fema_273-abstract_foundations and geotechnical hazards

7/24/2019 FEMA_273-Abstract_Foundations and Geotechnical Hazards

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FEMA 273 Seismic Rehabilitation Guidelines 4-1

4.Foundations and Geotechnical Hazards(Systematic Rehabilitation)

4.1 Scope

This chapter provides geotechnical engineeringguidance regarding building foundations and seismic-geologic site hazards. Acceptability of the behavior ofthe foundation system and foundation soils for a givenPerformance Level cannot be determined apart from thecontext of the behavior of the superstructure.

Geotechnical requirements for buildings that aresuitable for Simplified Rehabilitation are included inChapter 10.

Structural engineering issues of foundation systems arediscussed in the chapters on Steel (Chapter 5), Concrete

(Chapter 6), Masonry (Chapter 7), and Wood(Chapter 8).

This chapter describes rehabilitation measures forfoundations and geotechnical site hazards.Section 4.2provides guidelines for establishing site soilcharacteristics and identifying geotechnical sitehazards, including fault rupture, liquefaction,differential compaction, landslide and rock fall, andflooding. Techniques for mitigating these geotechnicalsite hazards are described in Section 4.3.Section 4.4presents criteria for establishing soil strength capacity,stiffness, and soil-structure interaction (SSI) parameters

for making foundation design evaluations. Retainingwalls are discussed in Section 4.5. Section 4.6containsguidelines for improving or strengthening foundations.

4.2 Site Characterization

The geotechnical requirements for buildings suitable forSimplified Rehabilitation are described inChapter 10.For all other buildings, specific geotechnical sitecharacterization consistent with the selected method ofSystematic Rehabilitation is required. Sitecharacterization consists of the compilation of

information on site subsurface soil conditions,configuration and loading of existing buildingfoundations, and seismic-geologic site hazards.

In the case of historic buildings, the guidance of theState Historic Preservation Officer should be obtained ifhistoric or archeological resources are present at thesite.

4.2.1 Foundation Soil Information

Specific information describing the foundationconditions of the building to be rehabilitated is required.Useful information also can be gained from knowledgeof the foundations of adjacent or nearby buildings.Foundation information may include subsurface soiland ground water data, configuration of the foundationsystem, design foundation loads, and load-deformationcharacteristics of the foundation soils.

4.2.1.1 Site Foundation Conditions

Subsurface soil conditions must be defined in sufficientdetail to assess the ultimate capacity of the foundationand to determine if the site is susceptible to seismic-

geologic hazards.

Information regarding the structural foundation type,dimensions, and material are required irrespective ofthe subsurface soil conditions. This informationincludes:

Foundation typespread footings, mat foundation,piles, drilled shafts.

Foundation dimensionsplan dimensions andlocations. For piles, tip elevations, vertical variations(tapered sections of piles or belled caissons).

Material composition/construction. For piles, type(concrete/steel/wood), and installation method (cast-in-place, open/closed-end driving).

Subsurface conditions shall be determined for theselected Performance Level as follows.

A. Collapse Prevention and Life Safety PerformanceLevels

Determine type, composition, consistency, relativedensity, and layering of soils to a depth at which the

stress imposed by the building is approximately 10% ofthe building weight divided by the total foundation area.

Determine the location of the water table and itsseasonal fluctuations beneath the building.
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4-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)

B. Enhanced Rehabilitation Objectives and/or Deep

Foundations

For each soil type, determine soil unit weight , soilshear strength c, soil friction angle , soilcompressibility characteristics, soil shear modulus G,

and Poissons ratio .

4.2.1.2 Nearby Foundation Conditions

Specific foundation information developed for anadjacent or nearby building may be useful if subsurfacesoils and ground water conditions in the site region areknown to be uniform. However, less confidence willresult if subsurface data are developed from anywherebut the site being rehabilitated. Adjacent sites whereconstruction has been done recently may provide aguide for evaluation of subsurface conditions at the sitebeing considered.

4.2.1.3 Design Foundation Loads

Information on the design foundation loads is required,as well as actual dead loads and realistic estimates oflive loads.

4.2.1.4 Load-Deformation CharacteristicsUnder Seismic Loading

Traditional geotechnical engineering treats load-deformation characteristics for long-term dead loadsplus frequently applied live loads only. In most cases,long-term settlement governs foundation design. Short-term (earthquake) load-deformation characteristics havenot traditionally been used for design; consequently,such relationships are not generally found in the soilsand foundation reports for existing buildings. Load-deformation relationships are discussed in detail inSection 4.4.

4.2.2 Seismic Site Hazards

In addition to ground shaking, seismic hazards includesurface fault rupture, liquefaction, differentialcompaction, landsliding, and flooding. The potential forground displacement hazards at a site should beevaluated. The evaluation should include an assessmentof the hazards in terms of ground movement. Ifconsequences are unacceptable for the desiredPerformance Level, then the hazards should bemitigated as described inSection 4.3.

4.2.2.1 Fault Rupture

Geologic site conditions must be defined in sufficientdetail to assess the potential for the trace of an activefault to be present in the building foundation soils. If thetrace of a fault is known or suspected to be present, the

following information may be required:

The degree of activitythat is, the age of mostrecent movement (e.g., historic, Holocene, lateQuaternary)must be determined.

The fault type must be identified, whether strike-slip, normal-slip, reverse-slip, or thrust fault.

The sense of slip with respect to building geometrymust be determined, particularly for normal-slip andreverse-slip faults.

Magnitudes of vertical and/or horizontaldisplacements with recurrence intervals consistentwith Rehabilitation Objectives must be determined.

The width of the fault-rupture zone (concentrated ina narrow zone or distributed) must be identified.

4.2.2.2 Liquefaction

Subsurface soil and ground water conditions must bedefined in sufficient detail to assess the potential forliquefiable materials to be present in the buildingfoundation soils. If liquefiable soils are suspected to be

present, the following information must be developed.

Soil type:Liquefiable soils typically are granular(sand, silty sand, nonplastic silt).

