-cement du ciment et la se´quence d’injection. Pour des argiles normalement surconsolide´es et des argiles le´gère-ment surconsolide´es, l’efficacite´ de la cimentation aug-mente lorsque la se´paration dans l’espace et dans le temps entre les injections est re´duite. On obtient une meilleure efficacite´ avec la cimentation de compaction qu’avec la cimentation de fracture. Les re´sultats des analyses d’e´le´ments finis des essais de laboratoire mon-trent que la magnitude et l’e´tendue de la pression inter-stitielle excessive baissent lorsque l’on effectue beaucoup d’injections simultane´es très rapproche´es dans l’espace. Quelques injections non simultane´es cre´ent de grandes concentrations de contrainte autour des points d’injec-tion, ce qui donne une consolidation du sol plus grande.INTRODUCTION Compensation grouting is a technique to offset subsidence caused during underground excavation and bored tunnelling. The basic principle is that grouts are injected in the zone between the tunnel and overlying buildings to compensate for the ground loss and stress relief induced by underground excavation (Mair & Hight, 1994). A common configuration of the compensation grouting operation is a fan array of grouting holes radiating horizontally from a vertical shaft as shown in Fig. 1. Grout injection is usually undertaken contemporaneously with tunnelling in response to detailedManuscript received 13 December 2001; revised manuscript accepted 12 September 2003. Discussion on this paper closes 1 September 2004, for further details see p. ii. * University of Cambridge, Department of Engineering, Cambridge CB2 1PZ, UK. † City University of Hong Kong, Department of Building and Construction, Hong Kong. STV Incorporated, New York USA. § University of Cambridge, Department of Engineering, Cambridge CB2 1PZ, UK.monitoring, so that settlements and distortions are limited to specified amounts. A common form of compensation grouting involves injec-tion of low-viscosity particulate grouts into the soil, creating hydrofractures (termed fracture grouting). The grout intrudes into the fractures, and the introduction of solids enables a compensation effect to be achieved in a limited number of fractures. Grout injection is commonly achieved through the use of a sleeved tube known as a tube a` manchette (TAM). A TAM is a plastic or steel tube with pairs of holes drilled at intervals of 0.3–1.0 m along the length of the tube, each pair being covered by a tight rubber sleeve. These ports act as one-way valves. The TAMs allow re-injection of grout from the same port; appropriate amounts of grout are injected at the right place and time. Successful applications of fracture grouting for compensation grouting are reported for the Viennese subway project (Pototschnik, 1992), the Saint Clair River tunnel project near the USA–Canada border (Kramer et al., 1994; Drooff et al., 1995), the Jubilee Line Extension project in London (Linney & Essler, 1994; Harris et al., 1996, 1999; Osborne et al., 1997), the Dock-land Extension Project in London (Sugiyama et al., 1999) and the Lisbon underground line (Schweiger & Falk, 1998).

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