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61 used. Very often the foundation size is restricted by the strength 3.1.7 Major Conclusions of the concrete, which is a limiting value (10 to 15 tsf) compared with the rock's strength. Inclination factors are not used because Major conclusions are the following: no load details are available from the structures group at the 1. In many states, the geotechnical aspects of the founda- time of the design. When designing foundations for retaining tion design (bearing capacity, settlement, and sliding) are wall, the maximum eccentricity is assumed. being evaluated before all the loading details are avail- able. As such, load inclination and/or eccentricity cannot 3.1.6.6 Washington--Interview with be directly accounted for. Several approaches are taken Jim Cuthbertson, Chief Foundations Engineer to resolve the situation including (1) providing effective Washington's questionnaire response indicated a relatively foundation sizes so that final design sizes will include the common use of shallow foundations on silts and clay (6%), eccentricity effect (i.e., B = B + 2e); (2) assuming highest the highest of all responders. It was clarified that those soils eccentricity; and (3) providing unit bearing values, nomi- are glacier, compacted, highly densified soils, with silts hav- nal and factored. ing SPT N values of 30 to 40 and clays having SPT N values of 2. The vast majority of the shallow foundations used to sup- 40 to 100. These materials are in some ways IGMs and, hence, port bridges are founded on rock. Only various references skew the statistics presented of foundations on silt/clay. When are currently available in the specification. A need for calculating bearing capacity, cohesion is neglected and only a specific, detailed methodology and its calibration was frictional component is assumed. Foundations on rock and advocated by most states and all those interviewed. IGM are common (about 30%) and the use of the classical 3. Although most states do not use inclination factors in bearing capacity analysis leads to unrealistically high values, design, they examine the resistance to sliding, and once which are then limited to about 80 tsf ultimate capacity based the final foundation size is established (after settlement on experience. consideration), they check again for bearing capacity with Similar to the problem presented by Tennessee, in Washing- or without inclination factor (depending on the state). ton the geotechnical analysis is carried out before eccentricity 4. New foundations on soft, cohesive soils are rarely being values are available. This is resolved by providing foundation constructed. Some of the statistics in that regard were dimensions (width and length) that are required to be main- skewed due to referencing highly compacted cohesive soils tained as effective foundation sizes. When the final design (which border on being IGM) as regular cohesive soils. accounts for eccentricity, it results in foundation sizes that, after being reduced for eccentricity, end with the originally 3.2 Assembled Databases provided effective foundation sizes. This effective foundation width is used for settlement analysis calculations and sliding 3.2.1 Overview resistance. As the foundations are cast on grade, a full mobi- Section 2.1 presents the research plan for establishing data- lization of the friction angle is assumed. bases for shallow foundation load tests. Two major databases were established: 3.1.6.7 Maine--Interview with Laura Krusinski, Senior Geotechnical Engineer UML-GTR ShalFound07, which incorporates Databases I The extensive use of shallow foundations in Maine can be and II. This database is based on a database originally assem- attributed to rock close to the ground surface (especially in bled for NCHRP Project 12-66 and in its current scope coastal areas) and economic considerations. The foundations contains 549 case histories of which 409 would conform to are sized first based on presumptive values and then are checked what is described as Database I and 140 case histories would against the factored resistance. Maine is making an effort to conform to Database II. UML-GTR ShalFound07 will be obtain the references mentioned in the code and study them as discussed in Section 3.2.2. no details are provided in the specifications. Laura Krusinski, UML-GTR RockFound07, which is presented as Database III Senior Geotechnical Engineer, finds it useful to provide details and contains 122 case histories, 119 of which were used in of recommended design methods and calibrate them against the calibration. UML-GTR RockFound07 will be discussed a database. As with other states, in Maine the foundation in Section 3.2.3. design is carried out before loading details are available; hence, eccentricity is assumed not to exist. However, the foundation A summary of the major attributes of each database is pre- is later checked as part of the structural design. Krusinski also sented below. Additional statistics are presented for relevant sees a need for guidelines for footing embedment in 100-year analyses (e.g., see Section 3.5 for centric vertical loading on and 500-year scour events. shallow foundations in/on granular materials).

