London W12 - The importance of monitoring older structures adjacent to deep excavations.
Tuesday 3rd August 2021
Carl O'Boyle BSc MRICS FCIOB MFPWS
Tayross chartered building surveyors were recently involved August 2021 in a large subterranean development in West London. An advising engineer was appointed and one of the areas that required special attention was the monitoring of ongoing excavation works. Damage can be predicted using computer modelling on older adjacent buildings where deep excavations are being carried out for subterranean developments.
One of the biggest causes of damage to adjacent properties is because of the lateral movement of concrete piles at the top capping beam once excavation gets fully on its way. The deeper the excavation holes the more lateral forces are placed on the top section of the piling. The recommended way of arresting this is to install lateral and diagonal bracing at the head of the pile. Contractors are reluctant to do this as it is very expensive, slows down the program and interferes with construction activities.
By carrying out daily monitoring it allows deep excavation to proceed without necessarily having to brace all of the piles on all sides. The monitoring acts as a safeguard and an early warning detection system giving the contractor time to install Props and shoring as and when required. By installing monitors on the actual capping beam movement can be detected early and propping installed to arrest this therefore preventing serious damage. The importance of monitoring helps ensure that serious damage can be avoided and intercepted before it occurs to older properties.
The sequence below details typical monitoring installation and recording:
A. Install the monitoring targets on the adjoining buildings (these are normally located on a drawing) and monitor the movements weekly as the piles are bored and cast.
B. Bore and cast the piles and install the capping beam.
C. Strike ( Remove) the capping beam formwork and install further monitoring targets onto the capping beam (Again as shown on drawings)
D. Monitor the targets on the buildings and those on the capping beam daily as the basement is excavated.
E. If any of the targets, particularly the capping beam targets, approach the 5mm trigger level The AE (advising engineer) to be notified.
F. If any of the targets, particularly the capping beam targets, show 5mm movements the works will be halted immediately, be notified and the contingency measures will be invoked. The contingency measure in this case will be to install a system of props to the capping beams.
G. Note that the design of the propping system will be undertaken, the props will be ordered and set aside on 'stand-by' in readiness to install if needed.
This preplanning is essential to help avoid damage - monitoring should be classified as Green <5mm, Amber <10mm and Red <15mm each with their appropriate movement defined and pre-action status. Targets can be electronically read remotely, using GPS coordinates. This type of monitoring is becoming more common.
Tayross chartered building surveyors would always recommend this type of monitoring when there are deep excavations occurring next to properties.