Soil density:Liquefiable soils are loose to mediumdense.

Depth to water table:Liquefiable soils must besaturated, but seasonal fluctuations of the water tablemust be estimated.

Ground surface slope and proximity of free-face

conditions:Lateral-spread landslides can occur ongently sloping sites, particularly if a free-faceconditionsuch as a canal or stream channelispresent nearby.

Lateral and vertical differential displacement:Amount and direction at the building foundationmust be calculated.


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Chapter 4: Foundations and Geotechnical Hazards

(Systematic Rehabilitation)


The hazard of liquefaction should be evaluated initiallyto ascertain whether the site is clearly free of ahazardous condition or whether a more detailedevaluation is required. It can be assumed generally thata significant hazard due to liquefaction does not exist ata site if the site soils or similar soils in the site vicinityhave not experienced historical liquefaction and if anyof the following criteria are met:

The geologic materials underlying the site are eitherbedrock or have a very low liquefactionsusceptibility, according to the relative susceptibilityratings based upon general depositional environmentand geologic age of the deposit, as shown inTable 4-1.

Table 4-1 Estimated Susceptibility to Liquefaction of Surficial Deposits During Strong Ground Shaking(after Youd and Perkins, 1978)

Type of Deposit

General Distributionof CohesionlessSediments inDeposits

Likelihood that Cohesionless Sediments, When Saturated,Would be Susceptible to Liquefaction (by Age of Deposit)

Modern< 500 yr.

Holocene< 11,000 yr.

Pleistocene< 2 million yr.

Pre-Pleistocene> 2 million yr.

(a) Continental Deposits

River channel

Flood plain

Alluvial fan, plain

Marine terrace

Delta, fan delta

Lacustrine, playa

Colluvium

Talus

Dune

Loess

Glacial till

Tuff

Tephra

Residual soils

Sebka

Locally variable

Locally variable

Widespread

Widespread

Widespread

Variable

Variable

Widespread

Widespread

Variable

Variable

Rare

Widespread

Rare

Locally variable

Very high

High

Moderate

High

High

High

Low

High

High

Low

Low

High

Low

High

High

Moderate

Low

Low

Moderate

Moderate

Moderate

Low

Moderate

High

Low

Low

Low

High

Moderate

Low

Low

Low

Very low

Low

Low

Low

Very low

Low

High

Very low

Very low

?

Very low

Low

Very low

Very low

Very low

Very low

Very low

Very low

Very low

Very low

Very low

Unknown

Very low

Very low

?

Very low

Very low

(b) Coastal Zone Deposits

Delta

Esturine

Beach, high energy

Beach, low energy

Lagoon

Foreshore

Widespread

Locally variable

Widespread

Widespread

Locally variable

Locally variable

Very high

High

Moderate

High

High

High

High

Moderate

Low

Moderate

Moderate

Moderate

Low

Low

Very low

Low

Low

Low

Very low

Very low

Very low

Very low

Very low

Very low

(c) Fill Materials

Uncompacted fill

Compacted fill

Variable

Variable

Very high

Low


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The soils underlying the site are stiff clays or clayeysilts, unless the soils are highly sensitive, based onlocal experience; or, the soils are cohesionless (i.e.,sand, silts, or gravels) with a minimum normalizedStandard Penetration Test (SPT) resistance, (N1)60,

value of 30 blows/foot for depths below thegroundwater table, or with clay content greater than20%. The parameter (N1)60is defined as the SPT

blow count normalized to an effective overburdenpressure of 2 ksf. Clay has soil particles withnominal diameters 0.005 mm.

The groundwater table is at least 35 feet below thedeepest foundation depth, or 50 feet below theground surface, whichever is shallower, includingconsiderations for seasonal and historic ground-water level rises, and any slopes or free-faceconditions in the site vicinity do not extend below

the ground-water elevation at the site.

If, by applying the above criteria, a possibleliquefaction hazard at the site cannot be eliminated,then a more detailed evaluation is required. Guidancefor detailed evaluations is presented in the Commentary.

4.2.2.3 Differential Compaction

Subsurface soil conditions must be defined in sufficientdetail to assess the potential for differential compactionto occur in the building foundation soils.

Differential compaction or densification of soils mayaccompany strong ground shaking. The resultingdifferential settlements can be damaging to structures.Types of soil that are susceptible to liquefaction (that is,relatively loose natural soils, or uncompacted or poorlycompacted fill soils) are also susceptible to compaction.Compaction can occur in soils above and below thegroundwater table.

It can generally be assumed that a significant hazarddue to differential compaction does not exist if the soilconditions meet both of the following criteria:

The geologic materials underlying foundations andbelow the groundwater table do not pose asignificant liquefaction hazard, based on the criteriain Section 4.2.2.2.

The geologic materials underlying foundations andabove the groundwater table are either Pleistocene in

geologic age (older than 11,000 years), stiff clays orclayey silts, or cohesionless sands, silts, and gravelswith a minimum (N1)60of 20 blows/0.3 m (20

blows/foot).

If a possible differential compaction hazard at the sitecannot be eliminated by applying the above criteria,then a more detailed evaluation is required. Guidancefor a detailed evaluation is presented in theCommentary.

4.2.2.4 Landsliding

Subsurface soil conditions must be defined in sufficientdetail to assess the potential for a landslide to causedifferential movement of the building foundation soils.Hillside stability shall be evaluated at sites with:

Existing slopes exceeding approximately 18 degrees

(three horizontal to one vertical)

Prior histories of instability (rotational ortranslational slides, or rock fall)

Pseudo-static analyses shall be used to determine sitestability, provided the soils are not liquefiable orotherwise expected to lose shear strength duringdeformation. Pseudo-static analyses shall use a seismiccoefficient equal to one-half the peak groundacceleration (calculated as SXS/2.5) at the site associated

with the desired Rehabilitation Objective. Sites with a

static factor of safety equal to or greater than 1.0 shallbe judged to have adequate stability, and require nofurther stability analysis.

