TR 334: Foundation Engineering I
2E Units – Core
By Dr. Siya Rimoy
University of Dar es Salaam
College of Engineering and Technology
Department of Transportation & Geotechnical Engineering
2014/2015.
•Course contents
1.
2.
3.
4.
5.
Vertical stresses within a soil mass
Deformation and settlements of soils
Bearing capacity of soils
Stability of soil slopes
Horizontal stresses within a soil mass
Chapter 2: Deformation and settlements of soils
Chapter contents
 General considerations
 Principles of settlements computation
 Differential settlements
 Allowable settlements
 Rate of settlements
Chapter 2: Deformation and settlements of soils
General considerations
Settlement is the vertical downward displacement of the ground
(cf. heave). Settlements can be caused by several aspects
including;
1) Compaction
2) Consolidation
3) Elastic (or immediate) movement
4) Moisture movement (Shrinkage and swelling)
5) Vegetation effects
6) Groundwater lowering
7) Temperature changes
8) Seepage and scouring (Erosion)
Chapter 2: Deformation and settlements of soils
Principles of settlement computation
Foundation engineers can avoid a number of the causes of
settlements but have to quantity the unavoidable ones, for
example, stresses changes in the ground as a result of engineering
structures.
produced by stresses changes are given by;
Total settlements,
Where;
- Immediate (elastic) settlement
- (Primary) Consolidation settlement
- Secondary settlement
Chapter 2: Deformation and settlements of soils
Principles of settlement computation
Chapter 2: Deformation and settlements of soils
Principles of settlement computation
Immediate (elastic) settlement,
In the case of uniform loading, the vertical surface displacement
of a soil layer of infinite depth is given by;
ν
Where q – intensity of contact pressure
B – least lateral dimension (breadth or diameter)
ν – Poisson’s ratio
E – Modulus of elasticity
- Influence factor for vertical displacement
Chapter 2: Deformation and settlements of soils
Principles of settlement computation
Immediate (elastic) settlement, (cont…)
Influence factors ( ) for immediate settlement are readable from
derived tables
Chapter 2: Deformation and settlements of soils
Principles of settlement computation
Immediate (elastic) settlement, (cont…)
Chapter 2: Deformation and settlements of soils
Principles of settlement computation
Consolidation settlement,
a) By oedometer load – settlement curves method
Chapter 2: Deformation and settlements of soils
Principles of settlement computation
Consolidation settlement,
a) By load – settlement curves method (cont…)
Chapter 2: Deformation and settlements of soils
Principles of settlement computation
Consolidation settlement,
a) By Oedometer mv method
Chapter 2: Deformation and settlements of soils
Principles of the settlement computation
Secondary settlements,
Occurs at the end of consolidation (primary) settlements
Chapter 2: Deformation and settlements of soils
Principles of the settlement computation
Secondary settlements,
Secondary settlements can then be
obtained from the laboratory test results
as
t1 – reference time, typically 1 year to
allow for construction period and primary
consolidation and,
t2 – is the design life of the structure
– can be obtained from oedometer test
by continued reading after EoP
Chapter 2: Deformation and settlements of soils
Settlements of Rigid Footings
Since rigid footings have infinite bending resistance, the
deformations of such footings doesn’t follow the deformation of
the subsoil.
Chapter 2: Deformation and settlements of soils
Settlements of Rigid Footings
To calculate the mean settlement Sm of a rigid footing we consider a
characteristic point Pc under the footing.
Pc is defined such that the settlement below Pc for a completely
flexible load causing the mean soil pressure qm is equal to the mean
settlement sm of the considered rigid building.
Chapter 2: Deformation and settlements of soils
Settlements of Rigid Footings
The vertical stress at a depth z below the
characteristic point Pc of a uniformly
loaded rectangular rigid footing with
dimensions l and b can be expressed as;
Where
is an influence factor from the
derived table adjacent
Chapter 2: Deformation and settlements of soils
Settlements of Rigid Footings
The vertical stress at a depth z
below different points of a
uniformly loaded circular
area;
Where
is an influence
factor from the derived table
adjacent
Chapter 2: Deformation and settlements of soils
Differential settlements
The magnitude of uniform settlements may not be as a
structurally distressing as differential settlements. The possible
causes of differential settlements include;
Chapter 2: Deformation and settlements of soils
Differential settlements
The possible causes of differential settlements include; (cont…)
Chapter 2: Deformation and settlements of soils
Differential settlements
Differential settlements can be assessed in terms of the angular
distortion of a structure or part of it.
Chapter 2: Deformation and settlements of soils
Effects of settlements
1) Affects the appearance of structures by cracks and tilting
2) May interfere with the function of utilities and machinery in a
structure
3) May cause structural failure of buildings even though the
factor of safety against shear failure has not been reached