Settlement Criteria

1
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

2
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

3
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

4
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

5
Plate Bearing Test

• used to simulate a foundation

• a 1.5 m square test pit is dug

Sowers Sowers, 3rd Ed.
6
Plate Bearing Test (Contd)

• then a 1 foot square (300mm x 300mm) steel plate
  is loaded in increments and the corresponding
  settlements measured

Peck, Hanson, Thornburn, 2nd Ed.
7
Plate Bearing Test Results

• a load-settlement curve is produced

Sowers Sowers, 3rd Ed.
8
Modulus of Vertical Subgrade Reaction, Kv

• Kv is taken from straight line portion of this
  curve

McCarthy, 6th Ed.
Splate Plate settlement
Qplate Plate Load
Aplate Area of plate
9
Design Kv Values
Condition Relative Density, Representative Values of Dry Unit Weight (kN/m3) Values of Kv (1000 kN/m3)
Loose lt 35 lt 14.0 15
Medium Dense 35 - 65 14.0 - 17.0 25 - 50
Dense 66 - 85 17.1 - 20.0 55 - 85
Very dense gt 85 gt 20.0 95 - 110
B
These are for the case where the water table is
at a depth greater than 1.5B. If the water table
is at the base of the foundation use 0.5Kv. Use
linear interpolation for intermediate locations
of the water table.
D
Dw
1.5B
McCarthy, 6th Ed.
10
Settlement Calculation
For cohesionless soils where D lt B lt 6.1m
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)
Remember, when calculating Kv from plate test
data, plate area (Aplate) is 0.09m2!
11
Beware!
The Plate Bearing Test results are extrapolated
for the design of the foundation!
Craig, 6th Ed.
12
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.
13
Split Barrel Sampler
Peck, Hanson, Thornburn, 2nd Ed.

• N-Value Standard Penetration Resistance

McCarthy, 6th Ed.
14
Sample Bore Hole Log
McCarthy, 6th Ed.
15
North American Equations
for B lt 1.2m
for B gt 1.2m
where Sa allowable settlement (mm) qa
allowable bearing pressure (kPa) B footing
width (m) N average corrected standard
penetration resistance

• a number of corrections are applied to N-Values
  (pore water pressure, overburden stresssee
  Craig)

16
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

B
D
Dw
Dw D B
B
water table correction