Factors Affecting Long-Term Efficiency of Compensation Grouting in Clays

Compensation grouting has been attracting attention in recent years to control ground settlement caused by underground construction. A successful and effective application of compensation grouting depends not only on the use of a good monitoring system in the field but also on a fundamental understanding of grout behavior in soils. For compensation grouting in clay, there is a need to consider the long-term effectiveness of compensation grouting. Grout injection tests were conducted in the laboratory to examine soil fracturing patterns and consolidation effects in relation to compensation grouting. Both compaction and hydrofracturing modes of grouting were tested on kaolin clay specimens with different overconsolidation ratios. Finite-element analysis was also conducted to simulate the balloon expansion tests ~ideal compaction grouting tests! and the results were compared with the data from the laboratory tests. Based on the research findings, new design criteria are proposed to improve the long-term efficiency of compensation grouting for clays. DOI: 10.1061/~ASCE!1090-0241~2003!129:3~254! CE Database keywords: Grouting; Laboratory tests; Finite element method; Fracture; Consolidation; Tunneling; Clays. 254 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN project ~Sugiyama et al. 1999; Essler et al. 2000! and the Lisbon underground line ~Schweiger and Falk 1998!. A summary of some of the past compensation grouting projects is listed in Table 1. The ‘‘compensation efficiency’’ in the Table 1 is defined as the ratio of the volume of heave obtained to the injected volume of grout. In general, low compensation efficiencies were calculated in soft clay conditions, whereas better compensation efficiencies were achieved in stiff clays. The field trial in Singapore soft clay reported by Shirlaw et al. ~1999! showed that the heave obtained immediately after grout injection reduced with time due to soil consolidation associated with the dissipation of the excess pore pressures generated during injection. In some cases, the surface level came back close to the original condition. Similar findings were obtained in the field trials reported by Ikeda et al. ~1996! and by Komiya et al. ~2001!, addressing the undesirable longterm effect of grouting in soft clay for settlement control purposes. Possible Mechanisms of Decreasing Compensation Efficiency Fig. 1 shows the conceptual modeling of compensation grouting in clays. At the initial stage of compensation grouting, the grout pushes the soil outward and deforms plastically, forming a ball. As the grouting pressure increases, the size of the ball increases rapidly until the grout pressure builds up to the fracturing pressure and a plane of weakness is formed by hydraulic fracturing as shown in Fig. 1~b!. As a result, the stress condition in the soil suddenly changes and the injection pressure drops. When grout with low viscosity and/or low solid content is used, it will intrude into planes of weakness to develop grout-filled fractures. When a grout with high viscosity and/or high solid content is used, the grout would not be able to penetrate into fractures and the grout ball simply continues to expand. In Europe, fracture grouting is often used in compensation grouting because it is possible to regrout from the same grouting port. The effectiveness of compensation grouting can be evaluated by the amount of soil heave obtained ~compensation effect! for a Lecturer, City Univ. of Hong Kong, Dept. of Building and Construction, Hong Kong. Senior Lecturer, Univ. of Cambridge, Engineering Dept., Cambridge CB2 1PZ, UK. E-mail: [email protected] Senior Geotechnical Engineer, STV Incorporated, New York. Professor, Univ. of Cambridge, Dept. of Engineering, Cambridge CB2 1PZ, UK. Professor, Chiba Institute of Technology, Dept. of Civil Engineering, Chiba, Japan. Note. Discussion open until September 1, 2003. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on December 26, 2001; approved on May 22, 2002. This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering, Vol. 129, No. 3, March 1, 2003. ©ASCE, ISSN 1090-0241/ 2003/3-254–262/$18.00. Introduction In recent years, compensation grouting has become a popular method for settlement control during underground construction. The basic principle is that grout is injected in the zone between underground openings and building foundations to compensate for the ground loss and stress relief caused by the underground excavation. Grout injection is often undertaken simultaneously with construction in response to detailed observations so that settlement and distortions are limited to specified amounts. The use of real-time monitoring of settlement, such as by electrolevels attached to buildings, is the key to success for an effective compensation grouting operation ~Mair and Hight 1994; Buchet et al. 1999; La Fonta 1999; Soga et al. 1999!. In many cases, the information gathered from a real-time monitoring system has forced changes to the original grouting plan. Successful applications of compensation grouting to tunneling are reported for the London Underground Jubilee Line extension project ~Harris et al. 1996; Osborne et al. 1997; Harris et al. 1999!, the Viennese subway project ~Pototschnik 1992!, the Saint Clair River tunnel project near the USA-Canada border ~Dramer et al. 1994; Droff et al. 1995!, the London Dockland extension EERING © ASCE / MARCH 2003 Table 1. Case Studies of Compensation Grouting Project Depth ~m! Soil type Tunneling method Grouting method Grout type Compensation efficiency Results