9.1. SR 01-07-2013 - Laserfiche WebLink

Home Browse Search

BITUMINOUS PAVEMENT SUBGRADE PREPARATION AND DESIGN GENERAL Bituminous pavem mts are considered layered"flexible" systems. Dynamic wheel loads transmit high local stresses throuah the bituminous/base onto the subgrade. Because of this, the upper portion of the subgrade requires high strength/stability to reduce deflection and fatigue of the bituminous/base system.The wheel load intensity dissipates throughthe subgrade such thatthe high level of soil stability is usually not needed below about Tto 4'(depending on the anticipated traffic and underlying soil conditions).This is the primary reason for specifying a higher level of compaction within the upper subgrade zone versus the lower portion.Moderate compaction is usually desired below the upper critical zone,primarily to avoid settlements/sags of the roadway.However,if the soils present below the upper 3'subgrade zone are unstable, attempts to properly compact the upper 3' zone to the 100% level may be difficult or not possible. Therefore, control of moisture just below the 3' level may be needed to provide a non-yielding base upon which to compact the upper oubgradosoils. Long-term pavement performance is dependent on the soil subgrade drainage and frost characteristics.Poor to moderate draining soils tend to be susceptible to frost heave and subsequent weakening upon thaw.This condition can result in irregular frost movernents and"popouts,"as well as an accelerated softening of the subgrade.Frost problems become more pronounced when the subgrade is layered with soils of varying permeability.In this situation,the free-draining soils provide a pathway and reservoir for water infiltration which exaggerates the movements.The placement of a well drained sand subbase layer as the top of subgrade can minimize trapped water,smooth frost movements and significantly reduce oob~rudcaofteoiu .ln wet,layered and/or poor drainage situations,�olong-term pedonnuoce8uin should 6euiAoi§cor�. � If sand subbase"is placed, we recommend it be o "Select Granular Bonn*` which meets Mn/DOT Spccicu1ion 2i402B2. PREPARATION Subgrade preparation should include stripping surficial vegetation and organic soils.Where the exposed soils are within the upper"critical"subgrade zone(generally 2 '/;z' deep for"auto only"areas and Tdeep for"heavy duty"areas),they should be evaluated for stability. Excavation equipment may make such areas obvious due to deflection and rutting patterns.Final evaluation of soils within the critical subgrade zone should be done by test rollingwith heavy rubber-tired construction equipment,such as a loaded dump truck.Soils which rut or deflect I"or more under the test roll should be corrected by either subcutting and replacement;or by scarification,drying,and recompaction.Reworked soils and new fill should be compacted per the"Specified Density Method"outlined in Mn/DOT Specification 2105.317 1 (a minimum of 100%of Standard Proctor density in the upper 3'subgrade zone,and a minimum of 95%below this). SuhArude preparation ochedulio8 can be an important consideration.Fall and Spring seasons usually have unfavorable weather for soil dryin'g.Stabilizing non-sand subgrades during these seasons may be difficult,and attempts often result in onmppominiugtbepuvemnot quality.Where construction scheduling requires subgrade preparation during these times,the use ofa sand subbase becomes even more 6eoefioiulfor oon*ruotubddyreasons. SUBGRADE DRAINAGE If � layer used, it should be provided with a means of subsurface drainage to prevent water build-up. This can be in the form ufdruind\u lines which dispose into storm sewer systems,oruudcm into ditches. Where sand subbase layers include sufficient sloping, and water can migrate to lower m `drui dle \incuouuh* bmitedtofiu8er drains at the catch basins. Even if a sand layer is not placed, strategically placed draintile lines can aid in improving pavement performance. This would be most important in meoa where adjacent non-paved areas slope towards the pavement. Perimeter edge drains can aid in intercepting water which may infiltrate below the pavernent. OIREP016(02/01) AMERICAN ENGINEERING TESTING, INC.