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FOUNDATION ALTERNATIVESTO MITIGATE EARTHQUAKE

EFFECTS

13.1 INTRODUCTION

If the expected settlement or lateral movement for a proposed structure is too large, thendifferent foundation support or soil stabilization options must be evaluated. One alterna-

tive is soil improvement methods, such as discussed in Chap. 12. Instead of soil improve-ment, the foundation can be designed to resist the anticipated soil movement caused by theearthquake. For example, mat foundations or post-tensioned slabs may enable the build-ing to remain intact, even with substantial movements. Another option is a deep founda-tion system that transfers the structural loads to adequate bearing material in order tobypass a compressible or liquefiable soil layer. A third option is to construct a floatingfoundation, which is a special type of deep foundation in which the weight of the structureis balanced by the removal of soil and construction of an underground basement. A float-ing foundation could help reduce the amount of rocking settlement caused by the earth-

quake. Typical factors that govern the selection of a particular type of foundation arepresented in Table 13.1.

13.2 SHALLOW FOUNDATIONS

A shallow foundation is often selected when the structural load and the effects of the earth-quake will not cause excessive settlement or lateral movement of the underlying soil layers.

In general, shallow foundations are more economical to construct than deep foundations.Common types of shallow foundations are described in Table 13.2 and shown in Figs. 13.1and 13.2.

If it is anticipated that the earthquake will cause excessive settlement or lateral move-ment, then isolated footings are generally not desirable. This is because the foundationcan be pulled apart during the earthquake, causing collapse of the structure. Instead, amat foundation (Fig. 13.2) or a post-tensioned slab is more desirable. This is becausesuch foundations may enable the building to remain intact, even with substantial move-ments.

CHAPTER 13

13.1

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13.3 DEEP FOUNDATIONS

13.3.1 Introduction

Common types of deep foundations are described in Table 13.3. Typical pile characteris-tics and uses are presented in Table 13.4. Figures 13.3 and 13.4 show common types ofcast-in-place concrete piles and examples of pile configurations.

Deep foundations are one of the most effective means of mitigating foundation move-ment during an earthquake. For example, as discussed in Sec. 3.4.2, the Niigata earthquakeresulted in dramatic damage due to liquefaction of the sand deposits in the low-lying areasof Niigata City. At the time of the Niigata earthquake, there were approximately 1500 rein-forced concrete buildings in Niigata City, and about 310 of these buildings were damaged,of which approximately 200 settled or tilted rigidly without appreciable damage to thesuperstructure (see Fig. 3.20). As noted in Sec. 3.4.2, the damaged concrete buildings werebuilt on very shallow foundations or friction piles in loose soil. Similar concrete buildingsfounded on piles bearing on firm strata at a depth of 20 m (66 ft) did not suffer damage.

Besides buildings, deep foundations can be used for almost any type of structure. Forexample, Fig. 13.5 shows concrete piles that were used to support a storage tank. The soil

13.2 CHAPTER THIRTEEN

TABLE 13.1 Selection of Foundation Type

Topic Discussion

Selection of foundation type Based on an analysis of the factors listed below, a specific

type of foundation (i.e., shallow versus deep) would be

recommended by the geotechnical engineer.

Adequate depth The foundation must have an adequate depth to prevent

frost damage. For such foundations as bridge piers, the

depth of the foundation must be sufficient to prevent

undermining by scour.

Bearing capacity failure The foundation must be safe against a bearing capacity

failure.

Settlement The foundation must not settle to such an extent that itdamages the structure.

Quality The foundation must be of adequate quality that it is not

subjected to deterioration, such as from sulfate attack.

Adequate strength The foundation must be designed with sufficient strength

that it does not fracture or break apart under the applied

superstructure loads. The foundation must also be properly

constructed in conformance with the design specifications.

Adverse soil changes The foundation must be able to resist long-term adverse soil

changes. An example is expansive soil, which couldexpand or shrink, causing movement of the foundation and

damage to the structure.

