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Monitoring Survey for Underpinning Projects: What You Need to Commission
Underpinning is one of the most intrusive structural interventions a building can undergo. It involves excavating beneath existing foundations and installing new structural elements to support the building. The process inevitably causes some ground disturbance, which can affect the building itself and neighbouring structures.
Monitoring surveys are essential for underpinning projects. They track movement during and after the works, protect the building and its neighbours, and provide the evidence needed for lender and insurance requirements. This article explains what monitoring you need to commission for an underpinning project.
Why Underpinning Requires Monitoring
Underpinning changes ground conditions beneath a structure. Excavation beneath foundations temporarily removes support. New underpinning is installed in stages to maintain support throughout the process. Each stage of excavation creates a period of reduced support.
This process causes movement. Some movement is expected and acceptable. Excessive movement indicates a problem. Without monitoring, you cannot distinguish between acceptable and excessive movement.
Monitoring also protects neighbouring structures. Underpinning affects ground conditions across a wider area than the footprint of the works. Neighbouring foundations may be affected by ground movement or vibration from the underpinning works.
Baseline Survey Before Underpinning
The first monitoring survey for an underpinning project is the baseline survey. This is conducted before any works begin. It establishes the condition of the structure being underpinned and any neighbouring structures that may be affected.
A thorough baseline survey includes:
Photographic record: Every elevation of the building and neighbouring structures, with detailed photographs of all existing cracks, service connections, and significant features.
Crack mapping: Location, width, and orientation of every crack in the structure. Cracks are marked on a plan and measured using crack gauges.
Level survey: Precise height measurements at monitoring points on walls and floors, referenced to stable benchmarks outside the zone of influence.
Tilt survey: Measurement of the inclination of walls and any tall structures, particularly retaining walls or structures adjacent to the excavation.
The baseline survey is documented in a formal report. This report becomes the reference point against which all subsequent movement is measured.
Monitoring During Underpinning
During the underpinning works, regular monitoring visits track any movement. The frequency depends on the stage of works and the level of risk:
During active excavation: Weekly monitoring is standard. In some cases — for deep underpinning or sensitive structures — twice-weekly or daily monitoring may be required.
During concrete curing: Reduced frequency is sometimes appropriate once a stage of underpinning has been completed and the concrete has gained strength.
After each stage completion: A monitoring visit following each stage of underpinning confirms that movement has stabilised before proceeding to the next stage.
Monitoring during underpinning typically includes crack monitoring, level monitoring, and visual inspections at each visit. Data is recorded and compared with baseline and previous readings.
Adjacent Structure Monitoring
Underpinning affects the ground across a wider area than the works footprint. Neighbouring structures may be affected by ground movement or vibration. Adjacent structure monitoring is often required under party wall awards or lender conditions.
Adjacent monitoring typically includes: crack monitoring on neighbouring properties, level monitoring where access is available, and vibration monitoring where heavy equipment is in use.
The extent of adjacent monitoring depends on the proximity of neighbouring structures and the sensitivity of those structures to movement. Victorian terraced houses are more sensitive than modern commercial buildings.
Trigger Levels for Underpinning
Trigger levels for underpinning monitoring are set based on the sensitivity of the structure and the requirements of stakeholders. Typical trigger levels for building movement during underpinning:
Amber alert: Movement of 1mm to 2mm since baseline. Triggers increased monitoring frequency and investigation by the structural engineer.
Red alert: Movement of 3mm to 5mm since baseline. Triggers works stoppage and structural assessment before works resume.
Trigger levels should be agreed with the structural engineer and party wall surveyor before works begin. They should be documented in the monitoring specification.
Post-Completion Monitoring
Underpinning does not stop affecting structures when the last concrete pour is complete. Ground conditions continue to adjust as the new structure integrates with the existing foundations. Post-completion monitoring confirms that movement has stabilised.
Post-completion monitoring typically runs for three to six months after the underpinning is complete. Monthly readings are sufficient for post-completion monitoring.
Monitoring reports during post-completion should confirm that movement has stabilised or ceased. If movement continues, further investigation and possible remediation may be required.
Surveyor Requirements for Underpinning Monitoring
Underpinning monitoring should be conducted by experienced surveyors with appropriate equipment. The surveyors should understand the risks associated with underpinning works and be able to identify concerning readings promptly.
icelabz surveyors are experienced in underpinning monitoring. We work with structural engineers and party wall surveyors to deliver monitoring programmes that satisfy all requirements.
Ask for: professional qualifications and insurance, examples of similar completed projects, the monitoring specification and reporting format, and the escalation protocol for trigger level exceedances.
Fixed-Fee Underpinning Monitoring
icelabz provides fixed-fee monitoring surveys for underpinning projects. Quotes confirmed before instruction. No hidden charges.
