The site was situated between residential blocks and the new railway, that was facing improvements and upgrading. The very close proximity with existing structures made extremely important the need to achieve soil stability at all moments during the execution of the trenches, each individual panel having 1m x 8.5m and 11m deep.
BAM Grondtechniek requested GEO support and assistance to assure stability of the Diaphragm Wall excavation for a minimum duration of 12 hours, using only ecological friendly technology. The products selected and the onsite technical support made possible the soil stabilization as per client demands and minimized fluid losses to the very porous sandy soil.
Challenges
Very constrained jobsite, reduced space available for slurry storage and the deployment of big excavation rigs – D-Wall clam-shell Rig was replaced by a long reaching excavator. Existing underground utilities crossing the initial/superficial 2 meters of the trench and a very granular geological soil profile, made out of Sands and railway ballast.
The D-wall panels were not very deep, but the soil profile included porous soil layers, with potential for fluid loss. These losses could difficult the slurry level maintenance and lead to collapse. It would also increase polymer volume needs and consumption.
Digging works were made during night shift and pouring concrete during day shift. This planification forced to keep excavations opened during a minimum period of 12 hours and in some cases until 36 hours.
Solution
PolyMud® slurry was used as an alternative to bentonite, as a more ecological friendly technology. All the diaphragm wall scope of work was concluded with the soil stabilization being assured with Polymer slurry combined with Sodium Silicate and Sodium Aluminate (slurry additives) poured to the trench as means of fluid loss control in the high porosity soil (Sands and railway ballast).
Effective soil stabilization was achieved successfully for periods of 12 to 36 hours. No substantial drop on slurry level occurred as the use of the slurry additives maintained the losses to a minimum and polymer consumption under control.
One of the reasons for the choice of the G3 System was the limited area available to install a traditional batch plant for bentonite. This type of technology requires less storage tanks and does not need a mixer nor a desanding unit. The simplified and fast mixing process of the polymer fluid also played an important role.
Key Figures
Scope:
A total of 170 lineal meters of Diaphragm Wall panels with a cross section of one meter divided in 22 elements of 8,5 m long and 11 meters deep.
Achievements:
D-wall panels completed with soil stability and held open for period up to 36h.
Simplified batching plant scheme and slurry being hydrated/ready-to-use in 15min.
Reduction of fluid losses to a minimum.
Preservation of the surrounding environment and slurry disposal onsite, after treatment.
Geology:
Top layer of organic soil with underground facilities crossing through in the first 3m. The sequent soil strata included successions of different extracts of thin sands and silty sand. The last 2m were pure gravels as there was a contiguous layer of railway ballast.
