settlement criteria f f f for clays, silty clays, plastic silts: chapter 5 (short term) chapter 7...
TRANSCRIPT
Settlement CriteriaSettlement Criteria
for clays, silty clays, plastic silts:for clays, silty clays, plastic silts:
Chapter 5 (short term)Chapter 5 (short term)
Chapter 7 (long term, i.e., consolidation)Chapter 7 (long term, i.e., consolidation)
in this module in this module SANDSSANDS (including gravelly (including gravelly sands, silty sands and non-plastic silts) are sands, silty sands and non-plastic silts) are consideredconsidered
The concern?The concern?
in most cases the maximum allowable in most cases the maximum allowable settlement will not be reached before shear settlement will not be reached before shear failure at a factor of safety of 3failure at a factor of safety of 3
the main concern is with narrow footingsthe main concern is with narrow footings
settlement in sands is rapid, occurring settlement in sands is rapid, occurring almost entirely during construction and almost entirely during construction and initial loadinginitial loading
dead load + max. live load are dead load + max. live load are considered to estimate settlementconsidered to estimate settlement
Maximum Allowable Maximum Allowable SettlementSettlement
Footing on SandFooting on Sand ~~ 25 mm25 mm this makes it likely that any this makes it likely that any differential settlement between differential settlement between footings will be less than 20 mmfootings will be less than 20 mm
Raft on SandRaft on Sand ~~ 50 mm50 mm corresponds tocorresponds to differential differential settlement between footings less than settlement between footings less than 20 mm20 mm
Pre-Construction TreatmentPre-Construction Treatment
for for loose sand depositsloose sand deposits, , compaction prior to construction is compaction prior to construction is recommended (vibroflotation, for recommended (vibroflotation, for example)example)
for for claysclays, if possible, , if possible, surchargingsurcharging with fill and with fill and vertical drainsvertical drains for several for several years prior to construction will reduce years prior to construction will reduce the ultimate settlement of the the ultimate settlement of the structure as most of the consolidation structure as most of the consolidation will have taken placewill have taken place
Plate Bearing TestPlate Bearing Test
used to simulate a foundationused to simulate a foundation
a 1.5 m square test pit is duga 1.5 m square test pit is dug
Sowers & Sowers, 3Sowers & Sowers, 3rdrd Ed. Ed.
Plate Bearing Test (Cont’d)Plate Bearing Test (Cont’d)
then a 1 foot then a 1 foot square (300mm square (300mm x 300mm) steel x 300mm) steel plate is loaded plate is loaded in increments in increments and the and the corresponding corresponding settlements settlements measuredmeasured
Peck, Hanson, Thornburn, 2Peck, Hanson, Thornburn, 2ndnd Ed.Ed.
Plate Bearing Test ResultsPlate Bearing Test Results a load-settlement curve is produceda load-settlement curve is produced
Sowers & Sowers, 3Sowers & Sowers, 3rdrd Ed. Ed.
Modulus of Vertical Modulus of Vertical Subgrade Reaction, KSubgrade Reaction, Kvv
KKvv is taken is taken from straight from straight line portion of line portion of this curvethis curve
mm
kN
S
A
Q
settlement
pressureK
plate
plate
plate
v
2
209.03.03.0 mmmAplate
McCarthy, 6McCarthy, 6thth Ed. Ed.
Aplate = Area of plate
Qplate = Plate Load
Splate = Plate settlement
Design KDesign Kvv Values Values
**These are for the case These are for the case where the water table is where the water table is at a depth greater than at a depth greater than 1.5B. If the water table 1.5B. If the water table is at the base of the is at the base of the foundation use 0.5Kfoundation use 0.5Kvv. . Use linear interpolation Use linear interpolation for intermediate for intermediate locations of the water locations of the water table.table.
ConditionRelative
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
McCarthy, 6McCarthy, 6thth Ed.Ed.
BB
1.5
B1
.5B
DD
DDww
vw K
DB
DD
5.1
5.05.0
vK
vK5.0
Settlement CalculationSettlement CalculationFor cohesionless soils where D < B For cohesionless soils where D < B << 6.1m: 6.1m:
2)3.0(
4
BK
QS
v
where S = expected foundation settlement (m)where S = expected foundation settlement (m) Q = column load (kN)Q = column load (kN) B = footing width (m)B = footing width (m) KKvv = modulus of vertical subgrade = modulus of vertical subgrade
reaction (kPa/m or kN/mreaction (kPa/m or kN/m33)*)*
plate
plate
plate
plate
plate
v S
Q
S
A
Q
K
09.0
*RememberRemember, when calculating Kv from plate test data, plate area (Aplate) is 0.09m2!
Beware!Beware!The Plate Bearing Test results are The Plate Bearing Test results are extrapolatedextrapolated for the design of the for the design of the
foundation! foundation!
Craig, 6Craig, 6thth Ed.Ed.
Standard Penetration TestStandard Penetration Test part of a part of a standard bore standard bore hole investigationhole investigation split barrel split barrel sampler is sampler is advanced by advanced by dropping a 64 kg dropping a 64 kg hammer 760 mmhammer 760 mm N-Values are N-Values are the number of the number of blows (hammer blows (hammer drops) to drops) to advance sampler advance sampler 300 mm300 mm
McCarthy, 6McCarthy, 6thth Ed.Ed.
Split Barrel SamplerSplit Barrel Sampler
N-Value = “Standard Penetration N-Value = “Standard Penetration Resistance”Resistance”
Peck, Hanson, Thornburn, 2Peck, Hanson, Thornburn, 2ndnd Ed.Ed.
McCarthy, 6McCarthy, 6thth Ed.Ed.
Sample Bore Hole LogSample Bore Hole Log
McCarthy, 6McCarthy, 6thth Ed.Ed.
North American EquationsNorth American Equations
for B for B << 1.2m 1.2m
NSq aa 21
for B > 1.2mfor B > 1.2m2
31 3.0
B
BNSq aa
where Swhere Saa = allowable settlement (mm) = allowable settlement (mm) qqaa = allowable bearing pressure (kPa) = allowable bearing pressure (kPa) B = footing width (m)B = footing width (m) N = average corrected standard N = average corrected standard
penetrationpenetration resistanceresistance a number of corrections are applied to N-a number of corrections are applied to N-
Values (pore water pressure, overburden Values (pore water pressure, overburden stress…see Craig)stress…see Craig)
Water Table CorrectionWater Table Correction
water table water table correction:correction:
BD
DC ww 5.05.0
we will be using average corrected N-we will be using average corrected N-ValuesValues Terzaghi and Peck proposed a correction, Terzaghi and Peck proposed a correction, CCww to the allowable bearing pressure, q to the allowable bearing pressure, qaa to to reflect the depth of water tablereflect the depth of water table
DDww =
D +
=
D +
BB
DD
DDww
BB
5.0wC
1wC
BB