19 June 2001
Often, control of water content with respect to the optimum value is left up to the
contractor, since he can elect to replace rigorous moisture control with increased
compactive effort. Where excess compactive effort could result in damage to the
structure, such as in quaywalls or cofferdams, the compactive effort should be
minimized and the moisture content controlled as well as is practicable. For these
latter types of structures, it is also very important that design densities not be
exceeded. Excessive compaction might result in undesirable lateral stresses in
10-3.2.4 Dewatering. Where excavation and replacement of soil takes place
below the water table, it may be necessary to dewater the site by using seepage
barriers, such as sheet piles. Where soil permeability is high, subsurface
drainage by well points or deep wells may be necessary. Prior to planning
dewatering procedures, it is necessary to determine permeability and piezometric
levels by field observations. The major concern is to avoid instability through
piping or heaving by controlling the upward hydraulic gradient at the base of the
excavation. Hydraulic gradients (head loss per unit length of flowpath) equal to
the ratio of buoyant density of the soil divided by the density of water lead to
immediate instability in all cases. This ratio varies from > 1 for dense soils to < 1
for loose soils. Exit gradients of 0.5 to 0.75 will cause unstable working
conditions even in clean sands. Silty materials are even more critical.
10-3.2.5 Sealing/Filling Voids.
10-22.214.171.124 Sealing. The loss of soil from behind quaywalls, or from within sheet
pile cofferdams, requires sealing the structure to prevent further loss of material
before replacing with a suitable backfill. Coarser-grained materials and/or
geotextile filters are preferred, since they are less subject to leaching or erosion.
Fine-grained materials are desirable only where very low permeability is required,
such as in the core of an earth dam. When fine materials are used, construction
of inverted geotextile filters and/or sealing of structural joints are necessary to
prevent erosion. Where materials consist of the very erodible silts or fine sands,
the structure must be sealed to prevent influx of surface water.
10-126.96.36.199 Grouting. In some cases, it may be more expedient to either seal or
repair a damaged structure by injecting grout. Grout may be used to reduce the
permeability of the soil fill or its foundation and, thereby, minimize erosion or
leaching. It may also be used to physically strengthen the structure to make it
more resistant to wave or ship loading, and superimposed dead loads.
Cavities or voids in the soil structure may be grouted using sand/water
mixtures, Portland cement, clay, chemical grouts, or a combination of these
materials depending on the size of voids. Sand/water mixtures are applicable
where large cavities are present and the paths of soil loss have been sealed off.
Cement grout is not applicable if the effective grain size of the in-place soils, D
(the sieve size through which only 10 percent of the soil would pass), is less than
0.5 mm (19.7 mils) for loose soils and 1.4 mm (55.1 mils) for dense soils.