Construction World May 2018

PROJECTS & CONTRACTS

STABILISING THE R75 SLOPE The project is located on the northwest quadrant of the R75 interchange with the R367 (Union Avenue) provincial main road and M6 municipal

The contract also included the construction of an earth catchwater bank at the summit of the cutting and a concrete-filled geocell-lined trapezoidal drain to prevent overland flow from the summit flowing down the cut face. Groundwater is intercepted by a network of sub- soil drains on the upper reaches of the cut face leading to manholes and then to the box drains. In order to ensure the integrity of the two electricity pylons that supply a large area of Uitenhage with electricity, a system of bored reinforced concrete piles was installed on the cut face side of the two pylons. This protection consisted of 20 piles 12 m long, at the northern pylon and 24 piles each 14 m long at the southern pylon, all 650 mm in diameter. The piles are anchored into the mudstone by 15 m long soils nails connected to galvanised steel waler beams along the front face of each contiguous pile wall. Corporate social investment It was a condition of the contract that a Contract Participation Goal (CPG) of 8% be achieved. The majority of the contract work was of a highly specialised nature (such as soil nails etc), the CPG could only be achieved by awarding several subcontracts to SMMEs to under- take work such as the construction of gabions, trapezoidal drains and subsoil drains. A total of 13 such SMMEs were engaged. 

T he existing cut slope has exhibited degrees of instability in the recent past resulting in slipping of the soft mudstone cut face material. This has resulted in the R75 interchange on-ramp being blocked on occasion; as well as endangering the integrity of two overhead power line pylons that are located immediately above the existing cut face. Topography and weather The topography of the site is such that the entire site is situated on an existing cut face with the height difference between the top of the face and the R75 on-ramp at the bottom being approximately 60 m. The mean annual precipitation for the Uitenhage area is 417 mm fall- ing generally throughout the year with, however, most rain falling in spring and autumn. During the year there are an average 81 days on which rain falls and 11 days where the rainfall is greater than 10 mm. The average maximum daytime temperature is between 30°C and 36,9°C and the average minimum night time temperature is between 1,3°C in winter and 12,2°C in summer. During the course of the contract the rainfall was generally greater than the average. Whenever rain fell, even in small quantities, the haul roads on site became very slippery because of the nature of the unconsolidated materials. The soft mudstone tended to return to its unconsolidated state and the haul roads became very muddy and generally of a clayey consistency, rendering them untrafficable in most instances. Description of the works The project entailed the stabilisation of the existing cut face by the installation of a gabion/soil nail combination system to stabilise the cut slope and to prevent soil debris slipping onto the interchange on-ramp below. The work also included the provision of subsoil drainage systems to control groundwater as well as a system of longitudinal trapezoidal drains to dispose of both groundwater and surface runoff. The two electricity pylons that were in danger of being under- mined were each stabilised by means of a contiguous pile wall below the pylons tied to soil nails. Major works The project entailed the excavation to spoil of 112 000 m³ of soil consisting in the main of soft mudstone with occasional bands of loosely consolidated fine-grained sandstone, the construction of 5 700 m³ of gabions in three continuous walls spanning the entire length of the project parallel to the R75 on-ramp, concrete-filled geocell-lined trapezoidal drains at the tops of the gabion walls and gabion and Reno Mattress-lined box drains conveying stormwater run-off down the cut face to the existing concrete lined drain on the western side of the on-ramp. The stabilisation of the cut face was achieved by drilling holes and inserting grouted soil nails into the soft mudstone, anchoring them to the gabion walls. main road roughly 3 km east of the town of Uitenhage within the Nelson Mandela Bay Metropolitan Municipality.

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CONSTRUCTION WORLD MAY 2018