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Technical Report Documentation Page
- Report No. FHWA-SA-02- 2. Government Accession No. 3. Recipient’s Catalog No. 5. Report Date September 2002
- Title and Subtitle
GEOTECHNICAL ENGINEERING CIRCULAR NO. 6
Shallow Foundations
- Performing Organization Code
- Author(s) Robert E. Kimmerling
- Performing Organization Report No.
- Performing Organization Name and Address 10. Work Unit No. (TRAIS) PanGEO, Inc. 3414 N.E. 55th^ Street Seattle, Washington 98105
- Contract or Grant No. DTFH61-00-C-
- Type of Report and Period Covered Technical Manual
- Sponsoring Agency Name and Address Office of Bridge Technology Federal Highway Administration HIBT, Room 3203 400 Seventh Street, S.W. Washington, D.C. 20590
- Sponsoring Agency Code
- Supplementary Notes Contracting Officer’s Technical Representative: Chien-Tan Chang (HIBT) FHWA Technical Consultant: Jerry DiMaggio (HIBT)
- Abstract This document is FHWA’s primary reference of recommended design and procurement procedures for shallow foundations. The Circular presents state-of-the-practice guidance on the design of shallow foundation support of highway bridges. The information is intended to be practical in nature, and to especially encourage the cost-effective use of shallow foundations bearing on structural fills. To the greatest extent possible, the document coalesces the research, development and application of shallow foundation support for transportation structures over the last several decades. Detailed design examples are provided for shallow foundations in several bridge support applications according to both Service Load Design (Appendix B) and Load and Resistance Factor Design (Appendix C) methodologies. Guidance is also provided for shallow foundation applications for minor structures and buildings associated with transportation projects.
- Key Words Shallow foundation, spread footing, abutment, mat foundation, bearing capacity, settlement, eccentricity, overturning, sliding, global stability, LRFD
- Distribution Statement No Restrictions. This document is available to the public from the National Technical Information Service, Springfield, Virginia 22161
- Security Classification (of this report) 20. Security Classification (of this page) 21. No. of Pages 310
- Price
Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
NOTICE
The information in this document has been funded wholly or in part by the U.S. Department of Transportation, Federal Highway Administration (FHWA), under Contract No. DTFH61-00-C- 00031 to PanGEO, Inc. The document has been subjected to peer and administrative review by FHWA and has been approved for publication as a FHWA document.
The contents of this report reflect the views of the author, who is responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the policy of the Department of Transportation. This report does not constitute a standard, specification or regulation. The United States Government does not endorse products or manufacturers. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by either the author or FHWA.
- Chapters 7 and 8 discuss the application and special spread footing design considerations for minor transportation structures and buildings.
- Chapters 9 and 10 provide guidelines for procurement and construction monitoring of shallow foundations.
- Appendix A provides recommended materials specifications for embankments constructed to support shallow foundations. The appendix also includes example material specifications used by state highway agencies that design and construct spread footings in compacted structural embankments.
- Appendix B includes five worked design examples of shallow foundations for highway bridges based on Service Load Design methodology.
- Appendix C includes practical guidance on the use of Load and Resistance Factor Design (LRFD) methodology for shallow foundation design and reworks two of the design examples from Appendix B using LRFD.
This Circular was developed for use as a desktop reference that presents FHWA recommended practice on the design and construction of shallow foundations for transportation structures. To the maximum extent possible, this document incorporates the latest research in the subject matter area of shallow foundations and their transportation applications. Attention was given throughout the document to ensure the compatibility of its content with that of reference materials prepared for the other FHWA publications and training modules. Special efforts were made to ensure that the included material is practical in nature and represents the latest developments in the field.
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ACKNOWLEDGMENTS
Special thanks are given to the state highway agency personnel who contributed to the collection of data regarding the state of the practice in using shallow foundations for support of highway bridges. The Nevada, Michigan and Washington State Departments of Transportation are especially acknowledged for providing the case history data included in this Circular.
Notable and much appreciated contributions to the design examples included in this Circular were provided by Messrs. George Machan, Will Shallenberger and Kenji Yamasaki of Cornforth Consultants, Inc., Portland, Oregon, and by Ms. Tiffany Adams and Mr. Siew Tan of PanGEO, Inc., Seattle, Washington.
