A new 200 KTPA aluminium recycling facility was planned within an existing industrial site to produce billets and T-bars using up to 75% post-consumer scrap. One of the facility’s critical structures was the Vertical Direct Chill casting pit beneath the casting machine. The pit provides the depth required for aluminium products, including round and square billets, rolling slabs and T-ingots, to move downwards and solidify continuously during casting.

The reinforced concrete pit measured approximately 12.5 m in diameter and 14 m in depth. A 2.6 m diameter steel caisson extended to approximately 24 m within the pit. The structure was enclosed by an 800 mm thick circular diaphragm wall, with panels extending to a depth of approximately 20 m. Together, the diaphragm wall and steel caisson were designed to withstand earth pressure and protect the pit from groundwater and aggressive chemicals in the surrounding soil and water.

TCE served as the EPCM consultant, while construction was carried out by the execution contractor and a specialist piling subcontractor. Close collaboration became critical when construction encountered subsurface conditions that had not been fully identified during the initial borehole investigation.

The Challenge

The geotechnical investigation identified a 9 to 11 m layer of loose to medium-dense silty sand, followed by extremely weak sandstone and calcarenite formations. Groundwater was encountered at a depth of approximately 3 to 4 m. Although the boreholes did not reveal any cavities, drilling-fluid losses had been recorded during parts of the investigation and piling work.

During construction of the diaphragm wall, sudden and unusually high losses of drilling fluid were observed. Concrete placed for some wall panels also began to sink unexpectedly. These conditions indicated the possible presence of localised voids or irregularities within the weak rock layers.

The immediate challenge was to stop the loss of drilling fluid, stabilise the excavation and complete the diaphragm-wall panels to their required depth. At the same time, the team needed to determine whether the subsurface conditions were limited to a few locations or represented a wider risk to the long-term stability of the casting pit.

Further challenges emerged as construction progressed. Gaps between adjacent diaphragm-wall panels led to water leakage outside the pit. After excavation, minor surface cracks, mud inclusions and bleed channels were observed in the primary and secondary panels. Water also seeped through cracks in the wall. These issues had to be addressed while safely excavating to the 14 m raft level in an area with shallow groundwater.

TCE’s Approach

TCE developed a phased response that addressed both the immediate construction challenges and the long-term performance of the structure. The approach combined ground sealing, targeted rework, geophysical investigation, continuous monitoring, controlled dewatering and permanent repairs.

1. Controlling Drilling-Fluid Loss and Stabilising the Excavation

Lean concrete was pumped through the drilling fluid to the bottom of the excavation. This created an immediate seal over suspected underground cavities or highly permeable areas and helped stabilise the panel excavation.

After 24 hours, the treated panel was redrilled to achieve the required diaphragm-wall depth. Once the method had been successfully tested at site, it was adopted for other primary and secondary panel excavations where similar drilling-fluid losses were encountered.

2. Understanding the Subsurface Conditions

Since the original borehole investigation had not identified any cavities, TCE recommended a geophysical investigation to examine the soft rock layers more closely and detect possible voids or irregularities.

The study confirmed that the cavities were localised and did not present a long-term risk to the stability of the structure. This allowed the team to focus treatment on the affected areas rather than approaching the condition as a site-wide ground problem.

3. Monitoring the Diaphragm Wall

TCE recommended installing two inclinometers within the diaphragm-wall panels and continuously monitoring them during execution.

These instruments enabled the team to measure wall movement as the excavation progressed. This provided direct evidence of the wall’s behaviour and an additional means of checking its performance during construction.

4. Strengthening Panel Connections and Watertightness

Investigations found that some of the leakage was caused by gaps between adjacent diaphragm-wall panels. To address this, a stop-end connection detail was developed for the junctions between the primary and secondary panels.

The detail improved the connection between neighbouring panels and strengthened the wall’s resistance to water ingress.

5. Repairing Cracks and Local Construction Defects

Visible points of water ingress were first plugged as an immediate measure. A permanent repair solution was then proposed using a polyurethane-based expanding resin to seal the cracks and restore the wall’s watertight integrity.

Minor surface cracks, mud inclusions and bleed channels exposed after excavation inside the pit were treated using shotcrete facing, bonding grouts and suitable admixtures.

6. Managing Groundwater During Deep Excavation

Deep dewatering points were installed within the soil and rock enclosed by the diaphragm wall. Where required, additional dewatering points were installed through the 800 mm thick wall panels.

These measures enabled the pit to be excavated safely and in a controlled manner to the 14 m raft level. Special drainage arrangements were also incorporated at the base of the pit, including dewatering pipes that could be sealed after construction of the raft.

Collaboration During Execution

The response required clear coordination between the design and construction teams. TCE assessed the conditions observed at site, developed or recommended the engineering solutions, and defined the investigation and monitoring requirements.

The execution contractor and specialist subcontractor implemented the agreed measures, including panel treatment, redrilling, repairs, instrumentation support and dewatering. This coordinated approach allowed site observations to be assessed quickly and translated into practical construction measures, while keeping the structure’s long-term performance in focus.

Outcome

The site trial confirmed the effectiveness of the lean-concrete bottom-sealing method. It was subsequently used wherever similar drilling-fluid losses were encountered during the construction of primary and secondary wall panels.

The geophysical investigation established that the underground cavities were localised and did not present a long-term stability risk. Continuous inclinometer monitoring also provided greater visibility into diaphragm-wall movement as the excavation progressed.

Improved panel connections, local sealing, resin injection, shotcrete treatment, bonding grouts, deep dewatering and drainage at the raft level collectively addressed the principal sources of water ingress. These measures enabled excavation to proceed safely to the required depth while protecting the stability and watertightness of the casting pit.

The project demonstrates the value of combining site observations with targeted investigation. Rather than relying only on the original ground model, the team used actual site behaviour to guide its engineering response. Immediate measures supported construction progress, while geophysical investigations, continuous monitoring and permanent repairs addressed the structure’s long-term requirements.

Key Engineering Takeaways

• Borehole investigations may not always identify small, localised cavities within weak rock formations, even when drilling-fluid losses indicate possible subsurface irregularities
• A site-tested bottom-sealing method can provide an effective immediate response to sudden drilling-fluid loss and unstable panel excavation
• Geophysical investigation can establish whether a subsurface condition is localised or part of a wider stability concern
• Continuous monitoring provides measurable evidence of diaphragm-wall behaviour as excavation progresses
• Watertightness in deep diaphragm-wall structures depends on panel connections, local repairs, dewatering and permanent drainage working together
• Close coordination between the EPCM consultant, execution contractor and specialist subcontractor is critical when actual ground conditions differ from the original investigation