Failure mechanism
Damage to adjacent structures from excavation
An excavation changes the stresses in the ground around it, and everything founded in that ground responds to the change. Damage appears when a footing is undermined, when the soil beneath it settles as the water table is lowered, or when the retained face moves — and the structure that suffers is normally the neighbouring one, not the one being built. The 45-degree load spread is a starting point for the geometry of that influence, not a substitute for assessing it.
How this failure works
Ground is in equilibrium before the dig: the soil on one side of the eventual excavation face is being held by the soil on the other. Removing that soil lowers the horizontal stress on the retained side, and the retained ground responds by moving towards the excavation. That is the active state, and the pressure the support must resist rises faster than the depth because the failure wedge behind the face grows with it. The movement is not confined to the face either. The surface behind settles into a trough whose width and depth depend on the soil, the retained height, the water table and the stiffness of whatever is holding the face, and the trough is where the neighbouring footings sit.
Every footing spreads its load into the ground through a bulb of increased stress that widens and deepens with distance below the base. An excavation that enters that bulb changes the stress the footing is founded on, and a footing whose load bulb meets the excavation face has lost bearing even if the digging never touched it. The 45-degree line is the customary first estimate of where the bulb reaches — projected down and out from the edge of the footing — and it is useful precisely because it is crude and quick. The real zone is wider in soft, loose or layered soils, and wider again wherever the water table is being drawn down, so the rule sets an inner boundary rather than a safe distance.
Dewatering is frequently the deciding factor rather than the digging. Lowering the water table increases the effective stress in every layer it passes through, and the consolidation settlement that follows can extend well beyond the excavation and continue for a long time after the digging has stopped. Seepage also carries fines, so ground can be lost without anything visibly collapsing above it, and a footing left sitting over a void is a settlement waiting for the next load or the next rain. In granular soils the same mechanism, uncontrolled, becomes a piping failure at the face, which is fast, and which takes the adjacent support with it.
Excavation plant, compaction, sheet driving and loaded vehicles all put vibration into the ground, and old masonry, cracked render, shallow drains and unbedded services respond to it before anything else does. Retaining structures deform rather than break, and it is that deformation which transfers movement into the adjacent structure — the wall moves a little, the ground behind it moves a little, and the footing further back moves enough to crack a wall that has stood for decades. This is why the control is monitoring rather than inspection: settlement points, crack gauges, inclinometers in the retained soil and precise levelling on the neighbouring structure, read against trigger levels agreed before work starts and often enough that the next stage can still be altered.
Illustration
Conceptual illustration of an excavation beside an existing footing, with the 45-degree load spread marked and the footing load bulb meeting the excavation face.
Typical causes
- Excavation inside the influence zone of a footing, removing the soil that was providing its bearing.
- Loss of passive resistance at the toe of a wall or footing on the excavation side, so the element tips towards the dig.
- Dewatering-induced consolidation settlement extending well beyond the excavation boundary.
- Movement of the excavation support — deflected shoring, sheet piles rotating, or an unsupported batter slumping.
- Surcharge from spoil, plant or stored material on the retained side, adding to the pressure the support has to hold.
- Vibration from breaking, compaction or plant traffic, loosening or softening the soil around existing foundations.
- Groundwater seepage carrying fines out from beneath a footing, so the support is removed from the inside.
Warning signs
- New cracks in a neighbouring wall, or existing cracks opening; the change in crack width is the most useful early indicator available.
- Doors and windows in the adjacent building binding, or a plumb line on the wall going off.
- A depression or a crack line in the surface between the excavation and the structure.
- Water running into the excavation from the face, or a patch of the face that keeps slumping to slurry.
- Floors in the adjacent structure out of level, or a slab that has become hollow-sounding under foot.
- Movement of the support itself — a shoring line deflecting, or a wale pulling away from the face.
Inspection methods
- 01Carry out a condition survey of adjacent structures and services before work starts, with photographs and crack widths recorded, so a later change is measured rather than argued about.
- 02Review the soil investigation and the groundwater level, and check the excavation support against the conditions that are actually there rather than the ones assumed at tender.
- 03Monitor settlement and lateral movement on the neighbouring structures against defined trigger levels, at a frequency that matches the rate of work.
- 04Inspect the excavation face and its support daily for seepage, loss of ground, deflection and surcharge in the wrong place.
- 05Check the depth and position of the excavation against the stand-off the assessment assumed, and record the difference.
- 06Inspect drains and buried services crossing the zone for disturbance, particularly any that might be leaking into the excavation.
Preventive measures
- Assess the influence of the excavation on adjacent structures before the design is fixed, using their actual footing positions and the real soil profile.
- Keep excavation outside the assumed load spread from adjacent footings where the stand-off can be achieved, and support the face where it cannot.
- Design and install the excavation support before digging, and do not let the excavation run ahead of the support.
- Control dewatering and monitor the water level, because settlement from drawdown reaches further than the dig does.
- Keep spoil, plant and materials out of the surcharge zone, and keep heavy plant off the ground behind the retained face.
- Set trigger levels and a response plan before monitoring begins, so that a reading changes the next action rather than being filed.
- Underpin, prop or temporarily support an existing structure before the excavation reaches the depth at which it can no longer span the loss.
Relevant standards
The codes and specifications a design is normally judged against. Naming them is not a claim that AKEN certifies, inspects or approves against them — the design authority for a project does that.
- IS 456
- IS 800
- EN 1997
Related AKEN services
Scope note
This article explains a failure mechanism for engineering education. It is general guidance, not a design check, an inspection report or a certification for any particular structure. Assessments of an actual structure must be carried out by the responsible design authority against the project’s own specifications.