(A) Unless specifically authorized in writing by the City Engineer as an exception for a specific project, all pavement sections shall be designed in accordance with the AASHTO Guide for Design of Pavement Structures, latest edition.
(C) A minimum design period (traffic analysis/forecast) of 20 years shall be used for pavement section design. A formal traffic study with projections and supporting data shall be submitted for all street classifications to provide minimum ESAL criteria.
(D) All street designs shall use a factor of four and one half for the Initial (present) serviceability index. All street designs shall use a factor of two and one half for the terminal serviceability index.
(E) Subgrade soils shall be all materials used for subgrade including in-situ materials and fill materials. The investigation and evaluation of subgrade soils shall be an integral component of all pavement designs and shall include the following minimum requirements:
(1) Geotechnical. All testing and geotechnical work shall be provided by a firm approved by the City Engineer and provided at the expense of the developer or the design engineer. The geotechnical firm shall provide copies of all test results, reports, soils classifications and subsurface drainage requirements directly to the City Engineer.
(2) Sampling and testing.
(a) The investigation and sampling of soils shall conform to AASHTO T 86 (ASTM D420) or latest revision and test procedures referenced therein. The sampling of in-situ subgrade soils may be accomplished by boring or excavation of test pits. The minimum sampling and testing frequency shall be one density test, one liquid limit, one plasticity index, one gradation and soils classification and one CBR for each 500 feet of street or section thereof, or change in subgrade material, with a minimum of three sets of tests per project. The minimum depth of boring or excavation for in-situ materials shall be four feet below the top of the elevation of the final compacted subgrade. Additional depth shall be required when deemed necessary by the design engineer or the City Engineer.
(b) For import material, the minimum sampling and testing frequency shall be one density test, one liquid limit, one plasticity index, one gradation and soils classification and one CBR per supplier. Said test results shall be within 90 days of import placement and initial testing. If date of information provided is more than 90 days prior to import placement, new samples and testing will be required and results from field density tests will not be accepted until updated information provided.
(c) Additional sampling and tests will be requested when deemed necessary by the City Engineer. The specific locations for all additional samples shall be determined by the City Engineer's representative.
(3) Soil classification. Subgrade soils shall be classified in accordance with AASHTO system and the Unified Soil Classification system. All tests required for the classification of the soils shall be performed and reported unless specifically waived by the City Engineer.
(4) Load bearing strength. Load bearing strength of soils shall be determined by the California Bearing Ratio (CBR) test in accordance with AASHTO T 193 or ASTM D 1883. The frequency and location for samples for CBR tests shall be as noted in § 90.500.8(B) above with the specific sample for the CBR test taken at the proposed finished subgrade elevation.
(F) Subgrade support capacity for all pavements (resilient modulus for flexible pavements and modulus of subgrade reaction for rigid pavements) shall be determined from the load bearing strength (CBR) of the soils based upon the correlation contained in the AASHTO guide for the design of Pavement Structures except where other correlation data are approved in writing by the City Engineer.
(Ord. 2009-11-347, passed 11-3-09; Am. Ord. 2010-06-356, passed 6-1-10; Am. Ord. 2018-11-816, passed 11-6-18)