Particular gratitude is extended to Mr. Ronald G. Chassie, formerly with FHWA, Mr. Monte J. Smith of Sargent Engineers, Inc., Olympia, Washington, and Mr. James L. Withiam of D’Appolonia, Monroeville, Pennsylvania for their extremely helpful technical review, input and commentary provided on the document.
For her superior technical editing and outstanding positive attitude during the final production of this document, the author extends heartfelt thanks to Ms. Toni Reineke of Author’s Advantage, Seattle, Washington.
Last but not least, the author wishes to acknowledge the members of the Technical Working Group, Mr. Mike Adams; Mr. James Brennan; Mr. Myint Lwin; Mr. Mohammed Mulla; Dr. Sastry Putcha; Mr. Benjamin S. Rivers; Mr. Jésus M. Rohena; Ms. Sarah Skeen; Mr. Chien-Tan Chang, Contracting Officers Technical Representative; and Mr. Jerry DiMaggio for their review of the Circular and provision of many constructive and helpful comments.
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CHAPTER 1
INTRODUCTION
1.1 PURPOSE AND SCOPE
This Geotechnical Engineering Circular (GEC) coalesces more than four decades of research, development and practical experience in the application of shallow foundations for support of transportation structures. The document is intended to be a definitive desk reference for the transportation professional responsible for design, procurement and construction of shallow foundations for bridges and other transportation-related structures.
This GEC draws heavily from previous published work by the Federal Highway Administration (FHWA), State (and Local) Highway Agencies (SHAs) and other authors of practical guidance related to shallow foundations. As such, this document generally does not represent “new” research but is intended to provide a single reference source for state-of-the-practice information on the design and construction of shallow foundations.
The one exception to the foregoing statement is in the area of bridge support on shallow foundations bearing on compacted structural fills. Special attention has been given to case histories and design examples on the use of shallow foundations to support abutments in compacted approach embankments.
1.2 BACKGROUND
Shallow foundations represent the simplest form of load transfer from a structure to the ground beneath. They are typically constructed with generally small excavations into the ground, do not require specialized construction equipment or tools, and are relatively inexpensive. In most cases, shallow foundations are the most cost-effective choice for support of a structure. Your house is most likely supported on shallow spread footings, and you probably supported that deck you constructed last year on pre-cast concrete pier blocks because they were inexpensive and easy to place.
Bridges, however, are frequently supported on deep foundations such as driven piling. This may be as much a result of the continued use of past practice than for any other reason and has its roots not in highway construction, but railroads. The need to maintain constant and reliable grades over vastly differing ground conditions and topography made the choice to support virtually all railroad bridges on piles rather obvious. At the time of rapid rail expansion in North America, and all over the world, the concepts of soil mechanics and geotechnical engineering had not even been conceived. Railroad engineers needed a reliable way to support bridges and trestles, and the available technology directed them to driven piles.
As the pace of highway construction increased and eventually passed that of rail construction, the knowledge base for construction of bridges passed from the rail engineers to the highway engineers. It is likely that most of the early highway engineers were, in fact, ex-rail engineers, so it is not surprising that piling would be chosen to support highway bridges.
In many cases, and for very good reasons, pile support of transportation structures is wise, if not essential. Waterway crossings demand protection from the potentially disastrous effects of scour and other water-related hazards. Poor ground conditions and transient load conditions, such as vessel impact or seismicity, also may dictate the use of deep foundations such as piles. In more recent times, congested urban and suburban environments restrict the available construction space. The use of deep foundations, such as shafts, that can be constructed in a smaller footprint may be preferable to the costs associated with shoring, subterranean utility relocation, and right- of-way acquisition that would be necessary to construct a shallow foundation.
However, many transportation bridges are associated with upland development and interchanges. These locations are frequently removed from waterway hazards and are in areas with competent ground conditions. The engineers responsible for these structures, both geotechnical and structural, should be constantly on the lookout for the potential prudent and cost-effective use of shallow foundations.