Sites with a static factor of safety of less than 1.0 willrequire a sliding-block displacement analysis(Newmark, 1965). The displacement analysis shalldetermine the magnitude of potential ground movementfor use by the structural engineer in determining itseffect upon the performance of the structure and thestructures ability to meet the desired PerformanceLevel. Where the structural performance cannotaccommodate the computed ground displacements,

appropriate mitigation schemes shall be employed asdescribed in Section 4.3.4.

In addition to potential effects of landslides onfoundation soils, the possible effects of rock fall or slidedebris from adjacent slopes should be considered.


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4.2.2.5 Flooding or Inundation

For Performance Levels exceeding Life Safety, siteconditions should be defined in sufficient detail toassess the potential for earthquake-induced flooding orinundation to prevent the rehabilitated building from

meeting the desired Performance Level. Sources ofearthquake-induced flooding or inundation include:

Dams located upstream damaged by earthquakeshaking or fault rupture

Pipelines, aqueducts, and water-storage tankslocated upstream damaged by fault rupture,earthquake-induced landslides, or strong shaking

Low-lying coastal areas within tsunami zones orareas adjacent to bays or lakes that may be subject toseiche waves

Low-lying areas with shallow ground water whereregional subsidence could cause surface ponding ofwater, resulting in inundation of the site

Potential damage to buildings from flooding orinundation must be evaluated on a site-specific basis.Consideration must be given to potential scour ofbuilding foundation soils from swiftly flowing water.

4.3 Mitigation of Seismic Site

Hazards

Opportunities exist to improve seismic performanceunder the influence of some site hazards at reasonablecost; however, some site hazards may be so severe thatthey are economically impractical to include in risk-reduction measures. The discussions presented beloware based on the concept that the extent of site hazardsis discovered after the decision for seismicrehabilitation of a building has been made; however, thedecision to rehabilitate a building and the selection of aRehabilitation Objective may have been made with fullknowledge that significant site hazards exist and mustbe mitigated as part of the rehabilitation.

4.3.1 Fault Rupture

Large movements caused by fault rupture generallycannot be mitigated economically. If the structuralconsequences of the estimated horizontal and verticaldisplacements are unacceptable for any PerformanceLevel, either the structure, its foundation, or both, might

be stiffened or strengthened to reach acceptableperformance. Measures are highly dependent onspecific structural characteristics and inadequacies.Grade beams and reinforced slabs are effective inincreasing resistance to horizontal displacement.Horizontal forces are sometimes limited by slidingfriction capacity of spread footings or mats. Verticaldisplacements are similar in nature to those caused bylong-term differential settlement. Mitigative techniquesinclude modifications to the structure or its foundationto distribute the effects of differential verticalmovement over a greater horizontal distance to reduceangular distortion.

4.3.2 Liquefaction

The effectiveness of mitigating liquefaction hazardsmust be evaluated by the structural engineer in thecontext of the global building system performance. If it

has been determined that liquefaction is likely to occurand the consequences in terms of estimated horizontaland vertical displacements are unacceptable for thedesired Performance Level, then three general types ofmitigating measures can be considered alone or incombination.

Modify the structure:The structure can bestrengthened to improve resistance against the predictedliquefaction-induced ground deformation. This solutionmay be feasible for small ground deformations.

Modify the foundation:The foundation system can bemodified to reduce or eliminate the potential for largefoundation displacements; for example, byunderpinning existing shallow foundations to achievebearing on deeper, nonliquefiable strata. Alternatively(or in concert with the use of deep foundations), ashallow foundation system can be made more rigid (forexample, by a system of grade beams between isolatedfootings) in order to reduce the differential groundmovements transmitted to the structure.

Modify the soil conditions:A number of types ofground improvement can be considered to reduce or

eliminate the potential for liquefaction and its effects.Techniques that generally are potentially applicable toexisting buildings include soil grouting, eitherthroughout the entire liquefiable strata beneath abuilding, or locally beneath foundation elements (e.g.,grouted soil columns); installation of drains (e.g., stonecolumns); and installation of permanent dewateringsystems. Other types of ground improvement that arewidely used for new construction are less applicable to


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existing buildings because of the effects of theprocedures on the building. Thus, removal andreplacement of liquefiable soil or in-place densificationof liquefiable soil by various techniques are notapplicable beneath an existing building.

If potential for significant liquefaction-induced lateralspreading movements exists at a site, then theremediation of the liquefaction hazard may be moredifficult. This is because the potential for lateralspreading movements beneath a building may dependon the behavior of the soil mass at distances wellbeyond the building as well as immediately beneath it.Thus, measures to prevent lateral spreading may, insome cases, require stabilizing large soil volumes and/or constructing buttressing structures that can reducethe potential for, or the amount of, lateral movements.

4.3.3 Differential CompactionThe effectiveness of mitigating differential compactionhazards must be evaluated by the structural engineer inthe context of the global building system performance.For cases of predicted significant differentialsettlements of a building foundation, mitigation optionsare similar to those described above to mitigateliquefaction hazards. There are three options: designingfor the ground movements, strengthening thefoundation system, and improving the soil conditions.

4.3.4 Landslide

The effectiveness of mitigating landslide hazards mustbe evaluated by the structural engineer in the context ofthe global building system performance. A number ofschemes are available for reducing potential impacts forearthquake-induced landslides, including:

Regrading

Drainage

Buttressing

Structural Improvements

Gravity walls

Tieback/soil nail walls

Mechanically stabilized earth walls

Barriers for debris torrents or rock fall

Building strengthening to resist deformation

- Grade beams

- Shear walls

Soil Modification/Replacement

Grouting

Densification

The effectiveness of any of these schemes must beconsidered based upon the amount of ground movementthat the building can tolerate and still meet the desiredPerformance Level.

