Highway Reinforcement with ParaLink

Ripon, an historic Cathedral City in the north of England, is situated on karstic terrain. The area has long been affected by surface instability due to sinkhole development emanating from the underlying gypsum strata. A general geological profile is shown below.

Much of the area is thought to be undermined by vast caves eroded in the two layers of gypsum running beneath the town. These interconnected caves are up to 200m long and 15m high and have been created by circulating groundwater dissolving the calcium sulphate within the soft gypsum rock. These large voids progressively migrate upwards eventually leading to sinkholes, sometimes up to 80m diameter, at the ground surface. Records showed that 7 major subsidence events had occurred in the vicinity of the proposed bypass since 1970.

After the investigation of a number of stabilization techniques the use of a ParaLink tension membrane was finally chosen as being cost effective while providing long term security to the road surface against the effects of catastrophic collapse.

The ParaLink tension membrane was designed using the FLAC computer program to model the road embankment and geosynthetic and the void geometry and subsoil conditions. The design objective was to ensure that the road surface remained in place for at least a 24 hour period after a major subsidence event at any time in the 120 year operational life span to enable the area to be secured and maintenance measures to be put in hand.

Two layers of ParaLink 700S, running parallel to the road centerline, and with staggered joints, were used to support the highest parts of the embankment at various points along the road length. Lower sections of the embankment were supported using ParaLink 325S.