The fortuitous combination of good, competent ground conditions and an available source of good-quality, granular fill material should always be seen as an opportunity to save bridge construction costs. By supporting the bridge abutments within the compacted approach fills, cost savings will be realized from the following:
- A shortened construction schedule
- Deletion of piles placed or driven through a good-quality, compacted fill to competent foundation materials
- Reduction in concrete and steel materials costs in the case where a spread footing was detailed, but bearing below the approach fill
Various studies on the use of shallow foundations yielded the following information, which underscores the potential cost savings that can result from the judicious use of shallow foundations:
- There are currently approximately 600,000 highway bridges in the United States. The average replacement cost is about $500,000 per bridge. About 50 percent of the replacement cost is associated with the substructure. Shallow foundations can generally be constructed for 50 to 65 percent of the cost for deep foundations (Briaud & Gibbens, 1995).
- The Washington State Department of Transportation (WSDOT) constructed more than 500 highway bridges between 1965 and 1980 with one or more abutments or piers supported on spread footings (DiMillio, 1982). WSDOT continues using shallow foundations supported in structural fills on a regular basis.
1.4 SHALLOW FOUNDATION SYSTEMS APPLICATIONS
Shallow foundations principally distribute structural loads over large areas of near-surface soil or rock to lower the intensity of the applied loads to levels tolerable for the foundation soils. Shallow foundations are used in many applications in highway projects when the subsurface conditions are appropriate. Such applications include bridge abutments on soil slopes or embankments, bridge intermediate piers, retaining walls, culverts, sign posts, noise barriers, and rest stop or maintenance building foundations. Footings or mats may support column loads under buildings. Bridge piers are often supported on shallow foundations using various structural configurations.
The next chapter describes common applications for transportation structure support on shallow foundations.
CHAPTER 2
TYPES OF SHALLOW FOUNDATIONS
2.1 I NTRODUCTION
The definition of a “shallow foundation” varies from author to author but generally is thought of as a foundation that bears at a depth less than about two times the foundation width. The definition is less important than understanding the theoretical assumptions behind the various design procedures. Stated another way, it is important to recognize the theoretical limitations of a design procedure that may vary as a function of depth, such as a bearing capacity equation. Common types of shallow foundations are shown in Figures 2-1 through 2-5.
2.2 I SOLATED SPREAD F OOTINGS
Isolated spread footings (Figure 2-1) are designed to distribute the concentrated loads delivered by a single column to prevent shear failure of the bearing material beneath the footing and to minimize settlement by reducing the applied bearing stress. The forces, strength and plan dimensions of the column may govern the minimum size of an isolated spread footing. For bridge columns, isolated spread footings are typically greater than 3 m by 3 m (10 feet by 10 feet). These dimensions will increase when eccentric loads are distributed to the footing. The size of the footing is a function of the loads distributed by the supported column and the strength and compressibility characteristics of the bearing materials beneath the footing. Structural design of the footing includes consideration for moment resistance at the face of the column and in the short direction of the footing, as well as shear and punching around the column.
Figure 2-1: Isolated Spread Footing
- Depending on the structure type, accommodate shrinkage and temperature movements within the superstructure
Spread footings with cantilever stemwalls are well suited to performing these multiple functions. The general arrangement of a bridge abutment with spread footing and cantilever stemwall is shown in Figure 2-3.
Figure 2-3: Spread Footing with Cantilever Stemwall at Bridge Abutment
2.4.2 Retaining Structures
The bases of semi-gravity concrete cantilever retaining walls (inverted “T” walls) are essentially shallow spread footings. The wall derives its ability to resist loads from a combination of the dead weight of the backfill on the heel of the wall footing and the structural cantilever of the stem (Figure 2-4).
2.4.3 Building Foundations
When a building stemwall is buried, partially buried or acts as a basement wall, the stemwall resists the lateral earth pressures of the backfill. Unlike bridge abutments and semi-gravity cantilever walls, the tops or the ends of the stemwalls are frequently restrained by other structural members (e.g., beams, floors, transverse interior walls, etc.) to provide additional structural resistance.
