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crow-�aescern <br />August 14, 19ti5 <br />e-z�oa <br />Page 5 <br />line to minimize the possiL*ility oE strucCural damage due to <br />diEferential settlement. Isolated s��read footings should be <br />extended to natural soils in the transilion zcne. <br />Lateral loads due to wind or seisinic Eorces may be resisted by <br />friction between the foundations and the supporting compacted Eill <br />subgrade or by passiva earth pressure on the foundations. For the <br />latter, the f.oundations must ne ooured "neat" against the existiny <br />soil or backEilled with a compacted fill meeting the requirements <br />of structural fill. A coefficient oE Eriction of 0,40 may be used <br />between the structural foundation concrete and the supporting <br />subgrade. The passive resistance �f undisturbed natural soils and <br />well compacted fill may be taken as equal Co the pressure of a <br />fluid having a density of three hundred Eif.Ly (350) pounds per <br />cuhic Eoot (pcf). <br />�4e recommend that drains t�e placed around all perimeter Eoot- <br />ings. The drains should be constructed with a Eour inch diamecer <br />perforated pipe bedJed and covered with Eree draining gr.avel. The <br />drains shoul�9 have a positive gradient towards siaitable discharge <br />facilities. The Eooting drainage system should not be tied into <br />the roof drainage sys[em until tlie drains are tightlined well away <br />from the building. The footing excavation should be backfilled <br />with yranular soil except Eor the top Eoot which should be <br />backfilled with a relatively impermeable soil such as silt, clay <br />or topsoil. Alternatively, tl�e surface can be sealed with as�halt <br />or concrete pavements. <br />Slab-on-Grade Floors <br />Slab-on-grade Eloors may La supported on the compacted compe- <br />tent native soil subgrade or on structural fill. Any disturhed <br />native soils must either be recompacted or replaced with <br />structural fill. Tha slab should be provided with a minimum of <br />four inches of Eree �iraining sand or gravel. We also recommend <br />that a vapor barrier such as a 6 mil plastic membrane be placed <br />beneath the slab to reduce waCer vapor transmission through the <br />slab an�i tlie resultant moisture accumulation. Two inches oE sand <br />may be placed over the membrane f.or protection during construction <br />and to aid in curing of the concrete. <br />Retaining and Fo�ndation Walls <br />ReCaining and foundation walls should he designed to resist <br />lateral earth pressures imposed by the soils retained by these <br />structures. Walls that are designed to yield an amount equal to <br />at least 0,002 times the wall heiyht can be designed to resist the <br />lateral earth pressure imposed by an equivalent Eluid with a unit <br />weight of tliirty (30) pcf. If walls are to be restrained at the <br />top Erom free movemunt, a uniform torce of one hundred (100) psf <br />Eanh Consuhants. Inc <br />!�` <br />