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<br />rock surface for massive rock formations (such as <br />g~anite) that are highly resistant to scou~. Small <br />embedments (Keying) should be avoided sinc~ blasting to <br />achieve keyi~g frequently damages the sUb-footing rock <br />structure and makes it more susceptible to scour. If <br />footings on smooth massive rock surfaces r~quire <br />lateral constraint, steel dowels should be drilled and <br />g~outed into the rock below the footing level. <br /> <br />c. Spread Footirqs On Erodible Rock. <br /> <br />Weathered or other potentially erodible rock formations <br />need to be carefully assessed for scour. An <br />engineering ~eologist familiar with the a~ea geology <br />should be cor,sul ted to determine if rock or soil or <br />other criteria should be used to calculate the support <br />for the sprecd footing foundation. The decision should <br />be based on en analysis of intact rock cores including <br />rock quality designations and local geology, as well as <br />hydraulic data and anticipated structure life, An <br />important ccnsideration may be the existence of a high <br />quality rock formation below a thin weathered zone. <br />For deep deposits of weathered rock, the potential <br />scour depth should be estimated (Steps 4 and 6) and the <br />footing base placed below that depth. Excavation into <br />weathered rock should be made with care. If blasting <br />is required, light, closely spaced charges should be <br />used to mininize overbreak beneath the foot~ng level, <br />Loose rock p~eces should be removed and the zone filled <br />with lean concrete. In any event, the fina: footing <br />should be poured in contact with the sides of the <br />excavation for the full designed footing thickness to <br />minimize water intrusion below footing level. The <br />excavation above the top of the spread footing should <br />be filled wi~h riprap sized to withstand flood flow <br />velocities. <br /> <br />d. Spread Footinas Placed On Tremie Seals And supported On <br />Soil. <br /> <br />o Place the tremie base three feet below the scour <br />line (ste? 4) if the tremie is structurally capable <br />of sustai.ing the imposed structural load without <br />lateral SJil suppo~t, <br /> <br />o Check the design for the superflood to insure a <br />safety fa=tor of one. <br /> <br />26 <br />