Novel culvert jacking project showcases creative thinking

One of BLOC’s recent milestone projects serves as a prime example of the expertise and precision required to install new subterranean services under live railway conditions.

Situated in Inyoni, KwaZulu-Natal, the project scope entailed the establishment of two pedestrian culvert underpasses beneath sections of Transnet lines to enable access to new RDP houses implemented by the KwaZuluNatal Department of Human Settlements. The culverts also needed to be wide enough to accommodate the movement of cattle owned by small-scale farmers in the area.

In conjunction with BVi Consulting Engineers, BLOC devised a design and construct culvert jacking solution to install the two 3.5 m wide by 2.5 m high culverts, one measuring 26 m and the other 28 m in length.

“Conventionally, a common methodology in congested urban settings is to advance the installation incrementally with precast culvert sections to complete the structure. However, since we had lots of space to work with in this rural setting, we opted to install each culvert as a preconstructed unit,” says Field.

“That required a lot of room for out-of-the-box thinking to perfect this trenchless approach, considering that each culvert weighs around 500 tonnes. But the weight was not the main challenge. The primary one was to manage the drag,” Field continues.

Preparing to launch

In preparation for the launch, a casting floor was constructed for each of the two culvert sites, incorporating an anchored thrust block purpose-designed by BLOC. To firmly secure the thrust block in position, anchor cables were installed by drilling down some 18 m to reach bedrock.

Then, once BLOC had completed the construction of the culverts and the required concrete strength had been attained, the jacking programme commenced in conjunction with progressive tunnelling operations through the railway embankments at a steady advance rate.

“We generally tunnelled for 0.5 m, and then jacked the culvert element forward in a constant cycle until we came out the other side,” Field explains, adding that it took around two and a half months to push the culverts through their respective sections.

Reducing the drag coefficient

One of the key concerns during the design stage was the limited overhead cover above the culverts, comprising engineered fill varying in depth from 1.5 m to 2 m. Since engineered fill is not as stable as in-situ natural ground, this had to be factored in due to the high drag loads generated during the jacking programme. In principle, the risk existed that the railway lines could be pulled sideways during the underground thrusting process.

To ensure that didn’t happen, a specialist HDPE liner sheet technology was employed. Fed from the front and over the top of the culvert during the tunnel advance, the continuous sheet acted as a membrane barrier between the culvert and the overhead fill, greatly reducing the degree of friction.

“The top side of the sheet incorporates anchor knobs that embed into the overhead ground, while the underside of the sheet is completely smooth. This ensures that the sheet remains static, while the culvert slides smoothly forward. The system worked perfectly,” Field expands.

Current work nationally Alongside its pipe- and culvert-jacking contracts, BLOC has been working extensively on lateral support projects focused on reinstating structures and embankments damaged by the widespread flooding that occurred in eThekwini and beyond during 2022.

In parallel are a series of national infrastructure projects that support the installation of water and sanitation pipeline services. Examples include a project in Fleurhof, Gauteng, where BLOC has installed the underground services for a private residential development in conjunction with municipal entity Johannesburg Water. The scope entailed six pipejacking phases over a five-month period.

Back in KwaZulu-Natal, BLOC recently mobilised on-site to complete a four-phase pipejacking project for a sewer outfall within Harry Gwala District Municipality.

Further afield in Mpumalanga, there are also a series of new projects underway. Current work includes a project with four pipejacked crossings within Gert Sibanda District Municipality, entailing the installation of bulk water pipeline services in Balfour.

BLOC also recently completed a stormwater upgrade project in eMalahleni Local Municipality in conjunction with toll road concessionaire TRAC N4.

Setting the scene, an existing 30 m long Armco stormwater pipe travelling under the N4 had been scavenged over time for scrap metal, potentially threatening the integrity of the road by creating an unsupported void. To remediate the situation, BLOC jacked through a precast concrete pipe. The existing voids were in turn filled with grouting as part of the stabilisation works.

“From a time and cost perspective, trenchless techniques are ideal for the installation of below-ground services because they greatly reduce the need for open trench excavation, especially beneficial in dense urban areas. However, in the case of projects like Inyoni and the N4, it’s also the only viable approach to ensure that existing infrastructure remains intact,” Field concludes.