Seismic forces The foundation must be able to support the structure during

an earthquake without excessive settlement or lateral

movement.

Required specifications The foundation may also have to meet special requirements

or specifications required by the local building department

or governing agency.

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beneath the storage tank liquefied during the Kobe earthquake. Concerning the perfor-mance of this deep foundation during the earthquake, it was stated (EERC 1995):

The tank, reportedly supported by 33 piles extending to depths of approximately 33 meters,was undamaged. The piles consist of reinforced-concrete sections with diameters of approximately

FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS 13.3

TABLE 13.2 Common Types of Shallow Foundations

Topic Discussion

Spread footings Spread footings are often square in plan view, are of uniform rein-

forced concrete thickness, and are used to support a single loaddirectly in the center of the footing.

Strip footings Strip footings, also known as wall footings, are often used to sup-

port load-bearing walls. They are usually long, reinforced concrete

members of uniform width and shallow depth.

Combined footings Reinforced concrete combined footings are often rectangular or

trapezoidal in plan view and carry more than one column load (see

Fig. 13.1).

Other types of footings Figure 13.1 shows other types of footings, such as the cantilever

(also known as strap) footing, an octagonal footing, and an eccen-

tric loaded footing with the resultant coincident with area so that

the soil pressure is uniform.

Mat foundation If at mat foundation is constructed at or near ground surface, then it

is considered to be a shallow foundation. Figure 13.2 shows differ-

ent types of mat foundations. Based on economic considerations,

mat foundations are often constructed for the following reasons

(NAVFAC DM-7.2, 1982):

1. Large individual footings: A mat foundation is often con-

structed when the sum of individual footing areas exceeds aboutone-half of the total foundation area.

2. Cavities or compressible lenses: A mat foundation can be

used when the subsurface exploration indicates that there will

be unequal settlement caused by small cavities or compressible

lenses below the foundation. A mat foundation would tend to

span the small cavities or weak lenses and create a more uniform

settlement condition.

3. Shallow settlements: A mat foundation can be recommended

when shallow settlements predominate and the mat foundation

would minimize differential settlements.

4. Unequal distribution of loads: For some structures, there can

be a large difference in building loads acting on different areas

of the foundation. Conventional spread footings could be sub-

jected to excessive differential settlement, but a mat foundation

would tend to distribute the unequal building loads and reduce

the differential settlements.

5. Hydrostatic uplift: When the foundation will be subjected to

hydrostatic uplift due to a high groundwater table, a mat foun-

dation could be used to resist the uplift forces.Conventional slab-on-grade A continuous reinforced concrete foundation consists of bearing

wall footings and a slab-on-grade. Concrete reinforcement often con-

sists of steel rebar in the footings and wire mesh in the concrete slab.

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35 centimeters. Twelve of the 33 piles were arranged in an outer ring near the perimeter of thetank; the rest are situated closer to its center [see Fig. 13.5]. Beneath the tank, the ground hadliquefied and settled 28 centimeters. Damage to the exposed portions of the piles appeared tobe relatively light. Several piles contained hairline cracks in the upper meter or two. At leastone pile contained intersecting cracks that could allow large pieces of concrete to spall out. Themost seriously damaged piles were located along the northwestern part of the perimeter. The pilesappeared repairable and thus were not classified as having failed.

For earthquake conditions, two of the most commonly used types of deep foundationsare the pier and grade beam system and prestressed concrete piles. These two foundationtypes are described individually in the next two sections.


TABLE 13.2 Common Types of Shallow Foundations (Continued)

Topic Discussion

Posttensioned slab-on-grade Post-tensioned slab-on-grade is common in southern California and

other parts of the United States. It is an economical foundationtype when there is no ground freezing or the depth of frost penetra-

tion is low. The most common uses of post-tensioned slab-on-

grade are to resist expansive soil forces or when the projected

differential settlement exceeds the tolerable value for a conven-

tional (lightly reinforced) slab-on-grade. For example, a postten-

sioned slab-on-grade is frequently recommended if the projected

differential settlement is expected to exceed 2 cm (0.75 in).

