Aberdeen sits on a complex geological interface between the Cairngorm granite pluton and the Dalradian metasediments. Most deep excavations within the city centre encounter granitic bedrock at depths between 2 and 15 metres, overlaid by highly variable glacial till. The 2015 Union Square basement excavation reached 11 metres through mixed ground conditions, and groundwater ingress from the River Dee paleochannel complicated the retaining system. Every deep excavation in Aberdeen requires a design that accounts for the transition from dense till to fractured rock. Our team prepares slope stability analyses for open-cut sections and full shoring designs where space is tight. We also integrate excavation monitoring specifications into every design package, because the proximity of granite tenements on Union Street leaves zero margin for movement.
Granite bedrock in Aberdeen is strong but the till above it is unpredictable — our designs handle both with the same rigour.
Our approach and scope
Site-specific factors
The till-bedrock interface is the primary risk driver in Aberdeen. A steeply undulating rockhead can leave one corner of an excavation in dense till while the adjacent corner hits fresh granite. Differential stiffness leads to uneven wall loads and unexpected prop forces. The till itself contains lenses of silty sand that act as confined aquifers; we have measured artesian pressures of 15 kPa in boreholes near Holburn Street. Basal heave in overconsolidated till is less common than in soft clays, but wedge failures along joint sets in the granite are a real hazard. Our designs include joint orientation surveys from oriented core or televiewer logs, so the excavation face orientation can be adjusted to avoid daylighting unfavourable discontinuities.
Regulatory framework
BS EN 1997-1:2004 Eurocode 7: Geotechnical design, BS 5930:2015 Code of practice for ground investigations, BS 8002:2015 Code of practice for earth retaining structures, BS 7385-2:1993 Evaluation and measurement for vibration in buildings
Complementary services
Deep Excavation Design
Full geotechnical design including wall type selection (sheet pile, secant, diaphragm), strut and waler sizing, base stability checks against heave and piping, and groundwater control system specification. Delivered as a design report with calculation pack and construction drawings.
Construction Phase Monitoring Plan
Design of the instrumentation and monitoring scheme required by the temporary works designer. Includes trigger values for deflection, settlement, and vibration. We specify inclinometer locations, survey points on adjacent buildings, and piezometer arrays to verify groundwater assumptions during dewatering.
Typical parameters
Common questions
How deep can you excavate in Aberdeen before hitting bedrock?
Rockhead depth varies significantly across the city. In the city centre, granite is typically encountered between 2 and 15 metres below ground level. Near the harbour and along the River Dee, alluvial deposits can push bedrock deeper, occasionally exceeding 20 metres. We determine the exact profile through rotary coring as part of the ground investigation.
What retaining wall types are suitable for Aberdeen's ground conditions?
Sheet piles work well in the till but struggle to penetrate fresh granite. Secant pile walls are more adaptable to variable rockhead because the piles can be terminated at different depths. Diaphragm walls are rarely justified in Aberdeen due to the shallow rock, except for very deep shafts near the harbour where the granite is deeper.
How much does a deep excavation design package cost?
A complete design package for a deep excavation in Aberdeen typically ranges from £1,520 to £6,800, depending on excavation depth, complexity of the retaining system, and the extent of ground investigation data already available. Projects requiring 3D finite element modelling will be at the upper end of this range.
Do you handle the temporary works design for the contractor?
Yes. We act as the temporary works designer and provide the full design package required under the CDM Regulations. This includes the retaining wall design, strutting and bracing calculations, dewatering specification, and the monitoring plan. We can also carry out design checks on the contractor's proposed methods if the design is already partially developed.