The Infrastructure sector:

A483 Highway deep depth GPR & ground remediation

A483 highways project case study

The A483 is a concrete slab highway located in North Wales. Restek were commissioned to carry out a GPR survey along a stretch of the A483 starting at Junction 6, where it was believed that a significant number of voids beneath the concrete slab carriageway were causing issues. Restek carried out a Deep Depth GPR (Ground Penetrating Radar) survey along the A483.  Resteks GPR surveys penetrate deep underground by using a wide band performance of up to1400MHz combined with blended multi-frequency antenna to eliminate blind spots.

The survey included both southbound / northbound carriageways, running ten parallel scans of each slab, to cover the full width of the carriageway. The objective of the GPR investigation was to use non-intrusive site investigation techniques to look at the subgrade deep below the slabs, to establish the extent of the areas requiring remediation and reduce the risk of further settlement if only the voids were addressed. The site investigation had successfully located the presence of voids directly beneath the slabs. Restek’s deep depth GPR survey confirmed anomalies at depth within the subgrade that can be attributed to the loss of fines, resulting in an ongoing reduction in bearing capacity. The GPR survey identified the most significant anomalies were located directly beneath the joints of the slabs, consistently to a depth of 3.9 meters below ground level (BGL).

The faults recorded in Restek’s deep depth GPR survey also  identified poor subgrade conditions beneath the concrete carriageway, associated with loss of fines through washout caused by the slab joint failure. These deteriorating ground conditions are exacerbated when we factor in the continued cyclic loading and vibration on such a busy carriageway. If left un-remediated, the issues would reduce the bearing capacity of the subgrade and in many cases cause further incipient voids to form, both below the concrete slab as well as at depth which would in turn begin to break out.

The aim of the GPR survey was to provide the scope for remedial action, to either stabilise through specialised chemical grouting or renewal by removing the slabs and compacting the subgrade to increase its bearing strength, before recasting using a traditional on-site cast concrete pour.

The GPR radargrams taken over two channels, both Northbound and Southbound of the A483, share very similar recurring anomalies. Specifically, these anomalies derive from the failed joints within the carriageway’s concrete slabs. Post analysis of Restek’s  survey and regular engagement with the client identified that specialist chemical grouting was the most suitable and efficient solution.

Restek value engineer chemical grouting programs that include permeation, coagulation and void grouting. Restek’s deep depth GPR validation of the injection process was used throughout the A483 project as ground condition changes were monitored both during and post remediation. The technology we use for our ground remediation projects is utilised to help mitigate the risk of heave or differential settlement.

Restek’s remediation injection plan targeted the known incipient voids that were likely to break out at depth, in the weaker subgrade. These areas of poor subgrade were stabilised prior to grouting voids directly beneath the slab to omit the risk of hinging. A standard grid formation of injections was designed to allow the quantified structural Geo Polymer grout to be delivered directly beneath the carriageway.

A483 Project & Ground Remediation Details:

• The carriageway was constructed from original concrete slabs

• The reason for remediation was due to large volumes of traffic and HGVs causing settlement and hydro-fracture

• The scheme brief was to stabilise the slabs by addressing voids beneath the slab & poor sub-base at depth

• Restek’s GPR, compaction, void and permeation grouting services were used to remediate the slabs & ground beneath the highway

• Our GPR surveys identified voids directly beneath the slab and loosely compacted sub-base beneath the carriageway

• GPR data showed a range of depths of injections were required, with many being much deeper than originally anticipated

• An individual injection grid pattern was produced, to ensure each slab was successfully remediated according to the GPR results

• We used our deep depth GPR system to verify the success of the remediation work, we then communicated the results to the client

• The project was completed on budget, as scheduled and the client was very happy with all aspects of the work carried out

A483 Highway Project Review

The A483 project was delivered on time, all materials were quantified, the remediation procedure was validated for every slab and as a result of the successful remediation, the A483 now has an extended life cycle of 30 years.