EurOCEAN 2000 - Vlaams Instituut voor de Zee

technology. Harbour works, wharves, piers and bridges all could benefit from capacity
increases with reduced pile lengths. In these cases the large number of piles required and
longer time between driving and completion of the structure could provide even greater
economic benefit.
Field Test
Field grouting trials were made at the Dunkirk site. The goals were a) to create shallow grout
bulbs to determine feasible diameters and properties of soil-cement and b) to create a soilcement
bulb beneath a test pile. Six shallow bulbs were formed to evaluate the effects of
grouting parameters on the size and strength of the bulbs. A soil-cement bulb was made below
the test pile by drilling through the inside of the pile. After the base bulb was constructed, jet
grouting was continued inside the pile, extending up to near the ground surface. This interior
plug served to create an effectively closed-end pile bearing on the soil-cement base.
During execution all drilling and jetting parameters were continuously recorded. The jet spoil
was collected for treatment, and was re-used for jetting on one of the bulbs. The cement
content in the spoil was determined by means of a laser granulometer.
Fig.1 shows one of the shallow bulbs after excavation. Diameters of 2.8 m were obtained and
the soil-cement was compact and high quality. As can be seen, the sides of the columns have
distinct ridges left from the passage of the grouting jets. This irregularity would contribute to
interlocking between the grout bulb and surrounding soil.
Laboratory Testing
After about 30 days of in-situ curing, samples were cored from the grout bulbs. The cores were
subjected to an extensive testing programme at Imperial College, including high pressure (100
MPa) triaxial stress-path testing, chemical and fabric analysis, unconfined strength as well as
typical index testing (moisture content, specific gravity, unit weight, etc.). Data from the cores
were complemented by a parallel testing programme on laboratory prepared specimens
examining effects of cement/water and cement/sand ratios on strength and soil-cement-steel
interface friction and bond capacity.
As one example, Fig.2 shows the variation in unconfined strength with depth and radial
distance from the centre of a grout bulb. While there is scatter of about ±30% about the mean
in the strength data the soil-cement is very strong. In fact, the jet grouted sands would qualify
as a sandstone of moderate strength.
Representative stress-strain curves from unconfined compression tests are given in Fig.3. This
series shows samples from various depths in one shallow grout bulb. Note that the soil-cement
fails in a brittle mode. The failure strain is of the order of 0.5% and there is a considerable
strength loss between 0.5 and 1.0% strain. This feature must be accounted for in design.
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