Soils and Foundations Handbook phần 10

LATERAL LOAD RESISTANCE Critical lateral load and moment shall include the Design Wind required by the Department Policies including the 30% gust increase. Under the critical lateral load (typically computed by Structural Engineers) the following requirements shall be met: Deflections of panels, posts or top of barrier and deflections at the top of the auger cast piles shall meet the requirements specified in Section 32.6 of the Plans and Preparation Manual, January 2004. The minimum length of the auger cast pile shall be computed as the one meeting these requirements plus five feet or 20% of computed length, whichever is less. Computer programs such as LPILE, or COM624 shall be used to determine the deflections and rotations. k values in Sands. k values input into LPILE, or COM624 shall not exceed the following values, without lateral load tests: N (blows/ft) k (pci) 0-4 0-10 5-10 10-20 11-20 20-30 21-30 30-60 30-40 60-90 40-50 90-125 >50 125 Note: No distinction will be made between dry and submerged conditions. Friction Angles in Sand The following typical correlation may be used to estimate the soil friction angle, Ф: Ф = N/4 + 28 As an alternative, the procedure described in 6.1.1.5 Friction Angle vs. SPT-N shall be used. The maximum Ф value shall be limited to 35 degrees for silty sand and 38 degrees for clean sand, unless higher friction angles are statistically supported by laboratory shear strength test results. Clay Use the LPILE or COM624 program guideline to determine k and ε50 values. However, limit the properties of clay to stiff clay or weaker (design values for undrained shear strength shall not exceed 2000 psf and the ε50 shall not be less than 0.007), unless laboratory stress-strain measurements indicate otherwise. 148 Rock Rock material with N-values less than 10 blows / foot shall be modeled as sand. Rock material with N-values between 10 and 30 blows / foot shall be modeled as sandy gravel: Friction Angle, Ф = N/4 + 33 The maximum friction angle value shall be limited to 40 degrees, unless higher friction angles are statistically supported by laboratory shear strength test results. Rock material with N-values of 30 blows / foot or more: • Use the LPILE or COM624 program guideline to model p-y curves of weak rock. Modeling rock as stiff clay will be acceptable, provided reasonable conservatism in the selection of k and undrained shear strength are adopted. AXIAL LOAD RESISTANCE (will not normally control the design) Side Resistance in Sands Side resistance in cohesionless soils shall be computed by the FHWA Method (Beta Method) specified in the Publication FHWA-IF-99-025 (August, 1999) for drilled shafts as follows: fs = P’v βc β = β * N/15 where β ≤ β β = 1.5 – 0.135 (z)0.5 (z, depth in ft) where 1.2 ≥ β ≥ 0.25 β = 1.5 – 0.245 (z)0.5 (z, depth in meters) where 1.2 ≥ β ≥ 0.25 where fs = Ultimate unit side resistance The maximum value of fs shall be limited to 2.1 tsf, unless load test results indicate otherwise. P’v = Effective vertical stress Side Resistance in Rock: When limestone and calcareous rock cores are obtained for laboratory testing, ultimate unit side resistance shall be estimated as discussed in Appendix A. When rock cores and laboratory testing are not available, use the following approach: • If SPT N-value in rock is less than 10 blows / foot, assume sand behavior. • If SPT N-value in rock is greater than or equal to 10 blows / foot, use the following: fs = 0.1 N (tsf) where fs ≤ 5.0 tsf 149 Side Resistance in Clay Model inorganic clays and silts in accordance with FHWA methods. Shear strength values should be estimated from UU tests, unconfined tests, vane tests, etc. If only SPT tests are available, Consultants are expected to use reasonable judgment in the selection of undrained shear strength from correlations available in the literature. The shear strength of clay estimated from SPT-N values or CPT results shall not exceed 2000 psf, unless laboratory stress-strain measurements indicate otherwise. Side resistance shall be computed by the FHWA Method (Alpha Method) specified in the Publication FHWA-IF-99-025 (August, 1999) for drilled shafts as follows: fs = α Su where Su = Design undrained shear strength of clay (psf) α = A dimensionless correlation coefficient as defined below: α = 0 between 0 to 5 feet depth α = 0 for a distance of B (the pile diameter) above the base α = 0.55 for 1.5 ≥ Su/Pa α = 0.55 – 0.1 (Su/Pa – 1.5) for 2.5 ≥ Su/Pa ≥ 1.5 for Su/Pa > 2.5, follow FHWA Manual Figure B.10 Pa = Atmospheric pressure (2116 psf at 0 ft Mean Sea Level) Organic Soils Side resistance in any soil with an organic content greater than 5.0% by ASTM D 2974 shall be neglected. End Bearing Capacity End bearing capacity shall be neglected Factors of Safety To compute an allowable axial load, a minimum factor of safety of 2.0 shall be used for overturning loads. The service axial load shall not exceed this allowable load. For LRFD design, use a Load Factor in accordance with the latest AASHTO LRFD Bridge Design Specifications and a Resistance Factor of 0.6. DESIGN WATER TABLE For structures where the design is controlled by hurricane force wind loads, the design water table shall be at the ground surface. For load conditions not associated with hurricane force wind loads, the seasonal high water table estimated for the location shall be the water table used for computation of axial capacity and lateral load analysis. If no information is available to determine the 150 seasonal high water table, the designer will assume the water table at the ground surface. The foundation analysis shall include a justification for the selected design water level. SPT ENERGY CORRECTIONS SPT N values from automatic hammers may be corrected to account for higher energy as compared with safety hammer. The energy correction factor shall not exceed 1.24. USE OF CONE PENETROMETER TESTS If cone penetrometer test (CPT) is used in the geotechnical investigation, the cone resistance data shall be converted to SPT N-values. The converted SPT N-values shall in turn be used in the foundation design according to the methods indicated in the previous sections of these design guidelines. The correlation presented in FIGURE B1 shall be used in the conversion of the CPT cone tip resistance, Qc (tsf) to SPT N-values, based on mean particle size, D50 (mm) of the material. The use of design parameters that are less conservative than the values obtained from cone tip resistance to N-value correlations, and other sections of this document, shall be statistically supported by the results of high-quality laboratory tests and/or in-situ tests for the specific soil/rock deposits. 151 Figure B 1 REQUIRED COMPUTATIONS FOR GEOTECHNICAL REVIEW Reports, Shop Drawings, VECP submittals, and Design-Build submittals, shall include calculations and numerical program outputs of all the cases and loadings considered in the design. Copies of structural calculations indicating wind loads computations and structural deflections at the top of the wall (due to pole and panel bending) shall also be included in the geotechnical package of computations. 152 ... - tailieumienphi.vn