DOUBLE TUBE GROUTING TO STOP WATER IN SALINE-ALKALI AREA

Sealing Technology For Serious Leakage Of Offshore Pile Cofferdam

1. Project Background

When constructing the foundation of a sea-crossing bridge, double-layer steel plate pile sandwich filled with low-strength backfill material (CLSM) was used as a temporary cofferdam. The piling operation conditions at sea are severe, so the plate-pile connection fastener and tenon lock could not be completely closed. 

As a result, multiple permeable paths and gaps were generated when CLSM was poured in the water, and the seawater inside the cofferdam could not be drained for subsequent site preparation and foundation construction work. 

In order to ensure the quality of the project and avoid delays in the construction period, the contractor commissioned our company to propose and implement a plan to plug the water leakage.

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2. Solution

Because the project site was located in the sea, in addition to the influence of tidal water level and pressure difference, the salt and alkalinity of the seabed formation and the influence of seawater on the grouting material will cause great challenges to plugging the water leakage.

After a detailed analysis of the geological conditions in the area, the leakage of plate piles and the CLSM backfill records by our company’s professional engineers, it was determined. 

A three-level planned project including blocking the permeable path with salt-alkali-resistant instant-setting chemical grout, filling holes with salt-alkali-resistant cement chemical composite grout to prevent leakage and to strengthen, and post-treatment of PC layer micro-foaming polyurethane grout.

After evaluation by the contractor party’s experts, the plan was deemed to meet the goals of safe, reasonable, economical, and rapid operation, and a contract was signed for execution.

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3. Works Design

Due to the geological conditions of the seabed in that area, the upper part being a mid-fine sand layer, and the lower part was a weathered rock layer with debris and local high plasticity silt, the layout of piles was designed in a fan-shaped arrangement, and it was not easy to set up the operating platform.

After the company’s on-site engineers studied and judged the works environmental conditions, they decided to use a hoisting method to erect a cantilever working platform, and use the double-tube method to carry out the work:

(1) Investigation of the weak surface of CLSM by vertical drilling.

(2) Instant-setting SSA chemical grout injection.

(3) Quick-setting/slow-setting FLW cement chemical compound grout injection.

The depth of each operation was about 12m~15m, and had to reach more than 2m into weathered rock under the seabed.

Then, after the serious plate pile water leakage eliminated, and after the PC layer in the cofferdam is poured, again use pneumatic medium flow micro-foaming PU resin grouting. Then carry out small leak points plugging work to achieve the working conditions requirements such as laying out, rebar tying and concrete pouring inside the cofferdam.

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4. Works Process

Construction work on the sea is difficult and risky. Before construction every morning, the company’s staff had to conduct rigorous construction safety drills and work assignments on the day to ensure that the tasks are successfully completed under safe conditions. 

Large-scale equipment such as for mixing, pumping, monitoring, and the generator of the dual-pipe injection method was arranged on offshore trusses as much as possible.

Due to the limited space of the construction platform, the electric drilling machine was modified with a gantry truss to replace the traditional tripod for grouting pipe hoisting. 

In addition, according to the salt-alkali characteristics of the on-site formation, SSA chemical grout and FLW cement chemical grout were deployed to meet the designed gelation and consolidation time. 

After the double-pipe grouting process was completed according to the design, the dewatering in the cofferdam was inspected, and then localized supplementary grouting was carried out according to the actual water inflow.

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5. Results

With effective control of the grout ratio and pouring position by experienced on-site engineers, the conditions inside the cofferdam after dewatering had reached the requirements for subsequent works. 

After the contracting party performed the ground preparation and pouring of the PC layer, and the remaining few pile connection surfaces and the concrete interface seepage points were sealed with PU resin, the serious water leakage on the site was completely eliminated.

The contracting party’s acceptance exceeded the contract requirements. The job was handed over for construction of the follow-up foundation platform. The serious leakage into the offshore pile cofferdam was successfully sealed.

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