Foundation Settlement

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 highcost 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

ρ

F

=ρ i+ρ

c

+ρ

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, (1 − μ2 ) S e = qn B

Es

If

Where Se = elastic settlement B = width of foundation E s = modulus of elasticity of soil μ = Poissn' s ratio , qn = net foundation pressure, I f = influence factor

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) S′

c

=µ

g

Sc

S′ c = corrected consolidation settlement µ g = correction factor for geological conditions Sc = settlement calculated from consolidation.

Sc = mv x∆ σ

z

xH

Where mv = average coefficient of volume compressibility obtained from the effective pressure increment in the particular layer under consideration. ∆ σ 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

H Sc = ( e1 − e 2 ) 1 + e1 H = As defined above e1 = initial void ratio , corresponding to initial overburden pressure po′ at the center of the layer

( read

from e − log p curve ) e 2 = final void ratio corresponding to a pressure po′ + σz .

• OR

po′ + σz H Sc = C c log10 1 + e1 po′

Where po′ = Initial effective overburden pressure . C c = compression index − slope of virgin compression curve e − logp.

Calculation of Cc (empirical eqs.) C c = 0.007( L .L − 10% )

for N .C .C

C c = 0.009( L .L − 10% )

for remolded clay

C c = 1.15( e o − 0.35 )

for all clays

C c = 0.75( e0 − 0.50 )

soils with low plasticity

C c = 0.141G s

1.2

 1 + eo     Gs 

Cc = 0.0115 wN

2.38

for all clays

Consolidation Settlement Cc H c po + Δpav Sc = log 1 + eo po

for N .C .C

Cs Hc po + Δpav Sc = log 1 + eo po

for O .C .C with po + Δpav < pc

Cs Hc pc C c H c po + Δpav Sc = log + log 1 + eo po 1 + e o pc for O .C .C with po + Δpav > pc > po or po < pc < po + Δpav

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 index Hc = thickness of clay layer.

• ∆ pav = 1/6 (∆ pt + 4∆ pm + ∆ 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 soil

Settlement occurs immediate ∴ Total settlement = immediate settlement p′ + σ H (CPT) Cone Penetration Test S = 2.3 log o

C

10

po′

z

Where c = constant of compressib ility c = 1.5

qc po′

qc = static cone penetratio n resistance , kPa po′ = Effective overburden pressure at point of measuremen t, kPa qc = upto 2 B

Standard Penetration Test for Shallow Foundations saturated sands and gravel qn B qn B S = 0.96 ≈ N N

Where qn = Net foundation pressure kPa N = Average corrected SPT blows within the seat of settlement .

For silty sands q

S=2 n N

If D > 4 B

B

( Deep Foundations )

qn B 1 S= 2 N

C = c

eo − e Δe = σ1′ Δ log10 log 10 σo′

C = compression index represents c the slope of the linear portion of the pressure − void ratio curve , and remains cons tan t for fairly l arg e range of pressure . eo − e Δσ σ′ − σo

Coefficient of compressibility = av = − Δe = Coefficient of volume change = m

v

=−

Δe 1 × 1 + e o Δσ′

av Δe  − = av , m v = ′ Δσ 1 + eo

• 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 , Δ H 1 Hence m =− . v

H o Δσ′

∴ ΔH = − mv × H o × Δσ′

Compressibility of Various Types of Clays Type

Qualitative Description

Heavily over Very low consolidated boulder compressibility clay

Coefficient of volume compressibili ty, mv m2 /MN Below 0.05

Normally High compressibility 0.3- 1.5 consolidated alluvial clays Very organic alluvial Very high clays and peats 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 2

Tv d t= cv

Or expressed in m/years units Tv d × 10 t ( years ) = 2 3.154 × c v ( m / s ) 2

−7

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 σ z is small say 10 to 20 kN/m2 for

Drainage in one direction. Or d = H/2 for drainage at top and bottom of clay stratum. Cv = Average coefficient of consolidation over k ( 1 + eo ) k the of pressure involved. c v range or

mv γw

U < 60 % U > 60 % or

 U  Tv = π/ 4   100 

av × γw

2

U   Tv = −0.9332 log10  1 −  − 0.851  100  Tv = 1.781 − 0.933 log10 ( 100 − U % )

Estimation of Final Settlement ρ f =ρ i + ρ ρ

oed

c

B

= mv x σ z xH

qn

= mv x 0.55q x1.5 B + immediate 1.5 B settlement

Average pressure in the center of layer = 0.55 qn

0.1qn

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

Differential settlement

Total settlement ∆ l

l

∆ /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

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 the following results.

TABLE 5.20 Recommendation of European Committee for Standardization on Differential Settlement Parameters

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

l

s S min (Uniform settlement)

S

(Tilt)

max

δ S S

max

min

(Nonuniform settlement)

∆ s = smax- smin = diff. settlement Δs δ Angular distortion l== l

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

Requires rigid raft Dense Gravel Compressible soil Differential settlement

Bowl shape

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 Measures Philosophy of remedial measures is to (a) reduce or eliminate settlement (b) design structures to withstand the settlement. (a) Reduction of Settlement To reduce or eliminate settlement, consider following: 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 pressure ii.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.