Numerical Assessment on the Influence of Various Factors for Subsidence at the Intersection of Expwy 78 and High Speed Rail of Taiwan

Choshui River alluvial fan-delta (CRAFD) has been and still is the single-largest subsiding area in Taiwan. Some key infrastructures, including Taiwan High Speed Rail (THSR) and Expressway 78 (Expwy78) that come across this area, are suffering serious problems by the subsidence. Due to complexity of the issue, causes of the subsidence and their influences are not easily identified and quantified. This paper tends to disclose the above puzzles through a numerical approach that would allow separate applications of various factors and examination on their individual effects. The intersection of THSR and Expwy 78 is our primary concern since the subsidence at this location has become most serious along the entire route of THSR and the induced differential settlements are threatening the safety of the transportation artery. An 8-year subsidence monitoring data at the study site is used for calibration of material data adopted in the subsequent analyses. Results indicate the computed subsidence is 147.0 cm, or a rate of 10.53 cm/year, for a period approximately starting with the Expwy78 construction (01/1998–12/2011) and covering complete ranges of construction of Expwy78 and THSR of the site. Contributions due to various factors would be the greatest by previous overpumping (55.2%), followed by soil creeping (14.1%), groundwater fluctuations (11.8%), Expwy78 loading (11.6%), and the least by THSR loading (7.3%). Assessments also indicate around 1/5 and 4/5 of the total subsidence, respectively, occurred as the compression in soils with depths <70 m and >70 m, where 70 m is the average installation depth of THSR piles. Compression at shallower depths would be the greatest by Expwy 78 loading, and thus may trigger a major part of negative skin frictions and discount bearing capacity of the piles. Compression of deeper soils would be caused primarily by the previous overpumping, and would hence lead to significant settlements and distortions of vertical alignment of THSR structures.