Keywords: pipejacking, pipe-soil interaction, finite element analysis, physical modeling, trenchless construction, underground excavation, geotechnical engineering.

Due to the rapid urban development, in recent years, there are more and more trenchless pipejacking constructions in Taiwan area. Different difficulties were encountered during those pipejacking constructions, however, due to the insufficiency of theoretical studies, most of the countermeasures are still on the empirical side. This study first reviewed the past pipejacking works in order to figure out the common difficulties during construction. Then, for the analysis, both physical and numerical models were adopted to investigate the behavior of pipe-soil interaction during pipejacking. After calibrating with the physical modeling results, the finite element software ABAQUS [1] was used to study the behavior, including surface subsidence, failure mechanism, and pipe-soil interaction of pipejacking excavations.

Two typical geomaterials, i.e., the loose and dense sandy soils, are used for both the physical and numerical modeling. And the material properties were obtained through laboratory tests. The analysis results reveal that the driving force at the mining face is critical for the behavior of pipe-soil interaction. Surface subsidence is mainly due to the lack of driving force, however, the excessive driving force could cause the unfavorable surface heaving (blow out) problem. The shape and range of surface subsidence due to underground tunneling is a three-dimensional problem, therefore, it is essential to have a three-dimensional analysis. Based on specific assumptions, three-dimensional theoretical analytical models were derived and presented by Leca [2], Anagoustou [3] and etc., however, their models do not consider the complicated pipe-soil interaction and are limited to collapse from surface to an open tunneling face.

In this study, the three-dimensional finite element analysis was applied after calibrated and compared with the three-dimensional physical modeling results and the above-mentioned theoretical analytical models. The comparisons show that different models fairly consistently predict the surface subsiding and heaving behavior but not the shape of surface subsidence. Through a series of analyses, a characteristic curve for the driving force is summarized. This curve reveals an upper bound, above which the blow up effect will occur, and a lower bound, below which surface subsidence will occur. Beside the important driving force itself, it shows that the depth of the pipe is critical to determine a proper driving force to stabilize the tunneling face. The friction between the pipe and soil does affect the required driving force but not necessary to be significant. However, it is important for the stress state near pipe surface. It also suggests more attentions should be given to the jacking process and the surface loading, especially for pipejacking with shallow overburden.

1

Hibbitt, Karlsson & Sorensen Inc., "ABAQUS Version 6.3 User's Manual", Volume 1-3, U.S., 1988.

2

E. Leca, L. Dormieux, "Upper and Lower Bound Solutions for the Face Stability of Shallow Circular Tunnels in Frictional Material", Geotechnique, 40:4, 31-41, 1990.

3

G. Anagnostou, K. Kovári, "Face Stability Conditions with Earth Pressure Balanced Shields", Tunnelling and Underground Space Technology, 11:2, 165-173, 1996. doi:10.1016/0886-7798(96)00017-X

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