Although belonging to the interchange complex, this viaduct is located slightly north from it. Creating a secondary alternative road link between Cornubia and Umhlanga, over the N2 Freeway.
Dura is a South African foundation company that belong to the Soletanche Bachy Group. It requested GEO support and onsite assistance to assure a good usage of polymer slurry, assuring soil stabilization, reducing concrete consumption and improving production. The use of G3 System had direct impact in temporary casing length reduction and in the time required to complete each new pile.
Work started at the NW embankment. The bored piles were executed with a telescopic profile, having 3 different diameters (1180, 1000 and 900 mm): 1180mm for the casing having 7 to 9 m long; 1000mm for the major pile length and, 900mm for a pile (rock) socket having 2.5 to 3m long. The final depth of these initial piles was around 22.5m.
Challenges
Not being allowed to use bentonite slurry the pile contractor was forced to initially use long casing. Besides the time required to insert and retrieve casing sections, this operation increase the bored area leading to a concrete over cost, as overbreaks were above 15%.
The initial usage of polymer fluid was not hassle free. The client had no previous experience on its usage and required technical assistance. The main issues identified, prior to GEO’s Engineering support, were: inefficient head pressure, leading to soil collapse and drilling buckets getting stuck inside the bore.
Solution
PolyMud® slurry was used as an alternative to full casing method and bentonite slurry. The initial methodology was to use casing sections to reach the soft rock layer (≈18.5m). Polymud® slurry was implemented to reduce the need of casing. It has successfully achieved that reduction to a single piece (guide casing Ø1180mm, with 6-7m). The remaining soil stabilization was efficiently assured by the slurry.
The slurry level was increased, to top of platform level, and maintained at all stages of the drilling, including while drilling the Ø 900mm rock socket, cleaning of pile toe and concrete pour. At the end of excavation, a cleaning bucket was used and no de-sanding or flushing was required.
The reduction of casing length had direct impact in the time needed to execute each pile. Time consumed to screw and unscrew the various casing sections was eliminated. As well, it had a major impact in pile concrete consumption, from 35m3 to 21m3.
Key Figures
Scope:
The project comprised the execution of 187 piles (103 vertical and 84 raked) divided in two embankments and two central piers groups. Piles of 900mm diameter and average depth of 32m. The rake piles have a declination of 1:6.
Achievements:
The G3 System slurry and the onsite guidance allowed the client to reduce the casing length, reducing operational timings (production rate increase from 0.8 piles per day to 1.5 piles per day using one drilling rig) and reducing pile concrete consumption from 35m3 to 21m3.
Soil Conditions:
Top soil layer (working platform) in the first 1.5m. Followed by red (iron oxide) Clayey Sand to 16m. Yellowish red clayey Sand, from 16 to 18.5m. Weathered soft clayey Sandstone from 18.5 to 20m, above the medium shale rock socket.