4.3.5 Flooding or Inundation

The effectiveness of mitigating flooding or inundationhazards must be evaluated by the structural engineer inthe context of the global building system performance.Potential damage caused by earthquake-inducedflooding or inundation may be mitigated by a number ofschemes, as follows:

Improvement of nearby dam, pipeline, or aqueductfacilities independent of the rehabilitated building

Diversion of anticipated peak flood flows

Installation of pavement around the building tominimize scour

Construction of sea wall or breakwater for tsunamior seiche protection

4.4 Foundation Strength and

Stiffness

It is assumed in this section that the foundation soils arenot susceptible to significant strength loss due toearthquake loading. With this assumption, the followingparagraphs provide an overview of the requirementsand procedures for evaluating the ability of foundationsto withstand the imposed seismic loads withoutexcessive deformations. If soils are susceptible tosignificant strength loss, due to either the direct effectsof the earthquake shaking on the soil or the foundationloading on the soil induced by the earthquake, theneither improvement of the soil foundation conditionshould be considered or special analyses should be


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carried out to demonstrate that the effects of soilstrength loss do not result in excessive structuraldeformations.

Consideration of foundation behavior is only one part ofseismic rehabilitation of buildings. Selection of thedesired Rehabilitation Objective probably will be donewithout regard to specific details of the building,including the foundation. The structural engineer willchoose the appropriate type of analysis procedures forthe selected Performance Level (e.g., SystematicRehabilitation, with Linear Static or DynamicProcedures, or Nonlinear Static or DynamicProcedures). As stated previously, foundationrequirements for buildings that qualify for SimplifiedRehabilitation are included in Chapter 10.

4.4.1 Ultimate Bearing Capacities and Load

CapacitiesThe ultimate load capacity of foundation componentsmay be determined by one of the three methodsspecified below. The choice of method depends on thecompleteness of available information on foundation

properties (see Section 4.2.1.1) and the requirements ofthe selected Performance Level.

4.4.1.1 Presumptive Ultimate Capacities

Presumptive capacities are to be used when the amount

of information on foundation soil properties is limitedand relatively simple analysis procedures are used.Presumptive ultimate load parameters for spreadfootings and mats are presented inTable 4-2.

4.4.1.2 Prescriptive Ultimate Capacities

Prescriptive capacities may be used when eitherconstruction documents for the existing building orprevious geotechnical reports provide information onfoundation soils design parameters.

The ultimate prescriptive bearing pressure for a spread

footing may be assumed to be twice the allowable deadplus live load bearing pressure specified for design.

(4-1)qc 2qallow.D L+=

Table 4-2 Presumptive Ultimate Foundation Pressures

Class of Materials2

Vertical Foundation

Pressure3Lbs./Sq. Ft.(q

c)

Lateral BearingPressureLbs./Sq. Ft./Ft. ofDepth BelowNatural Grade4

Lateral Sliding1

Coefficient5Resistance6Lbs./Sq. Ft.

Massive Crystalline Bedrock 8000 2400 0.80

Sedimentary and Foliated Rock 4000 800 0.70

Sandy Gravel and/or Gravel (GWand GP)

4000 400 0.70

Sand, Silty Sand, Clayey Sand, SiltyGravel, and Clayey Gravel (SW, SP,SM, SC, GM, and GC)

3000 300 0.50

Clay, Sandy Clay, Silty Clay, andClayey Silt (CL, ML, MH, and CH)

20007 200 260

1. Lateral bearing and lateral sliding resistance may be combined.

2. For soil classifications OL, OH, and PT (i.e., organic clays and peat), a foundation investigation shall be required.

3. All values of ultimate foundation pressure are for footings having a minimum width of 12 inches and a minimum depth of 12 inches into natural grade.Except where Footnote 7 below applies, increase of 20% allowed for each additional foot of width or depth to a maximum value of three times thedesignated value.

4. May be increased by the amount of the designated value for each additional foot of depth to a maximum of 15 times the designated value.

5. Coefficient applied to the dead load.

6. Lateral sliding resistance value to be multiplied by the contact area. In no case shall the lateral sliding resistance exceed one-half the dead load.

7. No increase for width is allowed.
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For deep foundations, the ultimate prescriptive verticalcapacity of individual piles or piers may be assumed tobe 50% greater than the allowable dead plus live loadsspecified for design.

(4-2)

As an alternative, the prescriptive ultimate capacity ofany footing component may be assumed to be 50%greater than the total working load acting on thecomponent, based on analyses using the original designrequirements.

(4-3)

4.4.1.3 Site-Specific Capacities

A detailed analysis may be conducted by a qualifiedgeotechnical engineer to determine ultimate foundationcapacities based on the specific characteristics of thebuilding site.

4.4.2 Load-Deformation Characteristics forFoundations

Load-deformation characteristics are required where theeffects of foundations are to be taken into account inLinear Static or Dynamic Procedures (LSP or LDP),Nonlinear Static (pushover) Procedures (NSP), orNonlinear Dynamic (time-history) Procedures (NDP).Foundation load-deformation parameters characterizedby both stiffness and capacity can have a significanteffect on both structural response and load distributionamong structural elements.

Foundation systems for buildings can in some cases becomplex, but for the purpose of simplicity, threefoundation types are considered in these Guidelines:

shallow bearing foundations

pile foundations

drilled shafts

While it is recognized that the load-deformationbehavior of foundations is nonlinear, because of thedifficulties in determining soil properties and staticfoundation loads for existing buildings, together with

the likely variability of soils supporting foundations, anequivalent elasto-plastic representation ofload-deformation behavior is recommended. Inaddition, to allow for such variability or uncertainty, anupper and lower bound approach to defining stiffnessand capacity is recommended (as shown in Figure 4-1a)to permit evaluation of structural response sensitivity.The selection of uncertainty represented by the upperand lower bounds should be determined jointly by thegeotechnical and structural engineers.

4.4.2.1 Shallow Bearing Foundations

A. Stiffness Parameters

The shear modulus, G, for a soil is related to themodulus of elasticity,E, and Poissons ratio, , by therelationship

where Qmax.= QD+ QL+ QS

Qc

1.5Qallow.D L+

=

Qc 1.5Qmax.=

Figure 4-1 (a) Idealized Elasto-Plastic Load-

Deformation Behavior for Soils

(b) Uncoupled Spring Model for Rigid

Footings

Upper bound

Lower bound

Deformation(a)

Loa

d

P

H

M

Foundation load Uncoupled spring model

ksh

ksr

ksv

(b)


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(4-4)

Poissons ratio may be assumed as 0.35 for unsaturatedsoils and 0.50 for saturated soils.

