The excavation of deep foundation pit (1.34 m from the
top of the tunnel) in water-rich round gravel stratum often
brings risks of foundation rebound, stress release, tunnel uplift, pipe
cracking, structural leakage, etc., for the
shield tunnel below. To solve these problems, the rebound amount at the bottom
of foundation pit and tunnel foundation is calculated theoretically. As the
foundation rebound amount caused by foundation pit exvacation is offset by the
settlement caused by loaded pressure, the loading weight per running meter
inside tunnel is calculated. And then a 3D numerical analysis model using MIDAS
GTS finite element software is built and three working conditions, i.e., the method without any control measures (condition 1), that with
bench pile foundation (condition 2), and that with bench pile foundation and
loading pressure inside tunnel (condition 3), are simulated respectively. The
results show that: (1)In conditions 1 and 2, the maximum uplifts are 16.8 mm
and 12.5 mm, which is close to the theoretical calculation result (14.95 mm).
(2)In working condition 3, the average uplift at tunnel bottom is 4.7 mm, which
is very close to the structural difference after theoretical calculation (3
mm). The method in working condition 3
is adopted in practical engineering, the vertical displacement of tunnel vault
and invert is 8.46 mm and 3.16 mm respectively, and after bench pile
constructed, the uplift of tunnel vault and invert is 7.64 mm and 2.56 mm
respectively, the differential settlement of structure is 2.96 mm, which can
meet the requirements of standard. It is concluded that: (1)Internal loading is
the best way to control uplift. (2)After the foundation pit excavation, the
upper part of the tunnel is free from load, its internal load and extrusion on
both sides cause the tunnel to become a "vertical ellipse" . (3)Bench
pile foundation has limited isolation effect, but if it rides cross the upper
part of shield tunnel, the disturbance from upper structure construction and
highspeed railway operation can be effectively
reduced.
DU Jianqiang, ZHU Zhaorong, WU Honggang
.
Influence of Deep Foundation Pit Construction on Near Subway
Shield Tunnel and Its Protection Technology
[J]. Tunnel Construction, 2021
, 41(S2)
: 236
-245
.
DOI: 10.3973/j.issn.2096-4498.2021.S2.030
