Building Design and Construction Handbook - Merritt - Ventech!

SOIL MECHANICS AND FOUNDATIONS 6.57
6.5.6 Settlement of Cohesive and Organic Soils
Cohesive and organic soil can be susceptible to a large amount of settlement from
structural loads. It is usually the direct weight of the structure that causes settlement
of the cohesive or organic soil. The settlement of saturated clay or organic soil can
have three different components: immediate (also known as ‘‘initial settlement’’),
primary consolidation, and secondary compression.
Immediate Settlement. In most situations, surface loading causes both vertical and
horizontal strains. This is referred to as two- or three-dimensional loading. Immediate
settlement is due to undrained shear deformations, or in some cases contained
plastic flow, caused by the two- or three-dimensional loading. Common examples
of three-dimensional loading are from square footings and round storage tanks.
Many different methods are available to determine the amount of immediate settlement
for two- or three-dimensional loadings. Examples include field plate load tests,
equations based on the theory of elasticity, and the stress path method (see R. W.
Day, ‘‘Geotechnical and Foundation Engineering: Design and Construction,’’ Mc-
Graw-Hill Publishing Co., New York).
Primary Consolidation. The increase in vertical pressure due to the weight of the
structure constructed on top of saturated soft clays and organic soil will initially be
carried by the pore water in the soil. This increase in pore water pressure is known
as an excess pore water pressure (ue). The excess pore water pressure will decrease
with time as water slowly flows out of the cohesive soil. This flow of water from
cohesive soil (which has a low permeability) as the excess pore water pressures
slowly dissipate is known as primary consolidation, or simply consolidation. As
the water slowly flows from the cohesive soil, the structure settles as the load is
transferred to the soil particle skeleton, thereby increasing the effective stress of
the soil. Consolidation is a time-dependent process that may take many years to
complete.
Based on the stress history of saturated cohesive soils, they are considered to
be either underconsolidated, normally consolidated, or overconsolidated. The overconsolidation
ratio (OCR) is used to describe the stress history of cohesive soil,
and it is defined as: OCR � � �vm/� �vo, where: ��vm or �p� � maximum past pressure
(� �vm), also known as the preconsolidation pressure (� p�),
which is equal to the
highest previous vertical effective stress that the cohesive soil was subjected to and
consolidated under, and ��voor � v�
� existing vertical effective stress. In terms of
the stress history of a cohesive soil, there are three possible conditions:
1. Underconsolidated (OCR � 1). A saturated cohesive soil is considered underconsolidated
if the soil is not fully consolidated under the existing overburden
pressure and excess pore water pressures (u e) exist within the soil. Underconsolidation
occurs in areas where a cohesive soil is being deposited very rapidly
and not enough time has elapsed for the soil to consolidate under its own weight.
2. Normally Consolidated (OCR � 1). A saturated cohesive soil is considered
normally consolidated if it has never been subjected to an effective stress greater
than the existing overburden pressure and if the deposit is completely consolidated
under the existing overburden pressure.
3. Overconsolidated or Preconsolidated (OCR � 1): A saturated cohesive soil is
considered overconsolidated if it has been subjected in the past to a vertical