Mamelodi Sewer Tunnel under construction
Varndell,P and Pequenino,F.P. April 2006. In SAICE Civil Engineering
Article discusses the investigation and design of a 3m diameter, 760m tunnel through the Magaliesberg in Pretoria. Extensive core drilling (holes up to 120m deep) and laboratory testing was carried out to investigate the geotechnical conditions and rock mass characteristics for the tunnel design. This project demonstrated the true value that a detailed geotechnical investigation can add to a project; reducing numerous variables that such a tunnelling project would encounter. These parameters were utilised in the tunnel design, inlcuding geological delinitation and rock mass characterisation.
This article discusses the investigation, design and construction of a 760m long, 2.7m ID horseshoe shaped tunnel through the Magaliesberg in Mamelodi, Tshwane. The project was undertaken between 2003 and 2007. The tunnel formed part of a major upgrade to the sewer network in the Mamelodi area that also includes 3.4km of 1.6m diameter pipeline.
The project forms part of the City of Tshwane Long Term Sanitation Masterplan. Parts of the existing system are more than 40 years old and nearing the end of their expected life while the capacity of the existing outfall-sewer system is decreasing due to the continued development of Mamelodi and eastern Tshwane.
GVM Incorporated was appointed as the lead consultant, with VKE International as the sub-consultant for the tunnel design together with Vela VKE who undertook the detailed geotechnical investigations which revealed fascinating results for the Magaliesberg quartzites.
Cerimele Phenduka JV was awarded the R74-million construction contract and Bomar Projects have been appointed as a sub-contractor to construct the R32-million tunnel section.
The initial design called for a series of retaining walls, bridges and a short tunnel which would have taken the pipeline through the near pristine Baviaanspoort which is one of the very few river gorges in the Pretoria area that does not have a major thoroughfare dissecting it. The gorge is largely undeveloped with the exception of the existing sewer pipeline which follows a zig-zag alignment through the river gorge. Following the findings of the Environmental Impact Assessment, it was decided to abandon the initial proposal and a sewer tunnel conduit was the preferred proposal. The tunnel solution has a more favourable hydraulic alignment and also ensures that the picturesque and environmentally sensitive valley region will be conserved.
The Pienaars River Outfall Sewer Augmentation Project has since developed into a multi-disciplinary project consisting of several kilometres of 1.6m diameter pipeline, pipe bridges, junction boxes, pipe-jacked road and railway crossings, and the 760-meter long tunnel. Geotechnical investigations for the route were undertaken from 2004 to early 2005. The investigations were primarily focused on determining the anticipated tunnelling conditions, where the largest safety and liability concerns lay, and where delicate engineering was necessary.
The extensive geotechnical investigations included core drilling (holes up to 120m deep), detailed joint surveys (both on natural exposures and on cores), water pressure tests and rock mechanics testing carried out in Rocklab’s state-of-the-art laboratory (see insert below). Triaxial tensile tests, unconfined compressive strength (UCS) tests and rock boreability assessments were undertaken to predict the behaviour of the rock mass.
The core drilling in itself posed tremendous challenges for the drilling contractor due to the strict environmental and safety constraints imposed by the engineering team, together with the difficult access to the drilling positions on top of the Magaliesberg.
The investigations showed that the tunnel project would need to contend with some challenging geological features. Approximately 90% of the tunnel drive will be located in the Magaliesberg Quartzites of the Transvaal Supergroup and a short section of the Silverton Shale Formation will be encountered at the Southern Portal.
The quartzites are extremely hard with UCS’s in excess of 400MPa recorded. The rock was found to be very abrasive, but at the same time brittle. The water table is generally located at tunnel level, but does increase to 20m above the tunnel in the central section.
The significant regional feature in the area is the presence of a down-faulted block of quartzite through which the southern half of the tunnel cuts. A major fault zone, about 20m wide, associated with the down-faulted block, occurs approximately at the mid-point of the tunnel, while a large number of major joints are present in the southern half of the tunnel.
Tunnel Support Design
Information from the investigations and analyses were used in determining the geomechanics rock mass rating and Q-values which were used for the tunnel support design. The initial tunnel support was designed using the Q-classification method. It is anticipated that pattern rockbolting alone will be required for the majority of the tunnel length, but provision has been made for reinforced shotcrete and steel arch support in the sections where poor ground conditions are expected. A 180mm thick permanent concrete lining will be constructed to ensure both structural stability and efficient hydraulic functioning of the tunnel over the full design life.
The tunnel is being constructed using drill-and-blast techniques. The tunnel sub-contractor, Bomar Projects, is using a rail-bound Montebert H181 jumbo for the drilling operations. This hydraulic drill rig has two booms and can drill to a depth of 3.7m. The mucking operations will be conducted using a rail-bound Atlas Copco LM71 loader in conjunction with a Hopper Loading System.
Tunnel advance of 95 metres has been achieved to date (as at end February 2006) and the ground conditions encountered have been as expected. The quartzite has proved to be extremely hard with UCS’s in excess of 300MPa but there has been limited over break. The majority of the support installed has been provided by individual spot rockbolts. A 5m section of steel arch support was installed in an area where a major joint was encountered.
This project has thus far shown the true value that a detailed geotechnical investigation can add to a project. Much of what is currently being revealed about the condition and nature of the rock during construction was predicted during the detailed geotechnical investigations. This has minimised the numerous variables that such a tunnelling project would encounter and provided the contractor with a good indication of the conditions that can be expected.