• Danielle Mohammed

Building in Reverse

CPML Contractors Ltd was requested by Alpha Engineering to provide shoring for the construction of a 32-foot high Reinforced Concrete Cantilever Retaining Wall. The original intent was to auger soldier piles as an alternative but this proved to be unattainable due to the presence of an outcrop of hard rock through most of the site. We requested the geotechnical report that showed the hard rock and immediately saw the opportunity to construct a permanent wall starting from the top and proceeding downwards as excavation advances incrementally.

The retaining link wall designed was a 0.25m thick x 7.1m tall wall stem tapered with Manta Ray earth anchors generally spaced at 1.25m centre to centre along the length of the wall with a few irregular spacings due to existing anchors. All the earth anchors were installed at 15 degrees to the horizontal and are vertically spaced generally at 1.0m c/c with some irregularities. There were five (5) manta ray earth anchors per vertical row and they were positioned on a 400mmW x 200mm D concrete plinths so as to facilitate shifting the anchor rods either to the left or the right should the anchors clash with the existing piles during installation.

All manta ray earth anchors used were type MR-4 with an ultimate strength of 16 kip (71

kN) and a working load strength of 8 kip (36 kN) [FOS = 2]. Maximum anchor rod

diameter ¾ inch (20mm) and all anchor rods were hot dip galvanized.

The anchored retaining wall stem were doubly reinforced with orthogonally placed high tensile (strength 60,000 psi) ribbed, uncoated rebar on both faces of the wall.

Despite many challenges, the team persevered and tried different methods of drilling until we were successful. The result was a strong Earth Retaining Structure and some very pleased clients.