(A) Subgrade soils shall be all materials used for subgrade including in-situ materials and fill materials. Subgrades for pavement shall be stabilized by mechanical compaction or by other methods approved in writing by the City Engineer. Stabilization methods such as fabrics and chemical stabilization may be submitted for approval when supported by engineering data and calculations to substantiate the adequacy of the stabilization procedure.
(B) The top 24 inches of the subgrade shall be a material not susceptible to frost action unless modified with cement, lime or another method approved specifically by the City Engineer to resist frost action (Soils classified as A-4 and A-5, including sandy silts, fine silty sand or lean clays are highly susceptible to frost action).
(C) In-situ soils meeting the requirements outlined in these specifications may be utilized as subgrade material. In-situ soils used as subgrade shall be scarified to a minimum depth of eight inches below finish subgrade, recompacted, and tested as described in § 90.800.5 of these specifications. Fill material for subgrade shall be placed in lifts not to exceed eight inches compacted depth.
(D) Methods and procedures for establishing the total depth of soil replacement and/or modification shall be specified by the design engineer and included in the project plans and specifications. The minimum depth of replacement shall be 24 inches in the absence of engineering data showing otherwise.
(E) A “bridge lift” is defined as material that meets the requirements of these standards and is utilized to span areas of unsuitable material that lie below the 24 inch subgrade requirement. Bridge lift depth shall be determined by a geotechnical firm but in no case shall the lift be less than 24 inches in depth. A bridge lift will be placed in one lift in its entirety or as otherwise directed by the geotechnical firm and approved by the City and will require a “wheel roll” test prior to construction of the final 24 inch subgrade. Additional bridge lift depth may be required by the city dependent on field conditions.
(F) The adequacy of in-situ soils and fill materials as pavement subgrade shall be evaluated based upon the soils classifications, liquid limit, plasticity index and California Bearing Ratio (CBR) values.
(G) All soils with a liquid limit greater than 40, or a plasticity index greater than 15, or a CBR value of less than eight shall be undercut and removed from the street section or improved by a designed method of stabilization accepted by the City Engineer.
(H) Soils with a CBR of eight or greater, and classified as GM or GC soil, shall be accepted as “Hillside” material and no further treatment or upgrade will be required.
(I) Subgrade compaction requirements including the moisture density requirements shall be shown both on the plans and in the specifications. (Compaction shall be a minimum of 95% standard proctor. Moisture content shall be + 3% optimum moisture unless otherwise supported by site specific geotechnical data and approved in writing by City Engineer).
(J) Sampling and testing of subgrade materials shall be as set forth in § 90.500.8 of these standards.
(K) Pavement designs that utilize a subbase course shall include test data and specifications for the subbase material in the calculations submitted to the City Engineer for review and approval.
(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)