The initial shear modulus, Go,is related to the shear

wave velocity at low strains, vs, and the mass density of

the soil, , by the relationship

(4-5)

(In the fonts currently in use in the Guidelines, theitalicized vis similar to the Greek .) Converting massdensity to unit weight, , gives an alternative expression

(4-6)

where gis acceleration due to gravity.

The initial shear modulus also has been related tonormalized and corrected blow count, (N

1)60

, and

effective vertical stress, , as follows (from Seed et

al., 1986):

(4-7)

where:

It should be noted that the Goin Equation 4-7is

expressed in pounds per square foot, as is .

Most soils are intrinsically nonlinear and the shear wavemodulus decreases with increasing shear strain. The

large-strain shear wave velocity, , and the effective

shear modulus, G, can be estimated based on theEffective Peak Acceleration coefficient for the

earthquake under consideration, in accordance withTable 4-3.

To reflect the upper and lower bound concept illustratedin Figure 4-1a in the absence of a detailed geotechnicalsite study, the upper bound stiffness of rectangularfootings should be based on twice the effective shear

modulus, G, determined in accordance with the aboveprocedure. The lower bound stiffness should be basedon one-half the effective shear modulus. Thus the rangeof stiffness should incorporate a factor of four fromlower to upper bound.

Most shallow bearing footings are stiff relative to thesoil upon which they rest. For simplified analyses, anuncoupled spring model, as shown in Figure 4-1b, maybe sufficient. The three equivalent spring constants maybe determined using conventional theoretical solutionsfor rigid plates resting on a semi-infinite elasticmedium. Although frequency-dependent solutions are

available, results are reasonably insensitive to loadingfrequencies within the range of parameters of interestfor buildings subjected to earthquakes. It is sufficient touse static stiffnesses as representative of repeatedloading conditions.

Figure 4-2presents stiffness solutions for rectangularplates in terms of an equivalent circular radius.

= Blow count normalized for 1.0 ton persquare foot confining pressure and 60%energy efficiency of hammer

= Effective vertical stress in psf

and

=

t = Total unit weight of soil

w = Unit weight of water

d = Depth to sample

dw = Depth to water level

GE

2 1 +( )--------------------=

Go vs2

=

Govs

2

g--------=

o

Go 20 000 N1( )601 3

o,

N1( )60

o

o td w d dw( )

o

Table 4-3 Effective Shear Modulus and ShearWave Velocity

Effective PeakAcceleration, SXS/2.5

0.10 0.70

Ratio of effective to initial shearmodulus (G/Go)

0.50 0.20

Ratio of effective to initial shearwave velocity ('

s/s)

0.71 0.45

Notes:

1. Site-specific values may be substituted if documented in a detailedgeotechnical site investigation.

2. Linear interpolation may be used for intermediate values.

v s


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Stiffnesses are adjusted for shape and depth usingfactors similar to those inFigure 4-3. Otherformulations incorporating a wider range of variablesmay be found in Gazetas (1991). For the case ofhorizontal translation, the solution representsmobilization of base traction (friction) only. If the sidesof the footing are in close contact with adjacent in situfoundation soil or well-compacted fill, significantadditional stiffness may be assumed from passivepressure. A solution for passive pressure stiffness ispresented in Figure 4-4.

For more complex analyses, a finite elementrepresentation of linear or nonlinear foundationbehavior may be accomplished using Winklercomponent models. Distributed vertical stiffnessproperties may be calculated by dividing the totalvertical stiffness by the area. Similarly, the uniformly

distributed rotational stiffness can be calculated bydividing the total rotational stiffness of the footing bythe moment of inertia of the footing in the direction ofloading. In general, however, the uniformly distributedvertical and rotational stiffnesses are not equal. The twomay be effectively decoupled for a Winkler model usinga procedure similar to that illustrated in Figure 4-5. Theends of the rectangular footing are represented by endzones of relatively high stiffness over a length ofapproximately one-sixth of the footing width. Thestiffness per unit length in these end zones is based onthe vertical stiffness of aBxB/6 isolated footing. Thestiffness per unit length in the middle zone is equivalent

to that of an infinitely long strip footing.

In some instances, the stiffness of the structuralcomponents of the footing may be relatively flexiblecompared to the soil material; for example, a slendergrade beam resting on stiff soil. Classical solutions forbeams on elastic supports can provide guidance onwhen such effects are important. For example, a gradebeam supporting point loads spaced at a distance ofLmight be considered flexible if:

(4-8)

where, for the grade beam,

E = Effective modulus of elasticity

I = Moment of inertia

B = Width

For most flexible foundation systems, the unit subgradespring coefficient, ksv, may be taken as

(4-9)

B. Capacity Parameters

The specific capacity of shallow bearing foundationsshould be determined using fully plastic concepts andthe generalized capacities ofSection 4.4.1. Upper andlower bounds of capacities, as illustrated in Figure 4-1a,should be determined by multiplying the best estimatevalues by 2.0 and 0.5, respectively.

In the absence of moment loading, the vertical loadcapacity of a rectangular footing of widthBand length

Lis

(4-10)

For rigid footings subject to moment and vertical load,contact stresses become concentrated at footing edges,particularly as uplift occurs. The ultimate momentcapacity,Mc, is dependent upon the ratio of the vertical

load stress, q, to the vertical stress capacity, qc.

