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Sanjivani Rural Education Society’s

Sanjivani College of Engineering, Kopargaon, 423603

An Autonomous Institute Affiliated to Savitribai Phule Pune University, Pune

Subject: Foundation Engineering

B. Tech. (Civil Engg)

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Unit-III: Settlement and Consolidation Settlement

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Department of Civil Engineering

Sanjivani College of Engineering, Kopargaon, 423603

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SETTLEMENT

• The vertical downward movement of loaded base is called settlement.

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• Every loaded structure will settle no matter what is its support, because all foundation materials are compressible (without exception)

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Settlement of base is mainly of two types of straining of underlying supporting material

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1.Elastic distortion Sd

2.Volume Change Sv

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• For an Elastic material the elastic distortion is almost instantaneous or immediate.
• It depends among other things on elastic properties of material soil undergoes volume reduction as a result of reduction in voids, for saturated soils it is a time dependent slow process, which depends on the perviousness of the soil

In sands and in partially saturated clay it is almost instantaneous thus

Total settlement

S=SD+ SV

For saturated compressible soils, Terzaghi’s theory of consolidation is used to estimate the volume change settlement hence assumptions are necessary to simplify the theory. As such settlement predictions gives only rough estimates.

Causes of Settlement

1.Static loads

i. Elastic compression

ii. Plastic flow

iii.Consolidation

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2.Dynamic loads

i. Liquefaction during earthquake

ii. Soil compaction due to shock and vibration

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3.Lowering of water table and dewatering

I. Stress increment

ii. Shrinkage

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4.Neighbouring construction

i. Excavation

ii. Heavy structure

iii. Pile driving, Mining

Contact Pressure

The normal stress at the plane of contact between the loaded base and foundation bed is known as contact pressure

OR

Pressure transmitted from the base of foundation to the soil is termed as the contact pressure

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It depends upon several factors such as flexural rigidity of the base, load distribution, type of soil and confinement

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a. Flexible base

b. Rigid base

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fig(a) sandy soil (b) clayey soil

TYPES OF SETTLEMENT

• Types with respect to Permanency.

a)

b)

Permanent settlement Temporary Settlement.

• Types with respect to Mode of Occurrence.
• Primary consolidation settlement (Sc)
• Secondary consolidation settlement (Ss)
• Immediate settlement (Si)

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• Types with respect to Uniformity.
• Uniform settlement
• Differential settlement

Types with respect to Permanency

• Permanent Settlement (Irreversible settlement):
• This type of settlement is caused due to distortion brought about by sliding and rolling of particles under the action of applied stresses.

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• The sliding and rolling will reduce the voids resulting in reduction of volume of soil deposit.

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• The increased in stresses may also crush the soil particles while alter the material and produce some settlement.

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• This type of settlement is permanent and undergoes in significant recovery upon removal of load.
• Settlement due to consolidation (both primary and Secondary) generally falls under this category.

TEMPORARY SETTLEMENT

• Settlement due to elastic compression of soil are usually reversible and recover a major part upon load release.

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• Immediate settlement falls under this category.

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• This settlement is generally small in soils.

TYPES WITH RESPECT TO MODE OF OCCURRENCE:

• PRIMARY CONSOLIDATION SETTLEMENT (Sc):

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• These settlements are time dependent or long term settlements and completion time varies from 1- 5 years or more.

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• This is also known as Primary consolidation (i.e the settlement caused due to expulsion of water from the pores of saturated fine grained soils (clays).

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• This type of settlement is predominant in saturated inorganic fine grained soil (clays).

SECONDARY CONSOLIDATION

• This is the consolidation under constant effective stress causing no drainage.

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• This is very predominant in certain Organic soils, but insignificant for inorganic soil.

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• This is similar to creep in concrete.

IMMEDIATE SETTLMENT

• This type of settlement is predominant in coarse grained soils of high permeability and in unsaturated fine grained soils of low permeability.

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• The completion time is usually few days (say about 7 days).

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• Usually this type of settlement is completed during construction period and is called build in settlement.

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• Also known as short term settlement.

• Settlement produced due to inadequate shear strength of the soil mass is caused due to bearing capacity failure of soil.
• Settlement due to lateral expulsion of soils from underneath the foundation is an example of this category.
• This settlement can not be estimated using present knowledge of soil mechanics but can be controlled easily by controlling bearing capacity.
• So the total settlement; St = Si + Sc + Ss

TYPES WITH RESPECT TO UNIFORMITY

UNIFORM SETTLEMENT:

• When all the points settle with an equal amount, the settlement is known as uniform settlement.
• This type of settlement is possibly only under relatively rigid foundation loaded with uniform pressure and resting on uniform soil deposit, which is a very rare possibility.
• This type of settlement may not endanger the structure stability but generally affects the utility of the structure by jamming doors/windows, damaging the utility lines ( sewer, water supply mains etc)

DIFFERENTIAL SETTLEMENT

• When different parts of a structure settle by different magnitude, the settlement is called differential settlement.
• This is very important as it may endanger the structural stability and may cause catastrophic failure.
• If soil is granular, then differential settlement will be 2/3 of the total maximum settlement.
• In case of cohesive soil, possible differential settlement is about 1/3 of the maximum settlement.

