UEA Trenchless has recently completed two on-grade sewer bores in hard rock in Kiama. Kiama is a township approximately 120km (2 hours) south of Sydney. Kiama was the site of two strong volcanic flows called the Gerringong Volcanics, which attributed to the local ground conditions, and UEA drilled through one of these rock formations, Latite. Latite is an igneous volcanic rock which forms during the cooling and solidification of lava. Due to the lava cooling in this process, the rock forms with no seams, making it difficult to excavate or drill. In this circumstance, the rock strength was tested as 250Mpa.
UEA was engaged by a developer to install a 180mm PN16 gravity sewer main from a new development to the existing gravity sewer system. The two bores were 180m and 230m in length. The project came with certain difficulties, including:
- 3% grade
- 250Mpa rock strength
- Peak of a hill which meant that there was no room to set up on either side of the project
- Constant horizontal curled bore
One challenge the client faced was in digging a lead-in trench. The depth of the trench was six metres, in the 250Mpa rock, with a 35T excavator. This took four weeks and the hammer used broke the chisel on three occasions. For the second bore, instead of trenching, UEA completed a lead-in bore with a Ditchwitch AT3020 which allowed this phase of the project to be completed in three days.
UEA worked with the client and Sydney Water to slightly change the design and set up locations to enable the best result. UEA utilised a Vermeer D100 with mud motor engaged for the entire bore.
Due to the prior knowledge of the rock strength, UEA was able to purchase specialised rock drilling heads and reamers. With UEA’s expertise and with the right tooling, the bore was completed to the exact grade and location and to the client’s satisfaction.