Injected ... a silicate gel solution to stabilise the soil for tunnelling.
Gel plays key role in soil stabilisation
01 October 2003
German civil engineering firm Bauer Spezialtiefbau recently used an accelerator additive in a silicate gel solution for soil stablisation prior to the tunnelling of 15 pedestrian subways in Abu Dhabi, UAE.
The contract specified that the work was to be carried out without slowing or holding up traffic on the street above.
Bauer engineers used an injectable silicate gel solution containing DuPont DBE dibasic esters, a method that is particularly suited for consolidating fine sand (grains up to 250 microns) of the kind common in the city. Its main advantage is that DBE is environmentally friendly because it is biologically degradable, according to its manufacturer Du Pont De Nemours.
Trials showed that injecting a combination of silicate gel and fine cement produced the best results, Du Pont says. This mixture consolidates disparate types of soil well: the fine cement consolidates the medium-grain sand (grains from 250 microns to 1 mm), while silicate gel is the material of choice for fine-grain (up to 250 microns) layers, it adds.
To introduce the consolidating liquid, Bauer's engineers drove several injection tubes spaced at 65 cm into the soil horizontally, between the road and the tunnel roof. Each tube has delivery valves spaced at about 75 cm intervals. First, the engineers injected 180 to 240 litres of fine cement suspension into the ground through every second valve or tube. Next, they injected 50 to 150 litres of silicate gel through the alternate valves or tubes. After about three days, the soil had hardened sufficiently to allow the tunnel to be driven safely.
The silicate gel mixture used for consolidating the fine sand layers consists of silicate, water and sodium silicate additive M -hardener for grouting, an accelerator based on DuPont DBE. This composition penetrates well into the layers of fine-grain sand, the company says. The silicate gel binds the grains of sand into a solid mass (firmness: 2 N/sq mm) without altering the natural structure of the soil. The gel time can be set at between 35 and 50 minutes.
