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AND THEY'RE OFF

SITE INVESTIGATION - Site investigation for Ascot's £180M redevelopment had to identify isolated clay layers.

Ascot Racecourse was founded in 1711 when Queen Anne came across an area of open heath, not far from Windsor Castle, that looked an ideal place for 'horses to gallop at full stretch' In August that year Ascot's first race meeting was run, for Her Majesty's Plate with a prize of 100 guineas.

As the sport blossomed in popularity Her Majesty's Plate moved to June and became the festival known as Royal Ascot. Last year it attracted a record 306,000 people.

To meet this growing demand, Ascot Racecourse is in the process of an ambitious £180M redevelopment programme that will include realignment of the track, construction of underpasses and redevelopment of the grandstands.

Buro Happold, geotechnical consultant on the scheme, appointed Lankelma CPT to carry out six weeks of intensive cone penetrometer testing (CPT) as part of the site investigation.

The new grandstand, which will be located over part of the track and lawns in front of its predecessor, will have a footprint about 400m long and 55m wide. Column loads are expected to vary between 500kN and 8,000kN.

Two vehicular underpasses are required beneath parts of the track. Disruption of the racing season had to be minimised. Programming of the 20-month closure was timed so that only one showpiece Royal Ascot meeting has to be relocated, leaving no room for error.

Studies showed the site is underlain by the Tertiary Bagshot Formatio, consisting of medium dense to very dense silty sand interbedded with occasional bands and layers of sandy clay. This was later confirmed in the first phase of site investigation, which was carried out in autumn 2001 using cable percussive drilling and machine excavated trial pitting. Access restrictions meant only a limited number of boreholes were possible beneath the footprint of the grandstand. The results from the first investigation showed evidence of both shallow and deep clay layers within the sand stratum.

Isolated clay layers beneath either shallow or piled foundations could cause unacceptable differential movements of the sensitive grandstand structure. As both shallow and piled foundations were being considered, detailed explorations were required to assess the presence of both the shallow and deep clay layers. Ideally, Buro Happold needed a method of investigation that gave a continuous ground profile to more than 20m below excavation level.

After looking at options available, Buro Happold opted for CPTs at every proposed column location to minimise the risk of future delays due to unexpected ground conditions.

Further CPT testing was carried out at the location of proposed vehicular underpasses to the east of the site. The main purpose of testing in this area was to investigte ground water levels (see box).

Lankelma was appointed to undertake testing during a six-week intensive period. It completed more than 200 cone penetration tests up to depths of 25m.The use of CPTs proved to be a quick and cost-effective method of site investigation.

Because the investigation was undertaken while the racecourse was still in use, no disturbance to the grounds was allowed. CPT measurements, recorded electronically insitu, remove the need to take samples and eliminate the production of waste spoil material.

Lankelma's 20t capacity Track-Truck was rolled out to undertake the tests.With its ability to use wheels or tracks, access problems and surface disturbance at Ascot were eliminated. To further guard against possible damage to the racetrack and lawns in front of the stand, plywood boards were placed under the tracks of the truck.

Because the underlying geology was composed of very dense sands, the CPT rig was used close to its limits. The full pushing force of up to 20t from the truck and a cone resistance of up to 90MPa was used to achieve scheduled depths of 25m.

The on-board, real time CPT display and preliminary plots were used on site to partially guide the investigation. Daily results were emailed and faxed from the truck to Lankelma's office. The results were returned by midday the following day as PDF interpreted plots and AGS digital data.To improve the accuracy of interpretation, the CPT tests were compared with adjacent cable percussion borehole logs.

Buro Happold analysed the AGS data and summarised the CPT tests into sections across each grid line of the structure using HoleBASE and KeyHOLE software supplied by Key Systems Geotechnical. This enabled the engineers to quickly study the results and assess ground conditions.

The purpose of the CPT work at Ascot was to increase understanding of the soils beneath the site, thereby reducing risk. This has been achieved with minimal disruption to the racecourse and at a speed and cost unachievable with standard drilling techniques.

Although CPT testing has not enabled sampling of the strata, the results have enabled a clearer understanding of the location and quantity of clay layers within it.The use of AGS data to analyse more than 200 CPT logs has allowed an efficient analysis of the results in a timeframe that would not have been possible with paper copies of the results. Piling at Ascot is due to start in October.

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