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57 CHAPTER 3 Findings 3.1 Design and Construction driven piles were used by 59%, and drilled foundations were State of Practice used by 24%. The use of shallow foundations was not changed overall relative to the last survey (conducted under NCHRP 3.1.1 Questionnaire and Interviews Project 12-66). There is a consistent trend, however, in the Code development requires examining the state of practice in decrease of the use of driven piles--75%, 62%, and 59% for design and construction in order to address the needs, research 1999, 2004, and 2007, respectively--and the increase of the the performance, and examine alternatives. The identification use of drilled foundations--11%, 21%, and 24% for 1999, 2004, of current design and construction methodologies was carried and 2007, respectively (1999 data from Paikowsky et al., 2004; out via a questionnaire. A six-page questionnaire concerning 2004 data from NCHRP Project 12-66). There is some dis- the design and construction practices of highway departments crepancy between the total foundation use and the percent- was developed and distributed in June 2007 to 161 state high- age of use specifically addressing piers and abutments. Some way officials, TRB representatives, state and FHWA bridge of this discrepancy can be attributed to the fact that all foun- engineers, and bridge engineers from Canadian Provinces. dations include non-bridge structures like buildings, posts, Appendix C provides a copy of the questionnaire. and sound barriers. The average use presented above changes significantly across the country as shown in Table C-2 that relates to bridge foundations only (with average use of 17.7% 3.1.2 Summary of the for abutments and piers). The use of shallow foundations in Questionnaire Response the Northeast exceeds by far the use of shallow foundations A total of 40 surveys was returned and analyzed (39 states in all other regions of the United States--40% in New York, and 1 Canadian province, see Table C-1 in Appendix C). The New Jersey, and Maine; 47% in New Hampshire; 50% in survey elicited information concerning foundation alternatives Vermont; 53% in Massachusetts; 65% in Pennsylvania; and and shallow foundation design. The questionnaire was fol- 67% in Connecticut. Other "heavy users" are Tennessee (63%), lowed by telephone interviews with geotechnical engineers of Washington (30%), Nevada (25%), and Idaho (20%). In selected states determined based on information gathered in contrast, out of the 39 responding states, 6 states do not use the responses. Appendix C provides a summary of the responses shallow foundations for bridges at all, and an additional obtained for the questionnaire in the form of two summary 8 states use shallow foundations in 5% or less of highway bridge tables and a summary of the responses. The original form was foundations. used as a basis for the summary encompassing all responses. The percent (%) values provided relate to the arithmetic aver- 3.1.4 Summary of Findings--Subsurface age of the responding states and province (Alberta, Canada) Conditions for Shallow Foundations for the specific item. The summary provided in Appendix C indicates that 55.8% of shallow foundations are built on rock (average of piers 3.1.3 Summary of Major Findings-- and abutments) with an additional 16.8% on Intermediate Foundation Alternatives Geomaterial (IGM); hence, 72.6% of foundations are built Among survey respondents, the use of foundations by on rock or cemented soils and only 27.4% are built on soils type was the following: shallow foundations were used by 17%, (24.2% are built on granular soils and 3.2% are built on clay
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58 or silt). A further breakdown is presented in Table C-2, allow- and Hoek and Marinos (2000). Two states commented on ing clarification of the practices of the different states. For using GSI instead of RMR. example, Michigan indicated that 50% of its shallow foun- 5. Sixty percent (60%) evaluate failure by sliding for footings dations at the piers' location are built on fine-grained soils; on rock. Seven states do not evaluate sliding because of however, Michigan is using only 5% of its pier foundations on a requirement to "wedge" the foundation into the rock shallow foundations; hence, only 2.5% of the pier foundations either by a key (Alabama--1 to 2 ft, Alaska--1.5 to 2.0 ft, are built on clay or silt. Examining all the states this way sug- North Carolina) or some other method (Iowa--notched in gests that the state leading in building bridge foundations on rock, Minnesota--using dowels, Pennsylvania--footings clay is Washington (6%) followed by Vermont (5%), Idaho embedded 1 ft below top of rock, and Maryland--"seat" (4%), and Michigan and Nevada (3.75%) each. Further exam- footings in the rock). Those that evaluate sliding use various ination of these facts (in a telephone interview) revealed that methods and margins of safety (): Idaho-- = 0.5, Ohio Washington's use of foundations on silt and clay refers to highly and Indiana--factor of safety = 1.5, New Hampshire-- densified glacial soils with SPT N values exceeding 30 for silts F.S. = 1.5 and = 0.8, Washington--F.S. = 1.5 and = 0.67, and between 40 to 100 for the clays. Alberta Canada-- = 0.8 (friction) and = 0.6 (cohesion). Twenty-eight states (out of 39) do not build shallow foun- Maine specified that sliding for Strength I is done by using dations for bridges on cohesive soils at all; hence, only 0.8% minimum vertical load and maximum horizontal load and of all bridge shallow foundations are built on clay or silt = 0.8 (based on footings on sand). Nevada specified that (including Washington), in comparison to 16.9% on rock, they use the limit equilibrium method per FHWA "Rock 5.4% on IGM, and 12.2% on frictional soils. The survey also Slopes" with superimposed foundation loading. F.S. = 1.5 suggests that only about 60% of the foundations on clay were for static conditions and F.S. = 1.1 for seismic. built without ground improvement measures; hence, only 6. Seventy percent (70%) of the states do not analyze lateral about 0.48% of the bridges were actually built on shallow displacement of shallow foundations on rock because they foundations on cohesive soils, practically a marginal number use limiting measures (key way, dowling, etc.) as described above. New York specifies geologic inspection during con- considering the state of these soils as described by Washington struction to ensure rock quality, and key way or dowelling State Department of Transportation (WSDOT). is ordered if necessary. 7. Seventy-five percent (75%) of the responding states limit the eccentricity of footings on rock. Most of the states 3.1.5 Summary of Findings-- follow AASHTO recommendations for e/B 3/8. Some Design Considerations (Idaho, Iowa, Michigan, North Carolina, Ohio, Wisconsin, 184.108.40.206 Foundations on Rock and Massachusetts) use e/B 1/4 based on the FHWA "Soils and Foundations Manual" that also meets the AASHTO Findings for foundations on rock are the following: standards specification. Wyoming, South Dakota, and Alberta (Canada) use e/B 1/6, with Alberta specifying that 1. About 90% of the states using foundations on rock either eccentricity is maintained within limits or an effec- obtain rock cores, evaluate RQD, and conduct uniaxial tive foundation size is used in which the dimensions are (unconfined) compressive strength tests. reduced by twice the eccentricity (e.g., B = B - 2e). 2. About 19% of the states using foundations on rock use 8. Seventy percent (70%) of the states do not analyze settlement presumptive values alone, 22% use engineering analyses of footings on rock as it is not seen as an issue of importance alone, and 59% use both when evaluating bearing capacity. and the settlement is limited to 0.5 in. Twenty-eight percent 3. Fifty-three percent (53%) of the states use AASHTO's pre- (28%) use AASHTO procedures for broken/jointed rock, sumptive values. Other states use or consult the Canadian with Nevada also using Kulhawy and Goodman (1987) Foundation Engineering Manual (2006), NY Building Code and the Army EM 110-1-2908 (1994). (International Code Council, 2008), or NAVFAC (1986), or base their capacity values on local experience (e.g., South 220.127.116.11 Foundations on Soil Dakota, Wisconsin, Oregon, Kansas, Iowa, and Arkansas). 4. Seventy percent (70%) of the responding states would like Findings for foundations on soil are the following: to see a specific analytical method presented for the eval- uation of the bearing capacity of foundations on rock. 1. All states using shallow foundations on soils follow either Twenty-five percent (25%) use the Kulhawy and Goodman AASHTO's LRFD or ASD guidelines. Only a small number (1987) analytical method and 33% use the Carter and of responders use presumptive values. Fifty-eight per- Kulhawy (1988) semi-empirical design method. Others cent (58%) use the theoretical general bearing capacity use Kulhawy and Goodman (1980), Hoek-Brown (1997), equation.
