26672_16a

STRUCTURAL FOUNDATIONSAND RETAINING WALLS

Robert W. Day, P.E.

ALLOWABLE FOUNDATION MOVEMENT 16.2FOUNDATION MOVEMENT AND SEVERITY OF DAMAGE 16.9FIELD OBSERVATION, INSTRUMENTATION, TESTING, AND ANALYSIS 16.13

Field Observation 16.13Instrumentation 16.14

Inclinometers 16.14Piezometers 16.14Settlement Monuments or Cells 16.14Crack Pins 16.14Other Monitoring Devices 16.15

Testing 16.15Nondestructive Field Testing 16.15Destructive Field Testing 16.19Laboratory Testing 16.19

Document Search 16.21Reports and Plans 16.21Building Codes 16.22Technical Documents 16.22

Analysis 16.23TYPES AND CAUSES OF COMMON NONPERFORMANCE AND FAILURE 16.23

Settlement 16.23Settlement of the Foundation Caused by Collapsible Soil 16.25Settlement of the Foundation due to Limestone Cavities and Sinkholes 16.26Settlement of the Foundation due to Consolidation of Soft and Organic Soil 16.28Settlement of the Foundation due to Collapse of Underground Mines andTunnels 16.29

Settlement of the Foundation due to Ground Subsidence from Extraction of Oil or Groundwater 16.31

Expansive Soil 16.32Expansive Soil Factors 16.32Laboratory Testing 16.34Surcharge Pressure 16.35

Lateral Movement 16.35Earthquakes 16.39

Surface Fault Rupture 16.39Liquefaction 16.40Slope Movement and Settlement 16.42

CHAPTER 16

16.1

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Deterioration 16.43Sulfate Attack of Concrete Foundations 16.43Frost 16.48

TEMPORARY AND PERMANENT REMEDIAL REPAIRS 16.48Reinforced Mat 16.49Reinforced Mat with Piers 16.51Partial Removal and/or Strengthening of Foundations 16.52Concrete Crack Repairs 16.54Other Foundation Repair Alternatives 16.55

RETAINING WALLS 16.57Common Causes of Failures 16.57

EXAMPLES OF CASE STUDIES OF NONPERFORMANCE AND FAILURE 16.58REFERENCES 16.70

ALLOWABLE FOUNDATION MOVEMENT

A foundation is defined as that part of the structure that supports the weight andloads acting on the structure and transmits this load to underlying soil or rock.Foundations are commonly divided into two categories: shallow and deep foun-dations. Table 16.1 presents a list of common types of foundations. The most fre-quently encountered conditions that cause damage to foundations and structuresare settlement, expansive soil, lateral movement, and deterioration. Table 16.21presents a list of the typical types of problems that affect foundations.

Because of the great diversity of foundation types (Table 16.1), there is no sin-gle code or standard in the United States that specifies the allowable movement offoundations. However, there is a considerable amount of data available on the sub-ject (e.g., Refs. 2 through 7). For example, it has been stated that the allowable dif-ferential and total settlement should depend on the flexibility and complexity ofthe structure, including the construction materials and type of connections.8

In terms of the allowable foundation settlement, Coduto9 states that it dependson many factors, including the following:

The type of construction. For example, wood-frame buildings with wood sidingwould be much more tolerant than unreinforced brick buildings.

The use of the structure. Even small cracks in a house might be considered unac-ceptable, whereas much larger cracks in an industrial building might not even benoticed.

The presence of sensitive finishes. Tile or other sensitive finishes are much lesstolerant of movements.

The rigidity of the structure. If a footing beneath part of a very rigid structuresettles more than the other footings, the structure will transfer some of the loadaway from the footing. However, footings beneath flexible structures must set-tle much more before any significant load transfer occurs. Therefore, a rigidstructure will have less differential settlement than a flexible one.

16.2 CHAPTER SIXTEEN

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TABLE 16.1 Common Types of Foundations

Category Common types Comments

Shallow foundations Spread footings (also called Spread footings are often square in plan view, are of uniform reinforced-concrete pad footings) thickness, and are used to support a single-column load located directly in the center

of the footing.Strip footings (also called Strip or wall footings are often used for load-bearing walls. They are usually long, wall footings) 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.Conventional slab-on-grade A continuous reinforced-concrete foundation consisting of bearing wall footings and

a slab-on-grade. Concrete reinforcement often consists of steel rebar in the footingsand wire mesh in the concrete slab.

