Settlement Criteria f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7...

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Settlement Criteria Settlement Criteria for clays, silty clays, plastic for clays, silty clays, plastic silts: silts: Chapter 5 (short term) Chapter 5 (short term) Chapter 7 (long term, i.e., Chapter 7 (long term, i.e., consolidation) consolidation) in this module in this module SANDS SANDS (including (including gravelly sands, silty sands and non- gravelly sands, silty sands and non- plastic silts) are considered plastic silts) are considered

Transcript of Settlement Criteria f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7...

Page 1: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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

Page 2: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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

Page 3: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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

Page 4: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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

Page 5: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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.

Page 6: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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.

Page 7: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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.

Page 8: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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

Page 9: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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

Page 10: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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!

Page 11: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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.

Page 12: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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.

Page 13: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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.

Page 14: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

Sample Bore Hole LogSample Bore Hole Log

McCarthy, 6McCarthy, 6thth Ed.Ed.

Page 15: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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)

Page 16: Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this.

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