Risk Mitigation and Construction Control for Effective Underwater Recovery of an EPB Shield: A Case Study of the First Metro Tunnel in Tel Aviv

Table 4

Comparison of different methods of shield recovery.

Recovery method	Applicable strata		Application effect	Evaluation
Shield recovery with receiving end reinforcement	Deep mixing cement pile	Silt, sandy soil, silty soil, peat soil	(1) Little soil disturbance; (2) no joint in the whole length of pile with full mixing of cement and soil; (3) good water sealing performance; (4) little environmental pollution, noise, and vibration during construction.	(1) Large site area required; (2) poor reinforcement effect when reinforcement depth is more than 15 m.
	High-pressure rotary jet grouting pile	Silt, sandy soil, silty soil, loess, plain fill	(1) Small footprint; (2) little vibration and noise; (3) potential for environmental pollution.	(1) Good control of position and scope of slurry injection; (2) tight connection between piles.
	Freezing	Water bearing strata	(1) High strength of soil reinforcement; (2) good water sealing performance; (3) no environmental pollution.	(1) Good strength and impermeability of soil is good, making it a relatively safe construction method; (2) produces a frost heaving and melting sinking effect, which has a great influence on the surrounding area; (3) long construction period and high cost.
	Rodin jet pile method	Various strata and complex site environment	(1) Large pile diameter; (2) pile of good quality; (3) small footprint; (4) low environmental impact.	(1) Guaranteed reinforcement quality with large depth; (2) high construction efficiency and low cost.
	Metro jet system method	Various strata and complex site environment	(1) Large pile diameter; (2) pile of good quality; (3) small footprint; (4) low environmental impact.	(1) Complex construction technology and low construction efficiency; (2) high construction cost.
Shield recovery with no receiving end reinforcement	Steel sleeve	Strata with high permeability and water content	(1) Safe construction; (2) short construction period; (3) reusability.	(1) Wide applicability; (2) avoids pipeline relocation; (3) provides environmental protection.
	Underwater recovery	Water bearing strata with high permeability and bearing capacity	(1) Short construction period; (2) environmental protection; (3) small environmental impact.	(1) Suitable for shafts that do not allow for end reinforcement owing to site constraints and that have strata with a degree of self-stabilization.