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Maritime victory

SITE INVESTIGATION: Pleasure craft traffic ruled out the use of jack-up platforms on a Devon dock, so Lankelma Piling and Engineering had to find another way of investigating ground conditions.

Exmouth Docks in Devon have changed dramatically in recent years, from a commercial port to a mainly recreational one.The dock area is changing too, as developer Rockeagle Homes' transforms it into prestigious housing.

Geotechnical consultant and site investigation firm Yeandle Geotechnical has been involved in various phases of development since 1997, carrying out a series of onshore investigations.

These have typically involved CPTs supplemented by cable percussion drilling and trial pitting because of the presence of deep estuarine alluvial deposits in the dock area.

Most properties have been piled, although ground conditions in some areas have allowed reinforced shallow foundations to be used, after analysis and prediction of foundation performance.

The latest phase of investigation work was for homes being built close to the dock entrance. A number of landbased CPTs and a few cable percussion boreholes were put down to refusal in the underlying Mercia Mudstone Group rock.

However, consultant Sands Chartered Engineers also needed to know the ground conditions on the foreshore to design new sea defences. Information was needed within the dock and from around the dock entrance for the design of a replacement dock wall.

Investigations for these areas posed particular problems, says Lankelma Piling and Engineering cone testing manager Andy Barwise.

'Jack-up platforms could not be used because the docks are in continual use by pleasure craft, ' he explains.

Lankelma's solution was to use a specially designed frame, cantilevered up to 1.5m over the dock wall, to allow cone testing of the sediments.

The frame was attached to the quayside using expanding bolts and cross members welded to the sheet pile wall to act as a reaction force for the testing. This meant 20t of thrust could be used, enabling tests carried out on the frame to be compared with those carried out by a CPT truck on land, says Barwise.

The cantilever system was designed to allow thick wall casing to be used to provide lateral support to the rods through the air and water. Barwise explains that at exposed locations water currents were up to 9km/h, so it was critical to plan the testing to coincide with slack water, to ensure the casing could be set vertically.

On average a 15t thrust was used to advance the cone to refusal in very dense gravels overlying the Mercia Mudstone.Testing was carried out at 13 locations along the wall with depths from dock to mudline of up to 15m and test depths of up to 25m.

After each test was finished, data was emailed back to Lankelma's offices for processing and interpretation, with test reports produced the same day.

The three-week investigation, worth £40,000, established the soil/rock profile and soil parameters to allow the new dock wall to be designed with a 50-year life.

Most of Lankelma's work in the UK is basic sleeve and friction and piezocone testing (for porewater pressure measurements) but it is also able to carry out near continuous sampling using the Mostap system, says managing director Eric Zon.

This produces a 66mm diameter Class 1 sample for testing.The firm also has seismic cones and can carry out Delft sampling, conductivity testing and water and gas testing, he adds.

Lankelma's trucks act like mobile offices, explains Barwise. Their electronic links mean that unless a client insists on having a full-time engineer on site, results can be sent directly by email or fax to the engineer's office at the end of each test.

Instead of waiting three to four weeks for a report, the client can have one the next day, Barwise says.This allows rapid interpretation of the results - and tailoring of the site investigation if anything unusual crops up.Output is on Key Systems' Holebase package, which supplies data in AGS format.

The firm is developing new techniques, including a smaller 20mm 2cone used with a mini-rig for embankment investigations. This is more convenient than cable tool investigations which need scaffolding, Barwise explains, and more information can be gained from the cone test than from window sampling.

Zon says a video cone is now available that should prove useful in environmental work, to see when the pollution starts in the soil profile.

While he admits that CPT is not the perfect tool for every site investigation in the UK - 'there isn't one, the geology is too varied'- some areas are suitable but do not use the system very often; East Anglia for example.

Zon says one of the main reasons for this is design procedures. In the Netherlands, where CPT has been used for more than 40 years in the country's more uniform soils, the method forms the basis for all site investigations, with a minimum of one test every 25m required by law.

'In Holland pile design is carried out directly from the cone test data, 'he says, but in the UK an empirical correlation is made from the CPT strength data to give a range of N values - 'which is never going to be perfect'Inevitably, the lower bound of calculated strength is used, which leads to a conservative design, Zon claims.

As a result, UK investigations rarely just include CPTs.

Often, Zon says, Lankelma is called in to confirm ground conditions following a conventional borehole survey, particularly on piling contracts before work starts 'to prevent mistakes' Even when CPTs form the bulk of an investigation, boreholes are put down close to the test sites to confirm the results. Zon attributes this to the varied ground conditions in the UK.

'We are also called in during piling works, for example on driven piling jobs where piles are reaching set at a shallower or deeper depths than originally planned or if things are not going as well as expected, ' he adds.

Barwise points out that some UK consultants are changing their attitudes. Lankelma has begun an 18month project to develop software that will allow clients to design foundations directly from the cone test data.

Dr Joshua Omer from Glamorgan University will carry out the work, partly funded by a Royal Industrial Society fellowship.

'The main advantages of cone testing is that it is fast, accurate and repeatable to within one or two percent, ' says Zon.'Often there are lots of boreholes confirmed by cone testing.My dream is to have cone tests confirmed by boreholes in the UK.'

Market potential

Dutch geotechnical firm Lankelma Group has been involved in site investigation and foundations since 1896, and is well known for cone penetration testing (CPT) across continental Europe.

While it has worked extensively in the UK and the Republic of Ireland in the past, it only established a permanent local base with the formation of Lankelma Piling and Engineering last year. Its head office is in Iden near Rye in East Sussex.

The firm was set up by managing director Eric Zon, who previously worked at Lankelma's geotechnics division in the Netherlands. He explains that while the parent company owns 50% of the UK firm, the latter operates independently, working exclusively in the UK and the Republic of Ireland.

Despite its name, the bulk of its work is in cone penetration testing (CPT).

'We saw more potential in cone testing and have invested about £400,000 this year in new equipment, ' says Zon. The latest addition to the fleet, which ranges from small mobile units up to cone testing trucks, is a wheeled Scania truck from Dutch manufacturer AP van de Berg. The £170,000 vehicle has tracks that can be lowered to allow it to cross almost any terrain.

This level of investment shows Lankelma is confident of the potential cone testing market in the UK.

Past projects include investigations on the N4 and N6 motorways in the Republic of Ireland.

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