Thukela River Mouth Retaining Wall, Kwazulu-natal

This concrete retaining block (CRB) wall was constructed near the Thukela River mouth to secure a five metre wide premix road which provides access to the Thukela Bulk Water scheme’s low lift pump station and associated works.

The CRB blocks used to face off the wall were selected due to their nib shear resistance and wall angle flexibility which ranged from 65˚ to 85˚.

Extensive cut operations into the steep valley embankments which run parallel to the Thukela River were specified by the main road contractor. This exposed large dolerite boulders which, in some instances, were greater than five metres in diameter. Blasting was the only means of removing them which meant that the wall was required as much to prevent the dislodgement of fractured boulders as it was to stabilise the entire embankment.

The 2 190m² wall was a cost-and-time efficient alternative to rock anchoring. It is 210m long and rises to 16m at its highest point.

Although a geotechnical investigation was undertaken for the road cutting, it proved difficult to obtain undisturbed samples. Besides the hard bluish-grey dolerite boulders, the cutting comprises a matrix of orange/brown highly weathered dolerite which in some cases presented as a sandy clay.

The geotechnical parameters were thus largely inferred, based on close observation as well as professional judgement. A nominal surcharge behind the backfill of 2kPa was used in the design of the wall. As a result of these assessments, the following geotechnical parameters were obtained:

In-situ Backfill : c’= 5 kPa ф=28° , ƴ=18 kN/m3

Reinforced Soil zone : c’= 10 kPa ф=35 °, ƴ=20 kN/m3

Foundation soil : c’= 50 kPa ф=40° , ƴ=20 kN/m3 UCS =3000kPa

The design was undertaken in compliance with the latest South African Code, SANS 207: 2006 “Design and Construction of reinforced soils and fills” and input from various research papers worldwide; in particular, Canadian engineers R.J Bathurst and M Simac, American engineers T.M Allen and B.Q Huang.

A non-woven high-strength polyester composite geotextile was used to inhibit excessive wall movement caused by frictional stresses in the backfill material. The geogrid tension requirement was for a 120 year design life and was calculated as 48 kN/m for the 18m portion of the wall. The geotextile offers a long-term design strength of 52 kN/m and a creep limiting strength of 60 kN/m over that period.

In calculating the overall stability of the wall, the reinforced soil block was regarded as a rigid mass.

Internal and external stability checks were undertaken and geogrid spacing was calculated using various commercially available computer programmes with limit equilibrium principles, such as Hand computations using the tie back wedge method was also used. Finite element analyses using PLAXIS 2D provided valuable input as a check on the serviceability criteria (I.e. movement of the wall).

Major drainage measures for the wall include a surface channel (SWC8) to collect the upper catchments stormwater runoff. Subsurface drainage behind the backfill consists of a 110mm diameter slotted pipe wrapped in 19mm stone and Bidim® . In addition, a series of 160 mm PVC pipe networks was constructed within the backfill for rapid dissipation of excess pore pressures.