Geoscience Reference
In-Depth Information
Another case history of the preloading technique was presented by Samson and
La Rochelle (1972) for an expressway construction in Quebec, Canada. The original
thickness of the peat varied from3 to 6m. The initial undrained strength of the peat was
low (10 kPa), organic content was 84-95% and bulk density was 0.87-1.04Mgm
−
3
.
The preload construction was carried out in four stages:
•
Stage 1: a 1.2m thick sand working platform was left for 10 days. This created a
large settlement of 1.2m.
•
Stage 2: fill was increased to 2.5-3.3m and left to consolidate for 6.5 months.
Settlement varied from 1.1 to 2.6m.
•
Stage 3: a preload surcharge of 1.0 to 1.5m was added and left for 1 year. The
total settlement varied from 1.4 to 3.3m.
•
Stage 4: the surcharge was removed, and the road was completed in about three
months. The embankment heaved for a period of 200-475 days. A total rebound of
43 to 79mm was observed. After this, slow settlement was observed at a constant
rate with respect to log time for 5.5 to 8 years, after which the rate was found to
increase somewhat. Total settlement over a period of 18 years after completion of
the highway was 270mm to 1.19m (Samson, 1985).
The preloading principle, often in conjunction with vertical drains, was also adopted in
the above-mentioned methods to accelerate settlement and minimize post-construction
settlement (see Figure 6.7). Where it is desirable to try to speed up the dissipation of
pore water pressures beneath an embankment and hence speed up the settlement pro-
cess, geosynthetic vertical drains have become a modern substitute for sand drains.
There are two basic reasons to wish for the quick dissipation of the pore water pres-
sures: in a stage construction, where the strength gain of the subsoil at each stage
is needed to ensure the stability of the next stage, quicker dissipation allows quicker
construction; while in preloading it reduces the time necessary for the preload fill to
Figure 6.7
Preloading (stage construction) and vertical drain.
