Compensation grout for soil stabilisation on London's Crossrail project

Crossrail, Europe’s largest construction and tunnelling project was a uniquely challenging project involving constructing around 26 miles of tunnels under central London and building ten new stations connecting to existing rail infrastructure. Many of these tunnels would run below London’s existing critical infrastructure and some of the highest value commercial and residential property in Europe. It was essential that the ground above was not disturbed. In two particularly challenging sections at Finsbury Circus and the Bond Street tube upgrade a sophisticated approach was required to monitor ground movement and stabilise by injecting a compensation grout at precise locations to compensate for this movement.

Our Solution

The technical services team at Pozament was asked to formulate a bespoke compensation grout to meet the unique challenges of the project. The grout would need to be highly fluid enabling it to be pumped over long distances through access tunnels and the pre-installed network of grouting tubes and to infiltrate voids in the ground. It needed to meet pre-defined minimum strength requirements to provide structural stability. Since the grout would be placed under pressure, it would need to resist pressure filtration and segregation while pressure was applied. It would also need to stay fluid in the lines for up to 45 minutes without solidifying or causing blockages in the pumps or pipes.

Bespoke Mix Design

The bespoke grout was based on Pozament’s proven GM grout range of cementitious grouts designed for void filling, piling and ground remediation. They have excellent pumpable qualities, a low heat of hydration a minimal plastic shrinkage, along with high resistance to sulphates. It would be further enhanced with a flow modifier to ensure ease of mixing and placing. Extensive testing was carried out during the design and development of the grout to make sure that it met the stringent performance requirements and would work successfully on site.

Project Implementation

Given the critical nature of the project Pozament’s Technical Services team worked closely with the contractor to ensure that the project was implemented successfully and to help answer any questions or manage any issues that arose. The grout was mixed above ground and pumped into the access shaft, down the tunnels into injection points where required, based on seismographic readings on the buildings. The specific properties of this bespoke grout allowed it to be pumped rapidly through gallery tunnels in response to signals from movement sensors and provided a jacking action wherever it was needed.

Results and Benefits

The special compensation grout formulation supplied by Pozament was a critical part of the settlement mitigation system used on the Crossrail project. It allowed tunnelling work to continue underground without undermining existing buildings or infrastructure assets and causing damage through vibration or movement. This formed a key element of risk management on this uniquely challenging infrastructure project. It also provided the client, main contractor and key stakeholders with the necessary reassurance that tunnelling work could be completed without risk of damage to existing buildings or infrastructure assets.