361-Sl10 Settlement of Soils

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ENCE 361ENCE 361

Soil MechanicsSoil Mechanics

Settlement and Volume Expansion:Overview

Settlement of Cohesionless SoilsSettlement of Cohesive Soils (Part I)


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Field TripField Trip

Date: 17 October 2001

Place:Qore Property Sciences

3903 Volunteer Drive Suite 400

Chattanooga, TN 37416(423) 499-0957


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OverviewOverviewSoil is a

nonhomogeneousporous materialconsisting of three

phases Solids

Fluid (normallywater)

Air

Soil deformation may

occur by change in: Stress

Water content Soil mass

Temperature


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T pe of SettlementType of Settlement

Elastic deformation (volume distortion)

Consolidation

Secondary compression and creep

Dynamic forces

Expansive soil

Collapsible soil

S tS vdS pcS sc


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Settlements in CohesionlessSettlements in CohesionlessSoilsSoils

Elastic or immediate deformation caused by

static loads is usually small, and it occursessentially at the same time these loads areapplied to the soil.

Consolidation occurs quickly in coarse-grained soils such as sands and gravels, andit is usually not distinguishable from elasticdeformation. This is due to their highpermeability


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Settlement in Cohesive SoilsSettlement in Cohesive Soils

Elastic or immediate deformation caused bystatic loads is usually small, and it occursessentially at the same time these loads areapplied to the soil.

Consolidation in fine-grained soils such asclays and organic materials can be significantand take considerable time to complete.

Secondary compression and creep areassociated with the compression and distortionat constant water content of compressible soils

such as clays, silts, organic materials, andpeat.


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Settlement of Cohesionless SoilsSettlement of Cohesionless Soils


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Schmertmann's MethodSchmertmann's MethodMethod for estimating the settlement of

foundations in sand

Especially useful when CPT data is available

This procedure provides settlementcompatible with field measurements in manydifferent areas

The analysis assumes that the distribution ofvertical strain is compatible with a linearelastic half space subjected to a uniform

pressure


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Schmertmann's MethodSchmertmann's MethodRequired Data

A profile of standardpenetration resistance N(blows/ft) versus depth,

from the proposedfoundation level to adepth of 2B, or to

boundary of anincompressible layer,whichever occurs first.

Soil Type Es/N

4

7

1012

Silts, sands silts, slightlycohesive silt-sandmixtures

Clean, fine to med,sands & slightly siltysandsCoarse sands & sands

with little gravelgravel


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Schmertmann's DataSchmertmann's DataRequired Data

Least width of foundation = B

Depth of embedment = D

Proposed average contact pressure = P

Approximate unit weights of surcharge soils, andposition of water table if within D

If the static cone bearing value qc, measuredcompute E

s, based on E

s, = 2 q

c.


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Schmertmann's MethodSchmertmann's Method

Analysis Procedure Divide the subsurface soil profile into a

convenient number of layers of any thickness,

each with constant N over the depth interval 0 to2B below the foundation.

Prepare a table using the indicated column

headings. Fill in columns 1, 2, 3 and 4 with thelayering assigned above.

Multiply N values in column 3 by the appropriate

factor Es/N (col. 4) to obtain values of Es; placevalues in column 5.


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Schmertmann'sSchmertmann's

MethodMethod Draw an assumed 2B-0.6

triangular distribution for thestrain influence factor Iz, along a

scaled depth of 0 to 2B belowthe foundation. Locate the depth

of the mid-height of each of thelayers assumed in Step 2, andplace in column 6. From thisconstruction, determine the I

z

value at the mid-height of eachlayer, and place in column 7.


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Schmertmann's Method ExampleSchmertmann's Method Example

(in 10 years)


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Schmertmann's Method ExampleSchmertmann's Method Example


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ExampleExample


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Notes on Schmertmann MethodNotes on Schmertmann MethodGenerally applies to normally loaded sand

deposits

Will overestimate foundation settlement ifsand has been previously compressed ordensified (such as with compaction)

When precompressed sands are present, the

settlement should be estimated as roughly halfof the computed settlement


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Methods of ComputingMethods of Computing

Settlement in Cohesive SoilsSettlement in Cohesive SoilsVolume distortion (immediate settlement)

Perloff's Method

Primary Consolidation

Terzaghi Consolidation Theory

Secondary Consolidation

C

Method

S tS vdS pcS sc


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Perloff's MethodPerloff's Method

Computes volume distortion according to theformula

Cs= shape and foundation rigidity factor

q = equivalent distributed load on foundation

B = plan width or diameter of the foundation

Eu

= undrained elastic modulus of clay

= Poisson's Ratio

S vdCs q B 12

Eu


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Notes on VariablesNotes on VariablesValues of C

swill vary with shape of

foundation, rigidity of foundation, andproximity of rigid substratum

Values of Es can be estimated from soilconsistency/undrained shear strength

Assume for Poisson's ratio for saturated

clays, slightly less for unsaturated


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Perloff Method ExamplePerloff Method Example


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Perloff Method ExamplePerloff Method Example

SolutionSolution


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Homework Set 5Homework Set 5Textbook Reading

Chapter 10 (pp. 347-391)

Laboratory Soils Testing

Appendix VIII (Consolidation Test)

Homework

10-7, 10-9, 10-19, 10-27, 10-34, 10-39, 10-40, 1044, 10-47, 10-49


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QuestionsQuestions

361-Sl10 Settlement of Soils

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