Post-tensioned slab-on-grade A continuous post-tensioned concrete foundation. The post-tensioning effect is createdby tensioning steel tendons or cables embedded within the concrete. Common post-ten-sioned foundations are the ribbed foundation, California slab, and PTI foundation.

Raised wood floor Perimeter footings that support wood beams and a floor system. Interior support isprovided by pad or strip footings. There is a crawl space below the wood floor.

Mat foundation A large and thick reinforced-concrete foundation, often of uniform thickness, that iscontinuous and supports the entire structure. A mat foundation is considered to be ashallow foundation if it is constructed at or near ground surface.

16.3

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TABLE 16.1 Common Types of Foundations (Continued)

Category Common types Comments

Deep foundations Driven piles Driven piles are slender members, made of wood, steel, or precast concrete, that aredriven into place by using pile-driving equipment.

Other types of piles There are many other types of piles, such as bored piles, cast-in-place piles, or com-posite piles.

Piers Similar to cast-in-place piles, piers are often of large diameter and contain rein-forced concrete. Pier and grade beam support are often used for foundation supporton expansive soil.

Caissons Large piers are sometimes referred to as caissons. A caisson can also be a watertightunderground structure within which construction work is carried on.

Mat or raft foundation If a mat or raft foundation is constructed below ground surface or if the mat or raftfoundation is supported by piles or piers, then it should be considered to be a deepfoundation system.

Floating foundation A special foundation type where the weight of the structure is balanced by theremoval of soil and construction of an underground basement.

Basement-type foundation A common foundation for houses and other buildings in frost-prone areas. The foun-dation consists of perimeter footings and basement walls that support a wood floorsystem. The basement floor is usually a concrete slab.

Note: Shallow and deep foundations in this table are based on the depth of the soil or rock support of the foundation.

16.4

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STRUCTURAL FOUNDATIONS AND RETAINING WALLS 16.5

TABLE 16.2 Problem Conditions Requiring Special Consideration

Problem type Description Comments

Soil Organic soil, highly Low strength and high compressibilityplastic soilSensitive clay Potentially large strength loss upon large

strainingMicaceous soil Potentially high compressibilityExpansive clay, Potentially large expansion upon wettingsilt, or slagLiquefiable soil Complete strength loss and high deformations

caused by earthquakesCollapsible soil Potentially large deformations upon wettingPyritic soil Potentially large expansion upon oxidation

Rock Laminated rock Low strength when loaded parallel to beddingExpansive shale Potentially large expansion upon wetting;

degrades readily upon exposure to air and waterPyritic shale Expands upon exposure to air and waterSoluble rock Rock such as limestone, limerock, and gyp-

sum that is soluble in flowing and standingwater

Cretaceous shale Indicator of potentially corrosive groundwaterWeak claystone Low strength and readily degradable upon

exposure to air and waterGneiss and Schist Highly distorted with irregular weathering

profiles and steep discontinuitiesSubsidence Typical in areas of underground mining or

high groundwater extractionSinkholes Areas underlain by carbonate rock (Karst

topography)Condition Negative skin friction Additional compressive load on deep founda-

tions due to settlement of soilExpansion loading Additional uplift load on foundation due to

swelling of soilCorrosive Acid mine drainage and degradation of soil environment and rockFrost and permafrost Typical in northern climatesCapillary water Rise in water level which leads to strength

loss for silts and fine sands

Source: Reproduced with permission from Standard Specifications for Highway Bridges, 16thed., AASHTO, 1996.

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Coduto9 also states that the allowable settlement for most structures, especiallybuildings, will be governed by aesthetic and serviceability requirements, not struc-tural requirements. Unsightly cracks, jamming doors and windows, and other sim-ilar problems will develop long before the integrity of the structure is in danger.Because the determination of the allowable settlement is so complex, engineersoften rely on empirical correlations between observed behavior of structures andthe settlement that results in damage.

