Modification of Peck Formula to Predict Ground Surface Settlement of Twin Tunnels in Low Permeability Soil

Table 2

Monitoring items and indexes.

No.	Item	Instrument	Accuracy (mm)	Frequency
1	Surface settlement	Levels	0.01	L > 5 S, 1 time/week
				L < 5 S, 1 time/2 days
				L < 2 S, 1∼2 times/day
2	Vault settlement	Levels steel rulers	0.01	After excavation
				1∼15 days, 1∼2 times/day
				15∼30 days, 1 time/2 days
				1∼3 months, 1∼2 times/week after 3 months, 1∼3 times/month

Note. L = interval of excavation face and measurement section, S = tunnel span.