15 AUGUST 2005
Type 1 portland cement, Type III portland cement, and processed bentonite cannot be
used to penetrate soils finer than 30, 40, and 60 mesh sieve sizes, respectively.
Different types of grouts may be combined to both coarse- and fine-grained soils.
Cement and Soil-Cement Grouting. See the American Society of Civil
Engineers Grouting Publications for cement and soil-cement grouting.
Chemical Grouting. To penetrate the voids of finer soils, chemical grout
must be used. The most common classes of chemical grouts in current use are
silicates, resins, lignins, and acrylamides. The viscosity of the chemical-water solution
is the major factor controlling groutability. The particle-size ranges over which each of
these grout types is effective is shown in Figure 8-7.6.
Preloading. Earth fill or other material is placed over the site to be
stabilized in amounts sufficient to produce a stress in the soft soil equal to that
anticipated from the final structures. As the time required for consolidation of the soft
soil may be long (months to years) and varies directly as the square of the layer
thickness and inversely as the hydraulic conductivity, preloading alone is likely to be
suitable only for stabilizing thin layers and with a long period of time available prior to
final development of the site.
Surcharge Fills. If the thickness of the fill placed for pre-loading is
greater than that required to induce stresses corresponding to structure-induced
stresses, the excess fill is termed a surcharge fill. Although the rate of consolidation is
essentially independent of stress increase, the amount of consolidation varies
approximately in proportion to the stress increase. It follows, therefore, that the
preloading fill plus surcharge can cause a given amount of settlement in shorter time
than can the preloading fill alone. Thus, through the use of surcharge fills, the time
required for preloading can be reduced significantly.
The required surcharge and loading period can be determined using
conventional theories of consolidation. Both primary consolidation and most of the
secondary compression settlements can be taken out in advance by surcharge fills.
Secondary compression settlements may be the major part of the total settlement of
highly organic deposits or old sanitary landfill sites.
Because the degree of consolidation and applied stress vary with depth, it
is necessary to determine if excess pore pressures will remain at any depth after
surcharge removal. If so, further primary consolidation settlement under permanent
loadings would occur. To avoid this occurrence, determine the duration of the
The rate and amount of preload may be controlled by the strength of the
underlying soft soil. Use berms to maintain foundation stability and place fill in stages to
permit the soil to gain strength from consolidation. Predictions of the rates of