Another major reference for the allowable settlement of structures is the 1956paper by Skempton and MacDonald entitled The Allowable Settlement ofBuildings.10 As shown in Fig. 16.1, Skempton and MacDonald defined the maxi-mum angular distortion /L and the maximum differential settlement for a build-ing with no tilt. The angular distortion /L is defined as the differential settlementbetween two points divided by the distance between them less the tilt, where tiltequals the rotation of the entire building. As shown in Fig. 16.1, the maximumangular distortion does not necessarily occur at the location of maximum differen-tial settlement.

Skempton and MacDonald studied 98 buildings, of which 58 had suffered nodamage and 40 had been damaged in varying degrees as a consequence of settle-ment. From a study of these 98 buildings, they concluded in part the following:

The cracking of the brick panels in frame buildings or load-bearing brick walls islikely to occur if the angular distortion of the foundation exceeds 1300. Structural


FIGURE 16.1 Diagram illustrating the definitions of maximum angular distortion and maximum dif-ferential settlement.

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damage to columns and beams is likely to occur if the angular distortion of thefoundation exceeds 1150.

By plotting the maximum angular distortion /L versus the maximum differen-tial settlement , such as shown in Fig. 16.2, a correlation was obtained that isdefined as 350/L (note that is in inches). Using this relationship and anangular distortion /L of 1300, cracking of brick panels in frame buildings orload-bearing brick walls is likely to occur if the maximum differential settle-ment exceeds 114 in (32 mm).

The angular distortion criteria of 1150 and 1300 were derived from an observa-tional study of buildings of load-bearing wall construction, and steel and rein-forced-concrete-frame buildings with conventional brick panel walls butwithout diagonal bracing. The criteria are intended as no more than a guide forday-to-day work in designing typical foundations for such buildings. In certaincases they may be overruled by visual or other considerations.

The 1974 paper by Grant et al.11 updated the Skempton and MacDonald datapool and also evaluated the rate of settlement with respect to the amount of dam-age incurred. Grant et al. in part concluded the following:

A building foundation that experiences a maximum value of deflection slope /Lgreater than 1300 will probably suffer some damage. However, damage does notnecessarily occur at the point where the local deflection slope exceeds 1300.

For any type of foundation on sand or fill, new data tend to support Skemptonand MacDonalds suggested correlation of 350/L (see Fig. 16.2).

Consideration of the rate of settlement is important only for the extreme situa-tions of either very slow or very rapid settlement. Based on the limited dataavailable, the values of maximum /L corresponding to building damage appearto be essentially the same for cases involving slow and fast settlements.

Data concerning the behavior of lightly reinforced, conventional slab-on-grade foundations have also been included in Fig. 16.2. These data indicate12 thatcracking of gypsum wallboard panels is likely to occur if the angular distortionof the slab-on-grade foundation exceeds 1300. The ratio of 1300 appears to be use-ful for both wood-frame gypsum wallboard panels and the brick panels studiedby Skempton and MacDonald.10 The data plotted in Fig. 16.2 would indicate thatthe relationship 350/L can also be used for buildings supported by lightlyreinforced slab-on-grade foundations. By using /L 1300 as the boundarywhere cracking of panels in wood-frame residences supported by concrete slab-on-grade is likely to occur and substituting this value into the relationship 350/L (Fig. 16.2), the calculated differential slab displacement is 114 in (32mm). For buildings on lightly reinforced slabs-on-grade, cracking of gypsumwallboard panels is likely to occur when the maximum slab differential exceeds114 in (32 mm).


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Another example of allowable settlements for buildings is Table 16.3.13 Inthis table, the allowable foundation displacement has been divided into threecategories: total settlement, tilting, and differential movement. Table 16.3 indi-cates that those structures that are more flexible (such as simple steel-framebuildings) or have more rigid foundations (such as mat foundations) can sustainlarger values of total settlement and differential movement.

Figure 16.3 presents data from Ref. 14. Similar to the studies previously men-tioned, this figure indicates that cracking in panel walls is to be expected at anangular distortion /L of 1300 and that structural damage of buildings is to beexpected at an angular distortion /L of 1150. This figure also provides other lim-iting values of angular distortion, such as for buildings containing sensitivemachinery or overhead cranes.


FIGURE 16.2 Maximum differential settlement versus maximum angular distortion. (Initial datafrom Skempton and MacDonald 1956, Table 1 in Day 1990.)

