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Jesse Jerabek JN 15044
<br /> March 30, 2015 Page 4
<br /> (percent passing the No. 200 sieve) of less than 3 percent and a sand content (percent passing the No. 4
<br /> sieve) of no more than 10 percent. As noted by the American Concrete Institute (ACI) in the Guides for
<br /> Concrete Floor and Slab Structures, proper moisture protection is desirable immediately below any on-grade
<br /> slab that will be covered by tile, wood, carpet, impermeable floor coverings, or any moisture-sensitive
<br /> equipment or products. ACI also notes that vapor retarders, such as 6-mil plastic sheeting, are typically used.
<br /> A vapor retarder is defined as a material with a permeance of less than 0.3 US perms per square foot(psf)per
<br /> hour, as determined by ASTM E 96. It is possible that concrete admixtures may meet this specification,
<br /> although the manufacturers of the admixtures should be consulted. Where plastic sheeting is used under
<br /> slabs, joints should overlap by at least 6 inches and be sealed with adhesive tape. The sheeting should
<br /> extend to the foundation walls for maximum vapor protection. If no potential for vapor passage through the
<br /> slab is desired, a vapor barrier should be used. A vapor barrier, as defined by ACI, is a product with a water
<br /> transmission rate of 0.00 perms per square foot per hour when tested in accordance with ASTM E 96.
<br /> Reinforced membranes having sealed overlaps can meet this requirement.
<br /> In the recent past, ACI (Section 4.1.5) recommended that a minimum of 4 inches of well-graded compactable
<br /> granular material, such as a 5/8 inch minus crushed rock pavement base, should be placed over the vapor
<br /> retarder or barrier for protection of the retarder or barrier and as a "blotter"to aid in the curing of the concrete
<br /> slab. Sand was not recommended by ACI for this purpose. However, the use of material over the vapor
<br /> retarder is controversial as noted in current ACI literature because of the potential that the protection/blotter
<br /> material can become wet between the time of its placement and the installation of the slab. If the material is
<br /> wet prior to slab placement, which is always possible in the Puget Sound area, it could cause vapor
<br /> transmission to occur up through the slab in the future, essentially destroying the purpose of the vapor
<br /> barrier/retarder. Therefore, if there is a potential that the protection/blotter material will become wet before the
<br /> slab is installed, ACI now recommends that no protection/blotter material be used. However, ACI then
<br /> recommends that, because there is a potential for slab cure due to the loss of the blotter material, joint
<br /> spacing in the slab be reduced, a low shrinkage concrete mixture be used, and "other measures" (steel
<br /> reinforcing, etc.) be used. ASTM E-1643-98 "Standard Practice for Installation of Water Vapor Retarders
<br /> Used in Contact with Earth or Granular Fill Under Concrete Slabs" generally agrees with the recent ACI
<br /> literature.
<br /> We recommend that the contractor, the project materials engineer, and the owner discuss these issues and
<br /> review recent ACI literature and ASTM E-1643 for installation guidelines and guidance on the use of the
<br /> protection/blotter material. Our opinion is that with impervious surfaces that all means should be undertaken
<br /> to reduce water vapor transmission.
<br /> EXCAVATIONS AND SLOPES
<br /> Excavation slopes should not exceed the limits specified in local, state, and national government safety
<br /> regulations. Temporary cuts to a depth of about 4 feet may be attempted vertically in unsaturated soil, if there
<br /> are no indications of slope instability. However, vertical cuts should not be made near property boundaries, or
<br /> existing utilities and structures. Based upon Washington Administrative Code (WAC) 296, Part N, the
<br /> unsaturated, dense native soil at the subject site would generally be classified as Type A. Therefore,
<br /> temporary cut slopes greater than 4 feet in height cannot be excavated at an inclination steeper than 0.75:1
<br /> (Horizontal:Vertical), extending continuously between the top and the bottom of a cut. For overall slope
<br /> heights of less than 12 feet, the cut can manifest itself as a 4-foot vertical cut at the toe of a 1:1 (H:V)slope.
<br /> The loose to medium-dense fill and near-surface soils would be considered Type B soil and should not be
<br /> excavated any steeper than a 1:1 (H:V)inclination.
<br /> The above recommended temporary slope inclination is based on the conditions exposed in our explorations,
<br /> and on what has been successful at other sites with similar soil conditions. It is possible that variations in soil
<br /> and groundwater conditions will require modifications to the inclination at which temporary slopes can stand.
<br /> Temporary cuts are those that will remain unsupported for a relatively short duration to allow for the
<br /> construction of foundations, retaining walls, or utilities. Temporary cut slopes should be protected with plastic
<br /> sheeting during wet weather. It is also important that surface water be directed away from temporary slope
<br /> cuts. The cut slopes should also be backfilled or retained as soon as possible to reduce the potential for
<br /> instability. Please note that sand and/or loose soil can cave suddenly and without warning. Excavation,
<br /> foundation, and utility contractors should be made especially aware of this potential danger. These
<br /> GEOTECH CONSULTANTS, INC.
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