This is where geotechnics comes in - helping project teams to understand the history beneath a project site. Geotechnical engineers drive our approach to navigating major projects by assessing possible impacts of underground conditions, ensuring site stability during construction and enabling us to anticipate future challenges regarding project growth or expansion.
From concept to detailed design, tendering processes to project procurement, and right through to construction, geotechnical engineers provide fundamental, even foundational data with implications for project feasibility, safety and budgets.
Settlement on the NSW Hunter Expressway
Large-scale transport projects often represent confounding geotechnical challenges. For example, the Hunter Expressway project involved treating soft soils in a time-critical part of the 40 km, four-lane motorway link between the M1 Pacific Motorway west of Newcastle and the New England Highway.
For the Buchanan project area, the paleochannel alluvial deposition pattern made treating the soft soils problematic. Stringent settlement criteria called for preloading with wick drains and surcharge, and installing multiple instrumentation arrays for monitoring. Once consolidation settlement seemed complete, engineers released the treatment preload.
But post-construction monitoring revealed that in less than three years, settlement had exceeded the criteria. Some locations recorded over half the permissible 40-year total settlements and more than the permissible 40-year longitudinal grade change. It became evident that remediation works were needed on an already, fully operational expressway.
So, what happened?
Robert Kingsland, Geotechnics Technical Executive at WSP remembers, “The situation confirmed just how sharply a complex sediment depositional environment can vary. Such situations warrant higher-density ground investigations and more conservative treatment extents, particularly if there are bridge abutments involved.
“Hindsight is a wonderful thing. Put simply, if we had delayed surcharge release, extra consolidation would have helped constrain these settlements.
“But, turns out, our weekly ‘hold point review’ meetings at the time were just too frequent for the team to detect subtle changes. If anybody had noticed, we’d have delayed the decision until we had enough extra data to challenge or validate the hold point release.
“In the end we used a targeted slab-jacking program with injected expanding resin to correct areas of grade non-compliance and lift areas of most concern.
“We learned, among other things, that with good pre-planning and safety protocols, a busy motorway carriageway can be shut down for pavement treatment with good productivity and safety outcomes.”
Woolgoolga to Ballina Pacific Highway upgrade
The Woolgoolga to Ballina Pacific Highway upgrade (W2B) project is duplicating about 155 km of single carriageway road on the Pacific Highway to a four-lane divided road. Roads and Maritime Services engaged Pacific Complete, a joint venture between Laing O’Rourke and WSP, as the delivery partner for the upgrade. The geotechnical team provided detailed design and construction phase geotechnical support to all seven major design and construction portions of the project.
Among the major project risks identified were several construction phase risks related to managing geotechnical aspects. The construction program depended on knowing how long ground consolidation would take at soft soil treatment sites – which turned out to be approximately 19 per cent of the project alignment.
The soft soil design experts reviewed daily monitoring data and grouped the sites into low risk and high risk, depending on characteristics, thickness and non-negotiable performance criteria.
Dr Sudarshan Aryal, Geotechnics Technical Executive at WSP says, “For the low risk sites, we released the hold points based on progressive monitoring data reviews, allowing pavement construction to progress as planned.
“For high risk sites, the team reviewed monitoring data to assess the treatment performance and any need to adjust or intervene. They worked with the detailed designers, the construction team and senior management to help decide on any measures before releasing the sites for pavement construction.”
Implementing foundation treatments under embankment areas posed another construction risk, since success depended heavily on each project area’s varying seasonal weather regimes. The geotechnical team devised several strategies including bridging layer treatments; field trials in small areas to see what worked best; completing foundation works in small sections at a time so larger chunks weren’t exposed to inclement weather; constructing embankment foundations in the dry season; and having a team of experienced geotechnical engineers on site full time to advise.
Big projects always offer lessons learnt. Sometimes they emerge from behind carefully managed processes that can hide unpredictable consequences.
“W2B taught us the importance of communication protocols, for example,” adds Dr Aryal. “These must incorporate all the people who need to receive critical technical information. Also, something as minor as differences between formats or document designs can sometimes confound users who miss important data because they weren’t immediately sure where to look. And tender briefs for procuring works need to clearly specify the geotechnical requirements so we can define clear scopes of services.”
Most importantly, all agree that project quality assurance systems need to specify the process and procedure for releasing hold points from site geotechnical inspections.
Robert Kingsland & Dr Sudarshan Aryal are part of WSP’s expert geotechnical team. They joined several speakers from WSP at the 13th Australia New Zealand Conference on Geomechanics held this week in Perth.
The Hunter Expressway Buchanan soft soil problem – what to do when settlement exceeds expectations
R. Kingsland, H. Zhang & V. Urbano, WSP; C. McCallum, Roads and Maritime Services; A. McCann, CPB
Role of the geotechnical engineer in a Delivery Partners’ team delivering a major infrastructure project - case example from Woolgoolga to Ballina Pacific Highway upgrade
S. Aryal, WSP and D. Groth, Roads and Maritime Services
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