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FOUNDATION MOVEMENT AND SEVERITY OFDAMAGE

Table 16.4 summarizes the severity of cracking damage versus approximatecrack widths, typical values of maximum differential movement and maximumangular distortion /L of the foundation.1517 The relationship between differentialsettlement and angular distortion /L was based on the equation 350/L(from Fig. 16.2).

When the severity of damage for an existing structure is assessed, the damagecategory (Table 16.4) should be based on multiple factors, including crack widths,differential settlement, and the angular distortion of the foundation. Relying ononly one parameter, such as crack width, can lead to inaccuracy in cases wherecracking has been hidden or patched, or in cases where other factors (such as con-crete shrinkage) contribute to crack widths.


TABLE 16.3 Allowable Settlement

Type of movement Limiting factor Maximum settlement

Total settlement Drainage 1530 cm (612 in)Access 3060 cm (1224 in)Probability of nonuniform settlement:

Masonry-walled structure 2.55 cm (12 in)Framed structures 510 cm (24 in)Smokestacks, silos, mats 830 cm (312 in)

Tilting Stability against overturning Depends on H and WTilting of smokestacks, towers 0.004LRolling of trucks, etc. 0.01LStacking of goods 0.01LMachine operationcotton loom 0.003LMachine operationturbogenerator 0.0002LCrane rails 0.003LDrainage of floors 0.010.02L

Differential High continuous brick walls 0.00050.001Lmovement One-story brick mill building,

wall cracking 0.0010.002LPlaster cracking (gypsum) 0.001LReinforced-concrete building frame 0.00250.004LReinforced-concrete building curtain walls 0.003LSteel frame, continuous 0.002LSimple steel frame 0.005L

Notes: L distance between adjacent columns that settle different amounts, or between any twopoints that settle differently. Higher values are for regular settlements and more tolerant structures. Lowervalues are for irregular settlement and critical structures. H height and W width of structure.

Source: From Sowers, 1962.13

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Foundations subjected to settlement can be damaged by a combination of bothvertical and horizontal movements. For example, a common cause of foundationdamage is fill settlement. Figure 16.4 shows an illustration of the settlement of fill ina canyon environment. Over the sidewalls of the canyon, there tends to be a pullingor stretching of the ground surface (tensional features), with compression effects nearthe canyon centerline. This type of damage is due to two-dimensional settlement,where the fill compresses in both the vertical and horizontal directions.18,19

Another common situation where both vertical and horizontal foundation dis-placement occurs is at cut-fill transitions. A cut-fill transition occurs when a build-ing pad has some rock removed (the cut portion), with a level building pad beingcreated by filling in (with soil) the remaining portion. If the cut side of the build-ing pad contains nonexpansive rock that is dense and unweathered, then very lit-tle settlement would be expected for that part of the building on cut. But the fillportion could settle under its own weight and cause damage. For example, a slabcrack will typically open at the location of the cut-fill transition, as illustrated inFig. 16.5. The building is damaged by both the vertical foundation movement (set-tlement) and the horizontal movement, which manifests itself as a slab crack anddrag effect on the structure (Fig. 16.5).

In the cases described above, lateral movement is a secondary result of the pri-mary vertical movement due to settlement of the foundation. Table 16.4 can there-fore be used as a guide to correlate damage category with and /L. In caseswhere lateral movement is the most predominate or critical mode of foundationdisplacement, Table 16.4 may underestimate the severity of cracking damage forvalues of and /L.


FIGURE 16.3 Damage criteria. (After Bjerrum 1963.)

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TABLE 16.4 Severity of Cracking Damage

Damage category Description of typical damage Approx. crack width /L

Negligible Hairline cracks. 0.1 mm 3 cm (1.2 in) 1300Very slight Very slight damage includes fine cracks that can be 1 mm 34 cm (1.21.5 in) 1300 to 1240

easily treated during normal decoration, perhaps an isolated slight fracture in building, and cracks in external brickwork visible on close inspection.

Slight Slight damage includes cracks that can be easily 3 mm 45 cm (1.52.0 in) 1240 to 1175filled and redecoration would probably be required; several slight fractures may appear, showing on the inside of the building; cracks that are visible externally and some repointing may be required; and doors and windows may stick.

