The A34-M4 Junction 13 improvements scheme was commissioned by the Highways Agency.
Located about 5km north of Newbury, the scheme involves construction of 2km of new dual carriageway beneath the M4 for through traffic on the A34, while retaining access to the existing, at grade junction. The landmark structure is an underpass, constructed top down beneath both the M4 and its sliproads, using large diameter bored piles and precast beams.
The scheme also includes seven other bridges, contiguous bored pile walls, conventional retaining walls, earthworks, reinforced earthworks, stabilised capping and a groundwater lowering system.
The site geology comprises chalk (Seaford Chalk Formation) overlain by soils of the Lambeth Group. The interface between the two units is typically present in the cuttings approaching the underpass to the north and south of the M4.
The site is on the feathering edge of the Lambeth Group deposits and adjacent areas of higher ground are capped by London Clay deposits. This combination brings a high risk of dissolution of the chalk.
Consequently, the design and site management of construction around solution features was a key feature of this design and build project.
The rapid construction programme also meant an innovative and flexible approach was required.
Jim Gelder, working for the designer Mott MacDonald, was involved in the geotechnical design of the scheme and development of the pre-construction ground model (Figure 1).
Design was based on the most credible ground conditions, to be confirmed on site.
Gelder was present on site during construction as a full time engineering geologist as part of actions to mitigate the risks posed by solution features, uncertainty over the level of the chalk/Lambeth Group boundary and high groundwater. His role specifically included:
l Verification of the solution feature design assumptions made in the design of bored pile walls and axially loaded piles l Determination of appropriate measures to be taken where solution features were encountered in structure foundations, cut slopes and highway formation (Figure 2), beneath embankments and in drainage structures l Verification of design assumptions made about the level of the Lambeth Group/chalk interface.
Gelder's observations on the size and distribution of solution features, along with observations on the genesis of the features and the relation of local topography and overlying lithologies, enabled the ground model to be refined and to feed into the risk management strategy (Figure 1).
Of particular value was logging of the Lambeth Group above the chalk interface to identify distinctive deposits that could be recognised in solution feature infill (Figure 3), where exposed and also in pile arisings. This added confidence to the verification of the design.
Features highlighted in Gelder's presentation included the close relationship between the incidence of solution features and the location of channels within the Lambeth Group or granular pedogenesis deposits.The effect of flint bands on groundwater flow and hence development of solution features was also mentioned.
Gelder concluded by discussing the benefits of having an engineering geologist present on site. It allows control and adaptation of the design model as they can influence changes to the model using observations made during construction.The risk posed by solution features was therefore managed and a value for money design implemented.