Before any new tunnels, tracks or stations can be constructed, it is vital that the Crossrail engineers understand the condition of the ground along the route so that suitable designs and construction methods can be employed.

To provide this information a dedicated team of Crossrail geologists and engineers, supported by external consultants Geotechnical Consulting Group (GCG), Norwest Holst Soil Engineering and Soil Mechanics, are working to analyse the ground along the entire length of the proposed Crossrail route.

The majority of these investigations consist of carefully drilling boreholes at regular intervals into the ground beneath London – even under the Thames.

Using two types of boring rig – light cable percussion rigs to start the boring and, in the case of the deepest holes, rotary rigs - the boreholes can reach down as far as 60 metres beneath ground level.

Once the boreholes have been drilled, soil samples are transported to the Crossrail laboratories for analysis.

To date, a diverse range of geology has been encountered on site investigations, ranging from deposits laid down between 10,000 to 65 million years ago.

These include River Terrace Deposits, Alluvium, London Clay and Chalk created by a variety of environments such as rivers, estuaries and shallow seas.

In addition to boreholes, the Crossrail team is also using geophysics to study the ground conditions. This non-intrusive technique includes using acoustic reflections to pick up underground structures and layering.

Once they have been processed and analysed, the results of these investigations are relayed to Crossrail's teams of tunnel engineers and surface structure designers to inform their plans for the construction of the Crossrail infrastructure.

The geological findings affect a range of decisions, including:

 The design of the boring machines that will be used to dig each tunnel
 The design of tunnel linings
 Any measures that are required to minimise the above-ground impact of tunnelling

The mission for Crossrail's engineers is to develop tunnelling and construction techniques to overcome the challenges presented by the ground conditions.

The methods they devise to overcome these obstacles will put Crossrail at the forefront of engineering, and geotechnical design.