Moderate Moderate damage includes cracks that require some 515 mm or a 58 cm (2.03.0 in) 1175 to 1120opening up and can be patched by a mason; number of cracksrecurrent cracks that can be masked by suitable 3 mmlinings; repointing of external brickwork and possibly a small amount of brickwork replacement may be required; doors and window stick; service pipes may fracture; and weathertightness is often impaired.

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16.12

CHA

PTER SIXTEEN

TABLE 16.4 Severity of Cracking Damage (Continued)

Damage category Description of typical damage Approx. crack width /L

Severe Severe damage includes large cracks requiring 1525 mm but also 813 cm (3.05.0 in) 1120 to 170extensive repair work involving breaking out and depends onreplacing sections of walls (especially over doors number of cracksand windows); distorted windows and door frames; noticeably sloping floors; leaning or bulging walls; some loss of bearing in beams; and disrupted service pipes.

Very severe Very severe damage often requires a major repair Usually 25 mm 13 cm (5 in) 170job involving partial or complete rebuilding; beams but also dependslose bearing; walls lean and require shoring; on number ofwindows are broken with distortion; and there is cracksdanger of structural instability.

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FIELD OBSERVATION, INSTRUMENTATION,TESTING, AND ANALYSIS

Field Observation

The purpose of the field observation is to evaluate the scope and nature of the fail-ure. The initial site visit should be performed by the forensic engineer, who maybe accompanied by assistants. For those structures having sudden damage or col-lapse, it is important to perform the initial site visit immediately after the assign-ment has been accepted. This is because evidence may be lost or disturbed as timegoes by. It is important to take photographs of the observed damage and to collectsamples at the initial site visit if it is likely that the samples will be lost ordestroyed before the investigation can be completed.


FIGURE 16.4 Fill settlement in a canyon environment.

FIGURE 16.5 Cut-fill transition lot.

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Numerous photographs should be taken of the damaged structure. It may beappropriate to take a professional photographer who has the proper equipment forlong-range and close-up photographs of the site. Sequentially numbering andmarking the location of the photographs on a site plan sketch may help refresh rec-ollection of where the pictures were taken.

Instrumentation

There are many types of monitoring devices used by forensic engineers. Some ofthe more common monitoring devices are as follows:

Inclinometers. The horizontal movement preceding or during the movement ofslopes can be investigated by successive surveys of the shape and position of flex-ible vertical casings installed in the ground.20 The surveys are performed by low-ering an inclinometer probe into the flexible vertical casing. The inclinometerprobe is capable of measuring its deviation from the vertical. An initial survey(base reading) is performed, and then successive readings are compared to thebase reading to obtain the horizontal movement of the slope. Figure 16.621 showsa sketch of the inclinometer probe in the casing and the calculations used to obtainthe lateral deformation.

Piezometers. Piezometers are routinely installed to monitor pore water pressuresin the ground. Several different types are commercially available, including bore-hole, embankment, or push-in piezometers. Figures 16.7 and 16.8 (from Ref. 21)show examples of borehole piezometers.

Settlement Monuments or Cells. Settlement monuments or settlement cells can beused to monitor settlement or heave. Figure 16.921 shows a diagram of the instal-lation of a pneumatic settlement cell and plate. More advanced equipmentincludes settlement systems installed in borings that not only can measure totalsettlement, but also can measure the incremental settlement at different depths.

Crack Pins. A simple method to measure the widening of a concrete or masonrycrack is to install crack pins on both sides of the crack. By periodically measuringthe distance between the pins, the amount of opening or closing of the crack canbe determined.

Other crack-monitoring devices are commercially available. For example, Fig.16.10 shows an Avongard crack-monitoring device. There are two installation pro-cedures: (1) The ends of the device are anchored by the use of bolts or screws, or(2) the ends of the device are anchored with epoxy adhesive. The center of theAvongard crack-monitoring device is held together with clear tape, which is cutonce the ends of the monitoring device have been securely fastened with bolts,screws, or epoxy adhesive.


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Other Monitoring Devices. There are many other types of monitoring devices thatcan be used by the forensic engineer. Some commercially available devicesinclude pressure and load cells, borehole and tape extensometers, soil strain-meters, beam sensors and tiltmeters, and strain gauges.

