M5 Junction 21 | Weston-Super-Mare



M5 Junction 21 | Weston-Super-Mare motorway construction

Leca® Lightweight Expanded Clay Aggregate has solved an embankment problem at the M5 Weston Super Mare Junction 21 improvement scheme which was designed and constructed to improve traffic flow and reduce congestion on the approach to and exit from the motorway interchange originally constructed in 1968.

The scheme entailed widening sections of the multi-lane roads within the interchange as well as parts of the A370 either side of the junction. These works required substantial remodelling of the embankments where geotechnical risks were established at the outset of the project.

CH2M HILL, a global leader in consulting and program management, was the designer of the project led by client and part land-owner of the site North Somerset Council (NSC).

Significant parts of the work fell on the Highways Agency’s network, also part land-owner. Principal contractor Costain worked with earthworks sub-contractor M J Church and project manager E C Harris. The overall project contract value was in excess of £3.5m. Key constraints to the scheme included heavy traffic, difficult access, working within existing highways boundaries, and the need to integrate the programme with environmental constraints. This involved badgers which were excluded from the earthworks area and existing setts temporarily closed. Road and lane closures were limited to night shifts only when traffic flows were lighter.

Ground conditions were onerous due to the thick deposits of soft compressible alluvial soils beneath the 10m high embankments. The original embankment construction comprised pulverised fuel ash fill overlying a 1m - 3m rockfill layer founded on 20m of normally consolidated clays and peat. Settlement of traditional fills was considered a major risk and the geotextile manufacturer, Tensar, worked with CH2M HILL in the design and development of reinforced soil elements leading to the specification of Leca® Lightweight Expanded Clay Aggregate.

The geotechnical design needed to deliver a scheme that met the serviceability requirements by limiting total and differential settlements. Careful investigation, analysis, monitoring, and innovative use of Leca® Lightweight Expanded Clay Aggregate within reinforced soil walls and behind reinforced concrete structures provided solutions limiting settlement risk.

To raise the ground levels by up to 6m providing a widened embankment within the footprint of the existing, an innovative solution was developed specifically for this project, combining existing reinforced soil techniques and the use of Leca® Lightweight Expanded Clay Aggregate . The solution which overcame any risk in settlement was technically acceptable to the Highways Authority.

A totally natural product, Leca® Lightweight Expanded Clay Aggregate is formed by heating and firing natural glacial clay in a rotary kiln at temperatures up to 1150ᴼC. This process transforms the clay into lightweight ceramic granules with a hard shell and porous core. With a bulk density of just 0.3 tonnes per cubic metre, Leca® Lightweight Expanded Clay Aggregate has excellent insulation properties, is free draining, fire resistant, frost resistant and chemically inert with no hazardous properties.

Used as a lightweight aggregate fill in many civil engineering and construction applications Leca® Lightweight Expanded Clay Aggregate reduces the weight on weak substrates and against retaining structures by up to 75% over traditional fill and eliminates expensive settlement delays, is easily handled and quickly installed. In other areas of the site, L-shaped reinforced concrete retaining walls were founded on embankment fill with Leca® Lightweight Expanded Clay Aggregate used as backfill to ensure that the additional loading on the slope was within acceptable limits. The use of Leca® Lightweight Expanded Clay Aggregate also permitted rapid infilling of excavations as it required only minimal compaction and was not overly sensitive to wet weather.

“The scheme design was constructed safely at a live motorway junction with minimal impact on traffic. Solutions used on this scheme satisfied all the adopting authorities’ technical requirements as well as safety and programme requirements,” says a spokesperson for CH2M HILL.

“Mitigation of geotechnical risks with Leca® Lightweight Expanded Clay Aggregate reduced the geotechnical works costs and risks. Alternatives included piled retaining structures to a 20m depth, lengthy land purchase negotiations to widen embankments and prolonged construction duration to allow for initial settlements. Future maintenance liabilities were reduced by the choice of solutions, including Leca® Lightweight Expanded Clay Aggregate within geogrid reinforced walls and behind reinforced concrete walls, and risk of future re-colonisation by protected mammals was achieved. Not only were the geotechnical risks successfully managed but the client’s, Highways Agency and the community’s needs were met in delivery of a sustainable scheme ahead of programme and within budget.”