Figure 2-4: Semi-Gravity Cantilever Retaining Wall
2.5 COMBINATION FOOTINGS
Combination, or combined, footings are similar to isolated spread footings except that they support two or more columns and are rectangular or trapezoidal in shape (Figure 2-5). They are primarily used when the column spacing is non-uniform (Bowles, 1996) or when isolated spread footings become so closely spaced that a combination footing is simpler to form and construct. In the case of bridge abutments, an example of a combination footing is the so-called “spill- through” type abutment (Figure 2-6). This configuration was used during some of the initial construction of the Interstate freeways on new alignments where spread footings could be founded on competent native soils. Spill-through abutments are also used at stream crossings to make sure foundations are below the scour level of the stream.
Due to the frame action that develops with combination footings, they can be used to resist large overturning or rotational moments in the longitudinal direction of the column row.
There are a number of approaches for designing and constructing combined footings. The choice depends on the available space, load distribution among the columns supported by the footing, variations of soil properties supporting the footing and economics.
2.6 M AT FOUNDATIONS
A mat foundation consists of a single heavily reinforced concrete slab, which underlies the entire structure or a major portion of the structure. Mat foundations are often economical when spread footings would cover more than about 50 percent of the footprint of the plan area of a structure (Peck et al. 1974). A mat (Figure 2-7) typically supports a number of columns and/or walls in either direction or a uniformly distributed load (i.e., tank). The principal advantage of a mat foundation is its ability to bridge over soft spots and reduce differential movement.
Structures founded on relatively weak soils or lightweight structures may be economically supported on mat foundations. Column and wall loads are transferred to the foundation materials through the mat foundation. Mat foundations distribute the loads over a large area, thus reducing the intensity of contact pressures. Mat foundations are designed with sufficient reinforcement and thickness to be rigid enough to distribute column and wall loads, minimizing differential settlements and tolerating larger uniform settlements. Often the mat also serves as the base floor level of building structures.
Mat foundations have limited applicability for bridge support, except where large bridge piers, such as bascules or other movable bridge supports, have the opportunity to bear at relatively shallow depth without deep foundation support. This type of application may arguably be a deep foundation, but the design of such a pier may include consideration of the base of the bascule pier as a mat. Discussion of large mat foundation design is included in Section 8.6.
A more common application of mat foundations to transportation structures might include lightly loaded rest or maintenance facilities such as small masonry block structures or sand storage bins or sheds, or box culverts constructed as a continuous structure.
REINFORCED CONCRETE MAT
Figure 2-7: Typical Mat Foundation
2.7 SHALLOW FOUNDATIONS IN PERFORMANCE APPLICATIONS
High quality case histories regarding shallow foundations in transportation applications are difficult to obtain, largely because settlement data following construction are often not recorded. This makes it difficult to evaluate the performance of the foundation with respect to the settlement predictions made during design. Several relevant case histories are documented in “Performance of Highway Bridge Abutments on Spread Footings” (DiMillio, 1982). These case histories are not reproduced here. However, three new case histories are included to highlight design and construction practices in use by various state highway agencies that take advantage of the cost savings available by constructing spread footings in compacted engineered fills and in competent natural ground.
2.7.1 Case History No. 1: I-5 Kalama Interchange, Washington
Project Background
During Interstate construction in the 1960s and 1970s, the Washington State Highway Commission, later to become the Washington State Department of Transportation (WSDOT), constructed numerous bridges with foundation support by shallow spread footings, many of which were built on structural fills (DiMillio, 1982). This case history demonstrates the significant cost savings achievable through the process of thorough site investigation, laboratory testing, application of sound geotechnical engineering and follow-through during construction with the observational method (Peck, 1969). The observational method in this application included measuring settlement as a function of time during a settlement delay period following construction of the approach embankment. These data were used to evaluate the feasibility of using shallow foundations at the abutments and to predict long-term settlement potential for the abutment footings.
The project included design and construction of a 71-m (233-ft) long by 8.5-m (28-ft) wide, four- span, prestressed concrete structure that provides east-west traffic access across Interstate 5 in Kalama, a town in southwestern Washington State. The foundation elements are numbered as Piers 1 through 5. Note that WSDOT calls out abutments as piers, so Piers 1 and 5 are the abutments. The bridge layout and the location of subsurface exploration borings are shown in Figure 2-8. The bridge was constructed in 1968 and 1969.