Installation and field inspection procedures for post-tensioned slab-

on-grade have been prepared by the Post-Tensioning Institute (1996).

Post-tensioned slab-on-grade consists of concrete with embedded

steel tendons that are encased in thick plastic sheaths. The plastic

sheath prevents the tendon from coming in contact with the concrete

and permits the tendon to slide within the hardened concrete during

the tensioning operations. Usually tendons have a dead end (anchor-

ing plate) in the perimeter (edge) beam and a stressing end at the

opposite perimeter beam to enable the tendons to be stressed from

one end. However, the Post-Tensioning Institute (1996) does recom-

mend that the tendons in excess of 30 m (100 ft) be stressed from

both ends. The Post-Tensioning Institute (1996) also provides typicalanchorage details for the tendons.

Because post-tensioned slab-on-grade performs better (i.e., less

shrinkage-related concrete cracking) than conventional slab-on-

grade, it is more popular even for situations where low levels of

settlement are expected.

Raised wood floor Perimeter footings support wood beams and a floor system. Interior

support is provided by pad or strip footings. There is a crawl space

below the wood floor.

Shallow foundation If the expected settlement or lateral movement for a proposed shal-alternatives low foundation is too large, then other options for foundation sup-

port or soil stabilization must be evaluated. Commonly used alter-

natives include deep foundations, grading options, or other site

improvement techniques. Deep foundations are discussed in this

chapter, and grading and other site improvement techniques are

discussed in Chap. 12.

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13.3.2 Pier and Grade Beam Support

The typical steps in the construction of a foundation consisting of piers and grade beamsare as follows:

1. Excavation of piers: Figures 13.6 to 13.8 show the excavation of the piers using atruck-mounted auger drill rig. This type of equipment can quickly and economically exca-vate the piers to the desired depth. In Figs. 13.6 to 13.8, an auger with a 30-in (0.76-m)diameter is being used to excavate the pier holes.

2. Cleaning of the bottom of the excavation: Piers are often designed as end-bearing

members. For example, there may be a loose or compressible upper soil zone with the piersexcavated through this material and into competent material. The ideal situation is to havethe groundwater table below the bottom of the piers. This will then allow for a visualinspection of the bottom of the pier excavation. Often an experienced driller will be able toclean out most of the bottom of the pier by quickly spinning the auger. A light can then belowered into the pier hole to observe the embedment conditions (i.e., see Fig. 13.9). Aworker should not descend into the hole to clean out the bottom; rather, any loose materialat the bottom of the pier should be pushed to one side and then scraped into a bucket low-ered into the pier hole. If it is simply not possible to clean out the bottom of the pier, thenthe pier resistance could be based solely on skin friction in the bearing strata with the end-bearing resistance assumed to be equal to zero.


FIGURE 13.1 Examples of shallow foundations. (a) Combined footing; (b) combined trapezoidal footing;(c) cantilever or strap footing; (d) octagonal footing; (e) eccentric loaded footing with resultant coincident

with area so soil pressure is uniform. (Reproduced from Bowles 1982 with permission of McGraw-Hill, Inc.)

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FIGURE 13.2 Examples of mat foundations. (a) Flat plate; (b) plate thickened under columns; (c) beam-

and-slab; (d) plate with pedestals; (e) basement walls as part of mat. (Reproduced from Bowles 1982 with per-

mission of McGraw-Hill, Inc.)

