Context

Koster was contracted to carry out a trial on a section of the Metropolitan Line on the London Underground. The brief was to provide an elastic, permanent repair solution which would not cause damage to the structure nor the trains passing through it.

The construction of the tunnel is multiple layers of brickwork with an assumed tar or bitumen waterproofing layer externally. There was evidence of water seeping from the brickwork, stalactites, heavy salt and soot crust formations throughout the tunnel and a previous installation of injection packers which lined the crown.

Koster sought to offer a robust system that would easily accommodate the stresses associated with tunnel waterproofing whilst being sympathetic to the aging construction.

Work undertaken

First stage of the trial

A close examination of the tunnel was undertaken. The brickwork at the tunnel's crown showed significant signs of water seepage.These areas of active leakage were initially sealed using Koster Waterstop and KD2 Blitz. Both products immediately prevented the passage of pressurised water.

Second phase of the trial

Once the area was free from running water, the borehole positions for Koster Superpackers were measured. Koster Superpackers are small injection lances which have been specially designed to give a superior grip (within the boreholes) whilst delivering a constant flow of grouting resin. Identifying the correct injection packer placement is an important process. Koster specified the injection ports to be placed at 400mm centres in a grid pattern, with a single packer positioned in the centre.

Boreholes were drilled to a predetermined depth which would facilitate the packers. Koster Superpackers were then inserted and the area was ready for the Koster G4 Gel injection, an
acrylic-based resin which has a similar initial viscosity to water). The gel fills the same minute tracks and veins that water follows when infiltrating the structure and cures to form an elastic, permanent gel. It is then able to move and flex within the brickwork, protecting against future water ingress and damage.

Following the injection phase, the Koster Superpackers were removed and the boreholes filled using Koster KB Fix 5 fastset mortar.

Outcome

An elastic seal within the structure had been achieved without compromising the tunnel lining. Within 15 minutes of the Koster G4 Gel being injected all water seepage from the brickwork slowed and ceased. Koster continued to observe the trial area, noticing that it was drying out. The water seepage stopping meant that the resin had “gelled” within the tunnel lining and sealed off all routes which had previously been allowing the passage of water.

The following night (during Koster’s follow-up visit) the trial area was shown to be completely dry whilst the untreated brickwork surrounding the trial area was still wet with signs of seeping water.