NEED FOR GEOTECHNICAL EVALUATIONS

ERDC TN-DOER-N5

July 2000

highlighted potential problems with slope stability and bearing capacity of the soft dredged material

(Clausner et al. 1998; Rollings and Rollings 1998a; Rollings and Rollings 1998b). The 1998 Boston

Harbor CAD demonstration project highlighted potential problems with sustaining a viable cap on

the surface of the recently deposited soft dredged material (Murray et al. 1998; ENSR 1997). Other

Districts have used or are considering using CAD as a disposal option (e.g., Los Angeles and Seattle).

With the popularity of and interest in CAD, a need was recognized for geotechnical design guidance.

Evaluation and understanding of consolidation and shear strength behavior of these soft deposits

will allow assessment of long-term site capacity and stability/physical isolation capabilities of the

cap, respectively. In CAD deposits, slope stability is of concern when the CAD cell is to be partially

filled across its aereal extent; it is also of potential concern regarding the side slopes of the CAD

cell. Bearing capacity, or the ability of the contaminated dredged material to support the capping

material, must be determined if the contaminated material is to be successfully isolated from the

overlying environment. It is essential that geotechnical design and analysis be conducted for these

CAD projects so that they function as sound engineering structures with behavior that can be

quantified and accurately predicted.

NEED FOR GEOTECHNICAL EVALUATIONS: Geotechnical engineering considerations are

important in design and construction of CAD sites, particularly since most contaminated sediments

are fine-grained silts and/or clays and have high water contents and low shear strengths in situ, a

most daunting combination of factors. As these sediments are dredged and placed at a subaqueous

containment site, the situation is often exacerbated by the mixing or inclusion of more water in the

sediments, resulting in even lower shear strengths. For normal geotechnical construction projects,

typical requirements are high shear strength and low moisture content, just the opposite of conditions

existing in recently deposited dredged material.

Soil shear strength is a complex function of many parameters including soil density, particle

distribution, moisture content, and plasticity characteristics. Assessment of the shear strength is

important because it controls (a) the bearing capacity of the dredged material, which then determines

the ability of the dredged material to hold up (sustain) a cap of clean sediments, and (b) the stability

of any slopes of dredged material that may form in the CAD, e.g., the stability of a mound of dredged

material in the bottom of the CAD when only a portion of the footprint of the CAD is covered with

material. Over time, as the dredged material undergoes consolidation, the shear strength of the

sediment will increase somewhat and its ability to maintain a viable sand cap of given thickness

will likely improve. The degree of change of the shear strength will depend upon material type, the

initial conditions of the dredged material, the thickness of the deposit, and the thickness of any

overlying cap that will act as a surcharge load. The most critical time with regard to slope stability

is immediately after disposal and/or immediately after capping when the pore pressures will be

greatest and excess pore-water pressure has not had time to dissipate. Likewise the most critical

time regarding bearing capacity is immediately upon placement of the cap while the shear strength

of the contaminated sediments is at its lowest.

Because contaminated sediments are usually fine-grained and have a relatively high water content,

they are often susceptible to large amounts of consolidation, a phenomenon that entails the

squeezing/pressing together of soil particles as pore water is expelled under sustained load.

Assessing consolidation potential of capped dredged material deposits requires consideration of the