Bearing Capacity Analysis (cont.)

ERDC TN-DOER-N5

July 2000

structures built on soft soils. However, there is no theoretical basis for this reduction, and the newer

version of Terzaghi, Peck, and Mesri (1996) does not include any reference to a reduced value.

Since any reduction in the soil strength value will result in a reduced bearing capacity and thus a

thinner cap that can be sustained on the surface of the deposit, it is advantageous to use the full value

of cohesion when evaluating feasibility of capping. Therefore, it is recommended that the value of

cohesion (strength) used in Equation 3 not be reduced unless and until data have been collected to

indicate a need for the reduction. It should also be noted that recent analyses of dredged material

deposits in New York District agreed well with field performance, and no reduction in the strength

value was assumed in these analyses (Rollings and Rollings 1998b).

Using the density of the sand cap, the factor of safety (FOS) against a bearing capacity failure of

the sand cap on the dredged material can then be calculated as

Available Bearing Capacity qult

FOS =

(4)

hγ s

Load Applied by Sand

where

= thickness of sand cap, ft

γs

= submerged unit weight of sand, lb/ft3

If no density data are available for the sand cap, a value of 1,650 kg/m3 (103 lb/ft3) (or 650 kg/m3

(40.6 lb/ft3) submerged unit weight) (Poindexter 1988) may be assumed for preliminary calculations.

Figure 1 shows the calculated factor of safety against bearing capacity failures for different dredged

material strengths for an initial cap thickness of 0.3 m (1 ft), an intermediate thickness of 0.6 m

(2 ft), and the final design thickness of 1 m (3.3 ft). Table 2 shows the strength that must be present

in the dredged material at a factor of safety of 1.0 when the cap is on the verge of precipitating a

bearing capacity failure and at 3.0, which would represent a conventional design factor of safety

against a bearing capacity failure.

As an example, assume the dredged material soil strength on a current project is in the range of 479

to 958 Pa (10 to 20 lbf/ft2). Enter Figure 1 with the strength, and move upward to intersect the

diagonal line representing a particular cap thickness. Then move horizontally to read the factor of

safety for bearing capacity. From this figure, it appears that the 0.3-m- (1-ft-) thick cap would be

stable (FOS > 1), but the 0.6-m- (2-ft-) thick cap would be more questionable (FOS ≤1 ). The final

1-m- (3.3-ft-) thick cap would be expected to trigger a bearing capacity failure (FOS < 1). None of

the cap thicknesses would produce a conventional level of protection against bearing capacity failure

(e.g., FOS of 3.0).

The preceding bearing capacity calculations reveal that rapid placement of thick sand caps could

engender bearing capacity failures. Such failures could result in upward displacements of dredged

material and ultimately in unanticipated reductions in the actual cap thickness or exposure of the

dredged material to the overlying ocean environment. These sand caps are most vulnerable to

bearing capacity failures immediately after they are placed.