COVER STORY
Figure 1: Site location.
and southern boundaries where full-height lateral support was impractical. These berms were subsequently removed using a staged, slotted excavation sequence. Construction and ground movement Construction commenced in January 2025. Ground movements were monitored using high-precision survey measurements taken at regular intervals. Given that permanent grouted anchors were detailed, rigorous quality assurance and quality control procedures were implemented to ensure that full double corrosion protection was achieved. Weekly inspections were conducted by Franki’s design team to confirm that excavation profiles matched the design intent and that corrosion protection measures were correctly applied. Movement in the range of (10-20 mm) was recorded, in line with our predictions and within acceptable limits. All anchors were acceptance-tested and approved in accordance with ISO 22477-5. Corestones in granite profiles – A practical challenge The presence of case-hardened corestones within granitic profiles presents a common challenge during excavation. When rock is encountered, construction teams often request permission to shorten anchor or nail lengths (this might occur at “current excavation level” in Figure 4). However, it is frequently unclear whether the intercepted material represents a corestone or continuous bedrock prior to further excavation or investigation. If the intercepted material is a corestone, its geometry and extent are unknown. Premature decisions to shorten anchors or replace them with rock bolts may result in an inadequately stabilised support system as excavation proceeds to greater depths. Figure 4 illustrates several potential scenarios encountered during excavation: • Figure 4a: Original postulated lateral support design. • Figure 4b: Corestone located within the failure wedge. • Figure 4c: Excavation below an inadequately stabilised corestone, potentially resulting in instability or rock fallout or the requirement for additional/increased density of geonails or grouted anchors below its base. • Figure 4d: Corestone partially intercepting the failure surface, with soil present behind. This might result in increased shear resistance on the failure plane, but the plane may also shift slightly and is difficult to assess. However when drilling a few anchors/geonails to the required depth would illustrate that there is soil behind the corestone. • Figure 4e: Large corestone extending to bulk excavation level but not fully intercepting the failure plane. This scenario is very misleading as the face would show a full rock profile but the anchors/geonails will still be required to stabilise the cut as the failure plane is located in soil.
Figure 2: SPT-N versus depth.
To mitigate this risk, it is recommended that at least two anchors or nails per level be drilled to the full design length as a form of supplementary subsurface investigation to confirm continuous rock conditions. Only once continuity of competent rock has been verified should reductions in anchor or nail length be considered. Conclusion The basement excavation at 115 Victoria Road provides a valuable case study in lateral support design within deeply weathered granite. The combination of a stressed shotcrete support system, real-time monitoring, and rigorous quality control ensured the safe and successful construction of this deep basement. This project highlights the importance of responsive geotechnical design, close collaboration between designers and contractors, and careful interpretation of subsurface conditions in complex geology.
Figure 3: Finite element model used to predict ground movements and structural forces.
Figure 4a: Original postulated lateral support design.
22 CONSTRUCTION WORLD JUNE 2026
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