The South African Society of Trenchless Technology (SASTT) Joop van Wamelen Award of Excellence has been was won by The ABSA Bank Energy Centre sleeves reticulation project, undertaken by Trenchless Technologies cc, and installed by horizontal directional drilling.
The works comprised connecting 10 of ABSA Bank’s buildings in Johannesburg’s central business district with 100 underground sleeves for water, electricity, gas and fiber optics.
Drilling took place from building basement to basement beneath busy roadways, at depths of up to 50 feet, using horizontal directional drilling. Sleeve diameters ranged in size from 6 inches to 8 inches using HDPE pipe. The 61⁄2-inch and 9-inch sleeves were installed in bundles of three, four or six pipes.
Challenges and SolutionsThe ABSA building’s basement floors originally were 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 7 feet 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 6 feet.
The majority of the installations took place in clay, while 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 12 inches to 520 inches 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 berms.
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 (up to 2,000 ft.) to remove the bentonite from basement to basement up the vehicular ramps. However, at 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 700 feet.
Time Out TakenFor the first 6 months of the 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 damages to cables and infrastructure during the busy end-of-month banking period. It resulted in a very tight work schedule, necessitating crews to work day and night shifts, 7 days a week.
The location of drilling equipment on either side of the walls was required through concrete and rock, at depths of up to 50 feet. Trenchless 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 20-foot lengths of HDPE pipe to be individually transported down into basements where they were butt-welded into long continuous lengths for installation.
While in one of the buildings, the only access was by means of a lift, and pipe lengths were limited to 10 feet. Here, and in other confined and storage areas and plant rooms, the bentonite was collected in tanks and wheeled out in 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.
Benefits of Trenchless TechnologyThe installations were exceptionally deep, precluding conventional excavation methodologies. There was no damage to roadways or existing buried infrastructure, nor was there any disruption to pedestrian and vehicular traffic in the busy commercial district. Cost savings also were significant when compared to other possible methodologies.
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 also is sufficiently compact and maneuverable to be able to operate in the confined basement spaces.
Trenchless Technologies cc was established in 1991, and to date, has completed more than 725,000 feet of trenchless pipeline rehabilitation and new installation works.