Geotechnical Engineering

Resins to the rescue

Left unattended, foundation failures eventually undermine their supporting structures, whether it’s a bridge, a building or a concrete slab. Traditionally, demolition and reconstruction were among the remediation options available. However, the latest geopolymer resin technologies offer a far more viable, non-disruptive alternative, says Tony Pappalardo, director, URETEK Geo-systems SA.

Water ingress is one of the main causes of undermining in any structure, whether it be a gradual penetration of the underlying soils or a sudden event. The latter was the case for the new Chuma Mall in Dieplsoot, Gauteng.

Just as it was nearing completion in November 2019, the first storm of the season caught the contractors unprepared. Some 30 mm of rain fell in just 20 minutes, with the bulk of the run-off accumulating between the north-east corner of the building and a retaining wall.

Subsequently, water infiltration caused the main corner concrete column at the Chuma Mall to settle by 60 mm, affecting seven additional columns, 25 m of strip footings,

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4 as well as some 200 m 2 of internal floor slabs. DCP (dynamic cone penetrometer) tests revealed that the layer works between the strip footing and the retaining wall were compromised to a depth of 4 m.

After considering their options, which included the possibility of extensive rebuilding, the client’s technical team decided to appoint URETEK Geo-systems SA to employ its revolutionary non-disruptive approach to complete the necessary ground support interventions. Remarkably, the columns, foundations and slabs were successfully resupported and relevelled in five days, ensuring that the contractor could meet the handover date.

“This result seems amazing, and it is. URETEK’s pioneering solution increases the load-bearing capacity of the subsoil and is a revolutionary technology that involves injecting the ground with structural resins to achieve permanent consolidation of the foundation soil below any concrete built structure,” Pappalardo explains, adding that different types of URETEK-patented resin are specified, depending on each project requirement. URETEK Geo-systems SA is a licensed partner of the URETEK network – spanning some 70 countries worldwide.

Deep injection and slab lifting

For the Chuma Mall project, the two URETEK methodologies employed were deep injection and slab lifting. The resins are injected via tubes inserted to predetermined depths through 14 mm diameter holes spaced in a grid pattern. As it expands, the resin immediately starts to improve the

MAYDON WHARF WAREHOUSE FLOOR REINSTATEMENT 1 Resin injection operations in progress, with the rising concrete slab level being verified by continuous laser measurement 2 Resin being extruded to the surface through one of the drilled holes as the floor nears its original level KHWEZELA COLLIERY 3 A side view of the dragline service pads 4 Drilling into the pad

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soil’s geomechanical properties, increasing the shear resistance by compacting the soils and filling in any macroscopic spaces present.

Expansion is greater in the direction in which the resin encounters the least resistance, which is where the foundation soil is the weakest. Hence, the weakest soil layers are compacted first. Soil bearing capacity improvements of 200% to 300% are common.

At Chuma Mall, a series of holes were drilled between the retaining wall and the strip footing. Each hole contained three injection tubes of varying lengths to evenly increase the bearing capacity of the fill material between the building and the retaining wall to a depth of 4 m.

During the slab lifting stage, 14 mm diameter holes were drilled in a grid pattern to cover the 200 m 2 of subsided surface. Resin was injected into 300 mm of the layer directly below the concrete, returning the slab to its original level.

“When traditional methods and piling are not suitable, URETEK is often the most cost-effective, least invasive and fastest remediation solution,” Pappalardo continues. Water sealing is another key service URETEK offers to prevent soil settlements and ground erosion.

Khwezela Colliery

Remediating dragline service pads truly underscores the success of URETEK’s proprietary technologies. Draglines are enormous excavation machines weighing thousands of tonnes, so the pads they rest on need to be very well founded, given the point loadings they’re subjected to.

“If our methods work here, then they can work anywhere and for any ground support application,” Pappalardo points out.

A recent example is URETEK’s project for Anglo American’s Khwezela Colliery in eMalahleni (Witbank). Hit by 2019’s unusually monsoon-like rainfall patterns, the site’s dragline service pads suffered severe settlement.

The structure comprises two thick concrete pads, each measuring 40 m x 1.5 m x 0.8 m in height. A crack developed in one of the pads and it started to sink.

“DCP tests revealed that the bearing capacity of the first metre of soil below the two pads was insufficient to carry the 3 600 tonne dragline,” he explains. “A further weak layer was also found at -3.5 m, having been compromised by water.”

The option of removing and replacing the two concrete pads and ground works to a depth of 4 m proved to be too time-consuming and

At Chuma Mall, a series of holes were drilled between the retaining wall and the strip footing ahead of the resin injection process

price-prohibitive. The delay to the dragline operations would be too costly.

Mobilising on-site, URETEK’s team drilled a series of 14 mm holes diagonally along the external face of the pads. Injection tubes of varying lengths were then inserted to evenly treat the weak soils under the 1.5 m width of the pads.

The structural geopolymer injected increased the bearing capacity of the weak top 1 m layer. In addition, injections at -3 m stabilised the weak layer identified at this depth.

“Expansion forces of up to 30 000 kPa can be achieved, depending on the requirements,” says Pappalardo. “The resin injections continue until the overlying structure begins to lift, as verified by continuous laser monitoring. This confirms the point at which the load bearing achieved at the treated point is now considerably greater than the static load.” The entire procedure at Khwezela Colliery was successfully completed in three days.

Maydon Wharf storage warehouse

Concrete slab settlement is a common occurrence, whether it’s an airport apron or a warehouse floor. When the construction occurs on reclaimed land, factoring in the ground conditions can be especially challenging.

This was the case for a storage warehouse floor at the Maydon Warf Port Terminal in Durban. The facility makes use of a high-density mobile racking system, which is electrically operated and runs on rails embedded in the concrete.

The 2 600 m 2 concrete warehouse floor is built on reclaimed land that, over the years, has developed problematic settlement issues. In the past, these problems were remedied by casting a new concrete floor over the existing one. However, the settlement continued by some 60 mm and the operation and safety of the mobile racking system became compromised.

Conventional options included the installation of additional piling or the construction of a new concrete floor slab. The latter would have been around 30% cheaper than the piling solution. However, the reinstatement of the embedded railing system would have caused further delays. In the end, going either route was ruled out from time and cost perspectives.

URETEK’s repair solution was completed in around seven days via the application of its slab lifting and deep injection technologies. Demonstrating the precision of these operations, the 2 600 m 2 floor was returned to its original level within a 3 mm tolerance.

The above case studies showcase the fact that there’s always a solution and, to date, the URETEK Group has introduced more than 30 resins to meet varying project and soil parameters.

“In the last 30 years or so, Group licensees have successfully executed more than 200 000 projects and we’ve certainly made our own unique contribution as the South African arm. Locally, URETEK technologies are rapidly becoming the mainstream choice in geotechnical engineering and ground support,” Pappalardo concludes.

www.uretek.co.za