a. Removal by Excavation. Organic swamp deposits with low shear
strength and high compressibility should be removed by excavation and
replaced by controlled fill. Frequently these organic soils are underlain
by very loose fine sands or silt or soft clayey silts which may be adequate
for the embankment foundation and not require replacement.
Topsoil is usually stripped prior to placement of fills; however,
stripping may not be required for embankments higher than 6 feet as the
settlement from the upper 1/2 foot of topsoil is generally small and takes
place rapidly during construction period. However, if the topsoil is left
in place, the overall stability of the embankment should be checked assuming
a failure plane through the topsoil using the methods of Chapter 7.
b. Displacement. Partial excavation may be accompanied by displacement
of the soft foundation by the weight of fill. The advancing fill should
have a steep front face. The displacement method is usually used for peat
and muck deposits. This method has been used successfully in a few cases
for soft soils up to 65 feet deep. Jetting in the fill and various blasting
methods are used to facilitate displacement. Fibrous organic materials tend
to resist displacement resulting in trapped pockets which may cause
differential settlement.
3. BALANCING LOAD BY EXCAVATION. To decrease final settlement, the
foundation of heavy structures may be placed above compressible strata
within an excavation that is carried to a depth at which the weight of
overburden, removed partially or completely, balances the applied load.
a. Computation of Total Settlement. In this case, settlement is
derived largely from recompression. The amount of recompression is
influenced by magnitude of heave and magnitude of swell in the unloading
stage.
b. Effect of Dewatering. If drawdown for dewatering extends well below
the planned subgrade, heave and consequent recompression are decreased by
the application of capillary stresses. If groundwater level is restored
after construction, the load removed equals the depth of excavation times
total unit weight of the soil. If groundwater pressures are to be
permanently relieved, the load removed equals the total weight of soil above
the original water table plus the submerged weight of soil below the
original water table. Calculate effective stresses as described in Figure
2, and consolidation under structural loads as shown in Figure 3.
4. PRECONSOLIDATION BY SURCHARGE. This procedure causes a portion of the
total settlement to occur before construction. It is used primarily for
fill beneath paved areas or structures with comparatively light column
loads. For heavier structures, a compacted fill of high rigidity may be
required to reduce stresses in compressible foundation soil (see DM-7.2,
Chapter 2).
a. Elimination of Primary Consolidation. Use Figure 17 to determine
surcharge load and percent consolidation under surcharge necessary to
eliminate primary consolidation under final load. This computation assumes
that the rate of consolidation under the surcharge is equal to that under
final load.
7.1-244
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