Geotech

Two water reservoirs on opposite sides of the N1 near Hammanskraal, north of Gauteng, are to be connected as part of the upgrade and extension of the Klipdrift WTW near Hammanskraal.  The two reservoirs will be connected with a 750mm steel pipe, which will be housed in 2.2m high, 2.8m wide box culverts which will have to be pipe jacked beneath the N1.

 Hard rock was found at some 4.5m below the natural ground level and therefore the pipe jacking was analysed to see whether it will be possible to avoid having to excavate within this rock.  The problem was analysed using Rocscience’s Phase2, a 2D Finite Element Analysis (FEA) software package.  The original plan was to have a 50-100mm gap between the top of the box culverts and the soil during construction (this gap will be grouted up with cement after construction).  The analyses showed that the soil overburden between the box culverts and the N1 will not be able to support itself during construction, and therefore a solution was required.

 The proposed solution was to use a pressurised bentonite slurry between the culverts and the soil overburden during construction.  The bentonite will support the soil overburden as well as act as a lubricant during the pipe jacking.  FEAs were conducted to find the optimum bentonite pressure.  A trade-off was made between the downward displacement experienced by the N1 pavement when a tridem axle unit of a truck is directly above the pipe jack compared to the upward displacement experienced when there is no load on the pavement with the bentonite slurry wanting to lift the pavement.

 After several analyses, an optimum bentonite pressure range was found that will give acceptable upward and downward deflections of the pavement for a pipe jacking depth to avoid excavation in the hard rock.  There are various construction challenges associated with this solution.  The contractor is currently busy formulating his construction methodology for discussion with ARQ and the project consultants, whereafter the pipe jacking will commence.

Project details