SETTLEMENTS OF FOUNDATIONS

NO SETTLEMENT * TOTAL SETTLEMENT * DIFFERENTIAL SETTLEMENT

Uniform settlement is usually of little consequence in a building, but differential settlement can cause severe structural damage

DIFFERENTIAL SETTLEMENT

DIAGONAL CRACKS IN BRICK WORK DUE TO DIFFERENTIAL SETTLEMENT

LEANING TOWER OF PISA TOWER

PLASTIC AND ELASTIC DEFORMATION

• All materials deform when subjected to an applied load.
• If all this deformation is retained when load is released, it is said to have experienced Plastic deformation.
• Conversely if the material returns to its original size and shape when the load is released, it is said to have experienced elastic deformation.
• Soil exhibits both plastic and elastic deformation.

ANGULAR DISTORTION

• Angular distortion between two points under a structure is equal to the differential settlement between the points divided by the distance between them.
• Angular distortion is also known as Relative Rotation.

Differential settlement = ∆S = Smax – Smin Angular Distortioin = ∆S /L

CAUSES OF SETTLEMENT

• Following are the major causes of settlement;
• Changes in Stress due:
1. Applied structural load or excavation
2. Movement of ground water table
3. Vibrations due to Machines, Earth quake
• Desiccation due to surface drying and/or plants life. (Desiccation = Removal of water from soil

Loss (evaporation) of water / effective stress(inter-granular stresses) increase /Mass shrinkage will start ) ( Reason= High fines content, Volume of water is the direct function of shrinkage)

• Changes due to structure of soil.
• Adjacent excavation.
• Mining Subsidence.
• Swelling and Shrinkage.
• Lateral expulsion of soil.
• Land slides.

Remedial Measures

• Philosophy of remedial measures is to;
• Reduce or eliminate settlement.
• Design structure to with stand the Settlement.
• To reduce or eliminate stresses following considerations can be followed;
• Reduce Contact pressure.
• Reduce Compressibility of soil deposits using various ground improvement techniques ( Stabilization, pre-compression, vibro-flotation etc).
• Remove soft compressible material such as peat, muck. etc

• Built slowly on cohesive soils to avoid lateral expulsion of soil mass, and to give time for pore pressure dissipation.
• Consider using deep foundation (piles or piers).
• Provide lateral restraint against lateral expulsion.
• To achieve uniform settlement one can resolve to;
• Design of footing for uniform pressure.
• Use artificial cushion underneath the less settling foundation parts of the structure.

• Built different parts of foundation of different weight on different soil at different depths.
• Built the heavier parts of structure first (such as towers) and lighter parts later.

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PRESSURE ISOBARS-Pressure Bulb

• An isobar is a line which connects all points of equal stress below the ground surface. In other words, an isobar is a stress contour. We may draw any number of isobars as shown in Fig. for any given load system.

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• Each isobar represents a fraction of the load applied at the surface. Since these isobars form closed figures and resemble the form of a bulb, they are also termed bulb of pressure or simply the pressure bulb.

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• Normally isobars are drawn for vertical, horizontal and shear stresses. The one that is most important in the calculation of settlements of footings is the vertical pressure isobar.

^•we may draw any number of isobars for any given load system, but the one that is of practical significance is the one which encloses a soil mass which is responsible for the settlement of the structure.

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Significant Depth

The depth of this stressed zone may be termed as the significant depth Ds which is responsible for the settlement of the structure.

Terzaghi recommended that for all practical purposes one can take a stress contour which represents 20 per cent of the foundation contact pressure q, i.e, equal to 0.2q.

^•Terzaghi's recommendation was based on his observation that direct stresses are considered of negligible magnitude when they are smaller than 20 per cent of the intensity of the applied stress from structural loading, and that most of the settlement, approximately 80 per cent of the total, takes place at a depth less than Ds.

^•The depth Ds is approximately equal to 1.5 times the width of square footing

Significant depths:

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Pressure Isobars for Footings

CONSOLIDATION (OEDOMETER) TEST

• This test is performed to determine the magnitude and rate of change in volume of a laterally confined soil specimen undergoes when subjected to different vertical pressure
• To compute the consolidation settlement in a soil we need to know stress- strain properties. (i.e relationship between( σZ & εz ).
• This normally involves bringing the soil sample to the laboratory, subjecting it to a series of loads and measuring corresponding settlements.

• This test is known as consolidation test or Oedometer test.
• We are interested in engineering properties of natural soils as they exist in the field, so consolidation tests are usually performed on high quality Undisturbed samples.
• It is also important for samples that were saturated in the field to remain so during storage and testing.
• If the sample is allowed to dry, a process we call Desiccation, negative pore pressure will develop and may cause irreversible changes in the in the soil.

Consolidation Apparatus

• The test begins by applying vertical normal load P. It produces a vertical effective stress of;

σ’Z = P/A - U

Where;

σ’Z = Vertical effective stress. P = Applied Load.

A = Cross sectional area of soil specimen.

U = pore water pressure inside the soil specimen.

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• The water bath barely covers the specimen , so the pore water pressure is very small as compared to the vertical stress and thus may be ignored;

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σ’Z = P/A

• The vertical strain εz is noted by monitoring the dial gage, for each corresponding increase in load.

εz = Change in Dial Gage Reading Initial height of the sample

• Increase the load to some higher value and allow the soil to consolidate again, thus obtaining a second value of (σZ ,εz).
• This process will continues until we have reached the desired peak vertical stress; from this loading sequence we obtain the loading curve ABC.
• We then incrementally unload the sample and allow it to rebound thus producing unloading curve CD. Shown in the figure presented in the next slide.
• Data is plotted on logarithmic scale.

AB representing the Recompression Curve

BC representing the Virgin Curve ,CD representing the Rebound Curve