Controlling Water for Cross Passage Excavation

Doha Metro project in 2015

During construction of the Doha Metro project in 2015, significant uncontrolled water ingress was encountered during cross passage excavation.

Cross passage excavation in segmentally lined tunnel construction will continue to challenge our engineering capabilities, especially in unpredictable geology. Pre-excavation grouting (PEG) into the rock formation or soils between the running tunnels using cement based materials such as microfine or ultrafine is common practice, i.e. to reduce rock hydraulic conductivity or permeability of soils. However, where PEG has not been employed and unexpected ground water is encountered, specialist injection materials are often the most suited systems to effectively control the water ingress.

Phase 1 of the Doha Metro project involved the use of EPB TBMs to excavate 55 km of 7.02-m OD twin-bore tunnels. The project is divided into four separate lines – Red Line North, Red Line South, Green Line and Gold Line, with the tunnel excavation operations completed in 2016.

In November 2015, a section of the tunnel lining was removed to allow excavation of the cross passage. Significant uncontrolled water ingress was encountered with an estimated water volume of 9 cubic meters per minute. The water entering the cross passage position originated from the water bearing Midra shale and Rus formation boundary. Apart from the issue of controlling and managing the water, this unexpected event had a knock-on effect with logistics and tunnel product supply, causing delays to the tunnel construction. It was therefore imperative that a fast and effective solution was employed to quickly control the water ingress to a manageable level, allowing tunnel construction to resume. Taking into account the levels of water observed, a fast-reactive chemical polyurethane in conjunction with a engineered delivery approach was considered the most optimum approach.

Phase 1 of the Doha Metro project

The type of resin chosen for any given project is always subject to many variables such as the volume of water ingress, hydrostatic pressure, water/resin temperature and resin reaction time to name a few. Although single component resin foams are effective in dealing with relatively high levels of water, on this occasion, a two-component system was chosen. Two component resins offer improved cell structure strength upon final foam development and are predominantly closed cell in nature. This provides improved resistance to hydrostatic pressure and superior water tightness. They also tend to be hydrophobic in nature, which avoids dilution with water in the injection zone, which can result in a weak foam cell structure. For the Doha Metro project, Normet‘s TamPur 125 was chosen as the most suitable resin system to control the water ingress. TamPur 125 is 1:1 ratio by volume consisting of MDI and Polyol with the accelerator and blowing agent incorporated into the polyol side of the resin. This factory produced system ensures full and controlled reaction after mixing.

RELATED: Ground Freezing for Cross Passage Construction

The injection process to the cross passage entrance was divided into four engineered steps. Step 1 was to systematically drill 50 mm diameter grout delivery holes through the concrete segmental lining up from the cross passage position. A Brokk drilling machine was used to drill the holes with each grout delivery hole positioned five rings apart. Each drill hole extended through into the water bearing rock formation at depths between 500 mm and 750 mm. Two additional drill holes where formed in the crown at 1 o’clock position, directly above the cross passage entrance. High pressure mechanical grout packers were then installed in the drill holes, with the ball valve assembly left open to allow water to flow through each packer unit. This had the effect of reducing the water volume at the cross passage position allowing for a more controlled grouting process.

Step 2 was to understand the direction of water flow into the cross passage entrance. This was achieved by systematically injecting a water-based dye through each packer location and observing the flow characteristics to the cross passage position. The grouting sequence and position was then determined from the results obtained (Step 3).

cross passage excavation

Following further preparation, injection of the TamPur 125 was undertaken (Step 4) using a Normet TP 4, a high-pressure, twin-piston pneumatic pump capable of delivering 36 liters of resin per minute. The pump delivery line mixing head was connected to the first positioned grout packer with all remaining adjacent packer valves left open. TamPur 125 was injected at a rate of approximately 15 liters per minute until resin flow was evident at the adjacent packer position. The injection was then transferred to the next grout packer position and injection resumed. This sequence continued until reacted resin was observed at the cross passage entrance. As the injection proceeded, it was clear that the water pressure and water volume at the cross passage entrance was being altered due to the effects of the resins volumetric change (foaming). This clearly indicated the water path had been intercepted. Injection continued systematically to all remaining injection packer positions until the water ingress was controlled. In total, 2,430 kg of TamPur 125 resin was successfully injected.

The injection proved extremely effective at controlling the water ingress into the cross passage entrance. As a result, time delays were minimized and an effective solution for similar future situations identified.

Lawrence Halls is Head of Global Marketing/Global Manager Ground Engineering for Normet. He is based in the United Kingdom.

Comments are closed here.