AT-GRADE CONSTRUCTION OVER DOLOMITES

Posted on Mar 5, 2009 in Media Releases

Background

Between Centurion and Pretoria, in the northern section of the Gautrain route, the rail alignment traverses about 16km of dolomitic ground, of which 5.8km will be on viaducts (elevated structures) with the remaining portion constructed at ground level.

This band of dolomites is part of a broad swathe of dolomite bedrock which surrounds the Johannesburg Granite Dome and which separates the granites of Midrand from the quartzites and shales of Pretoria. The dolomites are notorious for the occurrence of sinkholes which can be triggered by changes in the ground water regime and which, in the past, have resulted in sudden and tragic loss of life and property.

The challenge to construction

The dolomite areas traversed by the Gautrain are characterized by sub-surface bedrock pinnacles, the presence of highly compressible wad material as well as hard rock “floaters” and extremely strong chert layers. Added to this is the risk of cavities which may collapse and even develop into sinkholes which daylight on surface.

The design of foundation systems in these conditions requires a thorough understanding of the properties of the soils and rock. Obtaining reliable design parameters has been a unique challenge for the project team and Bombela mobilized a team of international and South African experts in the fields of geology, ground investigation, foundation design and construction, in order to accomplish this.

As a result of the extensive ground investigations , laboratory testing and full scale trials carried out, a suite of foundation solutions has been designed and validated. This Factsheet describes the foundation solution applied to the at-grade track bed in the “military area” north of Jean Avenue.

STATISTICAL ANALYSIS OF SINKHOLE EVENTS

A total of 458 historical sinkhole events were studied using records supplied by:

• Centurion Council database
• Desk study database (Kirsten, Wagener, Venter)
• Department of Public Works

From these, a probability distribution of likely sinkhole diameters was compiled (right). This indicated that, in the event of a sinkhole occurring, there is a 95% probability that it’s diameter will be smaller than 15m.

Figure 1 : Probablility distribution of sinkhole diameter.

On this basis, a design was developed which would enable the at-grade track structure to straddle a 15m diameter sinkhole without catastrophic failure.

TRACKBED PREPARATION

Based on the geotechnical investigation works carried out ahead of construction, the alignment was divided into different sections each requiring different construction techniques. The purpose of these different techniques was to provide a homogenous foundation under the railway platform by densifying the soils below the platform and to collapse any relatively shallow cavities.

Figure 2 : Division of the alignment into different treatment areas depending on underlying geology.

The methods employed included:

• Dynamic Compaction (DC), where thick layers of soft materials are located at relatively shallow depths;
• Impact Rolling, where there was a suitable layer of chert gravel and
• The construction of a U-shaped ground beam (in conjunction with DC and Impact Rolling) in areas of deep bedrock and where the risk of cavities and sinkholes was high.

Trackbed Preparation – Dynamic Ground Compaction

Dynamic Compaction involves repeatedly dropping 10-14ton steel weights from a crane onto the ground.  This collapses any cavities and densifies the soil to a depth exceeding 10m. The compaction points are arranged in a regular grid pattern to result in inverted cones of compacted material. The distance between the compaction points is decreased in subsequent passes and compaction is carried out in-between the previously compacted points to ensure that the entire area has been compacted. Compaction quality is controlled by means of monitoring the penetration of the pounder and by conducting plate load tests.

Figure 3 : Inverted cones of compacted material after application of
DC technique.

Figure 4 : “Print” cavities resulting from DC pounder.

Trackbed Preparation – Impact Rolling

Impact rolling is a South African developed technique where a specially shaped roller is towed behind a heavy tractor. The method is particularly suited to compacting large areas rapidly to a depth of about a meter. The impact roller is able to continuously measure the efficacy of the compaction being achieved by measuring the sub-grade response to the dynamic loading of the impact roller. Through linkage to a GPS receiver, the roller can produce a “map” of compaction quality.

Figure 5 : View of impact roller

Figure 6 : Map of compaction quality

Once the track-bed has been sufficiently compacted, it is graded to the correct levels and the construction of the compacted earthworks layers (which will eventually support the track itself) can begin.

U-Shaped ground bridge construction:

Where even the application of the above techniques is deemed insufficient to satisfactorily mitigate the risk of settlement, a U-shaped ground-beam or ground-bridge is constructed on the compacted track-bed.

The U-beam is exactly analogous to a conventional bridge except that, instead of spanning between piers, it sits on the ground and spans over soft spots and cavities. Based on the statistical analysis described above, the U-beams are designed to span a 15m diameter sinkhole without catastrophic failure.

The U-beams are typically cast in 180m long sections using a travelling shutter and casting 13m sections at a time. The structure is cast on a smooth concrete bedding surface to enable it to contract  during the post tensioning operation. Once the post tensioning is complete, the interlocking 180m “ground bridge” section effectively forms a continuous bridge on the ground.

Figure 7 : View of the U-beam under construction.

As a further precaution against subsidence, the ground below the U-beam structure will be continuously monitored and any untoward ground movements will immediately be registered for investigation and mitigation works.