Construction World December 2017
R75 SLOPE STABILISATION NEAR UITENHAGE
The project is located on the northwest quadrant of the R75 interchange with the R367 (Union Avenue) provincial main road and M6 municipal main road.
• Company entering: Penny Farthing Engineering • Client: SANRAL
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 subsoil 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. • Contract value: R74 012 878 • Start date: 31 October 2015 • End date: 28 February 2016 • Main contractor: Penny Farthing Engineering • Project manager: SANRAL • Consulting engineer: WorleyParsons
The 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. The topography of the site is such that the entire site is situated on an existing cut face. 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 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. The two electricity pylons
that were in danger of being undermined were each stabilized by means of a contiguous pile wall below the pylons tied to soil nails. 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 m³of 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.
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