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fissuring and sand boils, then this layer may provide passive resistance for the piles, caps,and grade beams.

4. Liquefaction of sloping ground: For liquefaction of sloping ground, there willoften be lateral spreading of the ground, which could shear off the piles. One mitigationmeasure consists of the installation of compaction piles (see Sec. 12.3.3), in order to create

a zone of nonliquefiable soil around and beneath the foundation.

13.4 FOUNDATIONS FOR SINGLE-FAMILY

HOUSES

In southern California, the type of foundation for single-family houses often consists ofeither a raised wood floor foundation or a concrete slab-on-grade.

13.32 CHAPTER THIRTEEN

FIGURE 13.31 The excavation for the grade beams is complete, and thetops of the prestressed piles are trimmed so that they are relatively flush.

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FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS 13.33

FIGURE 13.33 Close-up view of the top of a prestressed pile with the steel reinforcement from the gradebeam positioned on top of the pile. The strands from the pile are attached to the steel reinforcement in thegrade beam.

FIGURE 13.32 Close-up view of one of the prestressed piles showing a trimmed top surface with thestrands extending out the top of the pile.

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13.4.1 Raised Wood Floor Foundation

The typical raised wood floor foundation consists of continuous concrete perimeter foot-ings and interior (isolated) concrete pads. The floor beams span between the continuousperimeter footings and the isolated interior pads. The continuous concrete perimeter foot-ings are typically constructed so that they protrude about 0.3 to 0.6 m (1 to 2 ft) above theadjacent pad grade. The interior concrete pad footings are not as high as the perimeter foot-

ings, and short wood posts are used to support the floor beams. The perimeter footings andinterior posts elevate the wood floor and provide for a crawl space below the floor.

In southern California, the raised wood floor foundation having isolated interior pads iscommon for houses 30 years or older. Most newer houses are not constructed with thisfoundation type. In general, damages caused by southern California earthquakes have beenmore severe to houses having this type of raised wood floor foundation. There may be sev-eral different reasons for this behavior:

1. Lack of shear resistance of wood posts: As previously mentioned, in the interior, the

raised wood floor beams are supported by short wood posts bearing on interior concretepads. During the earthquake, these short posts are vulnerable to collapse or tilting.


FIGURE 13.34 Overview of the steel reinforcement positionedwithin the grade beam excavation.

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2. No bolts or inadequate bolted condition: Because in many cases the house is not ade-quately bolted to the foundation, it can slide or even fall off the foundation during theearthquake. In other cases the bolts are spaced too far apart, and the wood sill platesplits, allowing the house to slide off the foundation.

3. Age of residence: The houses having this type of raised wood floor foundation are

older. The wood is more brittle and in some cases weakened due to rot or termite dam-age. In some cases, the concrete perimeter footings are nonreinforced or have beenweakened due to prior soil movement, making them more susceptible to cracking dur-ing the earthquake.

4. Crawl-space vents: To provide ventilation to the crawl space, long vents are oftenconstructed just above the concrete foundation, such as shown in Fig. 13.37. Thesevents provide areas of weakness just above the foundation.

All these factors can contribute to the detachment of the house from the foundation. For

example, Fig. 13.37 shows the sliding of the house off the foundation caused by the SanFernando earthquake.


FIGURE 13.35 The top of the steel pile separated from the con-crete pile cap during the Kobe earthquake on January 17, 1995.

(Photograph from the Kobe Geotechnical Collection, EERC,University of California, Berkeley.)

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Besides determining the type of foundation to resist earthquake-related effects, the geo-technical engineer could also be involved with the retrofitting of existing structures. As previ-ously mentioned, the raised wood floor with isolated posts is rarely used for new construction.But there are numerous older houses that have this foundation type, and in many cases, thewood sill plate is inadequately bolted to the foundation. Bolts or tie-down anchors could

be installed to securely attach the wood framing to the concrete foundation. Wood bracingor plywood could be added to the open areas between posts to give the foundation greatershear resistance and prevent the house from sliding off the foundations, such as shown inFig. 13.37.

13.4.2 Slab-on-Grade

In southern California, the concrete slab-on-grade is the most common type of foundation

for houses constructed within the past 20 years. It consists of perimeter and interior con-tinuous footings, interconnected by a slab-on-grade. Construction of the slab-on-gradebegins with the excavation of the interior and perimeter continuous footings. Steel rein-forcing bars are commonly centered in the footing excavations, and wire mesh or steel barsare used as reinforcement for the slab. The concrete for both the footings and the slab isusually placed at the same time, to create a monolithic foundation. Unlike the raised woodfloor foundation, the slab-on-grade does not have a crawl space.

In general, for those houses with a slab-on-grade, the wood sill plate is securely boltedto the concrete foundation. In many cases, an earthquake can cause the development of an

exterior crack in the stucco at the location where the sill plate meets the concrete founda-


FIGURE 13.36 The top of the concrete pile separated from the concrete pile cap during the Kobe earth-

quake on January 17, 1995. (Photograph from the Kobe Geotechnical Collection, EERC, University ofCalifornia, Berkeley.)

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tion. In some cases, the crack can be found on all four sides of the house. The crack devel-ops when the house framing bends back and forth during the seismic shaking.

