In-Situ Monitoring of Ground Subsidence at the Intersection of Expwy 78 and High Speed Rail of Taiwan During 2003–2011

This paper discusses results of a long-term onsite monitoring on ground subsidence and soil compressibility at the intersection of Expressway (Expwy) 78 and the High-Speed Rail of Taiwan (THSR). The intersection area is located on the Chuoswei River Alluvial Fan-Delta (CRAFD), the largest and thickest alluvial deposit to the mid-west of the island. The CRAFD has been subjected to serious subsiding problems for decades because of excess extractions of groundwater for agricultural and industrial usages. The constructions of Expwy 78 and THSR in the late 1990s imposed additional loadings on the soft ground and accelerated the subsidence problem, which was becoming a threat to the safety of THSR. An 8-year onsite monitoring program at the intersection was conducted between 10/2003 and 12/2011. The subsidence and compression of soils were measured through multi-leveled magnetic rings installed in the ground along boreholes of 300-m deep, as well as a GPS station and several level-surveying benchmarks. Results indicate the ground subsidence in the intersection area was 55.7 cm for the entire deposit in the 8-year monitoring period without the loadings of Expwy 78 and THSR. The loadings of Expwy 78 embankment and THSR piers/viaducts would contribute additional subsidence of 9.4 cm and 5.5 cm, respectively, to the ground in the same period. The total subsidence in the 8-year period was 70.6 cm, with an average rate of 8.6 cm/yr. Further analysis of the compression in soils with depth <300 m indicated that the shallower deposit (depth <70 m; Aquifer F1 and Aquitard T1) was least compressible, with a strain rate of <0.01%/yr; while the deeper deposit (depth 220–300 m; Aquifers F3, F4 and Aquitards T3, T4) was most compressible, with a strain rate of 0.03–0.05%/yr. Higher compressive rates in deeper soils suggest the extraction of groundwaters has gone deeper in recent decades.