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62 3.2.2 UML-GTR ShalFound07 Table 22. Countries in which tests were The UML-GTR ShalFound07 database was expanded conducted and number from its original format of 329 cases (developed for NCHRP of test cases conducted Project 12-66) to contain 549 load test cases for shallow founda- in each country. tions, mostly on granular soils, and concentrating on load tests Country No. of cases to failure and/or loading other than centric vertical loads. The Australia 1 database was constructed in Microsoft Access 2003 format. Brazil 19 The bulk of the cases was collected and assembled from four Colombia 1 sources: (1) ShalDB Ver5.1 (updated version of Briaud and Croatia 1 France 60 Gibbens, 1997), (2) Settlement of Shallow Foundations on Gran- Germany 254 ular Soils, a report to the Massachusetts Highway Department India 6 by Lutenegger and DeGroot (1995), (3) a German test database Italy 56 Jamaica 1 compiled by DEGEBO (Deutsche Forschungsgesellschaft fr Japan 9 Bodenmechanik) in a set of volumes, and (4) tests carried out Kuwait 10 at or compiled by the University of Duisburg-Essen, Germany. Nigeria 3 Northern Ireland 1 Table 22 lists the countries in which the tests were carried Portugal 6 out and the number of related cases. The majority of cases South Africa 1 were tests carried out in Germany, the United States, France, Sweden 11 UK 14 and Italy. USA 84 Table 23 summarizes the database by classification based on Others 11 the foundation type, predominant soil type below the footing Total 549 base, and country. The foundation type was classified based on the footing width, which follows the convention utilized by Lutenegger and DeGroot (1995). The tests on footing widths less than or equal to 1 m (3.3 ft) were classified as plate load tests, A detailed list of input parameters in the database is presented widths between 1 m and 3 m (9.8 ft) were classified as small in Appendix D (see Table D-1). See Figure 47 for the site con- footings, widths between 3 m and 6 m (19.7 ft) were classified dition (e.g., a footing tested in an excavation or a footing on a as large footings, and widths greater than 6 m were classified as slope, etc.) and Figure 48 for the conventions of footing dimen- rafts and mats. "Mixed" refers to soil containing alternating sions and loading. Figures D-1 through D-13 in Appendix D layers of sand or gravel and clay or silt. "Others" refers to cases contain screen images of the UML-GTR ShalFound07 data- with either unknown soil type or with materials like loamy base in Microsoft Access. SearchModify, listed under Forms, scoria. The majority of the tests in the database are plate load allows the user to easily search/modify a footing case in the tests on granular soils. database. Table 23. Summary of UML-GTR ShalFound07 database. Foundation type Predominant soil type Total Country Sand Gravel Cohesive Mixed Others Germany Others Plate load tests 346 46 -- 2 72 466 253 213 B 3.3 ft (1m) Small footings 3.3 ft < B 9.8 ft 26 2 -- 4 1 33 -- 33 (3m) Large footings 30 -- -- 1 -- 31 -- 31 9.8 ft< B 19.7 ft (6m) Rafts & Mats 13 -- -- 5 1 19 1 18 B > 19.7 ft Total 415 48 0 12 74 549 254 295 Note: "Mixed" are cases with alternating layers of sand or gravel and clay or silt "Others" are cases with either unknown soil types or with other granular materials like Loamy Scoria 1m 3.3 ft

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63 SiteConditionID 40101 SiteConditionID 40102 SiteConditionID 40103 SiteConditionID 40104 SiteConditionID 40105 Figure 47. Footing dimensions and site details along with the associated SiteConditionID employed in database UML-GTR ShalFound07.

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64 x edge D = x2 edge A d = thickness Se-zz Ro-xx Ro-zz Di-yy edge B th Di-xx g Ro-yy len x3 = y = b2 edge C b3 = wid th x1 = z (a) x edge D = x 2 edge A d = thickness thickless FzSL MxxSL MzzSL FySL edge B th FxSL MyySL g len x3 = y = b2 edge C b3 = wid th x1 = z (b) Figure 48. Conventions for footing dimensions (a) and applied loads (b). 3.2.3 UML-GTR RockFound07 and/or square shapes. A majority of the circular footings are plates. All the rock sockets in the database are circular for which A database consisting of rock loading by small size inden- the end bearing capacity (tip resistance) could be isolated, sep- tation, shallow foundations, and drilled shafts (for which the arating it from the shaft resistance of the rock sockets. tip load-displacement relations were measured) was assembled. Figures 49 to 52 present the distributions of the foundation The database is composed of a total of 122 case histories from sizes for all cases--non-embedded and embedded footings 10 different countries. Thirty-nine of the cases were obtained and rock sockets, respectively. Table 24 presents a summary from a study by Zhang and Einstein (1998), and 31 cases were of the database cases used for the determination of the uncer- obtained from a study by Prakoso (2002) whereas the re- tainty of the bearing capacity analyses of foundations on rock. maining cases were searched for and found in the literature. Appendix E presents in detail the references that were used to In a final review, three of the footing cases were found to be build the rock foundation database along with the rock details tested over a rock that contained a clay seam and, hence, were and the foundation type. All 122 original cases are presented in excluded from the statistics used in the calibrations. The Appendix E with the three foundations omitted clearly marked. database developed for the study included footing field load The database has 30 non-embedded shallow foundations, tests conducted in pseudo rock, hardpan, fine-grained sedi- 28 embedded shallow foundations, and 61 rock sockets. Only mentary and igneous or volcanic rocks. The shallow foundation four of the shallow foundations have square shapes; the others case histories were subcategorized according to their embed- are circular. All 61 rock sockets are circular. The width or ment, differentiating between embedded (embedment depth diameter (B) of the shallow foundations range from 0.07 to D > 0) and non-embedded (D = 0) footings with circular 23 ft with an average (Bavg) of 1.98 ft. The Rock Sockets have a

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65 40 25 119 Rock sockets and Footing cases 28 Footing cases with D>0 Mean B = 2.58 ft 0.3 Mean B = 1.18 ft 0.8 COV = 1.594 COV = 0.926 20 30 0.25 0.6 No. of observations No. of observations 0.2 15 Frequency Frequency 20 0.15 0.4 10 0.1 10 0.2 5 0.05 0 0 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 0 1 2 3 4 5 Footing width, B (ft) Footing width, B (ft) Figure 49. Distribution of B (ft) for 119 case histories Figure 51. Distribution of B (ft) for 28 embedded in database UML-GTR RockFound07. footing case histories in database UML-GTR RockFound07. 10 30 Footing cases with D=0 12 Mean B = 4.20 ft 0.3 61 Rock socket cases COV = 1.799 Mean B = 2.47 ft 8 COV = 0.739 0.25 0.15 No. of observations 6 0.2 8 No. of observations Frequency Frequency 0.15 0.1 4 0.1 4 2 0.05 0.05 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 0 0 Footing width, B (ft) 0 2 4 6 8 10 12 Footing width, B (ft) Figure 50. Distribution of B (ft) for 30 non-embedded footing case histories in database UML-GTR Figure 52. Distribution of B (ft) for 61 rock socket RockFound07. case histories in database UML-GTR RockFound07.