For raised wood floor foundations and the slab-on-grade foundations subjected to sim-

ilar earthquake intensity and duration, those houses having a slab-on-grade generally havethe best performance. This is because the slab-on-grade is typically stronger due to steelreinforcement and monolithic construction, the houses are newer (less wood rot and con-crete deterioration), there is greater frame resistance because of the construction of shearwalls, and the wood sill plate is in continuous contact with the concrete foundation.

Note that although the slab-on-grade generally has the best performance, these housescan be severely damaged. In many cases, these houses do not have adequate shear walls,there are numerous wall openings, or there is poor construction. The construction of a slab-on-grade by itself is not enough to protect a structure from collapse if the structural frame

above the slab does not have adequate shear resistance.


FIGURE 13.37 Sliding of house off the foundation caused by theSan Fernando earthquake in California on February 9, 1971. The

house is located in the city of San Fernando, near Knox and GroveStreets. (Photograph from the Steinbrugge Collection, EERC,University of California, Berkeley.)

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13.4.3 California Northridge Earthquake

The Northridge earthquake, which occurred in California on January 17, 1994, struck anurban area that primarily contained single-family dwellings. The type of foundation for thesingle-family houses was a major factor in the damage caused by the Northridge earthquake.

Particulars concerning the Northridge earthquake are as follows (Day 1999, USGS 1994):

G The Northridge earthquake had a magnitude of 6.7 and occurred beneath the SanFernando Valley on a deeply buried blind thrust fault that may be an eastern extension ofthe Oak Ridge fault system. The fault plane ruptured from a depth of about 11 mi (17.5 km)upward to about 3 mi (5 km) beneath the surface. For 8 s following the initial break,the rupture propagated upward and northwestward along the fault plane at a rate of about2 mi/s (3 km/s). Fortuitously, the strongest seismic energy was directed along the faultplane toward sparsely populated areas north of the San Fernando Valley.

G The earthquake deformed the earths crust over an area of 1500 mi2 (4000 km2), forcingthe land surface upward in the shape of an asymmetric dome. The dome manifests fea-tures and consequences of blind thrust faulting that might lead scientists to the discoveryof similar faults elsewhere. The lack of clear surface rupture in 1994 may be explainedby fault movement terminating at depth against another fault that moved in the 1971 SanFernando event.

G Studies of more than 250 ground-motion records showed that peak accelerations duringthe earthquake generally exceeded those predicted. At several locations, horizontal peakswere close to or exceeded 1g, and at one station, vertical acceleration exceeded 1g.Ground motions both near and far from the fault contained consistent, high-energy pulsesof relatively long duration. Midrise to high-rise steel structures designed for lessermotions were particularly vulnerable to these pulses. In general, the ratio of horizontal tovertical shaking was similar to that of past earthquakes, and the motions, although strong,were not unusual.

G There was collapse of specially designed structures such as multistory buildings, parkinggarages, and freeways. In some areas, the most severe damage would indicate a modifiedMercalli intensity of IX, although VII to VIII was more widespread. Because theNorthridge earthquake occurred in a suburban community, damage to single-familyhouses was common.

G Numerous structural failures throughout the region were evidence of significant deficien-cies in design or construction methods. Steel frames of buildings intended for seismic resis-tance were cracked, and reinforced concrete columns were crushed. Most highwaystructures performed well, but freeways collapsed at seven sites, and 170 bridges sustainedvarying degrees of damage.

G Damage estimates varied considerably. For both public and private facilities, the totalcost of the Northridge earthquake was on the order of $20 to $25 billion. This makes the

Northridge earthquake Californias most expensive natural disaster. Given the significantdamage caused by this earthquake, the number of deaths was relatively low. This waspartly because most people were asleep at home at the time of the earthquake (4:31 a.m.).

The observed foundation damage caused by the California Northridge earthquake indi-cated the importance of tying together the various foundation elements. To resist damageduring the earthquake, the foundation should be monolithic with no gaps in the footings orplanes of weakness due to free-floating slabs. For new construction in southern California,many single-family houses are being constructed with post-tensioned slab-on-grade (seeFig. 13.38). This type of foundation has an induced compressive stress due to the tension-ing of the steel tendons embedded in the foundation concrete. Because of the compression


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stress and lack of free-floating slab elements, this type of foundation will probably performeven better during an earthquake than the conventional slab-on-grade.

13.5 PROBLEMS

13.1 Use the data from Prob. 9.13 and Fig. 9.39 and assume a level-ground site. A pro-posed building will have a deep foundation system consisting of piles that are driven into theFlysh claystone. Assuming that the piles are widely spaced and do not increase the liquefactionresistance of the soil, calculate the differential movement between the building and adjacentground. Answer: Using Fig. 7.1, differential movement 20 cm. Using Fig. 7.2, differentialmovement 14 cm.

13.2 Use the data from Prob. 13.1 and an effective friction angle between the pile sur-face and the surface soil layer and sand layer of 28. Assume that k0 0.5 and that the lastlocation for the earthquake-induced pore water pressures to dissipate will be just above theclayey fine sand layer. Further assume that the clayey fine sand layer and the silty fine sandlayer are not anticipated to settle during the earthquake. If the piles are 0.3 m in diameter, cal-culate the down-drag load on each pile due to liquefaction at the site. Answer: Down-dragload 61 kN.

13.3 Use the data from Prob. 6.12 and Fig. 6.13. To prevent liquefaction-induced set-tlement of the building, what is the minimum length of piles that should be installed at thesite? Answer: 20-m-long piles.


FIGURE 13.38 Construction of a post-tensioned foundation for a single-family residence.

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