Soil nailing and erosion control works
Phase 4 of the Worcester Southern Link Road (SLR4) will deliver a continuation of the dualling of the A4440 Southern Link Road between the Ketch and Powick Roundabouts. Works include dualling of the A4440, and construction of the new Carrington Bridge, Powick Common Viaduct and Hams Way Footbridge.
As part of the wider upgrade slope reinforcement measures were required to the southern extent of Ketch Slope, located at the eastern extent of the scheme, below the Ketch roundabout itself and to the South of the Carrington Bridge abutment A3. The slope, in its original condition, was not considered to meet the requirements of modern design standards and as a result needed to be strengthened.
The slope is located adjacent to the River Severn, within the flood plain and was comprised of weathered Mercia Mudstone. Mudstone of this nature has a tendency to suffer strain softening of the upper surface and localised slumping. As part of the wider project, the Severn way cycleway and footpath was to be reinstated at the bottom of the slope, which provided another reason to undertake the works.
As part of the works Griffiths completed temporary works design assessments relating to placing large excavators adjacent to slopes known to be prone to instability. Griffiths worked with our design partner Jubb Consulting Engineers to establish safe positioning of the plant to be able to install the required soil nails. This resulted in a requirement to ensure no load was placed within 2m of the slope crest which was demarcated on site by use of a temporary fence. Soil nails were applied on a staggered grid layout varying from 2m to 3m across the slope to provide reinforcement. The nails comprised 6m to 7.5m long 28mm bars installed in a 130mm diameter borehole and secured by cementitious grout. A total of 200 nails were installed, with 3 tests nails installed prior to the main works to verify the design.
The soil nails were installed using a combination of a 13T excavator working at the toe of the slope and a 36T long reach excavator working at the crest of the slope, both fitted with hydraulic drill attachments.
Soil nails were installed by rope access technicians in open holes, utilising centralisers and tremie pipes to ensure the nails were fully grouted from bottom up. Grout was mixed by a colloidal mixer located >25m away from the river stabled in a bund to ensure grout could not be accidentally lost to the river.
A flexible facing was required across the slope, to assist with holding back the face between each soil nail location. It will also act in conjunction with the soil nails to pin the slope behind the plates and provide a substrate for re-vegetation. A factor which was very important to the ecological design and impact of the project to replace the trees lost as part of the strengthening works.
The facing material was heavy, to reduce manual handling the rolls of facing material were mounted on spindles and lifted into position by the excavator. The rope access operatives then unrolled the required matting down the slope prior to fixing to the soil nails and the surrounding area using J pins.
Large 750mm square steel plates were fixed with wedge bosses to the soil nails and tensioned to the face. The plates were positioned utilising rope access haulage techniques to avoid manual handling concerns. The wedge booses were employed to provide the angle compensation between the soil nail and the plates.
A further 60 nails were installed to the same design as the main slope below and on the approach to the bridge abutment where the footpath passed beneath the road to provide edge reinstatement.
A short 10m long retaining wall was also installed at the transition between the soil nailed slope and the yacht club access to provide crest retention. The solution comprised T76 micro piles installed vertically in a 130mm diameter hole to 7m depth, tied with 6m long soil nails to the same detail as the main slope.
A reinforced capping beam was then formed around the head details of the soil nails and piles, before being shuttered and poured with C40 concrete.. Again, the micro piles and soil nails for the wall were installed by a hydraulic drill excavator attachment.
The entire slope reinforcement works were self delivered by Griffiths utilising its fleet of inhouse plant and rope access operatives. Early Contractor Involvement (ECI) was provided to the scheme designer by Griffiths geotechnical engineers to ensure a buildable solution was developed. Griffiths engineers also undertook, interpretated and reported the sacrificial nail tests prior to the main works commencing.