Assuming that contact stresses are proportional tovertical displacement and remain elastic up to thevertical stress capacity, qc, it can be shown that uplift

will occur prior to plastic yielding of the soil when q/qc

is less than 0.5. If q/qcis greater than 0.5, then the soil

at the toe will yield prior to uplift. This is illustrated inFigure 4-6. In general the moment capacity of arectangular footing may be expressed as:

(4-11)

where

P = Vertical load

q =

B = Footing width

L = Footing length in direction of bending

EI

L4

------ 10ksvB 0.5q

cq

M

M

Mc

Mc

A = Cross-sectional area of a pile

E = Modulus of elasticity of piles

L = Length of pilesN = Number of piles in group

kvn = Axial stiffness of the nth pile

Sn = Distance between nth pile and axis of rotation

ksr kvnSn2

n 1=

N

=


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manner analogous to that described for a footing inFigure 4-6.

The lateral capacity of a pile group is largely dependenton that of the cap as it is restrained by passive resistanceof the adjacent soil material. The capacity may beassumed to be reached when the displacement reaches2% of the depth of the cap in a manner similar to thatfor a shallow bearing foundation.

4.4.2.3 Drilled Shafts

In general, drilled shaft foundations or piers may betreated similarly to pile foundations. When the diameterof the shaft becomes large (> 24 inches), the bendingand the lateral stiffness and strength of the shaft itselfmay contribute to the overall capacity. This is obviouslynecessary for the case of individual shafts supportingisolated columns. In these instances, the interaction of

the soil and shaft may be represented using Winklertype models (Pender, 1993; Reese et al., 1994).

4.4.3 Foundation Acceptability Criteria

This section contains acceptability criteria for thegeotechnical components of building foundations.Structural components of foundations shall meet theappropriate requirements of Chapters 5through8.

Geotechnical components include the soil parts ofshallow spread footings and mats, and friction- and end-bearing piles and piers. These criteria, summarized inTable 4-4, apply to all actions including vertical loads,moments, and lateral forces applied to the soil.

4.4.3.1 Simplified Rehabilitation

The geotechnical components of buildings qualified forand subject to Simplified Rehabilitation may beconsidered acceptable if they comply with therequirements of Chapter 10.

4.4.3.2 Linear Procedures

The acceptability of geotechnical components subject tolinear procedures depends upon the basic modelingassumptions utilized in the analysis, as follows.

Fixed Base Assumption. If the base of the structure hasbeen assumed to be completely rigid, actions ongeotechnical components shall be as on force-controlledcomponents governed by Equation 3-15and componentcapacities may be assumed as upper-bound values. Afixed base assumption is not recommended for theImmediate Occupancy Performance Level for buildingssensitive to base rotations or other types of foundation

movement.

Flexible Base Assumption. If the base of the structureis modeled using linear geotechnical components, thenthe value of m, for use in Equation 3-18,for Life Safetyand Collapse Prevention Performance Levels may be

assumed as infinite, provided the resultingdisplacements may be accommodated within theacceptability criteria for the rest of the structure. For the

Table 4-4 Soil Foundation Acceptability Summary

Analysis ProcedureFoundationAssumption

Performance Level

Collapse Prevention and Life Safety Immediate Occupancy

SimplifiedRehabilitation

See Chapter 10 Not applicable.

Linear Static orDynamic

Fixed Actions on geotechnical components shallbe assumed as on force-controlledcomponents governed by Equation 3-15and component capacities may be assumedas upper bound values.

Not recommended for buildingssensitive to base rotation or otherfoundation movements.

Flexible m= for use in Equation 3-18 m= 2.0 for use in Equation 3-18

Nonlinear Static orDynamic

Fixed Base reactions limited to upper boundultimate capacity.

Not recommended for buildingssensitive to base rotation or otherfoundation movements.

Flexible Geotechnical component displacementsneed not be limited, provided that structurecan accommodate the displacements.

Estimate and accommodate possiblepermanent soil movements.
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Immediate Occupancy Performance Levels,mvaluesfor geotechnical components shall be limited to 2.0.

4.4.3.3 Nonlinear Procedures

The acceptability of geotechnical components subject to

nonlinear procedures depends upon the basic modelingassumptions utilized in the analysis, as follows.

Fixed Base Assumption. If the base of the structure hasbeen assumed to be completely rigid, then the basereactions for all geotechnical components shall notexceed their upper-bound capacity to meet CollapsePrevention and Life Safety Performance Levels. A rigidbase assumption is not recommended for the ImmediateOccupancy Performance Level for buildings sensitiveto base rotations or other types of foundationmovement.

Flexible Base Assumption. If the base of the structureis modeled using flexible nonlinear geotechnicalcomponents, then the resulting componentdisplacements need not be limited to meet Life Safetyand Collapse Prevention Performance Levels, providedthe resulting displacements may be accommodatedwithin the acceptability criteria for the rest of thestructure. For the Immediate Occupancy PerformanceLevel, an estimate of the permanent nonrecoverabledisplacement of the geotechnical components shall bemade based upon the maximum total displacement,foundation and soil type, soil layer thicknesses, andother pertinent factors. The acceptability of these

displacements shall be based upon their effects on thecontinuing function and safety of the building.

4.5 Retaining Walls

Past earthquakes have not caused extensive damage tobuilding walls below grade. In some cases, however, itmay be advisable to verify the adequacy of retainingwalls to resist increased pressure due to seismicloading. These situations might be for walls of poorconstruction quality, unreinforced or lightly reinforcedwalls, walls of archaic materials, unusually tall or thin

walls, damaged walls, or other conditions implying asensitivity to increased loads. The seismic earthpressure acting on a building wall retainingnonsaturated, level soil above the ground-water tablemay be approximated as:

(4-14)

where

The seismic earth pressure given above should be addedto the unfactored static earth pressure to obtain the totalearth pressure on the wall. The expression inEquation 4-14is a conservative approximation of theMononabe-Okabe formulation. The pressure on wallsduring earthquakes is a complex action. If walls do nothave the apparent capacity to resist the pressures

estimated from the above approximate procedures,detailed investigation by a qualified geotechnicalengineer is recommended.

4.6 Soil Foundation Rehabilitation

This section provides guidelines for modification tofoundations to improve anticipated seismicperformance. Specifically, the scope of this sectionincludes suggested approaches to foundationmodification and behavioral characteristics offoundation elements from a geotechnical perspective.

