12-12-1987 Planning Packet - Laserfiche WebLink

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activity has caused compaotlon, then the ability of this soil to transmit liquid will have been seriously reduced. For proper hydraulic performance, there should be at least 2 feet of natural or fill soil with a percolation rate between 5 and 120 minutes per inch. This could be 2 feet of natural soil above a saturated layer, it could be 1 foot of natural soil plus 1 foot of fill soil, or it could be 2 feet of fill soil, preferably of loamy sand texture. Unless the soil under the mound has the ability to transmit liquid both vertically and horizontally, the mound will not function properly. Another critical layer which is essential to proper mound performance is the soil surface upon which the clean sand layer is placed. Soil surface preparation which is described on page 9 should be carefully studied. Once the clean sand layer is in place, it will be extremely difficult for the inspector to determine how the soil surface was prepared prior to sand placement. A soil surface which has been smeared, compacted, and otherwise made unsuitable for the movement of liquid through it, will not recover that capacity after a period of time. Liquid will likely seep out of the mound at the toe of the dike or at the edge of the rock layer. Page 9 indicates that a crawler or track-type tractor shall be used for mound construction. Page 16 shows earth pressures of various sized track-type tractors. It is recommended that the earth pressure of the crawler tractor used for mound construction not exceed 1,000 Ibs/sq ft. The use of proper construction equipment along with proper construction techniques is essential to proper mound performance. The other layer over which the contractor has responsibility, but which can easily be checked by the inspector, is the texture of the clean sand layer. Clean sand is described on page 10 on the basis of a sieve analysis. Clean sand can also easily be determined by using the fruit jar test. Place exactly 2 inches of sand in the bottom of a quart fruit jar and then fill the jar three-fourths full of water. Place the cover on the jar and shake the contents vigorously. Allow the jar to stand for about an hour and observe if there is a layer of silt or clay on top of the sand. If the thickness of these fine particles is more than 1/8 inch, the sand is likely not suitable for use in the mound construction. The reason is that too many fine particles tend to cause the soil to compact during the construction process. Also, the long-term acceptance rate of this soil will be slower than the long-term acceptance rate of clean sand, which is used for sizing the rock layer. Troubleshooting a failed raound requires a soil auger which can collect a sample of the sand layer used by the contractor. The same soil auger can penetrate into the soil under the clean sand layer to determine if that layer is unusually compact. The soil 'uger will also allow one to determine if a barrier layer such as seasonally saturated soil is closer than 3 feet to the bottom of the rock layer. The soil sizing factor for the clean sand layer of the mound is 0.83 square feet per gallon of waste per day. Mote that this is the soil sizing factor for a soil having a percolation rate in the range of 0.1 to 5 mpi, as found -ii- r A r