TABLE 13.3 Common Types of Deep Foundations

Topic Discussion

Pile foundations Probably the most common type of deep foundation is the pile

foundation. Piles can consist of wood (timber), steel H-sections,

precast concrete, cast-in-place concrete, pressure-injected con-

crete, concrete-filled steel pipe piles, and composite-type piles

(also see Table 13.4). Piles are either driven into place or

installed in predrilled holes. Piles that are driven into place are

generally considered to be low displacement or high displace-ment depending on the amount of soil that must be pushed out of

the way as the pile is driven. Examples of low-displacement piles

are steel H-sections and open-ended steel pipe piles that do not

form a soil plug at the end. Examples of high-displacement piles

are solid section piles, such as round timber piles or square pre-

cast concrete piles, and steel pipe piles with a closed end.

Various types of piles are as follows:G Batter pile: A pile driven in at an angle inclined to the verti-

cal that provides high resistance to lateral loads.

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TABLE 13.3 Common Types of Deep Foundations (Continued)

Topic Discussion

Pile foundations G End-bearing pile: This piles support capacity is derived

(continued) principally from the resistance of the foundation material onwhich the pile tip rests. End-bearing piles are often used

when a soft upper layer is underlain by dense or hard strata. If

the upper soft layer should settle, the pile could be subjected

to down-drag forces, and the pile must be designed to resist

these soil-induced forces.G Friction pile: This piles support capacity is derived princi-

pally from the resistance of the soil friction and/or adhesion

mobilized along the side of the pile. Friction piles are often

used in soft clays where the end-bearing resistance is smallbecause of punching shear at the pile tip. A pile that resists

upward loads (i.e., tension forces) would also be considered

to be a friction pile.G Combined end-bearing and friction pile: This pile derives

its support capacity from combined end-bearing resistance

developed at the pile tip and frictional and/or adhesion resis-

tance on the pile perimeter.

Piles are usually driven into specific arrangements and are used

to support reinforced concrete pile caps or a mat foundation. For

example, the building load from a steel column may be supportedby a concrete pile cap that is in turn supported by four piles

located near the corners of the concrete pile cap.

Concrete-filled steel pipe piles Another option is a concrete-filled steel pipe pile. In this case,

the steel pipe pile is driven into place. The pipe pile can be dri-

ven with either an open or a closed end. If the end is open, the

soil within the pipe pile is removed (by jetting) prior to place-

ment of the steel reinforcement and concrete. Table 13.4 pro-

vides additional details on the concrete-filled steel pipe piles.

Prestressed concrete piles Table 13.4 presents details on typical prestressed concrete piles

that are delivered to the job site and then driven into place.

Other types of piles Table 13.4 provides additional details on various types of piles.

Piers A pier is defined as a deep foundation system, similar to a cast-

in-place pile, that consists of a columnlike reinforced concrete

member. Piers are often of large enough diameter to enable down-

hole inspection. Piers are also commonly referred to as drilled

shafts, bored piles, or drilled caissons.

Caissons Large piers are sometimes referred to as caissons. A caisson canalso be a watertight underground structure within which work is

carried on.

Mat or raft foundation If a mat or raft foundation is constructed below ground surface or

if the mat or raft is supported by piles or piers, then it should be

considered to be a deep foundation system.

Floating foundation A floating foundation is a special type of deep foundation where

the weight of the structure is balanced by the removal of soil

and construction of an underground basement.

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TABLE 13.4 Typical Pile Characteristics and Uses

Ca

p

Pile type Timber Steel

Maximum length 35 m Practically unlimited 45 m

Optimum length 920 m 1250 m 925

Applicable material ASTM-D25 for piles; P1-54 for ASTM-A36 for structural ACI

specifications quality of creosote; C1-60 for sections

creosote treatment (standards of ASTM-A1 for rail sections

American Wood Preservers Assoc.)