Monitoring packages for underpinning include: baseline survey, regular monitoring visits during works, adjacent structure monitoring where required, post-completion monitoring, and regular reporting.
Contact icelabz with your underpinning project details for a fixed-fee quote.
Underpinning Stages and Monitoring Points
Underpinning is typically carried out in short sections called bays. Each bay is excavated, reinforced, and concrete is poured. The next bay is not excavated until the previous bay has gained sufficient strength.
Monitoring points are installed at each bay to track movement during and after excavation. Points are typically installed at: the wall above each bay, at mid-bay positions, at the corner of the structure, and on adjacent structures where relevant.
The monitoring schedule is aligned with the construction programme. Monitoring visits are conducted before each bay excavation, after each concrete pour, and at regular intervals during curing.
Level Monitoring for Underpinning
Level monitoring is the primary method for tracking vertical movement during underpinning. Precision levelling measures height changes at monitoring points with accuracy of around 0.5mm to 1mm.
Level monitoring points are installed on internal walls and floors, typically at heights of around 1m above floor level. Points are referenced to stable benchmarks outside the zone of influence of the underpinning works.
Readings are taken before each stage of underpinning begins, after each stage is completed, and at regular intervals during the monitoring period.
Crack Monitoring for Underpinning
Crack monitoring tracks the opening and closing of existing cracks during underpinning works. New cracks may also appear as movement occurs.
Crack gauges are installed across existing cracks and measured at each monitoring visit. New cracks are identified through visual inspection and photographed.
Crack monitoring data shows whether movement is active and how quickly it is progressing. A crack that widens steadily during underpinning indicates that the works are causing movement. A crack that stabilises after a stage is complete is less concerning.
Vibration Monitoring
Underpinning works involve vibration from excavation equipment, concrete breaking, and heavy plant. Vibration can affect both the structure being underpinned and neighbouring structures.
Vibration monitoring uses accelerometers to measure ground vibration levels at monitoring points. Measurements are compared with trigger levels defined in the monitoring specification — typically based on BS 5228 guidance for construction vibration.
If vibration levels approach or exceed trigger levels, works may need to be paused and mitigation measures implemented to reduce vibration.
Reporting Requirements
Underpinning monitoring reports should be issued promptly after each monitoring visit. Reports should include: current readings at all monitoring points, comparison with baseline and previous readings, trend analysis showing movement over time, flagging of any approaching or exceeded trigger levels, and photographic record of current conditions.
Reports are typically copied to the structural engineer, the contractor, the client, and the party wall surveyor where relevant. All stakeholders should receive monitoring data promptly so that they can respond to any concerns.
icelabz monitoring reports are issued within 48 hours of each monitoring visit. Any urgent concerns are communicated immediately by phone.
Duration of Underpinning Monitoring
Underpinning monitoring extends from the baseline survey through the construction period and into post-completion. The total duration depends on the size and complexity of the underpinning works:
Small underpinning projects: Three to six months total monitoring duration.
Medium underpinning projects: Six to twelve months total monitoring duration.
Large or complex underpinning projects: Twelve months or longer, depending on the programme.
Post-completion monitoring typically continues for three to six months after the last underpinning bay is complete. Monitoring frequency reduces post-completion — from weekly to monthly.
Coordination With Party Wall Surveyors
Underpinning works typically fall under the Party Wall etc. Act 1996 if they affect a party wall or adjoining owner's property. The party wall award specifies monitoring requirements for the adjoining owner's property.
The monitoring surveyor coordinates with the party wall surveyor to ensure that monitoring satisfies award requirements. Monitoring reports are provided to the party wall surveyor so that they can fulfil their obligations.
icelabz works with party wall surveyors to deliver monitoring programmes that satisfy all award requirements.
Costs of Underpinning Monitoring
Underpinning monitoring costs depend on the scope of the monitoring programme:
Baseline survey: From around 300 to 500 pounds depending on property size and number of monitoring points.
Monitoring visits: From around 100 to 200 pounds per visit depending on number of points.
Monthly monitoring package: From around 400 to 800 pounds per month for weekly monitoring.
Adjacent structure monitoring: Additional cost depending on number of neighbouring properties and monitoring points.
All icelabz quotes are fixed-fee with no hidden charges. Contact icelabz with your underpinning project details for a fixed-fee quote.
Choosing a Monitoring Surveyor
For underpinning monitoring, choose a surveyor with specific experience of underpinning projects. Underpinning monitoring has particular requirements — alignment with construction stages, coordination with party wall surveyors, and prompt escalation of concerns.
Ask for examples of similar completed projects. Verify professional qualifications and insurance. Confirm the reporting format and escalation protocol.
icelabz surveyors are experienced in underpinning monitoring. We provide fixed-fee monitoring for underpinning projects across London.