Kimball.GEOreport 1.pdf

This is a Geotechnical Evaluation Report prepared for WSN & Associates, Inc. by Braun Intertec Corporation dated December 5, 2011, addressing the USFS 2011 Aquatic Passages project—specifically the reconstruction of Forest Road 304 crossing over Kimball Creek northeast of Grand Marais, Minnesota. The report documents subsurface conditions through two penetration test borings (KC-1 and KC-2) conducted to depths of 15.2 and 14.1 feet respectively, with groundwater measured at approximately 10 and 11 feet. The general geologic profile consists of poorly graded sand fill materials (existing road embankment) to approximately 10 feet depth, underlain by poorly graded sand with silt glacial till to termination depths. Fill penetration resistances ranged 11-52 blows per foot (BPF), indicating moderate to good compaction, while underlying sands showed medium dense to very dense characteristics with resistances between 18 and over 50 BPF. Auger refusal was encountered at 15.2 feet in KC-1 and 14.1 feet in KC-2, attributed to bedrock.

The report provides comprehensive design recommendations for the proposed bottomless box culvert structure with spread footings and walls. Key recommendations include: embedding footings 60 inches below channel bottom for scour protection; dewatering footing excavations; sizing spread footings for net allowable bearing pressure of 3,000 psf with 3.0 safety factor, anticipating total and differential settlements of less than 1 and 1/2 inches respectively; removing topsoils and organic materials from widening areas prior to embankment fill placement; designing pavements for R value of 70; installing subdrains behind abutment walls with perforated pipes in washed gravel; and configuring wall design for 50 pcf active earth pressure (allowing rotation) or 70 pcf at-rest conditions. The report also emphasizes that scour analysis should account for highly scourable poorly graded sand with silt soils, and recommends close field observations during construction, additional density testing (one test per 100 cubic yards of fill), compliance with Mn/DOT specifications and OSHA excavation requirements, and retention of geotechnical engineers for plan review and construction observation.