Testing

There are two general categories of field testing for the forensic investigation offoundation failures: nondestructive testing and destructive testing.

Nondestructive Field Testing. After the initial site visit, there are usually follow-up visits in order to prepare field sketches and field notes, conduct interviews,


Lateral Deviation(L Sin )

Verti

cal

Angle ofInclination

() MeasurementInterval (L)

InclinometerCasing

FIGURE 16.6 Inclinometer probe in a casing. (Reprintedwith permission from the Slope Indicator Company.)

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perform nondestructive testing, and install monitoring devices such as piezome-ters and inclinometers.

As the name implies, nondestructive testing does not cause any damage or dis-ruption to the site. An example of nondestructive testing is geophysical techniquesthat can be used to locate underground voids or buried objects, such as oil tanks.Similar to geophysical techniques is acoustic emission, which uses high-fre-quency sound waves to detect flaws in engineering structures. For example,Kisters and Kearney22 state that acoustic emission is presently being used to mon-itor crack propagation in bridges. This technique can be easily adapted to steel


FIGURE 16.7 Pneumatic piezometer installed in aborehole. (Reprinted with permission from theSlope Indicator Company.)

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lock and dam structures. According to Rens et al.,23 other nondestructive testingincludes thermal, ultrasonic, and magnetic methods. Thermal techniques havebeen applied to several civil engineering projects such as asphalt and pavementcondition assessments.23

Another example of nondestructive testing is a manometer survey, which isalso referred to as a floor-level survey. It is a nondestructive means of findingflaws or design defects in foundations. A manometer survey is commonly used todetermine the relative elevation of a concrete slab-on-grade or other foundationelement. The survey consists of taking elevations at relatively close intervalsthroughout the interior floor slab. These elevation points are then contoured, muchlike a topographic map, to provide a graphical rendition of the deformation con-dition of the foundation. Soil movement can cause displacement of the foundation,


Vented Cap

PiezometricWater Level

Groundwater Level

Bentonite/CementGrout

Bentonite Seal

Filter Tip

Standpipe

FIGURE 16.8 Standpipe (Casagrande) piezometerinstalled in a borehole. (Reprinted with permissionfrom the Slope Indicator Company.)

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Reservoir

Tubing, Liquid Filledand Pneumatic

Liquid Head

Settlement CellOptional

Settlement PlateS-LSG.cdr

FIGURE 16.9 Pneumatic settlement cell installation. (Reprinted with permission from the SlopeIndicator Company.)

FIGURE 16.10 Avongard crack monitoring device.

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and the manometer survey can detect this deformation. For example, if one side orcorner of a concrete slab is significantly lower than the rest of the foundation, thiscould indicate settlement or slope movement in that area. Likewise, if the centerof the foundation bulged upward, it would be detected by the manometer surveyand this could indicate expansive soil. Other soil phenomena or slab conditionscan also be detected by the manometer survey. For example, close deformationcontours indicate a high angular distortion, which in many cases corresponds tothe location of foundation cracks.

Destructive Field Testing. Concerning destructive testing, Miller24 states:

To document and cover the extent of resultant damages, it is often necessary toperform extensive destructive testing to determine the source and extent of the dam-age. Destructive testing can involve removing limited sections of the building so thataccess may be gained to concealed areas. If destructive testing is undertaken, it isimperative that it be performed in a logical and methodical fashion. Extensive doc-umentation should be made during all portions of the destructive testing because itis a costly process, and repetition should be avoided if at all possible.

Examples of destructive testing include subsurface exploration, such as theexcavation of test pits and borings.

The test pits or borings are used to determine the thickness of soil and rockstrata, estimate the depth to groundwater, obtain soil or rock specimens, and per-form field tests such as sand cone tests or standard penetration tests (SPTs). TheUnified Soil Classification System (USCS) can be used to classify the soilexposed in the borings or test pits.25 The subsurface exploration and field samplingshould be performed in accordance with standard procedures, such as those spec-ified by the American Society for Testing and Materials2628 or other recognizedsources.2932 An example of field exploration and testing is shown in Fig. 16.11,where a test pit has been excavated into an airport runway and a sand cone test isbeing performed on the runway base material.