These must be used in conjunction with appropriatestructural material provisions from other chapters.Additionally, the acceptability of a modified structure isdetermined in accordance withChapter 2of theGuidelines.

4.6.1 Soil Material Improvements

Soil improvement options to increase the verticalbearing capacity of footing foundations are limited. Soilremoval and replacement and soil vibratorydensification usually are not feasible because theywould induce settlements beneath the footings or be

expensive to implement without causing settlement.Grouting may be considered to increase bearingcapacity. Different grouting techniques are discussed inthe CommentarySection C4.3.2. Compaction groutingcan achieve densification and strengthening of a varietyof soil types and/or extend foundation loads to deeper,stronger soils. The technique requires careful control toavoid causing uplift of foundation elements or adjacentfloor slabs during the grouting process. Permeation

p 0.4khtHrw=

p = Additional earth pressure due to seismicshaking, which is assumed to be a uniformpressure

kh

= Horizontal seismic coefficient in the soil,which may be assumed equal to SXS/2.5

t = The total unit weight of soil

Hrw = The height of the retaining wall
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grouting with chemical grouts can achieve substantialstrengthening of sandy soils, but the more fine-grainedor silty the sand, the less effective the techniquebecomes. Jet grouting could also be considered. Thesesame techniques also may be considered to increase thelateral frictional resistance at the base of footings.

Options that can be considered to increase the passiveresistance of soils adjacent to foundations or gradebeams include removal and replacement of soils withstronger, well-compacted soils or with treated (e.g.,cement-stabilized) soils; in-place mixing of soils withstrengthening materials (e.g., cement); grouting,including permeation grouting and jet grouting; and in-place densification by impact or vibratory compaction(if the soil layers to be compacted are not too thick andvibration effects on the structure are tolerable).

4.6.2 Spread Footings and MatsNew isolated or spread footings may be added toexisting structures to support new structural elementssuch as shear walls or frames. In these instances,capacities and stiffness may be determined inaccordance with the procedures ofSection 4.4.

Existing isolated or spread footings may be enlarged toincrease bearing or uplift capacity. Generally, capacitiesand stiffness may be determined in accordance with theprocedures of Section 4.4; however, consideration ofexisting contact pressures on the strength and stiffnessof the modified footing may be required, unless auniform distribution is achieved by shoring and/orjacking.

Existing isolated or spread footings may beunderpinned to increase bearing or uplift capacity. Thistechnique improves bearing capacity by lowering thecontact horizon of the footing. Uplift capacity isimproved by increasing the resisting soil mass abovethe footing. Generally, capacities and stiffness may bedetermined in accordance with the procedures ofSection 4.4. Considerations of the effects of jacking andload transfer may be required.

Where potential for differential lateral displacement ofbuilding foundations exists, provision ofinterconnection with grade beams or a well-reinforcedgrade slab can provide good mitigation of these effects.Ties provided to withstand differential lateraldisplacement should have a strength based on rationalanalysis, with the advice of a geotechnical engineerwhen appropriate.

4.6.3 Piers and Piles

Piles and pile caps shall have the capacity to resistadditional axial and shear loads caused by overturningforces. Wood piles cannot resist uplift unless a positiveconnection is provided for the loads. Piles must be

reviewed for deterioration caused by decay, insectinfestation, or other signs of distress.

Driven piles made of steel, concrete, or wood, or cast-in-place concrete piers may be used to support newstructural elements such as shear walls or frames.Capacities and stiffnesses may be determined inaccordance with the procedures ofSection 4.4. Whenused in conjunction with existing spread footingfoundations, the effects of differential foundationstiffness should be considered in the analysis of themodified structure.

Driven piles made of steel, concrete, or wood, or cast-in-place concrete piers may be used to supplement thevertical and lateral capacities of existing pile and pierfoundation groups and of existing isolated andcontinuous spread footings. Capacities and stiffnessesmay be determined in accordance with the proceduresof Section 4.4. If existing loads are not redistributed byshoring and/or jacking, the potential for differentialstrengths and stiffnesses among individual piles or piersshould be included.

4.7 Definitions

Allowable bearing capacity: Foundation load orstress commonly used in working-stress design (oftencontrolled by long-term settlement rather than soilstrength).

Deep foundation: Piles or piers.

Differential compaction: An earthquake-inducedprocess in which loose or soft soils become morecompact and settle in a nonuniform manner across asite.

Fault: Plane or zone along which earth materials onopposite sides have moved differentially in response totectonic forces.

Footing: A structural component transferring theweight of a building to the foundation soils and resistinglateral loads.


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Foundation soils: Soils supporting the foundationsystem and resisting vertical and lateral loads.

Foundation springs: Method of modeling toincorporate load-deformation characteristics offoundation soils.

Foundation system: Structural components(footings, piles).

Landslide: A down-slope mass movement of earthresulting from any cause.

Liquefaction: An earthquake-induced process inwhich saturated, loose, granular soils lose a substantialamount of shear strength as a result of increase in pore-water pressure during earthquake shaking.

Pier: Similar to pile; usually constructed of concreteand cast in place.

Pile: A deep structural component transferring theweight of a building to the foundation soils and resistingvertical and lateral loads; constructed of concrete, steel,or wood; usually driven into soft or loose soils.

Prescriptive ultimate bearing capacity:Assumption of ultimate bearing capacity based onproperties prescribed in Section 4.4.1.2.

Presumptive ultimate bearing capacity:

Assumption of ultimate bearing capacity based onallowable loads from original design.

Retaining wall: A free-standing wall that has soil onone side.

Shallow foundation: Isolated or continuous spreadfootings or mats.

SPT N-Values: Using a standard penetration test(ASTM Test D1586), the number of blows of a 140-pound hammer falling 30 inches required to drive astandard 2-inch-diameter sampler a distance of

12 inches.

Ultimate bearing capacity: Maximum possiblefoundation load or stress (strength); increase indeformation or strain results in no increase in load orstress.

