In construction process of large underground caverns, huge quantity computation is involved in feedback analysis of loosened zone of surrounding rock, corresponding estimate values are needed for keeping in step with excavation schedule. At first, elastic modular estimate type of surrounding rock at measure point is deduced from the classic deformation formula of circular tunnel, and an estimation method for correlation parameter is confirmed according to data reading characteristic of multipoint extensometers. Then, after excavation of top layer in main cavern, when loose zone hasn't been measured, initial test of the algorithm is carried out and its feasibility is verified. Furthermore, computation way of displacement pattern is improved. Finally, the method is applied to feedback analysis of main cavern after excavation of next layers, and it is shown that the method's computation precision meets demand of practical engineering. So that, the estimation method with brief computation and reliable estimate results, will be hopefully applied in more other large underground cavern projects in the future.
Loosened zone model is a kind of basic cavern supporting theory (Dong et al. 1994, Meng et al. 2010), in which loosened rock mass around cavern is regarded as a part of supporting structure, can availably reduce supporting cost of cavern. One of important parameter among loosened zone parameters is its depth, which is belong to key parameters for design, such as bolt length and spout layer thickness. Its concrete numerical value should be confirmed by field test, with familiar method such as ultrasound, seismic wave and acoustic emission as well as comprehensive analysis (Peng et al. 2008, Yang et al. 2012, Toshio et al. 2003, Fan et al. 2011, Zhou et al. 2011, Liu et al. 2006). Embedded multipoint extensometers are installed in advance of excavation, interior distortion in surround rock of cavern can be then perfectly measured, so loosened extent and degree of surround rock can be reflected in principle at last. But, for limitation on measure point arrange density and distribution of space, formerly corresponding method is auxiliary (Toshio et al. 2003) or for qualitative analysis (Liu et al. 2006). It is difficult to give satisfactory estimate value individually because of limited precision and reliability.