Another common type of destructive testing is the coring of foundations. Bycoring the foundation, the thickness of concrete, reinforcement condition, and anydeterioration can be observed. Also, soil samples can be obtained directly beneaththe foundation. Figure 16.12 shows a photograph of a slab-on-grade foundationthat has been cored.

Numerous other types of destructive testing can be performed by the forensicengineer. Examples include field load tests, cone penetration testing, and in-placetesting, such as determining the shear strength of in-place soil or rock.

Laboratory Testing. Laboratory tests are commonly used to determine the classi-fication, moisture and density, index properties, shear strength, compressibility,and hydraulic conductivity of the soil. Soil, groundwater, or foundation samplesrecovered from the site visits can be sent to the laboratory for testing.


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FIGURE 16.11 Test pit excavation.

FIGURE 16.12 Coring of a concrete slab-on-grade.

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Usually at the time of the laboratory testing, the forensic engineer will havedeveloped one or more hypotheses concerning the cause of the foundation failure.The objective of the laboratory testing is to further investigate these failurehypotheses. It is important that the forensic engineer develop a logical laboratorytesting program with this objective in mind. The laboratory tests should be per-formed in accordance with standard procedures, such as those recommended bythe American Society for Testing and Materials (ASTM) or those procedureslisted in standard textbooks or specification manuals (e.g., Refs. 33 to 36).

For some foundation investigations, it may be important to determine thepotential for future soil movement and damage. In these cases, the laboratory test-ing should model future expected conditions so that the amount of movement orstability of the ground can be analyzed.

For forensic investigations involving civil litigation, samples that are notirrecoverably damaged or destroyed during the laboratory testing should be savedand preserved so that they do not become contaminated. This is because otherforensic experts involved in the case may want to observe or test the specimens.Also, at the time of trial, the specimens may need to be admitted as evidence.

Document Search

Table 16.5 presents a list of typical documents that may need to be reviewed for aforensic investigation. The following is a brief summary of these types of documents:

Reports and Plans. The reports and plans that were generated during the designand construction of the project may need to be reviewed. The reports and plans canprovide specific information on the history, design, and construction of the proj-ect. These documents may also provide information on maintenance or alterationsat the site.

As part of the discovery process for projects dealing with civil litigation, attor-neys commonly subpoena, or the judge may order, that the entire records of theowner, designers, and contractors be placed in a document depository. Once thedocuments are submitted to the document depository, they will be available to allparties involved in the lawsuit.

The judge assigned to the case will normally issue an order detailing the pro-cedures to be followed concerning the document depository. Once documentshave been submitted to the document depository, the depository coordinator willplace a bate-stamp, which is a prefix code and number, on each individual pageor map. (The bate-stamp is named after the person who invented the sequential-numbering apparatus.) Many documents in the depository will be irrelevant interms of the cause of the failure, and the forensic engineer must be able to distin-guish the important applicable documents from useless data. For example,Matson37 describes a case where the opponents flooded the document depositorywith irrelevant papers:


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The approach was the needle in the haystack in which the opponents suppliedtons of paper so overwhelming that my eyes became bloodshot reading only boxlabels. I spent all my time searching for meaningful documents in the flood of paper.

Building Codes. A copy of the applicable building code in effect at the time of con-struction should be reviewed. It has been argued that the standard of care is simplyto perform work in accordance with the local building code. While performingwork in accordance with building codes may reduce potential liability, it is still pos-sible that in a court of law a design engineer will be held to a higher standard. Forexample, Shuirman and Slosson38 state that, in many jurisdictions, the buildingcodes and code enforcement may not meet current professional standards, anddesign engineers cannot rely on building codes to indemnify them from liability.

