Foundation Settlement

Settlement is the vertical component of soil deformation beneath the load under consideration.

All imposed loads on soils will cause some settlement due to “elastic compression” of the foundation soils. This settlement occurs relatively rapidly

and is termed “elastic” or “immediate” settlement.

1. Immediate, or those that take place as the load is applied or within a time period of about 7 days. The water in the voids is expelled simultaneously with the application of load and as such the immediate and consolidation settlements in such soils are rolled into one.2. Consolidation, or those that are time-dependent and take months to years to develop. The Leaning Tower of Pisa in Italy has been undergoing consolidation settlement for over 700 years.

Foundation settlements must be estimated with great care for buildings, bridges,

towers, power plants , and similar high-cost structures.

The stress change q from this added load produces a time-dependent accumulation

of particle rolling, sliding, crushing, and elastic distortions in a limited influence zone

beneath the loaded area. The statistical accumulation of movements

in the direction of interest is the settlement. In the vertical direction the settlement will

be defined as H.

Many engineers seemed to have the misconception that any footing designed with an adequate factor of safety against a bearing capacity failure would not settle excessively. Independent settlement analyses also need to be performed

Settlement frequently controls the design of spread footings, especially when B is large, and that the bearing capacity analysis is, in fact, often secondary.

In saturated silts and clays, particularly those which are normally consolidated,

the settlement will be dominated by consolidation, as water slowly drains from these soils to reduce the pore water pressures to the original levels.

Settlement of cohesionless soil primarily occur from the re-arrangement of soil

particles due to the immediate compression from the applied load

To enable settlements to be calculated we have to calculate the change in stresses

within a soil mass, due to imposed external loads on the soil.

Elastic stress distributions within the soil are usually based on the theory of

Boussinesq and so methods of computing “elastic settlements” usually assume that Boussinesq theory is applicable.

Causes of Settlement

i. Static loads, such as those, imposed by the weight of a structure or an embankement.

ii. Dynamic or transient loads, such as those produced by machinery or moving loads on roads or airfield pavements, pile driving, blasting, etc

iii. Changes in moisture content, for example from seasonal fluctuations in the water table

iii Rainfall, and evaporation or the absorption of the water by the rots of larger trees.

iv the effects of nearby construction(e.g. excavation, pile driving, subsidence of mines and dewatering) may also be significant.

v Ground movement on earth slopes, e.g. surface erosion, landslide or slow creep.

Components of Settlement• Immediate (or undrained) settlement, which

occurs immediately upon application of the load, and which in a saturated soil arises from shear deformations under constant volume conditions. ( without change of water content)

• Consolidation settlement, which occurs primarily because of the dissipation of excess pore pressures in the soil and is therefore time dependent. This component of settlement arises mainly from volumetric deformation although shear deformations are also involved.

• Creep settlement(frequently termed secondary consolidation) which most frequently manifest itself as a time dependent settlement after the completion of excess pore pressure dissipation, however, significant creep settlements can also occur undrained conditions. Creep settlements generally involve both shear and volumetric deformations. (only for clay)

Total Settlement or Final Settlemnt

STF = Si + ScF + SSF

Where STF = Total final settlement

Si = Immediate settlement

ScF = Final consolidation settlement

SsF = secondary consolidation

Or F = i + c +s

• In case of foundation on medium dense to dense sands and gravels , the immediate and consolidation settlements are of relatively small order and take place almost simultaneously and a high portion of settlement is almost completed by the time the full loading comes on the foundations.(High permeability)

• Similar in the case of loose sands, where as the settlements on the compression clays are partly immediate and partly long term movements. The later takes long time(period of years) and is of greater proportion. (low permeability)

• Settlement of foundation are not necessarily confined to very large and heavy structures.

• In soft clays and silts appreciable settlements can occur under light loadings. (may be in two storey building cracks can occur or are observed).

• Differential or relative settlements are of greater importance to the stability of the structure.

• If a uniform settlements occur under the whole area of foundation, it may not be dangerous, but if differential settlement takes place, the stresses will develop, serious cracks or even collapse of the structure will occur if differential settlements are excessive.

• Skempton and McDonald have divided damages resulting from settlements into three categories.

ELASTIC SETTLEMENT BENEATH THE CORNER OF A UNIFORMLY LOADED FLEXIBLE AREA

BASED ON THE THEORY OF ELASTICITY• The net elastic settlement equation for a

flexible surface footing may be written as,

factorinfluence

pressure,foundationnet

ratiosPoissn'

elasticityofmodulus

foundationofwidth

settlementelastic

1 2

f

n

s

e

fs

ne

I

q

,

soilofE

B

SWhere

IE

BqS

μ

μ

Evaluation of Undrained modulus of Deformation of Elasticity

Eu = 500 Su (soft sensitive clay Nc)

1000 Su (firm to stiff clay OCR< 2)

1500 Su (very stiff clay OCR> 2)

Approximately. Si = 0.1Sc for N.C

Si = 0.5Sc for O.C

Settlement of Saturated Clays(NC)Sc = g Sc

Sc = corrected consolidation settlement

g = correction factor for geological

conditionsSc = settlement calculated from

consolidation.

