Application of the Upper-Bench CD Method in Super Large-Span and Shallow Tunnel: A Case Study of Letuan Tunnel

Table 1

Field test items.

Serial number	Test item	Instrument	Specification	Test frequency
1	Crown settlement	Total station	Accuracy: 1 mm	1 day–30 days	1 time/day
				30 days later	2-3 times/week
2	Horizontal convergence	Total station	Accuracy: 1 mm	1 day–30 days	1 time/day
				30 days later	2-3 times/week
3	Shotcrete stress	Concrete strain gauge	C25	1 time/day
4	Stress of steel frame	Steel string surface strain gauge	2000 με	1 time/day
5	Secondary lining concrete stress	Concrete strain gauge	C30	1 time/day
6	Secondary lining reinforcement stress	Steel stress meter	HRB400	1 time/day