Recommended Measured at midpoint of length: fs 65 to 140 MPa 0.33fc

maximum stresses 46 MPa for cedar, western fs 0.35fy0.5fy shell

hemlock, Norway pine, spruce, thickand depending on code

58 MPa for southern pine,

Douglas fir, oak, cypress, hickory

Maximum load for 270 kN Maximum allowable 900 kN

usual conditions stress cross section

Optimum load 130225 kN 3501050 kN 4507

range

Disadvantages Difficult to splice Vulnerable to corrosion Hard t

Vulnerable to damage in hard driving HP section may be damaged Consi

Vulnerable to decay unless treated, or deflected by major

when piles are intermittently obstructions

submerged

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TABLE 13.4 Typical Pile Characteristics and Uses

Cas

pi

Pile type Timber Steel w

Advantages Comparatively low initial cost Easy to splice Can be

Permanently submerged piles High capacity Shell n

are resistant to decay Small displacement

Easy to handle Able to penetrate through

light obstructions

Remarks Best suited for friction pile in Best suited for end bearing Best su

granular material on rock of me

Reduce allowable capacity

for corrosive locations

Typical illustrations

Notes: Stresses given for steel piles and shells are for noncorrosive locations. For corrosive locations estimate possi

corrosion.

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TABLE 13.4 Typical Pile Characteristics and Uses (Continued)

Concrete-filled Precast concrete

Pile type steel pipe piles Composite piles (including prestressed)

Maximum length Practically unlimited 55 m 30 m for precast

60 m for prestressed

Optimum length 1236 m 1836 m 1215 m for precast

1830 m for prestressed

Applicable material ASTM A36 for core ACI Code 318 for concrete ASTM A15 for reinforcin

specifications ASTM A252 for pipe ASTM A36 for structural steel

ACI Code 318 for concrete section ASTM A82 for cold-draw

ASTM A252 for steel pipe wire

ASTM D25 for timber ACI Code 318 for concret

Recommended 0.40fy

reinforcement Same as concrete in other 0.33fc unless local buildin

maximum stresses 205 MPa piles code is less; 0.4fy for 0.50fy for core175 MPa Same as steel in other piles reinforced unless

0.33fc for concrete Same as timber piles for prestressed

wood composite

Maximum load for 1800 kN without cores 1800 kN 8500 kN for prestressed

usual conditions 18,000 kN for large sections 900 kN for precast

with steel cores

Optimum load range 7001100 kN without cores 250725 kN 3503500 kN

450014,000 kN with coresDisadvantages High initial cost Difficult to attain good Difficult to handle unless

Displacement for joint between two prestressed

closed-end pipe materials High initial cost

Considerable displacemen

Prestressed difficult to splic

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TABLE 13.4 Typical Pile Characteristics and Uses (Continued)

Concrete-filled Precast concrete

Pile type steel pipe piles Composite piles (including prestressed)

Advantages Best control during installation Considerable length High load capacitiesNo displacement for can be provided at Corrosion resistance ca

open-end installation comparatively low cost be attained

Open-end pipe best Hard driving possible

against obstructions

High load capacities

Easy to splice

Remarks Provides high bending The weakest of any material Cylinder piles in particu

resistance where used shall govern allowable are suited for bending

unsupported length stresses and capacity resistance

is loaded laterally

Typical

illustrations

*ACI Committee 543, Recommendations for Design, Manufacture, and Installation of Concrete Piles,JACI, August 1Sources: NAVFAC DM-7.2 (1982) and Bowles (1982).

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3. Steel cage and concrete: Once the bottom of the pier hole has been cleaned, a steelreinforcement cage is lowered into the pier hole. Small concrete blocks can be used to posi-tion the steel cage within the hole. Care should be used when inserting the steel cage so thatsoil is not knocked off the sides of the hole. Once the steel cage is in place, the hole is filledwith concrete. Figure 13.10 shows the completion of the pier with the steel reinforcementextending out the top of the pier.

4. Grade beam construction: The next step is to construct the grade beams that spanbetween the piers. Figure 13.11 shows the excavation of a grade beam between two piers.Figure 13.12 shows the installation of steel for the grade beam. Similar to the piers, small

concrete blocks are used to position the steel reinforcement within the grade beam. Avisqueen moisture barrier is visible on the left side of Fig. 13.12.


