5627 1ST AVE SE 2016-01-01 MF Import

.-. Geotechnical Consultation Services Page 4 _ 93-1397 Hannabrook Rock Wall - Lots 39 through 45 July 26, 1993 Nevertheless, the placement of large rocks simply by their weight in-place tends to provide some, undetermined, degree of gravity retention. Rock wall construction is a craft that is highly dependant on ihe coniractor's experience, capabilities and equipment. Because of this, and considering each rock used has a different shape and "ftt" to its neighbor, it is virtually impossible to "design" a rock wall. However, providing the recommendations contained in this letter and the attached Associated Rockery Contractors(ARC) Standard Rockery Construction Guidelines are adhered to,by virtue of the mass of the rock used and its mechanically interlocking nature, some undetermined degreo of gravity retention is achieved. In our experience, sound construction practice typically results in the production of a competent and long lasting protective rock wall. To help achieve this goal, we recommend , the ARC Standard Rockery Construction Guidelines be closely adhered to during construction. A copY oi the curreM Guidelines is attached for your information. If there should be a conflict between the recommendations in the guideline and this letter, this letter should govern. Excavatlon: As mentioned above, before starting any rock placement, the contractor will need to perform sufficient excavation of the soils along the toe of the slope to allow for the installation of the rock wall. Typical practice is to excavate a keyway of about eighteen (18) inches in depth, as described below. The.excavated keyway material, as weil as the slough from in front of the cut face, will need to be removed from the sRe and disposed in accordance with the appropriate local regulations. Alternatively, it may be used elsewhere on the property as a compacted struc[ural fill, if desired. Keyway: The keyway excavation for the rock wall should extend at least eighteen (18) inches below the final site grade down into the underlying competent native soil. k should be wide enough to place the basai rock and the filter drain rock layer, in this case a distance of between about five and six feet. The surface should slope gently back to the rear of the keyway to provide some additional stabiliry and drainage control. The keyway subgrade should be proofrolled to tighten up the exposed surface. Any soft or potentially unstable areas should either be overexcavated and replaced with a compacted structural fill (described Iater in this letter) or, the area shouid be overlain with a layer of about ten (10) to twelve (12) inches of quarry spalls (two to four inch size coarse crushed rock or crushed concrete). The spalls should then be tamped firmly into the subgrade to tighten it sufficiently to support the rock wall load. Basal Rock: Once the keyway has been prepared,the basal row of rock for the lower stage of the wall should be set and firmly tamped into place. Because of the location of this protective rock wall,considering the wall's proposed maximum height, and given that the area .