The old building was partially demolished and the previous (direct) foundation had to be removed. The previous structure didn’t require deep foundations. The removal of these and other underground interferences created excavation pits up to 3 m depth, that had to be backfilled to create the working platform. The executed bored piles (58 units) were clustered in a small area, limited to the scenic and technical boxes, that was encircled by existing structures.
The Ø800mm piles are located in the core of the building and should provide support to the new, heavier structure. While boring these pies, fluid was essential as means of soil stabilization and counterbalance the tidal influence in the relatively shallow water table level.
Challenges
Space restrictions were several: related with the partially demolished building, neighboring buildings and the stockage of R-bar cages and excavated soil. Being located on a central urban area (old town) it was not possible to have lorries to remove the excavated soil daily. As well, the close proximity of the various piles, limited the operation.
Alluvial soft marine clay and the shallow water table level (<2m), with tidal influence, created a challenging combination of factors. Maintaining slurry level was also an essential part of the soil stabilization but a big fluctuation was observed, during the removal of the drilling. The reduced Ø800mm bucket had also the risk to generate suction while being extracted.
Solution
GLine products were selected by the client by their versatility and for being a good cost-effective solution. Furthermore, the simplicity of the mixing and preparation of the slurry was ideal for a small size batching plant: two 32m3 tanks, one for water storage and the other for slurry mixing/storage. The 3 products (GMud, G+ and GNet) were all directly applied into a mixing tank, with close circulation and air agitation. The mixture was ready in 30min, perfectly homogeneous and without any fisheyes, and could be immediately supplied to pile. During concrete pour, the used slurry was retrieved directly to the same tank, where more water and products were added in a simple manner: adjusting volume and parameters according to the needs.
Soil stabilization was assured at all times by the GLine slurry. The more sensitive layer of soft marine clay required the maintenance of the slurry level, that was assured by means of a compensation basin. The drilling bucket was also provided with extra external cutting capacity to increase bore annulus and avoid suction (external venting system).
When all the piles were completed, fluid was treated and disposal directly on site, through the city sewer system.
Key Figures
Scope:
A total of 58 Bored Piles with Ø800mm and reaching 29m depth.
Only guide casings of 6 to 8m were used.
Achievements:
Very simple setup for slurry mixing and management. Easy to be accommodated in a small jobsite. Combined GLine product were effective in assuring complete soil stabilization. Production of 2.5piles/day in a reduced working area.
Soil Conditions:
The site soil profile was composed by backfilled soil on the top 3m; soft marine clay from 3 to 6.5m; followed by Sands with shells and intercalations of marine clay to 16m. From 16m to 28m we found clayey Marls and, below it, Limestone, where the base of the pile was set.
