Project

This project was a site investigation carried out 154 steps  (about 30m with no lift!) beneath London in an underground tunnel.

The work was carried out in partnership with one of the underground operators as part of their tunnel assessment programme.  Lankelma is experienced in carrying out CPTs in basements of buildings beneath road level but this particular job had a small twist!

The twist was that the CPTs were not required to be taken vertically down as usual but at a nearly horizontal incline through the sidewall of an underground tunnel.  This was an interesting challenge and many novel and exciting ideas were drawn up.


Solution

The chosen solution was the most simple and involved attaching Lankelma’s special ‘basement’ set of CPT rams (UK9) to the tunnel wall lining (UK10 was developed following this site investigation).  Simple, but not easy.  Many different constraints were put on the testing due to the special working environment.

A core in the tunnel wall had to be drilled to allow the cone to penetrate.  Once it was drilled the rams were attached to the tunnel wall in line with the hole.

As part of the development program different types of testing and sampling were carried to in order to assess how they behave in such an environment so as to work out the best suited techniques.

Sampling

Two different sampling techniques were carried out, the first being MOSTAP soil sampling. This is the standard system used on land with ‘normal’ CPT rigs.  It gives you 65mm diameter by 1m long samples. Two meters were taken with this system.

Secondly, a thin wall shelby sampler was pushed in.  This technique is commonly used on offshore CPT systems.  Again, two samples were taken at the same location giving samples of 65 mm and 1m in length.

Cone penetration tests

After a successful 100% recovery from all four of the samples, the rig was moved to a new position to carry out the testing.  Again two different methods were used, the first being the seismic piezocone.  This was set up with two seismic modules each with three geophones inside; two to measure ‘S’ waves and one to measure ‘P’ waves.  In front of these modules was a piezocone measuring the excess porewater pressure in the shoulder (U2) position.

This was pushed into a depth of 5m taking seismic measurements every half  metre.  The seismic results were not so good, but this was expected due to the nature the of the source.  The piezocone (CPTU) results were very good, showing the expected results.

The second type of testing that was carried out was a pressuremeter cone.  This was again pushed in using the CPT rams which had been moved to a new position.  Three tests were carried out down the same hole.  This gave excellent results allowing us to gain small strain stiffness and many other parameters.

This site investigation shows how versatile CPT’s are and how they can be adapted to any location and any situation.  It also shows that by working together with the client at an early stage, cost effective solutions can be arrived at.