South Shields, Commercial Road

Menard was approached by John F Hunt Regeneration to increase the performance of the ground between two retaining walls. On the edge of the River Tyne, John F Hunt has built a new retaining wall 1.8m from the existing retaining wall. The locked-in Marine Silt between those walls needs treatment to reach the bearing loads needed and to control residual and differential settlements, upon placement of the upfill to final formation level and construction of the permanent structure, a riverside walkway. The solution proposed by Menard was approved by the Engineer for the project, Mott MacDonald.

Locked-in Marine Silt is present over the bedrock, a layer between 11 and 15m deep from the Working Platform level. The top 5 to 8 metres have been mixed with crushed concrete / stone using a long-reach excavator, before the arrival of Menard. Testing on this improved ground has not been deemed sufficient, and therefore the intervention of Menard was sought after. The layer treated by Menard is therefore up to 15 metres deep, to reach bedrock.

The outer retaining wall is tied with thick steel tie-rods placed every 2.5m, approx. 2m underneath the platform level.

Menard has proposed a combined solution of Controlled Modulus Columns (CMCs) and Prefabricated Vertical Drains (PVDs). The CMC treatment assists in controlling residual and differential settlements, upon placement of the upfill to final formation level and construction of the permanent walkway. The PVDs are placed before the CMCs and act as drainage paths to take pore water out of soft compressible soil, thus preventing significant pore pressure build-up during our operations. They also act to increase the consolidation rate of the soft Marine Silt under load conditions (approx. 3m upfill and permanent walkway).

CMCs are installed along 155m of wall, in areas where testing has shown a remaining presence of soft Marine Silt. PVDs are installed along the full 200m length of wall to ensure a uniform decrease of pore pressure behind the wall extent under load conditions.

Due to the presence of dense and obstructed material in the ‘Marine Silt mixed with stone’ layer, pre-augering is required to enable full penetration of the PVDs to the required target treatment depth.

Menard has worked in tight coordination with John F Hunt and Mott MacDonald, both during design to find an appropriate solution to the specific challenges of this site, and during operations. Installation of the PVDs and CMCs was particularly challenging owing to the presence of the 3m-high capping beam along the narrow treatment strip and the need for pre-augering. The number of PVDs and CMCs installed in such a tight area and the presence of buried tie-rods required extra precision and carefulness from the operators and banksmen.