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59 2. Fifty-three percent (53%) of the responders find it reason- mented that the resistance factors are in line with the able to omit the load inclination factors and 63% limit factor of safety range (2.5 to 3.0) used in the ASD method- the eccentricity of the footing mostly with e/B 1/6 to 1/4 ology and hence result in a design similar to that obtained (standard specifications e/B = 1/6, LRFD specifications using ASD. e/B = 1/4). Massachusetts responded that load inclination 8. Seventy percent (70%) evaluate failure by sliding, with factors must be used in the final design of the footing. about half (33%) using the full foundation area and 30% Pennsylvania commented that when inclination factors using the effective foundation area. were considered together with factored loads, it resulted in 9. Only 13% consider passive resistance for the lateral resist- an increased footing size; hence, unfactored loads are used. ance of the shallow foundations and all utilize a limited 3. Forty-five percent (45%) do not decrease the soil's strength value due to a limited displacement. Many responding parameters considering punching shear, while 23% do so. states expressed concern with a long-term reliance on a Seven states commented that punching shear is not a viable passive resistance. Washington commented that it is rarely option as foundations are not built on loose soil conditions used to meet the sliding criterion of extreme events, and or, alternatively, settlement criteria prevail, especially Minnesota commented it is used in front of shear keys only. under such conditions. 10. Traditionally no safety margin is provided to settlement 4. Fifty-eight (58%) use the AASHTO procedures presented analysis although it typically controls the size of shallow for footings on a slope. Nevada, Idaho, and Michigan foundations. When asked about it, 35% answered that commented that the charts are not clear and need to be the issue should not be of concern and 25% answered that improved. Washington and North Carolina commented it should. Of those who responded, some recognized on the use of Meyerhoff's method, also presented by the that safety margin needs to be researched (Connecticut, Navy Design Manual (NAVFAC, 1986), essentially iden- Michigan, and Tennessee) while others hold the notion tical to the AASHTO presentation. Oregon commented that a safety margin on bearing capacity already addresses that the provided foundations on slope analysis result in the issue (Hawaii, Maine, New Jersey, North Carolina, and a reasonable approach (somewhat conservative) while Washington) or that settlement calculations are conserva- Pennsylvania commented that experience shows that tive to begin with (New Hampshire and North Carolina). sometimes this analysis results in a drastically larger 11. Only two states stated that they conduct plate load tests: footing. one state (Connecticut) referred to tests from over 20 years 5. Thirty percent (30%) of the responding states do not use ago, and the other state referred to three recent tests the AASHTO procedures for footings on a layered soil, (Massachusetts). while 38% of the responders do use these procedures. 12. When asked to comment on any related subject, 13 states Eighteen states commented on the procedures. Idaho, responded. A major concern expressed by Michigan was Michigan, Vermont, and Wisconsin commented that they written by a bridge designer referring to the difficulties in calculate the bearing capacity for the layer with the lower using effective width for bearing capacity calculations strength. Iowa and Oregon commented that under such as it requires iterations for each load case for service and conditions alternative foundation solutions are examined. strength. Moreover, the division of responsibilities between 6. Only 28% (with 40% responding "No") of the respon- the geotechnical section (providing allowable pressure) and ders use the semi-empirical procedures described in Sec- structural section (examining the final design iteratively) tion 10.6.3.1.3 of AASHTO's LRFD Bridge Specifications is a source for problems. The engineer proposes allowable for evaluation of bearing capacity. The majority of the contact stresses for service and strength based on gross states that commented on the procedure expressed the footing width and eccentricity limited to B/6. (The issue opinion that the method is used for a rough evaluation, of "allowable" to ULS is not so clear and the engineer was only as an initial estimation and/or in comparison to other contacted.) methods. Oregon commented that the SPT method usually yields higher capacity and settlement controls the design. 3.1.6 Telephone Interviews 7. Nineteen states responded when asked for comments 18.104.22.168 Overview about the currently existing resistance factors being all about the same value. Some states stated that they don't Engineers of seven states were interviewed to obtain com- have enough experience with LRFD to judge the resistance plementary information and enhance understanding of the factor values. North Carolina and New Hampshire sug- state of practice of shallow foundation design and construction. gested combining all resistance factors to be 0.45, while All the interviewed states were selected due to their exten- Oregon, Pennsylvania, Vermont, and Washington com- sive use of shallow foundations and/or specific usage that
