Geotechnical Investigation <br />Capital Hill PUD <br />Eugene, Oregon <br />5.6 Pavement Design Recommendations <br />The correlated California Bearing Ration (CBR) for the clay below the topsoil is 3, which is a "poor" classification. <br />However, following the completion of roadway subgrade preparation as described in Section 5.1 a correlated CBR of <br />at least 5 is expected. Based on calculated roadway sections for similarly sized developments and the guidance of the <br />1993 American Association of State Highway and Transportation Officials (AASHTO) Guide for Design of Pavement <br />Structures, and the 2011 ODOT Pavement Design Guide we recommend a pavement section of a minimum of 4-inches <br />of asphalt concrete (AC) pavement over a minimum of 12-inches of compacted base rock for roadway sections. We <br />understand that the minimum new public street section for public roadways in the City of Eugene is 6-inches of AC <br />over 12-inches of base rock which will be adequate for the public roadways within the site. <br />For private driveway and shared roadways accessing lots not adjacent to Capital Drive the structural roadway section <br />may be decreased to 3-inches of AC over a minimum of 10-inches of compacted aggregate. <br />The above recommended structural pavement sections are designed for the type of vehicle use on the site after <br />construction completion, not for construction vehicle traffic which is generally heavier, occurs over a short time, and <br />impacts the site before full pavement sections are constructed. The construction traffic may cause subgrade failures <br />and the site contractor should consider over-building designated haul routes through the site to mitigate soft areas at the <br />time of final paving. <br />Pavement subgrades shall be observed and proof-rolled prior to placement of base rock, and the base rock shall be <br />compacted to at least 95% of the material's maximum dry density as determined by AASHTO T-180/ASTM D-1557 <br />(modified Proctor). The base rock shall be tested to measure compliance with this compaction standard prior to <br />placement of asphalt concrete. <br />5.7 Wet Weather/Dry Weather Construction Practices <br />The site materials are moderately moisture sensitive and will soften with prolonged exposure to precipitation. BEI <br />recommends that foundation subgrade preparation and general site earthwork be performed during the dry season, <br />generally May through October. Construction during the wet season may require special drainage considerations, such <br />as covering of excavations, pumping to mitigate standing water in footing excavations, and/or over-excavation of soft <br />soils. Control of surface water run-off during construction and moisture conditioning of structural fill material will <br />require additional measures during the wet season. <br />5.8 Seismic Site Classification <br />Based on the soil properties encountered in our site test pits and on-site well log information for structures founded on <br />the sandstone underlying the near surface clay, Site Class C (Table 20.3-1 ASCE 7) is recommended for design of site <br />structures. If the clay soil is to be left in place and foundations to bear on the soil overlying the sandstone material we <br />recommend using Site Class D (Table 20.3-1 ASCE 7). <br />Branch Engineering, Inc. <br />11 <br />