Sc = mv x z x H

Where mv = average coefficient of volume compressibility obtained from

the effective pressure increment in the particular layer under

consideration. z =average effective vertical stress imposed on the particular layer resulting from the net

foundation pressure qn

H = Thickness of the particular layer under consideration.

OR

.p

pressureatoingcorrespondratiovoidfinale

curveplogefromread

layertheofcentertheatppressureoverburden

initialtoingcorrespond,ratiovoidinitiale

abovedefinedAsH

)ee(e

HS

zo

o

c

σ

2

1

21

11

• OR

logp.ecurvencompressio

virginofslopeindexncompressio

pressureoverburdeneffectiveInitial

1 101

c

o

o

zocc

C

.p

Where

p

plogC

eH

Sσ

Calculation of Cc (empirical eqs.)

claysallforG

eG.C

plasticity

lowwithsoils.e.C

claysallfor.e.C

clayremoldedfor%L.L.C

C.C.Nfor%L.L.C

.

s

o.sc

c

oc

c

c

38221

0

11410

500750

350151

100090

100070

Cc = 0.0115 wN

Consolidation Settlement

avoco

ocavo

c

avo

o

cc

o

c

o

csc

cavoo

avo

o

csc

o

avo

o

ccc

ppppor

ppppwithC.C.Ofor

p

pplog

e

HC

p

plog

e

HCS

pppwithC.C.Oforp

pplog

eHC

S

C.C.Nforp

pplog

e

HCS

Δ

Δ

Δ

11

ΔΔ

1

Δ

1

Where po = Average effective pressure on the clay layer before the construction of the

foundation. pav = Average increase of pressure on the

clay layer caused by the foundation construction.

pc = pre-consolidation pressure.eo = initial void ratio of the clay.Cc = compression index.Cs = swelling indexHc = thickness of clay layer.

• pav = 1/6 (pt + 4pm + pb)

• Where pt ,pm and pb are the pressure increases at the top ,middle and bottom of the clay layer caused by the foundation construction.

Settlement of Cohesioless soilSettlement occurs immediate Total settlement = immediate settlementCone Penetration Test (CPT)

B2upto

kPat,measuremenof

pointatpressureoverburdenEffective

kPa,resistancenpenetratiocone static

51

ilitycompressibof constantWhere

32 10

c

o

c

o

c

o

zo

q

p

q

p

q.c

c

p

plog.

CH

Sσ

Standard Penetration Test for Shallow Foundations

NBnqS

N

Bn

qS

NBnq

NBnq.S

)sFoundationDeep(BDIf

.settlementofseatthe

withinblowsSPTcorrectedAverageN

kPapressurefoundationNetqWhere

sandssiltyFor

gravelandsandssaturated

n

21

2

960

4

o

vvv

o

v

o

o

o

o

e

am,a

e

ee

m

eeev

a

.pressureofrangeeargl

fairlyforttanconsremainsand

,curveratiovoidpressuretheof

portionlineartheofslopethe

representsindexncompressioc

C

loge

log

eec

C

changevolumeoftCoefficien

ilitycompressiboftCoefficien

1ΔΔ

Δ1

1Δ

ΔΔ

ΔΔ

10101

σ

σ

σσσ

σσ

• When the soil is laterally confined, the change in the volume is proportional to change in thickness H and the initial volume is proportional to initial thickness Ho , Hence

σ

σ

ΔΔ

Δ1Δ

ov

o

v

HmH

.HH

m

Compressibility of Various Types of Clays

Type Qualitative Description

Coefficient of volume compressibility, mv - m2 /MN

Heavily over consolidated boulder clay

Very low compressibility Below 0.05

Normally consolidated alluvial clays

High compressibility 0.3- 1.5

Very organic alluvial clays and peats

Very high compressibility

Above 1.5

Estimation of Rate of Consolidation

• May be required to know the rate of settlement of foundation during the long process of consolidation. This is normally calculated as the time period required for 50% or 90 % of the final settlement. The time required is given by

v

v

c

dTt

2

Tv =Time factor(Theoretical time factor, a pure number that has been determined for all conditions of importance and is given in terms of u d = H (Thickness of compressible stratum measured from foundation level for point which z is small say 10 to 20 kN/m2 for

s/mc.

dTyearst

v

v2

72

1543

10

unitsm/yearsinexpressedOr

Drainage in one direction. Or d = H/2 for drainage at top and bottom of clay stratum.Cv = Average coefficient of consolidation over

the range of pressure involved.