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60 required further investigation. Six of the interviews are sum- Chapter 5, Figures 5.2 to 5.4). No settlement on rock is eval- marized below. uated; anchors and dowels are being used but not keys. 22.214.171.124 Connecticut--Interview with Leo Fontaine, 126.96.36.199 Pennsylvania--Interview with Beverly Miller, Transportation Principal Engineer Bureau of Design Connecticut is the leading state among the responding The extensive use of shallow foundations in Pennsylvania states (39) in the use of shallow foundations (66% of bridge (65%) is attributed to the combination of subsurface conditions foundations). This fact was attributed by Transportation Prin- (rock or stiff soil at a shallow depth) and economic competitive- cipal Engineer, Leo Fontaine, to the longstanding high-quality ness. The design is commonly based on an in-house design engineering traditions established by Phillip Keene and Lyle manual (Pennsylvania DOT, Publication Number 15M, April Moulton that, along with sound economics, lead to the pre- 2000 edition, Part 4, Volume 1 of 2) and a software package vailing use of shallow foundations. Connecticut design practice (ABLRFD by PDT and Ibsen & Assoc., Inc.). for foundations on rock include unconfined rock testing, RQD About 60% of shallow foundations are built into rock, evaluation, and bearing capacity calculations followed by the embedded 1 ft into the rock. As a result, it is not required that use of predominantly presumptive values (typically 5 to 6 tsf), sliding be checked. About 33% of the foundations are built on mostly due to lack of confidence in the rock variability. Hence, granular material with no shallow foundations being built Fontaine sees a great need for the calibration of the design on cohesive soils. Cohesive soils would be either excavated methods based on a database. (approximate depth of up to 10 ft) or penetrated by piles. Connecticut's design practice for foundations on soil refers The bearing capacity of foundations on rock is calculated mostly to frictional soils as the construction schedule prevents utilizing Goodman (1989) and Carter & Kulhawy (1988) with = 0.55, relying on good past experience with both methods. building foundations on soft soils using the conventional Pennsylvania, according to Beverly Miller at the Bureau of approach (e.g., preloading), and the use of ground improve- Design, would very much like to see the methods being cali- ment techniques was found to be less attractive than the use brated. Presumptive values are rarely used and only used for of deep foundations in such cases. The design process of the comparison. Inclination factors are not used, and the design shallow foundations mostly includes SPT, internal friction is based on unfactored loads because the use of factored loads angle, bearing capacity analysis without inclination factors, resulted in unreasonably large foundations compared to past and then settlement evaluation that controls the foundation experience. Pennsylvania makes use of shallow foundations size. The procedure is completed by checking bearing capacity in water using protective measures. Abutments are built below again with the foundation size dictated by the settlement construction scour, and piers are built below construction analysis. For settlement analysis, service load is used without a scour and use rip rap to mitigate for half of the local scour. safety margin, and based on past performance, Connecticut feels comfortable with the process. 188.8.131.52 Tennessee--Interview with Edward Wasserman (Director of Structures Division), 184.108.40.206 Massachusetts--Interview with Nabil Hourani, Len Oliver, and Vanessa Bateman Chief Geotechnical Engineer (Soils and Foundations) Massachusetts is one of three states using the highest portion A large portion of Tennessee has relatively shallow soil depth of shallow foundations in bridge structures (53%), along with to rock. Similar to Pennsylvania, the practice in Tennessee is Connecticut (66%) and Pennsylvania (65%). When design- to excavate foundations to a depth of about 10 ft and use end ing foundations on rock, Massachusetts uses Goodman's bearing piles for soil depths exceeding 12 ft. method, which according to the accumulated experience, cor- The practice in Tennessee is to use capacity analysis on relates well with both test results, unconfined and point load rock based on AASHTO's Standard Specifications for Highway tests. Massachusetts does not use presumptive values and Bridges (1997), utilizing unconfined test results and being would like to see the uncertainty of the design methodology sensitive to the large variation in limestone strength and (i.e., Goodman) evaluated and calibrated for LRFD. possible karst phenomena. Presumptive values are used in Foundation design follows the AASHTO recommendations locations where good data are not available (e.g., drilling is for the range of eccentricity limitation. The values, according not possible) or the tests are inconclusive. The Navy Design to Nabil Hourani, Chief Geotechnical Engineer, were obtained Manual (NAVAC, 1986) values are then used, being overall from the load factor design methodology as presented in similar to AASHTO's values. When the rock is highly fractured NCHRP Report 343 (Barker et al., 1991, Part 3 [Kim et al., 1991], such that it controls the strength, shallow foundations are not