Development and experiment study on seawater-based slurry for subsea tunnels in slurry shield tunneling

In the construction of subsea tunnels by slurry shield tunneling method, the salt-resisting slurry which can satisfy both environmental and practical application is required. The intrusion of seawater can significantly decrease the physical stability of freshwater-based slurry. In this study, the HPMC, Na2CO3 and TFLS were selected as the additives to develop seawater-based Na-bentonite slurry. The effect of seawater content on the deterioration of the slurry colloid ratio and funnel viscosity before and after the addition of different additive contents was investigated by slurry mixing ratio and particle size analysis. In addition, the degradation mechanism of Nabentonite slurry and the anti-deterioration mechanism of slurry with different additives in seawater environment were revealed respectively by permeability tests and SEM tests. The test results indicated that the Nabentonite slurry significantly segregate when the seawater content is more than 30 %. To control the viscosity of slurry in the reasonable range, Na2CO3 is determined as the hydrolytic agent of HPMC, which can turn the molecular chain of HPMC gradually stretched from tight curl. And the optimum amount of Na2CO3 is 0.03 %. TFLS was validated as a stable dispersant for Na-bentonite slurry when the content is 3 %. The hydrolysates -SO3H, Fe3+ and Ti4+ can form complexes with the cations in seawater and bentonites respectively. The interaction between the complexes ensures that the TFLS slurry can maintain an appropriate colloidal ratio. Both the results of permeability tests and SEM tests showed that the salt resistance ability and the application performance of HPMC is superior to HPMC+Na2CO3, and superior to TFLS. The research results and findings could provide references for the preparation of slurry for the subsea slurry shield tunneling.