Seismic risk in Britain is low, but certain key installations such as heavy industrial plants and power generating facilities are designed to seismic requirements. Vibro treatment is a proven technique for reducing the risk of liquefaction in fine sandy soils, and CPT testing has been used on every contract of this type in Britain to date.
Contracts are usually for the gas or nuclear industry, where adequate densification of the sandy soils had to be proven. The CPT test examines layers as thin as 2mm, reports at 20mm and, by means of the friction ratio, reports the soil type.
At a site in southern England two 18.4m diameter cofferdams were being constructed in 17m of water, then filled with nominal 6mm to 3mm aggregate to minimum 80% relative density, proven by CPT testing. Placement of this fill in water resulted in low relative density and the contractor elected to hire a hydraulic vibrator and operators from a specialist contractor. Three unsuccessful attempts were made to meet the specified requirements.
Keller was then approached and proposed using a 125kW S-type vibrator suspended from a large pontoon crane. The post-treatment CPT tests demonstrated average relative densities in excess of 87%. The results reveal excellent consistency and show that experience, combined with construction control, is vital in ensuring the end product is fit for purpose, beyond simply using a powerful vibrator.