Site testing was needed to prove the alternative design for a Forth Replacement Crossing would work. GE reports.
One of the reasons the Forth Crossing Bridge Constructors (FCBC) joint venture comprising Hochtief Solutions, Dragados, American Bridge International and Morrison Construction secured the work on the Forth Replacement Crossing was price.
The bid made by the team was not only significantly lower than those submitted by other tenderers but also below the figure forecast by client Transport Scotland itself.
FCBC’s major cost saving was based on the decision to replace the specimen piled design for the Ferrytoll northern approach viaduct with a soil embankment up to 20m high.
But the formation underlying the proposed Ferrytoll embankment includes a wide range of potentially compressible subsoils, including up to 20m of quarry waste material. For Transport Scotland to accept the proposed embankment design, a robust foundation solution was required.
This, combined with uncertainty about the quarry waste stiffness, meant that ground improvement through high energy impact compaction (HEIC) was being considered unless acceptable embankment settlement could be reliably demonstrated to help avoid the additional costs.
The key to any meaningful settlement prediction is a realistic assessment of the operational bulk stiffness of the subsoil, a particularly difficult task for soils such as the quarry waste material present at the Ferrytoll site - sandstone and dolerite boulders in a matrix of gravelly clay.
For soils such as this it is not possible to obtain representative samples for laboratory testing.
Insitu penetration testing by SPT or CPT, when possible, reveals only the insitu density of the matrix soil and provides no evidence of its bulk stiffness. Large-scale load testing or cross-hole seismic techniques can be used to measure bulk stiffness but are both very costly and time consuming to undertake.
FCBC’s design joint venture geotechnical team leader (network connections) David Reynolds asked RJM Ground Solutions to find out if non-intrusive ground stiffness profiling by continuous surface wave (CSW) testing could deliver the information needed (see box).
The Ferrytoll Embankment site is the responsibility of FCBC senior site agent Iain Simpson. He says: “With initial earthworks having commenced at the site it was vital to provide our designers with the necessary information to complete their design of the new embankment while minimising disruption to the ongoing works.
“The CSW testing was completed in a couple of days and although we had to stop some compaction plant working near to test locations this was only for five minutes or so and consequently disruption to our works was minimal.”
“The CSW testing was completed in a couple of days and disruption to our works was minimal”
A total of 12 CSW profiles were obtained during the two-day site period earlier this year and these were successful in obtaining layered stiffness profiles at all locations to depths of 8m to 10m.
Stiffnesses measured by the CSW technique are small strain values and therefore require adjustment for strain level (see Figure 1).
This is readily achieved through the application of a “softening” function which is remarkably independent of soil type. Some Geotechnical software such as Plaxis can make this adjustment automatically.
Alternatively, the “softening function” can be applied manually to data either prior to analysis for predicted strain levels or through iterative analysis.
This means that CSW data is equally useful whether undertaking advanced finite element analysis or more conventional simple onedimensional layered elastic analysis.
While not designed to be a profiling technique, the results of testing at the Ferrytoll embankment site generally showed good agreement with nearby borehole data with the rock head clearly defined by a sharp increase in stiffness (see Figure 2).
The stiffness of the quarry fill was found to vary between 100MPa and 400MPa (E0.1%) and 40MPa and 140MPa (E1%), these strain levels representing typical levels beneath embankments and as determined by conventional laboratory testing respectively.
According to Reynolds, the CSW stiffness data proved very useful. “Anticipated settlements were calculated using a simple 1D linear elastic model,” he says. “Based on the results of this analysis we were able to satisfy ourselves and Transport Scotland that embankment settlements on the quarry backfill material would be within those limits set out in the requirements.
“The testing also helped to confirm our interpretations of rockhead levels.”
Transport Scotland geotechnical manager Paul Mellon was on site to witness the CSW testing. “We support the innovative approach to characterising the bulk properties of this difficult quarry fill material,” he says.
“This is a significant embankment and a robust settlement monitoring regime will be incorporated into the design - this should serve not only to validate the approach in this case, but also to give confidence in its use elsewhere.”
Continuous surface wave assessment
“The difficulties faced in obtaining realistic measurements of ground stiffness are frustrating,” says RJM director John Rigby-Jones. “We are often provided with nothing more than SPT results and asked to undertake accurate assessments of foundation settlement or complex soil structure interaction.
“Although engineering judgement suggests the movement will be small, the absence of reliable stiffness measurements and the constraints of codified design means we are forced to adopt an overly conservative approach.” Rigby-Jones was introduced to the continuous surface wave (CSW) concept by an old colleague, Gerhard Heymann, now professor of geotechnical engineering at South Africa’s University of Pretoria.
Rigby-Jones says the principle of CSW testing is simple and surprisingly not new, having been contemplated by Terzaghi and Hvorslev in the 1940s.
The test uses a frequency controlled vibratory source (or “shaker”) to generate surface waves which propagate across an array of surface geophones at a known spacing.
The velocity of the surface waves at a range of frequencies is measured and converted to stiffness using a relationship for which the only variables are Poisson’s ratio and soil density, both generally known with reasonable accuracy.
The dispersive nature of surface waves (their attenuation with depth) means that their depth of penetration is related to frequency. Using this dispersive behaviour, a vertical stiffness profile can be generated by calculating the relevant depth of influence for each test frequency.
“The advent of modern computers combined with a growing appreciation of the relationship between soil stiffness and strain in the late 1980s resulted in the rediscovery of the CSW technique,” says Rigby-Jones. “We worked with Heymann to develop our system and it provides a significant improvement on previous CSW systems through the use of a more powerful mechanical source with a sophisticated data processing technique which allows the production of layered stiffness profiles and is able to deal with complex modes of ground resonance.
“We can now reliably obtain stiffness profiles to 8m depth and depending on ground conditions as deep as 12m using our 80kg shaker. Heymann has developed a 250kg shaker which can extend the profiling depth to around 20m and is something we hope to be able to offer in future.”