FIGURE 13.3 Common types of cast-in-place concrete piles. (a) Uncased pile; (b) Franki uncased-pedestal

pile; (c) Franki cased-pedestal pile; (d) welded or seamless pipe pile; (e) cased pile using a thin sheet shell;(f) monotube pile; (g) uniform tapered pile; (h) step-tapered pile. (Reproduced from Bowles 1982 with per-mission of McGraw-Hill, Inc.)

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Figure 13.13 shows a pier located at the corner of the building. The steel reinforcement

from the grade beams is attached to the steel reinforcement from the piers. Once the steel rein-forcement is in place, the final step is to place the concrete for the grade beams. Figure 13.14shows the finished grade beams. The steel reinforcement protruding out of the grade beamswill be attached to the steel reinforcement in the floor slab.

5. Floor slab: Prior to placement of the floor slab, a visqueen moisture barrier and agravel capillary break should be installed. Then the steel reinforcement for the floor slab islaid out, such as shown in Fig. 13.15. Although not shown in Fig. 13.15, small concreteblocks will be used to elevate the steel reinforcement off the subgrade, and the steel will beattached to the steel from the grade beams. The final step is to place the concrete for the

floor slab. Figure 13.16 shows the completed floor slab.6. Columns: When the building is being designed, the steel columns that support the

superstructure can be positioned directly over the center of the piers. For example, Fig. 13.17shows the location where the bottom of a steel column is aligned with the top of a pier. A steelcolumn having an attached baseplate will be bolted to the concrete. Then the steel reinforce-ment from the pier (see Fig. 13.17) will be positioned around the bottom of the steel column.Once filled with concrete, the final product will be essentially a fixed-end column conditionhaving a high lateral resistance to earthquake shaking.

A main advantage of this type of foundation is that there are no open joints or planes ofweakness that can be exploited by the seismic shaking. The strength of the foundation is


FIGURE 13.4 Typical pile configurations. (Reproduced from Bowles 1982 with permission ofMcGraw-Hill, Inc.)

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FIGURE 13.5 Storage tank supported by concrete piles. The soil underneath the tank liquefied during the

Kobe earthquake on January 17, 1995. The soil around the piles was removed in order to observe the condi-tion of the piles. (Photograph from the Kobe Geotechnical Collection, EERC, University of California,Berkeley.)

FIGURE 13.6 Truck-mounted auger drill rig used to excavate piers.

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due to its monolithic construction, with the floor slab attached and supported by the gradebeams, which are in turn anchored to the piers. In addition, the steel columns of the super-structure can be constructed so that they bear directly on top of the piers and have fixed end

connections. This monolithic foundation and the solid connection between the steelcolumns and piers will enable the structure to resist the seismic shaking.


FIGURE 13.8 Close-up of auger being extracted from the ground with soil lodged within its grooves.

FIGURE 13.7 Close-up of auger being pushed into the soil.

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FIGURE 13.10 The pier hole has been filled with concrete. The steel reinforcement from the pier will be

attached to the steel reinforcement in the grade beam.

FIGURE 13.9 A light has been lowered to the bottom of the pier to observe embedment conditions.

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Usually this foundation system is designed by the structural engineer. The geotechnicalengineer provides various design parameters, such as the estimated depth of the bearingstrata, the allowable end-bearing resistance, allowable skin friction in the bearing material,allowable passive resistance of the bearing material, and any anticipated down-drag loadsthat could be induced on the piers if the upper loose or compressible soil should settle underits own weight or during the anticipated earthquake. The geotechnical engineer also needsto inspect the foundation during construction in order to confirm the embedment conditionsof the piers.

13.3.3 Prestressed Concrete Piles

Introduction. Common types of prestressed concrete piles are shown in Fig. 13.18.Prestressed piles are typically produced at a manufacturing plant. The first step is to set upthe form, which contains the prestressed strands that are surrounded by wire spirals. The


FIGURE 13.11 Excavation for the grade beam that will spanbetween the two piers.

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