Settlement Criteria
 for clays, silty clays, plastic silts:
Chapter 5 (short term)
Chapter 7 (long term, i.e., consolidation)
 in this module SANDS (including gravelly sands,
silty sands and non-plastic silts) are considered
The concern?
 in most cases the maximum allowable
settlement will not be reached before shear
failure at a factor of safety of 3
 the main concern is with narrow footings
 settlement in sands is rapid, occurring almost
entirely during construction and initial loading
 \ dead load + max. live load are considered to
estimate settlement
Maximum Allowable Settlement
Footing on Sand
~ 25 mm
 this makes it likely that any differential
settlement between footings will be less
than 20 mm
Raft on Sand ~ 50 mm
 corresponds to differential settlement
between footings less than 20 mm
Pre-Construction Treatment
 for loose sand deposits, compaction
prior to construction is recommended
(vibroflotation, for example)
 for clays, if possible, surcharging with fill
and vertical drains for several years prior to
construction will reduce the ultimate
settlement of the structure as most of the
consolidation will have taken place
Plate Bearing Test
 used to simulate a foundation
 a 1.5 m square test pit is dug
Sowers & Sowers, 3rd Ed.
Plate Bearing Test (Cont’d)
 then a 1 foot
square (300mm x
300mm) steel
plate is loaded in
increments and
the corresponding
settlements
measured
Peck, Hanson, Thornburn, 2nd Ed.
Plate Bearing Test Results
 a load-settlement curve is produced
Sowers & Sowers, 3rd Ed.
Modulus of Vertical Subgrade
Reaction, Kv
 Kv is taken
from straight line
portion of this
curve
McCarthy, 6th Ed.
 Q plate 

  kN 
 2


A
pressure  plate   m 
Kv 


settlem ent S plate
m
Splate = Plate settlement
Qplate = Plate Load
Aplate = Area of plate
Aplate  0.3m  0.3m  0.09m2
Design Kv Values
Condition
Relative Density,
%
Representative
Values of Dry Unit
Weight (kN/m3)
Values of Kv*
(1000 kN/m3)
Loose
< 35
< 14.0
15
Medium Dense
35 - 65
14.0 - 17.0
25 - 50
Dense
66 - 85
17.1 - 20.0
55 - 85
Very dense
> 85
> 20.0
95 - 110
1.5B
Dw
where the water table is at
a depth greater than 1.5B.
0.5K v
If the water table is at the
base of the foundation use 
 Dw  D  
0.5Kv. Use linear
0.5  0.5 1.5B  D  K v


interpolation for

intermediate locations of
Kv
the water table.
th
McCarthy, 6 Ed.
B
D
*These are for the case
Settlement Calculation
For cohesionless soils where D < B < 6.1m:
4Q
S
K v  ( B  0.3) 2
where S = expected foundation settlement (m)
Q = column load (kN)
B = footing width (m)
Kv = modulus of vertical subgrade
reaction (kPa/m or kN/m3)*
 Q plate 


A 
*Remember, when calculating Kv from
Q plate
plate 


plate test data, plate area (Aplate) is 0.09m2! K v  S
0.09  S plate
plate
Beware!
The Plate Bearing Test results are extrapolated
for the design of the foundation!
Craig, 6th Ed.
Standard Penetration Test
 part of a standard
bore hole
investigation
 split barrel
sampler is advanced
by dropping a 64 kg
hammer 760 mm
 N-Values are the
number of blows
(hammer drops) to
advance sampler
300 mm
McCarthy, 6th Ed.
Split Barrel Sampler
Peck, Hanson, Thornburn, 2nd Ed.
 N-Value = “Standard Penetration Resistance”
McCarthy, 6th Ed.
Sample Bore Hole Log
McCarthy, 6th Ed.
North American Equations
for B < 1.2m
for B > 1.2m
 B  0.3 
qa  S a N 
qa  S a N

B


where Sa = allowable settlement (mm)
qa = allowable bearing pressure (kPa)
B = footing width (m)
N = average corrected standard penetration
resistance
2
1
2
1
3
 a number of corrections are applied to N-Values
(pore water pressure, overburden stress…see
Craig)
Water Table Correction
 we will be using average corrected N-Values
 Terzaghi and Peck proposed a correction, Cw to
the allowable bearing pressure, qa to reflect the
depth of water table
Dw = D + B
B
Dw
D
B
C w  0.5
Cw  1
water table correction:
 Dw 
C w  0.5  0.5

D

B

