Deep excavation projects in London, Ontario demand a rigorous approach grounded in the National Building Code of Canada and CSA A23.3, but the real challenge lies in interpreting the city's complex glacial stratigraphy. The St. Joseph's till plain, deposited by the Huron lobe of the Laurentide Ice Sheet, creates a layered profile of silty clay till overlying fractured limestone—a combination that behaves unpredictably when unloaded. In our experience, the interface between the till and the bedrock is where most design assumptions fail; water migrates along the rockhead, softening the clay and triggering instability. A desk study alone won't capture this. The design process must integrate field investigation, laboratory testing, and a clear understanding of how pore pressures redistribute during staged excavation, particularly when working near the Thames River valley where the water table sits within three metres of grade. For sites where the till is unusually stiff, we often recommend supplementing the investigation with in-situ permeability testing to calibrate seepage models before finalizing the shoring geometry.
The interface between the glacial till and the fractured limestone is where most deep excavation designs in London succeed or fail—it controls both base stability and groundwater inflow.
Our approach and scope
Site-specific factors
The single most common mistake we see contractors make in London is assuming that the silty clay till can stand unsupported on a near-vertical cut for even a few hours. This till is heavily overconsolidated and looks competent when first exposed, but it relaxes rapidly when unloaded—within twenty-four hours, negative pore pressures dissipate and the face starts spalling. The result is a progressive sloughing that undermines the shoring installation sequence and creates dangerous overbreak behind the wall. On one project near Oxford Street, a contractor delayed installing the first row of walers by two days; the subsequent movement triggered a partial collapse that took three weeks to remediate. The lesson is clear: the design must specify a strict timeline for support installation tied to each lift, and the instrumentation plan must include real-time inclinometer readings to catch the early stages of creep before they become a safety incident.
Applicable standards
NBCC 2015 (National Building Code of Canada), CSA A23.3-14 (Design of Concrete Structures), ASTM D7181 (Drained Triaxial Compression Test for Soils), Canadian Foundation Engineering Manual (4th Edition), Ontario Regulation 332/12 (Building Code Act)
Other technical services
Shoring System Design
Development of shoring schemes including soldier pile and lagging, secant pile walls, and diaphragm walls, with structural analysis per CSA A23.3 and geotechnical input calibrated to London's glacial till properties.
Dewatering and Groundwater Control
Design of deep well, wellpoint, and ejector systems to manage the high water table near the Thames River and control seepage at the till-bedrock interface, including settlement assessment for adjacent structures.
Base Stability and Heave Analysis
Limit equilibrium and finite element analysis of excavation base stability in soft to stiff clay, evaluating the risk of bottom heave and hydraulic uplift using undrained shear strength data from local Shelby tube samples.
Construction-Phase Monitoring Plans
Specification and layout of inclinometers, vibrating wire piezometers, and total station survey points to track wall deflection and groundwater response during staged excavation, with trigger values for contingency measures.
Typical parameters
Quick answers
What is the typical cost range for deep excavation design in London, Ontario?
Professional fees for geotechnical design of deep excavations in London generally range from CA$2,790 for smaller cut-and-cover projects to CA$10,990 for complex multi-level excavations requiring detailed FEA modeling, tieback design, and instrumentation planning. The final cost depends on excavation depth, proximity to adjacent structures, and the number of shoring alternatives analyzed. We provide a fixed-fee proposal after reviewing the site investigation data and structural drawings.
How does the glacial till in London affect the choice of shoring system?
The St. Joseph's till in London is a stiff, overconsolidated silty clay that can exhibit significant relaxation upon excavation. This means the shoring system must be installed quickly after each lift and preloaded adequately. Soldier pile and lagging is common for shallower cuts, but for excavations exceeding eight metres in this till, we often recommend secant pile walls because they provide continuous lateral support and better control of groundwater ingress at the joints.
What instrumentation is required during deep excavation in London?
At minimum, we specify inclinometers installed behind the shoring wall to monitor lateral deflection, vibrating wire piezometers to track pore pressure changes in the till, and regular optical survey of wall alignment. For excavations adjacent to heritage masonry buildings—common in downtown London—we add crack monitors and settlement points on the neighboring structures, with daily readings during the first week of each major excavation stage.