wv

o

wv

v a

ekor

mk

cγγ

1

%Ulog..Tor

.U

log.T%U

U/T%U

v

v

v

10093307811

8510100

19332060

100460

10

10

2

π

Estimation of Final Settlement

f =i + c

oed = mv x z xH

= mv x 0.55q x1.5 B

+ immediate settlement

1.5 B

B

qn

0.1qn

Average pressure in the center of layer = 0.55 qn

1. Structural damages which involves only frame, i.e. stanchions and beams.2. Architectural damage involving only the panel walls, floors

or finishes. 1. Visual appearance 2. Serviceability or function 3. Stability

3. Combined structural and architectural damage. A study has shown that structural damage is likely to take

place when the angular distortion(/L) of the span(l) between adjacent column or along a given length of load bearing walls exceeds 1/150 and that architectural. Damage is likely to occur when the angular distortion exceeds 1/300

ll

Differential settlement

Total settlement

/l = angular distortion

Influence of structural rigidity on differential settlement(a) very flexible structure has little load transfer, and thus could have larger differential settlements; (b) a more rigid structure has greater capacity for load transfer, and thus provides more resistance to excessive differential settlement

Skempton and MacDonald(1956)

Soviet Code of Practice(1955)

Bjerrum [27] recommended the following limiting angular distortion ($max)for various structures

Grant et al.[28] correlated ST(max) and $max for several buildings with

thefollowing results.

TABLE 5.20 Recommendation of European Committee for

Standardization onDifferential Settlement Parameters

Table 9.1 Tolerable differential settlement of buildings, in inches, recommended maximum values in

parentheses

s

S min

l

S max

S min

S max

l

(Uniform settlement) (Tilt)

(Nonuniform settlement)

s = smax- smin = diff. settlement

Angular distortion = lls δΔ

Causes of differential settlements

1. Variation in soil strata one part of structure may be founded on a

compressible soil and the other part on incompressible material. Like (i) glacial deposits. Lenses of clay in sandy materials. (ii) Irregular bed rock surface (good rock, weathered compressible rock) (iii) Wind laid or water laid deposits of sands and gravels varying in density.

2. Variation in foundation loading: Some parts heavy load and other light. For example, (i) Building consists of high central tower, low projecting wings, (ii) factory- heavy and light items of machinery.

3. Large loaded areas on flexible foundations. (i) Large flexible raft foundation

Bowl shapeDifferential settlement

Compressible soil

Dense Gravel

Requires rigid raft

4. Difference in time of construction of adjacent parts of structure.

This is the case when extension of a structure is to be done after many years.(then the completion of original). Long term consolidation settlement of built structure may be complete, but the new structure(if of the same foundation loading as the original) will eventually settle an equal amount. Special provisions in the form of vertical joint are needed to prevent distortion and cracking.

5. Variation in site conditions (History)one part of building area may be

occupied by heavy structure which had been demolished or on sloping site it may be necessary to remove considerable thickness of overburden to form a level site. This variation results in different stress conditions.

Following are the major causes of settlement:(1) Changes in stress due to:

a. Applied structural load or excavations.b. Movement of ground water table.c. Glaciation; and d. Vibration due to machines and earthquake etc

(2) Desication due to surface drying and/or plant life.

(3) Changes due to structure of soil (secondary compression)

(4) Adjacent excavation(5) Mining subsidence(6) Swelling and Shrinkage(7) Lateral expulsion of soils(8) Land slides.

Compression of foundations soils under static loads.

Compression of soft clays due to lowering ground water table.

Compression of cohesionless soils due to vibrations

Compression of foundation soils due to wetting.

Shrinkage of cohesive soils caused by drying

Loss of foundation support due to erosion. Loss of foundation support due to

excavation of adjacent ground

Loss of support due to formation of sink holes

Loss of support due to thawing of permafrost

Loss of support due to partial or complete

liquefaction. Down drag on piles driven through soft

clay.

Methods of Preventing Excessive Differential Settlement

Remedial MeasuresPhilosophy of remedial measures is to (a) reduceor eliminate settlement (b) design structures towithstand the settlement.(a) Reduction of SettlementTo reduce or eliminate settlement, considerfollowing:1. Reduce the contact pressure.2. Reduce compressibility of the soil deposits using

various ground improvement techniques(stabilization, precompression, vibroflotation etc.)

(3) Remove soft compressible material such as peat, muck etc

(4) Build slowly on cohesive soils to avoid lateral expulsion of a soil mass, and to give time for pore pressure dissipation.

(5) Consider using deep foundations (piles and piers)

(6) Provide lateral restraint or counterweight against lateral expulsion.

To achieve uniform settlement one can resolve to:

i. Design footings for uniform pressureii. Use of artificial cushion underneath the less

settling foundation parts of the structure.iii.Build different parts of foundations of

different weight and on different soil at different depths.

iv.Build the heavier parts of the structure first (such as towers, and spires for example), and the lighter parts later.