OWNER:
Prologis
ENGINEER:
RPS
GENERAL CONTRACTOR:
Volker Fitzpatrick
MAIN FIGURES:
c.3,500 CMCs
Project Details
Menard was appointed by Volker Fitzpatrick to undertake ground improvement beneath industrial units of Prologis Park on Green Lane, Hemel Hempstead. Controlled Modulus Columns (CMC) have been installed under ground bearing slabs and shallow foundations to reduce settlement and to enhance the bearing capacity of the natural soil. As CMC holes are either filled with the auger or with concrete, there is little chance of water infiltration and creation of dissolution features in the chalk layers.
Ground Conditions
The soil profile was homogeneous across site with a variation in the fill placed on top of the natural soil. The natural soil was essentially chalk layers up to 16m (various grade) beneath topsoil and site upfill. Hence, treatment was required through the site upfill, topsoil and weak chalk layers.
Solution
3,500 Controlled Modulus Columns (CMC) were installed across the 5 Units, to depths ranging from 3m up to 13m, depending on the local conditions and on the site upfill.
CMC is an ideal solution to treat chalk layers where there is a chance of encountering dissolution features, as the process is sensitive to anomalies, and can easily pick up on anything unusual within the treated zone. During the works, a loss in the grout pressure and a high moving speed during penetration, together with slumping of the installed concrete was encountered in an isolated location beneath one of the proposed unit footprints. These parameters in combination are likely to indicate the presence of a void, which was borne out by a review of a nearby borehole, whereby ground conditions consistent with a potentially infilled void had been indicated. After liaising with Volker Fitzpatrick, the CMC rig was used to pour concrete to fill the void, closely observing to ensure no further slumping. Additional CMCs were then installed around the infilled void, with drilling parameters closely monitored to ensure no further loss in grout pressure. Additional plate load testing was undertaken for verification purposes.
Sustainable development
Controlled Modulus Columns were adopted as an alternative to piled solution, which resulted in carbon-efficient foundations and thinner slabs, plus less materials were used in the CMC technique i.e. less depth. Further environmental benefits resulted from the use of HVO (hydrotreated vegetable oil) fuel and opting for low carbon concrete in the CMC installation.