Technical Documents. During the course of the forensic investigation, it may benecessary to check reference materials, such as geologic maps or aerial pho-


TABLE 16.5 Typical Documents That May Need to Be Reviewed for a ForensicInvestigation

Project phase Type of documents

Design Design reports, such as geotechnical reports, planning reports, andfeasibility studiesDesign calculations and analysesComputer programs used for the design of the projectDesign specificationsApplicable building codesShop drawings and design plans

Construction Construction reports, such as inspection reports, field memos, labora-tory test reports, mill certificates, etc.Contract documents (contract agreements, provisions, etc.)Construction specificationsProject payment data or certificatesField change ordersInformation bulletins used during constructionProject correspondence between different partiesAs-built drawings (such as as-built grading plans and foundation plans)Photographs or videosBuilding department permits and certificate of occupancy

Postconstruction Postconstruction reports, such as maintenance reports, modificationdocuments, reports on specific problems, and repair reportsPhotographs or videos

Technical data Available reports such as weather reports and seismic activityReference materials, such as geologic and topographic maps and aer-ial photographsTechnical publications, such as journal articles that describe similarfailures

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tographs. Other useful technical documents can include journal articles that maydescribe a failure similar to the one under investigation. For example, the ASCEJournal of Performance of Constructed Facilities deals specifically with con-struction-related failures or deterioration.

Analysis

Calculations or computer analyses may be needed to help evaluate different failurehypotheses or the potential for future damage. It may even be appropriate to modelthe failure, such as by using finite-element analyses (e.g., Refs. 39 and 40). A thor-ough analysis is especially important with projects involving lawsuits because oneobjective is usually to determine proportional responsibility. Based on the cause ofthe failure, the forensic engineer will be able to offer an opinion on who is respon-sible for the failure and proportion the responsibility between different parties.

In performing the analysis and in the development of conclusions, the forensicengineer may need to rely on the expertise of other forensic specialists. For exam-ple, geological studies are often essential in the investigation of landslides, rock-fall, and seismic activity.41

There can be many different causes of failure. Some of the more commoncauses of failure are related to inadequate subsurface exploration and laboratorytesting, technical deficiencies or design errors, specification mistakes, improperconstruction, and defective materials.42 The failure theory must be well thoughtout and based on the facts of the case. It is not unusual that the forensic engineersinvolved in a case will disagree on the cause of the failure. Common reasons for adisagreement on the cause of the failure include the loss of evidence during thefailure, the presence of conflicting test data, substantially differing eyewitnessaccounts of the failure, or just differences of engineering opinions.

TYPES AND CAUSES OF COMMONNONPERFORMANCE AND FAILURE

As previously mentioned, the most frequently encountered geotechnical condi-tions that cause damage to foundations and structures are settlement, expansivesoil, lateral movement, and deterioration. Each is discussed in this section.

Settlement

Settlement can be defined as the permanent downward displacement of the foun-dation. There are two basic types of settlement:

Settlement due directly to the weight of the structure. For example, the weight ofa building may cause compression of an underlying sand deposit or consolidationof an underlying clay layer.


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Table of ContentsPart III. Engineering Analysis of Structural Defects and Failures10. Loads and Hazards 11. Steel Structures 12. Concrete Structures 13. Masonry Structures 14. The Building Envelope15. Timber Structures 16. Structural Foundations Allowable Foundation Movement Foundation Movement and Severity of Damage Field Observation, Instrumentation, Testing, and Analysis Field Observation Instrumentation Inclinometers Piezometers Settlement Monuments or Cells Crack Pins Other Monitoring Devices

Testing Nondestructive Field Testing Destructive Field Testing Laboratory Testing

Document Search Reports and Plans Building Codes Technical Documents

Analysis

Types and Causes of Common Nonperformance and Failure Settlement Settlement of the Foundation Caused by Collapsible Soil Settlement of the Foundation Due to Limestone Cavities and Sinkholes Settlement of the Foundation Due to Consolidation of Soft and Organic Soil Settlement of the Foundation Due to Collapse of Underground Mines and TunnelsSettlement of the Foundation Due to Ground Subsidence from Extraction of Oil or Groundwater

Expansive Soil Expansive Soil Factors Laboratory Testing Surcharge Pressure

Lateral Movement Earthquakes Surface Fault RuptureLiquefaction Slope Movement and Settlement

Deterioration Sulfate Attack of Concrete Foundations Frost

Temporary and Permanent Remedial Repairs Reinforced Mat Reinforced Mat with Piers Partial Removal and/or Strengthening of FoundationsConcrete Crack Repairs Other Foundation Repair Alternatives

Retaining Walls Common Causes of Failures

Examples of Case Studies of Nonperformance and FailureReferences

17. Temporary Structures Index

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