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<br /> Robert and Lisa Flake JN 16005
<br /> January 27, 2016 Page 6
<br /> SLABS-ON-GRADE
<br /> The building floors can be constructed as slabs-on-grade atop existing non-organic soils, or on structural fill.
<br /> The subgrade soil must be in a firm, non-yielding condition at the time of slab construction or underslab fill
<br /> placement. Any soft areas encountered should be excavated and replaced with select, imported structural fill.
<br /> Even where the exposed soils appear dry, water vapor will tend to naturally migrate upward through the soil to
<br /> the new constructed space above it. All interior slabs-on-grade must be underlain by a capillary break or
<br /> drainage layer consisting of a minimum 4-inch thickness of gravel or crushed rock that has a fines content
<br /> (percent passing the No.200 sieve)of less than 3 percent and a sand content(percent passing the No.4 sieve)
<br /> of no more than 10 percent. As noted by the American Concrete Institute (ACI) in the Guides for Concrete
<br /> Floor and Slab Structures, proper moisture protection is desirable immediately below any on-grade slab that
<br /> will be covered by tile,wood, carpet, impermeable floor coverings, or any moisture-sensitive equipment or
<br /> products.ACI also notes that vapor retarders,such as 6-mil plastic sheeting,are typically used. A vapor retarder
<br /> is defined as a material with a permeance of less than 0.3 US perms per square foot (psf) per hour, as
<br /> determined by ASTM E 96. It is possible that concrete admixtures may meet this specification, although the
<br /> manufacturers of the admixtures should be consulted. Where plastic sheeting is used under slabs,joints should
<br /> overlap by at least 6 inches and be sealed with adhesive tape. The sheeting should extend to the foundation
<br /> walls for maximum vapor protection. If no potential for vapor passage through the slab is desired,a vapor barrier
<br /> should be used. A vapor barrier, as defined by ACI, is a product with a water transmission rate of 0.00 perms
<br /> per square foot per hour when tested in accordance with ASTM E 96. Reinforced membranes having sealed
<br /> 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 retarder
<br /> is controversial as noted in current ACI literature because of the potential that the protection/blotter material
<br /> can become wet between the time of its placement and the installation of the slab. If the material is wet prior
<br /> to slab placement,which is always possible in the Puget Sound area, it could cause vapor transmission to occur
<br /> up through the slab in the future, essentially destroying the purpose of the vapor barrier/retarder. Therefore, if
<br /> there is a potential that the protection/blotter material will become wet before the slab is installed, ACI now
<br /> recommends that no protection/blotter material be used. However, ACI then recommends that, because there
<br /> is a potential for slab cure due to the loss of the blotter material, joint spacing in the slab be reduced, a low
<br /> shrinkage concrete mixture be used, and "other measures" (steel reinforcing, etc.) be used. ASTM E-1643-98
<br /> "Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under
<br /> Concrete Slabs"generally agrees with the recent ACI 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 to
<br /> 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,temporary
<br /> 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 heights
<br /> 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. The loose
<br /> GEOTECH CONSULTANTS,INC.
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