EXC/Hazards/TBT-EXC-005

Excavation Near Structures

Excavations › Hazards › Excavation Near Structures

Presenter		Project
Location		Date

Excavation near existing structures removes the ground that supports their foundations, risking settlement, cracking, and collapse.
The zone of influence extends diagonally from the base of the structure's foundation — any excavation within this zone is high risk.
A structural assessment must be carried out before excavating within the zone of influence of any existing building or wall.
Monitoring instruments such as tell-tales, crack gauges, and level surveys detect movement before it becomes dangerous.
Trigger levels must be set for monitoring readings — exceeding a trigger requires immediate investigation and possible work stoppage.
Temporary support such as shoring, propping, or underpinning may be required before excavation can proceed safely.
Vibration from plant, piling, and compaction near structures can cause additional settlement and damage to foundations.
Dewatering during excavation near structures can lower the water table, causing ground settlement beneath adjacent foundations.
Historical structures, party walls, and listed buildings require additional care and specialist engineering advice.
The excavation method statement must specifically address the sequence, support, and monitoring for work near structures.

Structural collapse	Removing ground support from foundations causes buildings to crack, lean, and collapse — structural assessment before digging is mandatory.
Third-party damage	Damaging a neighbouring property through careless excavation creates legal liability, compensation claims, and project delays.
Progressive failure	Foundation settlement starts slowly but accelerates — monitoring catches early movement before it becomes catastrophic.

Don't

Carry out a structural assessment before excavating near any existing building or wall.
Install monitoring instruments and set trigger levels before excavation begins.
Follow the excavation method statement for sequence, depth, and support near structures.
Install temporary support such as shoring or propping before undermining foundations.
Monitor readings daily and investigate immediately if trigger levels are approached.
Control vibration from plant and piling near existing structures throughout the works.
Assess dewatering effects on adjacent foundations before lowering the water table.
Seek specialist advice for work near historical, listed, or party wall structures.
Photograph and record the condition of adjacent structures before excavation starts.
Stop work and seek engineering advice if unexpected movement or cracking is detected.

DON'T excavate within the zone of influence without a structural assessment first.
DON'T proceed without monitoring instruments in place and trigger levels agreed.
DON'T deviate from the method statement sequence when excavating near structures.
DON'T undermine foundations without installing temporary support beforehand.
DON'T ignore monitoring readings approaching trigger levels — investigate immediately.
DON'T use heavy vibrating plant near existing structures without assessing the impact.
DON'T lower the water table near buildings without assessing the settlement risk.
DON'T treat party walls and listed buildings the same as new construction — seek advice.
DON'T skip the pre-condition survey — without it, existing damage cannot be distinguished.
DON'T continue excavating if cracking, tilting, or settlement is observed on adjacent structures.

See also: Excavation Safety Awareness | Adjacent Structure Monitoring

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