Subsurface Conditions
The bridge alignment is underlain by approximately 30 m (100 ft) of alluvial sediments, consisting of interbedded loose to medium dense sand, soft to medium stiff silt and occasional peat layers. Bedrock was encountered directly below the alluvium. Summary logs of three exploratory borings drilled along the bridge alignment are shown in Figure 2-9. Standard Penetration Test (SPT) N-values and moisture content test results are also indicated on the profile.
Note : Dimensions in feet (1 ft = 0.305 m) Figure 2-9: Subsurface Profile, I-5 Kalama Interchange
TABLE 2-1: RESULTS OF ONE-DIMENSIONAL CONSOLIDATION TESTS,
I-5 KALAMA INTERCHANGE
Sample Soil Type Test Results
Test Hole 24U @ 2.9 m (Pier 1/West Abutment)
Gray sandy silt
Moisture = 46%; Moist unit weight = 16. kN/m^3 ; Compression Index Cc = 0.
Test Hole 28U @ 5.2 m (Pier 5/East Abutment)
Gray clay- silt
Moisture = 50%; Moist unit weight = 17. kN/m^3 ; Compression Index Cc = 0.
Test Hole 28U @ 16.8 m (Pier 5/East Abutment)
Gray clay- silt
Moisture = 49%; Moist unit weight = 16. kN/m^3 ; Compression Index Cc = 0.
The specification for Gravel Borrow included in Appendix A is currently used by WSDOT for granular fills beneath footings. This specification has not changed significantly since the time this bridge was built. A maximum allowable bearing pressure of 3 tsf was used for sizing the footings. This is the presumptive value used by WSDOT for fills constructed using Gravel Borrow. The actual design bearing pressures were 280 kPa (2.9 tsf) for Piers 1 through 4, and 244 kPa (2.55 tsf) for Pier 5.
Construction Sequencing and Performance
The preload fill was placed in two stages such that the foundation soils would gain strength during the Stage 1 preload operation and to reduce the potential for embankment instability. In mid-October 1967, Stage 1 preload fill was completed, and as much as 7.6 m (25 ft) of fill was placed. Stage 2 preload fill was placed about 6 months after completion of Stage 1. The total preload fill thickness was about 13 m (43 ft) and 14.6 m (48 ft) thick at abutment Piers 1 and 5, respectively. At the interior piers, the preload fill was about 7.6 m (25 ft) thick. Available data indicated that, in late June 1968, 0.34 m (1.1 ft) and 0.9 m (3.0 ft) of settlement had occurred under the preload fill at Piers 1 and 5, respectively. Settlement data for the interior piers preload fill are not available.
Subsequent to the preloading, footings and columns at Piers 3 through 5 were completed in August 1968. Construction at Piers 1 and 2 was delayed until November 1968 because 3 months earlier, when Piers 3 through 5 were constructed, settlement data indicated that the rate of settlement of the approach fills was still excessive.
The bridge deck was completed in December 1968. Settlement monitoring of the bridge deck was performed on a monthly basis for the following 4 months. At the end of the monitoring period in April 1969, 37 to 49 mm (0.12 to 0.16 ft) of settlement was measured at the abutments and 21 to 34 mm (0.07 to 0.11 ft) at the interior piers. Detailed settlement data are shown in Table 2-2 and are plotted in Figure 2-10. The settlement data show that vertical deflections at each pier are relatively uniform and differential settlement is small. Based on this data, the maximum angular distortion (δ′/l) between adjacent piers is less than 0.002.
The project was completed in June 1969, and the bridge has performed well since its completion more than 30 years ago. The bridge as it appears today is shown in Figure 2-11.
Costs
Using an inflation-adjusted cost of $20 per square foot for a prestressed girder bridge in a non- water crossing situation (source: WSDOT price records, 1975 data), the estimated contract amount for the EB-line under crossing was about $130,000. According to a project memorandum, the use of the spread footings instead of timber piles represented savings of approximately $25,000. The estimated savings were based on the assumption that a total of 124 timber piles 15 to 18 m (50 to 60 ft) in length would be adequate for supporting the bridge. The cost savings of deleting the piling therefore represented about 20 percent of the total cost of the structure.
