Evaluation of Cavity Exploration Technology to deal with Ground Subsidence in Urban Area

Ground subsidence in downtown is attributable to various factors such as the impact by excavation, soft ground settlement, deteriorated sewer and poor compaction. Road sinking resulting from deteriorated sewer is mostly by the soil penetrated into the damaged sewer. Thus to prevent ground settlement in downtown, it's necessary to develop the detection technique that will evaluate the mechanism and predict the ground settlement by leaked sewer. GPR exploration is used to detect the cavity under the ground but when the cavity is deep or boundary between cavity and loosened soil is unidentified or complicate utilities and cavity and loosened soil are mixed, it's difficult to detect them. This study, as part of the way to prevent ground settlement by deteriorated sewer, was intended to identify the cause of cavity and soil relaxation and evaluate the cavity exploration technique using GPR exploration method. To simulate the cavity and ground relaxation caused by sewer leak, field test was conducted. 450 mm in diameter and 2,500 mm long concrete hume pipe was used install the sewer for the test and to evaluate the exploration performance by depth, sewer was buried at 1.0 m and 1.5 m depth. After putting the ice seized 1 mx1.0 mx0.3 m on top and at the bottom of the sewer, hose was connected to promote the thawing. To measure the cavity caused by ice thawing, GPR exploration was carried out in a day, 5 days and 15 days. In the result of test, Cavity and ground relaxation caused by sewer leakage were simulated using GPR exploration and the cavity and ground relaxation by sewer leakage were monitored. According to GPR analysis, ice thawing caused ground relaxation and the cavity. Given the reflective surface under the ground over time, cavity appeared to have developed upward. Though ground settlement by cavity was expected, no ground settlement occurred which was attributable to bearing capacity of the well-compacted ground.