4.8 Symbols

A Footing area; also cross-section area ofpile

B Width of footing

D Depth of footing bearing surfaceE Youngs modulus of elasticity

G Shear modulus

Go Initial or maximum shear modulus

H Horizontal load on footing

Hrw Height of retaining wall

I Moment of inertia

KL Passive pressure stiffness

L Length of footing in plan dimension

L Length of pile in vertical dimension

M Moment on footing

Mc Ultimate moment capacity of footing

N Number of piles in a pile group

(N1)60 Standard Penetration Test blow countnormalized for an effective stress of 1 tonper square foot and corrected to anequivalent hammer energy efficiency of60%

P Vertical load on footing

QD Dead (static) load

QE

Earthquake load

QL Live (frequently applied) load

Qallow.D+L Allowable working dead plus live load fora pile as specified in original designdocuments

Qc Ultimate bearing capacity

QS Snow load

R Radius of equivalent circular footing

SXS Spectral response acceleration at shortperiods for any hazard level or damping, g

Sn Distance between nth pile and axis of

rotation of a pile groupSS Spectral response acceleration at short

periods, obtained from responseacceleration maps, g

c Cohesive strength of soil, expressed in

force/unit area (pounds/ft2or Pa)


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4.9 References

ASTM, 1994, Standard Test Method for PenetrationTest and Split-Barrel Sampling of Soils: TestDesignation D1586-84, ASTM Standards, AmericanSociety for Testing Materials, Philadelphia,Pennsylvania.

BSSC, 1992,NEHRP Handbook for the Seismic

Evaluation of Existing Buildings,developed by theBuilding Seismic Safety Council for the FederalEmergency Management Agency (Report No. FEMA178), Washington, D.C.

BSSC, 1995,NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Part 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report Nos. FEMA222A and 223A), Washington, D.C.

Gazetas, G., 1991, Foundation Vibrations,

Foundation Engineering Handbook, edited by Fang,H. Y., Van Nostrand Reinhold, New York, New York,pp. 553593.

ICBO, 1994, Uniform Building Code, InternationalConference of Building Officials, Whittier, California.

Lam, I. P., Martin, G. R., and Imbsen, R., 1991,Modeling Bridge Foundations for Seismic Design andRetrofitting, Transportation Research Record,Washington, D.C., No. 1290.

NAVFAC, 1982a, Soil Mechanics: Naval Facilities

Engineering Command Design Manual, NAVFACDM-7.1, U.S. Department of the Navy, Alexandria,Virginia.

NAVFAC, 1982b, Foundation and Earth Structures:Naval Facilities Engineering Command DesignManual, NAVFAC DM-7.2, U.S. Department of theNavy, Alexandria, Virginia.

d Short side of footing lateral contact area

d Depth to sample

dw Depth of ground-water level

g Acceleration of gravity (386.1 in/sec.2, or9,800 mm/sec.2for SI units)

kh Horizontal seismic coefficient in soilacting on retaining wall

ko Stiffness coefficient for equivalentcircular footing

ksh Winkler spring coefficient in horizontaldirection, expressed as force/unitdisplacement/unit area

ksr Winkler spring coefficient in overturning(rotation), expressed as force/unitdisplacement/unit area

ksv Winkler spring coefficient in vertical

direction, expressed as force/unitdisplacement/unit area

kvn Axial stiffness of nth pile in a pile group

l Long side of footing lateral contact area

m A modification factor used in theacceptance criteria of deformation-controlled components or elements,indicating the available ductility of acomponent action, and a multiplier ofultimate foundation capacity for checkingimposed foundation loads in Linear Staticor Dynamic Procedures

q Vertical bearing pressure

qallow.D+L Allowable working dead plus live loadpressure for a spread footing as specifiedin original design documents

qc Ultimate bearing capacity

vs Shear wave velocity at low strain

Shear wave velocity at high strain

p Additional earth pressure on retainingwall due to seismic shaking

Foundation shape correction factor Embedment factor

Unit weight, weight/unit volume (pounds/

ft3or N/m3)

t Total unit weight of soil

w Unit weight of water

v s

c Pile compliance

Shape factor for lateral stiffness

Poissons ratio

Soil mass density

Effective vertical stress

Angle of internal friction, degreeso


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NAVFAC, 1983, Soil Dynamics, Deep Stabilization,and Special Geotechnical Construction, NAVFACDM-7.3, U.S. Department of the Navy, Alexandria,Virginia.

Newmark, N. M., 1965, Effect of Earthquake on Damsand Embankments, Geotechnique, Vol. 15,pp. 139160.

Pender, M. J., 1993, Aseismic Pile Foundation DesignAnalysis,Bulletin of the New Zealand NationalSociety for Earthquake Engineering, Vol. 26, No. 1,pp. 49161.

Reese, L. C., Wand, S. T., Awashika, K., andLam, P.H.F., 1994, GROUPa Computer Program forAnalysis of a Group of Piles Subjected to Axial andLateral Loading, Ensoft, Inc., Austin, Texas.

SBCCI, 1993, Standard Building Code, SouthernBuilding Code Congress International, Birmingham,Alabama.

Secretary of the Interior, 1993, Standards andGuidelines for Archeology and Historic Preservation,published in the Federal Register, Vol. 48, No. 190,pp. 4471644742.

Seed, H. B., Wong, R. T., and Idriss, I. M., 1986,Moduli and Damping Factors for Dynamic Analysesof Cohesionless Soils,Journal of the GeotechnicalEngineering Division, American Society of CivilEngineers, New York, New York, Vol. 112, pp. 10161032.

Wilson, J. C., 1988, Stiffness of Non-skewedMonolithic Bridge Abutments for Seismic Analysis,Earthquake Engineering and Structural Dynamics,Vol. 16, pp. 867883.

Youd, T. L., and Perkins, D. M., 1978, Mapping

Liquefaction Induced Ground Failure Potential,Journal of the Geotechnical Engineering Division,American Society of Civil Engineers, New York, NewYork, Vol. 104, No. GT4, pp. 433446.


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4 22 Seismic Rehabilitation Guidelines FEMA 273


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