The Joop van Wamelen SASTT Award was presented to Managing Member Sam Efrat and General Manager Marco Camarda of Trenchless Technologies cc at the SASTT Annual General Meeting by the outgoing SASTT President Johann Wessels. The award ceremony took place at the regional offices of Johannesburg Water in Midrand on 2 February 2010.

The project

The works involved connecting ten of ABSA Bank’s buildings in Johannesburg’s CBD with 100 underground sleeves for electricity, gas, fibre optic, as well as low temperature hot water and chilled water.

Drilling took place from building basement-to-basement beneath the busy CBD roadways, at depths of up to 16 metres, using horizontal directional drilling.

Sleeve diameters ranged in size from 160, 225, 450, 500, 560 and 710 mm using PE 100 PN 8 HDPE pipe. The 160 mm and 225 mm sleeves were installed in bundles of three, four or six pipes. The ten month contract had a value of R13 million ($US1.7 million). The type of contract document used was the NEC3.

Challenges and solutions

The ABSA building’s basement floors were originally designed to support the weight of a typical sedan vehicle. Consequently the Terra-Jet 7520 automatic rod loading system was removed to decrease the machine weight and propping was undertaken from the floor below to support all areas over which the drill needed to travel into the required positions within basements.

A floor to sprinkler height restriction required that all equipment be less than 2.1 metres in height. Consequently the Terra-Jet 7520 was further modified by removing the operator cabin, rebuilding the hydraulic oil tank at a lower position and lowering the encapsulating body work such that the remodelled drill resembled a tank with a height of only 1.8 metres.

The majority of the installations took place in clay whilst the remaining 20 were in rock. The presence of the rock necessitated the use of percussive hammer and rock reamers to expand the holes to the required diameters.

Lateral support ground anchors were encountered during drilling on 26 of the crossings. The presence and position of these were unforeseen, and resulted in the entanglement of drilling equipment in several instances. Drilling tools were pulled, pushed and rotated until they became free or broke the ground anchoring cables. In two instances the entangled cable would not break and an acetylene oxygen cutter was modified and inserted within the bore to cut the cable and free the drilling tools.

In order to gain access to the drill face, coring to remove concrete lateral walls from 300 – 500 mm thick was required. Containment of the large volumes of bentonite and spoil was a necessity as the basements are maintained in a pristine condition and in daily use. This was made possible using specially constructed tanks and brick burms.

The removal of bentonite to the surface was one of the greatest challenges the project faced. Initially pumping, which is by far the preferred method of spoil removal did not look feasible. This was due to requiring extremely long pipe lengths – of up to 600 metres – to remove the bentonite from basement-to-basement up the vehicular ramps. However, at the planning stage the idea was put forward to core holes through the slabs and enable vertical pumping along the shortest route to reduce pumping lengths to less than 200 metres.

For the first six months of contract no drilling was allowed during the ‘freeze period’ from the 26th of the preceding month to the 5th of the following month. This was to prevent any potential damage to cables and infrastructure during the busy end of month banking period. This resulted in a very tight work schedule necessitating crews to work day and night shifts, seven days a week.

The location of drilling equipment on either side of the walls was required through concrete, rock and at depths of up to 16 metres. Trenchless Technology cc made use of the radio detection i-track system and tracked the equipment horizontally from the basements on either side of each crossing.

Confined access into basements via vehicular ramps required short six metre lengths of HDPE pipe to be individually transported down into basements where they were butt-welded into long continuous lengths for installation.

Whilst in the museum the only access was by means of a lift and pipe lengths were limited to three metres. Here, and in other confined and storage areas and plant rooms, the bentonite was collected in tanks and removed in ‘wheelie’ bins.

In two instances the HDPE pipe was hammered into the completed bore from the drill machine side in a pipe ramming operation, as there was no access to pull the piping in from the opposite side of the bore.

The benefits of using Trenchless Technology for the ABSA project

The installations were exceptionally deep, precluding conventional excavation methodology. There was no damage to roadways and existing buried infrastructure, nor was there any disruption to pedestrian and vehicular traffic in the busy CBD area. Cost savings were also significant when compared to other possible methodologies.

Why HDD?

HDD allowed the insertion of the preferred HDPE sleeves without requiring the installation of a temporary pipe or permanent rigid pipe, followed by the HDPE pipe installation. HDD required no thrust abutment wall, and also allowed for adequate steering accuracy. The equipment is self- propelled and capable of drilling in a wide range of soil conditions including clay and rock. It is also sufficiently compact and manoeuvrable to be able to operate in the confined basement spaces.

Trenchless Technologies cc was established in 1991 and to date have completed over 220,000 metres of trenchless pipeline rehabilitation and new installation works. They have been the Southern African agent for Terra AG since 1993.

Previous awards include the SAICE Pretoria Branch Award for Technical Excellence in 2000, along with the City of Tshwane and consultants Bigen Africa, for the rehabilitation of sewers in Mamelodi. The SASTT Award of Excellence in 2007, along with Johannesburg Water and Consultants Vela VKE for the rehabilitation of sewers pipes in Klipspruit Basin.