The challenge of Contract 250 of the CTRL was to mitigate settlement of Ripple Road Bridge during and following twin TBM drives directly beneath the central pile founded piers.
The difficulties were that pile depths and construction were unknown, there was potential for the TBMs to clash with the piles, and finally there was extreme programme pressure if over-engineering was to be avoided.
The geotechnical options were first to cut and remove piles clashing with the tunnel alignments and to provide alternate foundations and second, where pile clash was avoided, to provide measures to mitigate the reduction in pile factor of safety and to unacceptable movements that could generate structural damage.
Geotechnical contractor Bachy Soletanche's appointment was based, unusually, on indicative costings only but supported with detailed submissions and proposals demonstrating that the firm had a full understanding of the overall geotechnical behaviour.
A strategy was developed around four key principles: parallel development work; staged verification; adaptability; and robustness.
Three dimensional tomography and magnetometry determined the tunnel alignment was clear of pile clash, but that clearance as low as 1m in places.
Programme, flexibility and cost favoured a ground improvement solution over a structural one. A three-element approach was adopted:
lIncreasing soil stiffness and pile shaft friction by permeation grouting of Terrace Gravels;
lProviding additional capacity and transmitting more load into the upper soils using short, high capacity, post-grouted minipiles;
lContemporaneously compensating for unavoidable settlement using compaction grouting.
Design was advanced and verified at several stages through site investigation (namely pile construction details, Terrace Gravel granulometry and soil parameters); laboratory testing to select optimum grout composition and confirm post-grouted soil properties and likely grouted soilconcrete bond; and site trials to verify insitu grouted soil behaviour and pile response.
Innovative solutions were devised to combat difficulties as work progressed, including:
lA switch from laboratory testing to insitu site trials because of the gravel's unexpectedly high insitu variability;
lAdoption of novel grout mix and 'style' of permeation grouting;
lThe use of Statnamic pile tests on minipiles in grouted soil to prove the shaft bond enhancelA pile load test was carried out on an original insitu working pile to obtain ultimate capacity and enhancement following grouting of surrounding soil.
Design and site verification works confirmed that reduction of pile settlement and an adequate factor of safety could be achieved by permeation grouting alone.
Implementing the solution also proved challenging, with a complex three dimensional borehole layout in extremely constrained and obstructed environment.
Remote movement and grout detection instrumentation was installed to monitor works, much of which had to be carried out during night time track possessions.
Project team Project manager (and Category 3 checker): Rail Link Engineering Main contractor: Nuttall-Wayss & Freytag-Kier JV Designer: Donaldson Associates Specialist contractor: Bachy Soletanche