Construction World August 2016

PRECAST

completes retail centre TERRACED RETAINING WALL

The Grove Mall, an upmarket shopping precinct developed by Resilient Africa and situated off Lynwood Road in eastern Pretoria, than the stabilisation of earthen embankments. They also give full reign to the creative talents of landscape architects and retaining wall builders. > illustrates this point in spectacular fashion. Its 420 m long multi-faceted retaining wall structure on its southern and western boundaries has transformed a six metre high erosion-prone earthen embankment into a terraced wall garden, comprising three, and in one section four layers, of retaining walls interspersed and overhung with abundant plant and flower life. Despite their eye-catching attributes, public safety rather than aesthetics was the prime motivator for shoring the embankment with terraced walls which were designed, built and engineered by Engineered Interlock Solutions to provide many years of trou- ble-free service. Reasons for the choice According to Engineered Interlock Solutions, owner, Manie Troskie, before the retaining wall option was considered, gabions were the The reason why concrete block retaining walls are used so widely is that they offer much more

Detailed logistical planning was required to counter the restricted working space and a ramp, which provided access to the upper wall sections, had to be built. Besides exca- vating and stockpiling the soil, an existing precast concrete drainage channel, which ran along the bottom of the embankment, had to be temporarily filled in to provide adequate and unimpeded access for building the lower wall. Provisional alternative drainage was installed in this section during construction. Dust was another problem and the on-site water was used to damp down the soil. In order to ensure that the walls were structurally sound and well drained, Engi- neered Interlock Solutions was obliged to use low-fines concrete as the backfill material for the first wall. This wall varied in height between one and three metres and in one three metre section the wall was built around a substantial quantity of rock. The upper walls ran between one and two metres in height. All the walls were built with concrete foundations and the excavated soil was used as backfill material for the upper terraces. All the walls were built at an angle of 80˚ with Maccaferri WG4 geosynthetic reinforcement. In addition, a layer of bidem was installed under the second to last layer of blocks on all the walls for erosion protection. Drainage was a very important facet of the project, given the large volume of water on site. Sub-soil pipes were placed at the bottom of the cut face and outlet pipes were run from the cut faces to the bottom of the

front-running reinforcement solution. “We were approached by Axiom

Consulting Engineers to provide an alter- native terraced concrete block retaining wall design using a cut-and fill construction technique. We submitted our design, which was based on Aveng Infraset’s very attractive and natural looking split-face retaining block, the Infrablok™ 350, to BJV Quantity Surveyors. “This solution not only proved less expen- sive to build than the gabion-based alterna- tive, but offered the opportunity of creating a wall garden. Over 46 000 block were supplied by Aveng Infraset to cover a total wall-face area of 2 570 m². “We incorporated terracing for two reasons. It was a far more attractive option than a single wall and it provided the parking basement with more natural light. The individual walls were built in S shapes with varying wall heights to lend further aesthetic appeal,” said Troskie. Actual construction was complicated because it commenced while the centre was fully functional. “This meant we had to work in a public space and this restricted our available working area. In addition, a section of the parking basement adjacent to the embank- ment had to be used for temporary storage of the excavated soil. Moreover, although the soil was sufficiently stable for backfill purposes, there was so much water in one section of the embankment that it ran contin- uously, and we had to pay particular attention to drainage” said Troskie. The project The walls were completed at the end of 2014 and took six months to build. The bulk of the structure comprised three walls, apart from the south western corner where a fourth wall was added.

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The lower wall under construction at The Grove.

CONSTRUCTION WORLD AUGUST 2016