When it comes to construction projects requiring deep excavations, there is always a requirement for a significant movement of material, and consequently the potential for disruption to that vital layer of support. Basement construction works are often also carried out in highly and densely populated urban areas, so it is crucial that an accurate geotechnical assessment of the ground conditions and surrounding infrastructure is carried out to understand the conditions at play prior to works commencing.
A fit-for-purpose, well managed and executed basement monitoring campaign will create value, aid best practice and deliver real benefits for engineers, contractors and the client in terms of safety, assurance and programme. On the other hand, monitoring campaigns that aren’t well-considered or lack the appropriate level of detail can prove costly in terms of wasted time, effort and money and also in terms of the safety risk to both the public and the construction workforce.
The team at SOCOTEC Monitoring has extensive experience in the monitoring of piles, diaphragm walls, temporary propping systems and neighbouring structures for changes in inclination, strain, water level, pore water pressure, vibration and more. Here, Jon Scott – managing director at SOCOTEC Monitoring – shares the knowledge and experience his team has gained from monitoring large excavations and underground works over the past 40 years, helping contractors and clients achieve best value from their monitoring campaigns.
Lessons learnt in deep excavations and basement monitoring
In SOCOTEC Monitoring’s experience, successful basement monitoring campaigns share three key approaches that ensure activity is carried out to best practice standards.
1) The specification is prescriptive about what needs to be achieved, and I&M specialists are trusted to advise on how it is done
Firstly, why is monitoring required? Of course, monitoring is important for contractors in relation to compliance with a Section 61 consent or any other potential third-party agreements that set out legal requirements in relation to construction activity. Other motivating factors include monitoring’s role in supporting Value Engineering and insurance stipulations. However, major failures where fatalities have occurred and where poorly managed monitoring schemes were involved underline the real critical nature of the issue, ensuring the safety of the public and the workforce.
The range in quality of specifications the team at SOCOTEC Monitoring sees is amazing, from the briefest of outlines to those that are incredibly prescriptive and provide little flexibility for the site team to develop a more pragmatic solution. We have even seen specifications which include systems and techniques that have been superseded by new, evolving technologies.
The challenge is to develop specifications that hit the sweet spot in the middle. This is best achieved through a collaborative relationship between the client or contractor and the instrumentation and monitoring (I&M) specialist. When the client or contractor’s specification clearly outlines what needs to be monitored, how frequently and for how long, the I&M specialist can then use this as the basis for proposing the best system to deliver these requirements, drawing on their experience, expertise and knowledge of the latest innovative technologies.
One issue SOCOTEC Monitoring commonly encounters in specifications for basement monitoring campaigns is the installation depth of inclinometers, which are used to monitor lateral movement of ground or retaining structures associated with underground excavations. On over 90% of projects where inclinometers are required to be installed in embedded retaining walls, they are specified to go to the full length of the piled or diaphragm wall. However, these installations sometimes do not go to a depth sufficient to guarantee that the toe of the inclinometer is fixed in place, leaving the instrument effectively floating in space and resulting in poor quality data.
One common way to ‘fix’ the inclinometer is to use a 3D target at the top of the inclinometer and manually survey the top. The problem with this approach is that the accuracy of the survey is often not precise enough and, as such, can lead to misleading data. Furthermore, being located at capping beam level, the targets themselves are vulnerable and invariably get damaged or become inaccessible during the course of the project.
Whilst using a drill rig might require a higher initial investment, because it can reach another five metres or more through the base of the void former into hard strata, this will improve the value of the resulting data no end. The potential for replacing in-place inclinometers with remote, wireless loggers also highlights the importance of specifications including newer, better monitoring techniques. The cost benefit for both of these recommended approaches is well worth considering, saving time and money later in the project.
Get the specifications right and the result is a reliable stream of accurate, actionable information.
2) Work is sequenced in the right order, baseline monitoring is in place early and people are accountable
The planning stages of any monitoring campaign have a crucial influence on the success of the outcomes. Typically, a project involving basements and underground excavations adopts the following sequence:
- Site Investigation
- Demolition/Enabling Works
- Substructure construction
- Superstructure construction
The value in monitoring is based on an understanding of risk, which itself is based on an assessment of any variance from ‘normal’ conditions. In order to establish a clear view of what ‘the norm’ is, the process should be incorporated into the early stages of any project.
