Agenda - Council - 10/26/2021 - Laserfiche WebLink

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cracking are cracks perpendicular to the roadway centerline or laydown direction. These are often caused by shrinkage of the pavement surface, reflective cracking from an underlying layer or top - down cracking. Alligator or fatigue cracking can be symptomatic of poor subgrade soils and/or inadequate pavement thickness. Photographs of the bituminous cores are presented in the Appendix. Aggregate Base An apparent aggregate base layer was observed below the pavements at each boring location. The apparent aggregate base appeared to contain little gravel and because of that it is identified as Possible Aggregate Base on the boring logs. Based on our observations the aggregate base or Possible Aggregate Base may not meet MN/DOT gradation specifications for Class 5 aggregate base. It is possible that the Possible Aggregate base was initially placed as new or virgin Class 5 aggregate base but has degraded over time. Subgrade Soils The borings generally encountered sandy subgrade soils including; poorly graded sand, poorly grades sand with silt and silty sand which correspond to the ASTM Classifications SP, SP-SM, SM, respectively. The sand soils (SP, SP-SM, SM) encountered in the borings are generally well suited for pavement and/or pipe support and the soil classified as SP and SP-SM are generally considered non -frost susceptible soils and are also free draining materials. The soil classified as SM can be moderately to highly frost susceptible. Frost susceptibility or frost heave refers to the soils ability to heave when frozen. Heave results from frost penetration and the formation of ice lenses within the soil. Heave can result in cracks in the pavement and reduced pavement life. The amount of heave depends, in -part on the available moisture in the subgrade and the subgrade soil type. Silt and clay rich soil are more likely to form ice lenses because of their high capillarity which enables them to draw up moisture. These soils are also slow draining materials and retain moisture. The frozen soil can also experience detrimental strength loss and settlement when they thaw. Soil borings SB-09 and SB-12 encountered buried topsoil and/or relatively thin layers of buried topsoil. The "buried topsoil" is generally a poor -quality soil for pavement support and typically do not recommend supporting pavements on topsoil, buried topsoil or other organic soils. The "buried topsoil" at soil boring SB-09 was encountered at a depth of about 91/2 feet below the ground surface and the laboratory organic content test yielded an organic content of 1 percent. Organic contents less that 2 percent are generally considered non -organic. Based on the organic content result and the depth below the pavement surface, it our opinion that the buried topsoil need not necessarily be removed. The apparent buried topsoil encountered near the surface in soil boring SB-12 may not necessarily need to be removed. However, if new storm sewer utilities will be installed in this area, we anticipate that the buried topsoil will be removed incidental to pipe installation. We do not recommend that the buried topsoil be reused as fill or backfill. Groundwater Ground water was encountered in 11 of the soil borings at depths ranging from about 5 to 10 1/2 feet below the ground surface. Groundwater was not encountered in the remaining borings while drilling and sampling or after removing the augers from the boreholes. We generally do not anticipate that groundwater will be encountered during shallow utility construction or reconstruction and generally do not anticipate that dewatering will be required. 9