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Deep Deep Down

Recently, Lankelma CPT Ltd has again demonstrated its ingenuity and versatility of the CPT by carrying out a very ambitious development program. The work was carried out 154 steps down (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 program. Although carrying out CPT’s in basements of buildings beneath road level is something that Lankelma is very experienced in, this particular job had a small twist!

That twist was that the CPT’s 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; but nevertheless one that Lankelma raised to- drawing up many different novel and exciting ideas.

The chosen solution was the most simple which involved attaching Lankelma’s special ‘Basement’ set of CPT rams to the tunnel wall lining. Simple. But not easy. Many different constraints were put on the testing due to the special environment that it was working in.

Firstly, a core in the tunnel wall needed to be drilled to allow for 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.

Firstly, 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 sample of 65 mm and 1m in length.

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 a meter. The seismic results were not so good, but this was expected due to the nature the of the source. The PCPT results were very good, showing the expected results.

The second type of testing that was carried out was a push-in Pressuremeter. 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 example shows how versatile CPT’s are, 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 whilst at the same time pushing back the boundaries of innovation.

Dr. Wouter Broere has written a dissertation on horizontal cpt. Click here to read his article.

Restricted Access Contracts

During December 2003, Lankelma CPT Ltd mobilised its ‘Mini-Crawler’ restricted access unit to complete three jobs that required its specialist capabilities.

1) Sheltered Housing

The Mini-Crawler went on to perform tests at an extremely difficult access location for Mouchel Parkman at Gale Farm, Sheltered Housing complex near York.

The rig was required to manoeuvre through a 1.2m wide passageway to reach a central courtyard where it performed five tests (using its ground anchors to produce a 20 Tonne reaction force) to a depth of ≈18.0m. Piezometer standpipes were pushed to a depth of 7.0 and 8.0 metres respectively to allow measurement of ground water levels.

2) Flood Defences

At the request of Norwest Holst, The rig performed tests to a maximum requested depth of ≈15.0m on a flood defence embankment at Goole on Humberside.

Scaffold platforms had been built to allow testing ‘over water’ from a stable, level platform. The reduced weight of the rig was ideal for access to this narrow platform and performance was outstanding giving maximum penetration with minimum disruption to other site works.

3) Lock Farm

The third job reverted back to flood defences, this time at the request of Allied Exploration & Geotechnics Ltd at Lock Farm near Pontefract, at an embankment.

Access to this embankment was narrow but well within the capabilities of the rig. Lankelma produced ≈45.0m of quality data, identifying various units along the profile of the embankment that correlated well with samples performed by others.

Basement Work

Lankelma CPT has recently had to carry out several basement ground investigations within the London area. With our specialised basement equipment we are able to work in headroom as low as 1.80 metres (6 foot).

The basement rig can get its reaction from concrete floors, walls or ceilings. On a job in January 2004 depths of up to 20 metres were reached through London gravels down into London clay. Our electrical rig keeps the sound down to a minimum and the basement tidy.

green field sites brown field sites motorways railways waterways and flood defences marine

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