Ideally, SOCOTEC Monitoring installs baseline monitoring well in advance of any significant work, providing a stable set of data which reflects the existing environmental conditions such as diurnal and seasonal temperature, tidal effects and other external influences. Being involved late can mean too short a baseline monitoring period and little or no continuity between the activities of the various contractors on each stage of the project, leading to a lack of continuity and poor monitoring results.
For example, on a recent project SOCOTEC Monitoring installed some magnetic extensometers, which are particularly useful in monitoring the heave induced by the unloading of over-consolidated clays such as London Clay to confirm basement slab design. Initially, these were installed as part of the site investigation where they had a drilling rig on site to drill the deep boreholes that were needed. The demolition contractor then came in and the installations were lost as the demo contractor had no responsibility for them.
The extensometers should have been replaced at least as soon as the demolition had been completed, although it would have been interesting to know how the London Clay had responded to the demolition of the existing high-rise building on the site and the subsequent unloading.
Through a sequence of changes in contractors and project managers, and despite the SOCOTEC Monitoring team raising the issue at many meetings, the decision was made to install the new extensometers using a cut-down rig through the base slab of the basement, after the dig had taken place. In cases like this, a lack of ownership and accountability is often the reason that planning and programme issues are common I&M delivery culprits.
By prioritising the planning and development of the programme, it means that all aspects have been properly considered, from power supply, remote communications, cabling specifications and routing through to environmental issues and protection of equipment. It also allows for equipment and systems to be tested by the monitoring contractor prior to deployment, and ensures the designs, programmes, licences, permits and systems will be compliant with all relevant standards, avoiding possible costly problems at a later stage.
3) Value, innovation and collaboration is prioritised over cost when it comes to procurement
Procurement can be a delicate area when it comes to monitoring. A thorough and robust procurement process, conducted by people with the right knowledge and skills, ensures the activity is optimised to deliver not only the most valuable data but the most value to the project.
Timing is also vital. Just as the strength of preparation and planning correlates directly to the overall success of monitoring campaigns, there is also real benefit in considering, specifying and procuring monitoring at the earliest stage possible. This ensures that the right data will be recorded at the right point as the project develops.
Another common issue is that procurement must be structured for the benefit of the client. If this is not the case and procurement processes are skewed for the convenience of the Quantity Surveyor, for example, then it can hinder best practice. Performance specifications are important here to introduce the necessary accountability and provide assurance
It is also important that procurement is conducted by those with the necessary depth of knowledge. Achieving the maximum return on investment from monitoring means bringing together and balancing out the various requirements into an optimised schedule of activity. Procurement teams must have a precise understanding of what is being set out in front of them if they are to make an accurate assessment of whether it represents good value. If this is not the case, and decisions are made simply on the basis of the lowest price, then the objectives of the monitoring campaign can be compromised.
The quality of the proposed system, the monitoring team’s experience and credentials and their ability to think around problems is often of greater value than the lowest price.
Damac Tower: a case study in collaboration
One example where best practice in monitoring has been followed with great success is DAMAC Tower, formerly AYKON Tower, which forms part of the New Bondway development in Nine Elms, London. The site is adjacent to the railway viaduct carrying several very busy tracks into Vauxhall and Waterloo stations, and so a major driver for the monitoring was to satisfy any possible concerns from Network Rail that the demolition, piling, basement dig and construction was not going affect the viaduct structure and impact on the safe running of trains.
The monitoring specifications developed by the consultant, WSP, were well-considered and outlined the requirements with the appropriate level of detail. Elements covered within the plan for this project included:
- Track trolley
- Vibration monitors
- Electrolevel’ beam arrays
- along the length of the parapet
- vertical face of the viaduct
- in the crowns of the viaduct arches
- on the bridge abutments
Having won the contract, SOCOTEC Monitoring was employed directly by the client, although it was necessary to work collaboratively with WSP and Network Rail to produce a practical Monitoring Design Plan, based on WSP’s original specification.
Another success story is the monitoring of the basement and sub-structure elements during the construction phase of the Soho Place Over Site Development (OSD), safeguarding the London Underground and Crossrail platforms and running tunnels directly below the site.
A better understanding and adherence to the three best practice approaches outlined here, and closer collaboration between everyone involved – from the monitoring contractor to geotechnical engineers, quantity surveyors, project engineers, main contractors and consultants – can help deliver best value on a basement monitoring project, and any other monitoring project.
If you would like to speak to the SOCOTEC Monitoring team about a best practice approach to your next basement monitoring project, please get in touch.
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