Stefanutti Stocks Geotechnical was given the opportunity by Growthpoint Properties to be part of a geotechnically challenging project, which included the design, supply and construction of lateral support, bulk earthworks as well as ground improvement for a commercial property located across the road from the main entrance of the Centurion Gautrain Station.
Centurion is well known for its challenging geology which includes dolomites and problem clays. The initial geotechnical report for this site identified three areas as a Class 8 risk dolomite area, with various unknowns, cavities and sinkholes. “When the project started in February 2017 we did extensive work to ensure that we complied with the recommended guidelines for working in a high-risk dolomite area and implemented a dolomite risks management plan,” says Thys Maree site agent for Stefanutti Stocks Geotechnical.
After the completion of the demolition works, and in order to ensure that the project was completed on time, Stefanutti Stocks’ three main activities of lateral support (to excavated faces), bulk earthworks and dynamic compaction, needed to be run concurrently. “Our activities had to be coordinated daily to ensure continuity of work for all the plant and to stay on track with the tight programme,” says Maree.
The scope of work for the bulk earthworks required Stefanutti Stocks to excavate 17 700m3 to the bulk earthworks level, as well as ensuring that all dolomite pinnacles, clay and WAD were removed to a bulk earthworks level of minus five metres. The holes were then backfilled using 23 740m3 of a blend of 50 per cent selected earth filling (G6 to G8) and 50 per cent hard rock (dolomitic material). Both of these were obtained from the bulk excavations on site. Based on the initial geotechnical report the consultants measured 6 426m3 for hard rock excavation, and the bill of quantities specified that all the hard rock dolomite pinnacles were to be removed by means of restricted blasting.
As the bulk earthworks progressed the blasting commenced. This was restricted to smaller blasts due to the proximity of the site to the Gautrain and the neighbouring buildings, sound and vibration was monitored constantly. “We soon discovered that the amount of rock on site was much more than originally expected” says Maree. “By the end of the project we had done 34 blasts to remove a total of 14 168m3 of hard rock dolomite pinnacles.” This almost doubled the volume of rock, which had a huge impact on the programme and Growthpoint Properties requested an acceleration of the works.
Once the area was rehabilitated and all the dolomite pinnacles, clay and WAD removed the dynamic compaction started. Dynamic compaction is achieved by dropping a 14.5 ton weight, known as a pounder, from a considerable height onto the soil, with an impact energy of 35 t.m/m3 to consolidate the soil. This is done in three different phases, known as the primary, secondary and ironing phase.
The secondary phase mainly achieves compaction in the intermediate layers. The ironing phase ensures overlapping of the initial phase by compacting the shallow layers between the initial prints. This method of compaction is applied to increase the bearing capacity of the in-situ soil and to reduce settlement potential. This method also reduces the liquefaction potential of soil and increases its stiffness. By removing all the rock and problem clay to a level of minus five metres, the effect of the dynamic compaction was designed to compact the soil to a depth of eight metres.
This soil improvement method created a five-metre thick soil mattress for the main building contractor to construct a raft foundation, followed by a nine-storey building.
As the project progressed two areas were identified as potential sink holes, and were rehabilitated as though they were actual sinkholes, using restricted dynamic compaction to avoid any possible risks.
The lateral support undertaken at the Lakeside Development site consisted of temporary lateral support next to West Avenue. “With all the vibration from the blasting and the dynamic compaction activities on site, the lateral support had to be designed to withstand a number of tests,” says Maree. “At the end of the project the wall is still intact with no significant movement.”
Stefanutti Stocks Geotechnical was faced with a number of difficult challenges over the six-month
project duration, and its success was driven by the engagement of the full team – from tender, to planning and execution stages. “Our ability to accelerate the work and complete the project ahead of schedule, without any safety incidents and delivering a final product that complies to international standards, is a testament to the Geotechnical division’s character and perseverance!” says Maree.
Source: Thys Maree, Stefanutti Stocks Geotechnical site agent