COVER STORY
WHERE THE MOUNTAIN MEETS THE SEA: ENGINEERING A DEEP BASEMENT IN BAKOVEN, CAPE TOWN A new luxury apartment development, The Solace , is currently under construction in the scenic Bali Bay area between Camps Bay and Bakoven. The five-storey building required the formation of a level construction platform through a deep excavation into a steep hillside. The maximum excavation height is 14,5 m on a roughly 23 m × 23 m footprint.
T he site is underlain by deeply weathered granite of the Peninsula Pluton, part of the Cape Granite Suite, presenting several geotechnical and construction challenges typical of this coastal setting. A stressed shotcrete lateral support system was implemented by Franki Africa, to provide both temporary and permanent excavation support. Location and problem statement The site is located immediately above Victoria Road in the Bali Bay area, between Camps Bay Beach to the north and Bakoven to the south (Figure 1). The development required a deep basement excavation extending approximately 12 m to 14,5 m below an existing retaining wall system. The existing retaining structures comprised a permanent anchored reinforced concrete retaining wall surmounted by a gabion wall and a three-storey residential structure above. The combined height of the existing retaining system is approximately 10 m. The proximity of these structures, together with the narrow site footprint, difficult access for rigs and adjacent properties, necessitated a carefully designed and executed lateral support system. Site geology and subsurface profile Geotechnical investigations were undertaken to inform the design and comprised three rotary core boreholes drilled to depths of between 16 m and 25 m. Standard Penetration Tests (SPTs) were carried out in soil horizons. The site is located within a coastal bay formed in closely jointed, north‑northwest‑trending granite, with hard rock promontories on either side. The subsurface profile is dominated by deeply weathered granite of the Peninsula Pluton. A typical soil and rock profile encountered on site can be summarised as follows: • 0 – 2,5 m: Sandstone colluvium • 2,5 – 3,8 m: Very stiff, fissured clayey silt, reworked residual Granite • 3,8 – 6,4 m: Very stiff, slightly clayey silt, residual Granite • 6,4 – 18 m: Very stiff, Clayey silt with scattered quartz grit, grading into very soft rock, residual Granite • > 18 m: Very hard Granite bedrock SPT results confirmed the progressive increase in strength with depth, transitioning from colluvial soils and weathered granite into competent granite bedrock (Figure 2). Site geohydrology and constraints Groundwater levels were recorded at depths ranging between approximately 9,7 m and 17 m below natural ground level. During excavation, only minor groundwater seepage was encountered, which significantly simplified construction and eliminated the
need for active dewatering measures. The project was subject to several constraints, including tight erf boundaries, neighbouring property agreements, and the narrow zone of soil remaining between the excavation face and the adjacent property’s basement. These challenges were further compounded by the presence of the existing retaining wall system supporting a three-storey residential structure upslope on the eastern hillside face. The existing retaining wall on the eastern boundary had previously been stabilised using grouted anchors. Permission was obtained to install additional permanent anchors extending into the neighbouring property to resist unbalanced earth pressures. Temporary anchors and soil nails were installed along the southern and northern faces. Lateral support design Back-analyses were carried out using slope stability methods to establish representative shear strength parameters. The lateral support design was undertaken using two-part wedge analyses in accordance with SANS 8006-2, supplemented by slope stability analyses and finite element modelling (Figure 3) to assess stability and predict ground movements. The back-analyses considered the existing sloped concrete structure at the front of the site, which remained, at the time, from a previous development. The final system comprised a stressed shotcrete lateral support system with both temporary and permanent grouted anchors, geonails, and locally thickened shotcrete pads at anchor head locations. Temporary berms were retained along sections of the northern
20 CONSTRUCTION WORLD JUNE 2026
Made with FlippingBook flipbook maker