GEOTECHNICAL ENGINEERING ANALYSIS

ERDC TN-DOER-N5

July 2000

of height H, void ratio e, and coefficient of compressibility αv over each load increment are used

in the calculations. More detailed information on use of this equation is contained in Lambe (1951)

and Terzaghi, Peck, and Mesri (1996).

GEOTECHNICAL ENGINEERING ANALYSIS: The elevation changes in a CAD deposit could

conceptually arise from consolidation of the dredged material as soil particles are rearranged to a

more dense state accompanied by an expulsion of pore water or from shear displacements within

the material. Consolidation of dredged materials is recognized as an important phenomenon

affecting site capacity (Poindexter 1988; Rollings 1994; Rollings and Rollings 1998a); however, it

is a long-term issue. Conversely, shear displacements are the more likely cause of large, rapidly

occurring elevation changes, particularly soon after placement of the dredged material and/or

capping material. These shear displacements may develop from collapse, bearing capacity failure,

or slope failures within the dredged material.

The process of consolidation occurs in fine-grained soils as soil particles are pressed together under

load. Consolidation may occur in the capping material (if a compressible material is used for the

cap), in the contaminated sediments, and/or in the underlying foundations soils. In the capping

material, consolidation will result from self-weight of the material, while in the underlying

contaminated sediment, consolidation will occur firstly from self-weight and secondly as the result

of the load imposed on it by the capping material. Consolidation of the natural bottom underlying

the recently constructed CAD will occur only if the load caused by the contaminated sediments and

cap exceeds any load previously placed on the foundation soils. For example, if the weight of the

material excavated during construction of a depression for a CAD exceeds the weight of the dredged

material and cap, then little or no foundation consolidation would be expected.

It should be noted that some rebound may occur upon unloading of the underlying soil (by

excavating the cell). Rebound would be indicated by an upward movement of the bottom surface

of the CAD cell. In most instances, this effect would likely be negligible. However, in some cases,

it might affect pay yardage for excavation of the cell as well as the holding capacity of the CAD

cell. The magnitude of any possible rebound can be determined during the consolidation test by

unloading the specimen after each consolidation load increment. Procedures for determining

rebound are given in the consolidation test procedures (ASTM 1999c; Headquarters, USACE,

1986).

Consolidation will generally occur in most fine-grained soils, although the amount and rate can vary

greatly depending upon a number of factors including the particle type (e.g., clay versus silt, high

versus low plasticity), moisture content/density, and permeability of the deposit, combined with the

loading conditions and thickness of the compressible layers. All of these factors interact to affect

the compressibility of sediment layers significantly. Consolidation is a slow process, sometimes

taking years or decades to reach completion. Coarse-grained sediments, e.g., sands and gravels,

will not consolidate appreciably.

Collapsible soils are formed in nature by various geologic processes in a variety of deposition

environments and can also be found in engineered earthworks (Mitchell 1993; Rollings and Rollings

1996). In simple terms, the particles of collapsible soils are deposited in approximately point-to-

point contact in a meta-stable condition. Upon loading or after dissolution of any bonding agent,