Challenges when building on dolomite

Instead of piling, a combined soil and concrete raft approach was used

Found in large areas of Gauteng, dolomite is widely used in the manufacture of roads, concrete and paving materials. But it is a complicated rock to build on.

Kim Timm, structural engineer at infrastructure consulting firm AECOM, explains how and why dolomite needs to be accommodated when designing and constructing foundations, services and the superstructure of a building.

“With dolomite, there is often a massive amount of variation in the bedrock. “It will, for example, be just below the surface on one part of the site then fall away to 50 m below ground just a few metres away. This is challenging, as it is difficult to apply a uniform approach to foundations,” explains Timm. Therefore, Timm advises that clear and detailed geotechnical information is the best way to be able to plan for surprises that might occur. Money spent on geotechnical investigation frequently leads to money being saved during design and construction. Good geotechnical information minimises the negative consequences of problematic ground conditions and enables projects to run smoother.

“As a structural engineer, it is possible to build upon any foundation, as long as you know and understand what is below the ground,” adds Timm.

The biggest concern for any structural engineer when building on dolomite is the potential of sinkholes.

Dolomite can be soluble. Rainwater and percolating groundwater can gradually dissolve the dolomite over time as it seeps through joints, fractures and fault zones in the rock. The dissolution of the dolomite gives rise to cave systems and voids in the rock. Soils covering the rock can collapse into these caves or voids, resulting in catastrophic ground movement on the surface such as sinkholes.

“This is hugely problematic, especially in built-up areas like Vereeniging and Centurion,” adds Timm.

Exxaro Head Office in Centurion

According to Timm, AECOM encountered challenging dolomitic ground conditions at the site of the new Exxaro Head Office in Centurion, Pretoria. “We had to deal with potential sinkholes or pockets on-site before construction commenced. Soil was removed to reach an appropriate foundation level and dolomitic pinnacles within this zone were blasted. Dynamic compaction was then used to increase the density of the underlying material and provide a working platform. This

allowed us to pre-collapse high-lying potential sinkholes in certain areas. Weak material like weathered altered dolomite (WAD) was excavated and replaced with a blended, crushed, blasted rock from site and then recompacted. This process enabled us to alter the nature of the underlying layers up to 10 m below ground.”

However, dolomite is often found deeper than 10 m below the ground, meaning that sinkholes may still develop below a building. “With the Exxaro building, we therefore had to reshape the entire building concept to account for the highly variable ground conditions. We ultimately assessed 16 different variations of the structure with the assistance of the AECOM’s quantity surveyors and AMA Architects to mitigate the underlying foundation risk in the most cost-effective manner possible,” says Timm.

Instead of piling, AECOM adopted a combined soil and concrete raft approach. The structural foundation system was a 2.25 m reinforced concrete raft, designed to span a 15 m sinkhole. The raft was designed on a mattress of variable spring stiffness, adjusted for the depth to bedrock, the anticipated depth of the WAD, and the enhanced soil mattress zone created by dynamic compaction. The concrete raft mitigated against sinkholes deeper than 10 m and reduced the potential of associated cracking in the upper structure.

New sinkholes

Timm adds that it is also important to avoid causing any new sinkholes. “This can be done by making sure that all services are constructed above the ground where possible. The structure needs to accommodate considerably more complicated services routing and the trenches, sleeves and voids associated with such. All this needs to be designed in the structure up front.”

Since sinkholes are often water driven, it is important to avoid a large influx of water in and around the building due to leaks. In high-risk areas, Timm advises that if pipes have to be placed within the ground, a double sleeved system should be used where high-density polyethylene (HDPE) pipes are placed within PVC pipes. “With the Exxaro building, these pipes run to a double manhole system. This prevents leaks from seeping into the ground and the entire system is fitted with sensors that alert the building monitoring system (BMS) of the location of the leak. This allows for the rapid identification and remediation of any leaks that might otherwise have led to the development of sinkholes.”

Another complexity experienced with the Exxaro building was the attenuation of the stormwater – especially in Centurion, where there are always flooding concerns. “However, large attenuation tanks that are embedded in the ground may leak and cause sinkholes. Therefore, with the Exxaro building, we built the attenuation tank on a separate platform with bund walls. If the attenuation tank were to burst, water would fill up the bund wall area first, where sensors would trigger an alarm. There is also an overflow route, in the event of accidents, that would direct any excess water into the floodplain area at Centurion Lake,” says Timm.

AECOM also installed a water cut-off drain that would deal with potential water leaks from the road or neighbouring properties. The water would run into the subsoil drain where a sensor would again activate an alarm.

As an extra precaution, three sets of three rod extensometers were placed in locations where sinkholes were likely to develop. These were embedded into the ground at three different heights to provide an early warning system of encroaching sinkholes. When the surrounding soil holding the rod in place falls away, the rod slips and triggers an alarm on the BMS. This provides an indication of the depth and location of the sinkhole.

Kim Timm, executive: Structures, Buildings and Places at AECOM

An artist’s impression of the new Exxaro Head Office in Centurion, Pretoria