Watercrest Mall Retaining Walls – Hillcrest, Kwazulu-natal

This project boasts the highest concrete retaining block (CRB) wall to have been built using these concrete blocks in KwaZulu-Natal. It was one of five CRB walls which formed part of the civil and earthwork preparations of the 43 000m² development. Begun simultaneously with the demolition of an existing shopping centre, the mall is Durban’s third largest.

Designed for a 120-year lifespan, the walls were built to shore up level platforms for the construction of the multi-level mall. The two largest are each 120m long, one topping out at 15m and the other at nine metres. The three remaining walls are all 60m with maximum heights ranging between eight and 10m.

The CRB blocks were chosen for this project because they cater for a vertical (90˚) wall configuration, unlike most other retaining wall blocks which only go up to 70˚. Extensive geotechnical testing was undertaken before the walls were designed to establish drained cohesion and drained friction angles. The friction angles were between 32-35 degrees and cohesion was of the order of five to 10KPa.

The walls were designed in compliance with SANS 207: 2006 at angles of 70˚ to 83˚ to maximize the amount of level ground for building purposes. They were reinforced with a new generation high-tensile composite geotextile which offers the strength characteristics of woven geotextiles and the favourable hydraulic qualities of non-woven geotextiles.

In calculating the overall stability of the wall, the reinforced soil block was used as a rigid mass. Geogrid spacing was calculated using various software programmes on limit equilibriums. (SRWall, MacStars, PCStabl, Snail and others). Finite element analyses were also undertaken as a check on serviceability criteria.

The backfill was benched in steps and seven metre geotextile tiebacks were used at every third course of blocks. However, in some areas geogrid was used in combination with steel anchors for additional reinforcing.

The backfill consisted of weathered Natal Group sandstone and approximately 30 000m3 was used in the reinforced zone. The high-tensile composite geotextile was used to anchor the facing blocks and inhibit wall movement caused by frictional stresses in the backfill.

In order to prevent crushing of the lower blocks, they were filled with 25MPa concrete as opposed to the upper blocks which were filled with soilcrete. The foundations were based on conventional strip footing using steel reinforcement, but in areas of rock deposits, the foundations were slightly smaller.

Drainage was installed at regular intervals using clean Umgeni sand (100 x 100 wick drains) horizontally.