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HomeMy WebLinkAboutSWG Onsite Survey - 2/9/1989 M ENGINEERING REPORT LARGER ON-SITE SEWAGE SYSTEM BELFAIR ELEMENTARY SCHOOL BELFAIR, WASHINGTON D Owner: North Mason School District P.O. Box 167 U Belfair, WA 98528 Ltd Attn: 7Chuck Jeu Y 1989 C''eAleo Engineer: Art Anderson Associates / SE 148 First Street RVjC�S Bremerton, WA 98310 Attn: James R. Levey, P.E. (206) 479-5600 Project Description: Modernization and additions to Belfair Elementary School Project Location: State Hwy 3, Belfair, Washington Tax Parcel No. 12332-40-60000 Existing System: The school presently consists of two buildings, each with its own septic tank and gravity fed drainfield. One is located southeast of the school and has a 3758 gallon septic tank with 556 L.F. of drainfield trench. The other is located northwest of the school and contains a 3600 gallon septic tank and 556 feet of drainfield trench. The existing system serves an occupant load of 460 students and staff. There are no showers in the building and the kitchen is only a serving kitchen so the design flow would be 10 gal/day/person or 4600 gallons per day. The required septic tank capacity is 6900 gallons compared to the combined capacities of the existing tanks of 7358 gallons. The combined length of drainfield trench for the two systems is 1206 feet. The loading rate per foot of trench length is 3.8 gal/lin ft. Domestic Water Source: Belfair Water District provides water to the school and surrounding properties. Land use: Present land use is residential on the north and south, public use on the west, and undeveloped on the east //,/ a_S_ 9 Area Wells: The 1" = 2000' area map shows the site d drainfield locations. Plan sheets Cl and C2 show the existing and proposed drainfield areas, structures, roads, parking, storm drainage system, site contours and water and sewer lines and drainage patterns. There is a small creek east and south of the new drainfield site. Geology and Soils: Detailed descriptions of the area geology and soils were prepared in reports by Grover Way dated 16 Sep 88 and 27 Nov 88. The soils are principally sandy, glacial tills. There is some evidence that periodic ground water seepage will occur during wet conditions. Location: The proposed drainfield area is located southwest of the school building. The drainfield is 150 feet from the school in an area with average slope of 1.5%. Design Criteria: The design flow for the new additions is 10 gals. per person per day. The addition has no showers or kitchen. The occupancy of the new addition is 84. The total daily design flow will be 840 gallons. The effluent loading rate for the drainfield is 0.8 gal/day/sq. ft. for the silty fine sand. The trenches will be 3 feet wide, and the total length will be 354 feet. The septic tank is sized for 1500 gallons with a scum storage area of 300 gallons. The baffles would permit buildup of sludge in the bottom before failure. Operation and Maintenance: This system will be operated and maintained by North Mason School District and district personnel will respond to emergencies, when warranted. Routine maintenance as outlined in O/M manual will be integrated into the district's planned maintenance program. ;C 0 t}�t FfSs•.or114�� NEW BELFAIR ELEMENTARY SCHOOL, PHASE II SPECIFICATIONS Septic Tank and Drain Field GENERAL The Contractor shall be responsible for establishment of elevations on all inverts and tfie bottoms of the septic tanks. The Contractor shall place the septic tank so as to allow flow into the tank from the sewage line, as installed by the mechanical contractor. The drainfield is to be installed in accordance with the "Manual of Septic Tank Practice" written by the U.S. Department of Health, Education and Welfair and any applicable county requirements. 1 . SEPTIC TANK AND PUMP CHAMBER The Contractor shall furnish all labor, materials, equipment, machinery, skills, and tools necessary to fabricate and erect one (1 ) 3760 gallon septic tank with attached pump chamber, furnish and install one (1) duplex pump system and controls, furnish and install all piping, and appurtenances required and specified and any miscellaneous site work. All materials and work shall conform to the contract drawings and to "Specifications for Structural Concrete" (ACI 318-71) . All concrete to be "Working Stress" type; fc @ 28 days = 3000 psi , fc — 1350 psi. Proportion concrete in accordance with Chapter 4, using 3/4 inch maximum aggregate for slabs and 1 1/2 inch maximum aggregate for walls. Refer to Chapter 5 of ACI 318-71 for curing and to Chapter 6 for joints. All reinforcing steel to conform to ASTM Specifications A615-40, fy = 40,000 psi, fs = 20,000 psi. Bending and placing to be in accordance with Chapter 7 of the 'Building Code Requirements for Reinforced Concrete (Act 318-71) . Access holes to the septic tank and pump chamber shall be sized and covered, as specified on the drawing. Pumps- Pumps shall be Hydromatic Model OSP25M submersible pumps or equal . Each pump shall have a capacity of 50 GPM at a total head of 5.0 feet. Pump motor shall be 1/4 horsepower, single phase, 115 volts. Electrical Control Panel - The control panel shalt have a NEMA I enclosure for mounting inside the School . A circuit breaker shall be provided for each pump and a magnetic starter with overload protection shall be supplied for each pump. An alternating relay shall be provided to alternate pumps on each successive cycle of operation. Starters shall have auxiliary contacts to operate both pumps on override condition. An interlock relay shall be provided to automatically reconnect the control circuit in case of circuit breaker trip on one pump. H-O-A switches and run lights shall be supplied for each pump. A high water alarm shall be supplied. Sump Level Controls - Sealed float type mercury switches shall be _ supplied to control sump level and alarm signal . The mercury tube switches shall be sealed in a solid polyurethane float for corrosion and shock resistance. The support wire shall have a heavy Neoprene jacket and a weight shall be attached to cord above float to hold switch in place in sump. Three float switches shall be used to control level . One for pump turn on, one for pump turnoff, and one for both pumps turn on. A fourth switch shall be provided for alarm control . A NEh1A 4 weather proof junction box shall be used inside the sump to connect pump and control cords to the incoming power and control wires from the remote control panel . Operation of System - On sump level rise lower mercury switch shall first be energized, then upper level switch shall next energize and start lead pump. With lead pump operating, sump level shall lower to low switch turn off setting and pump shall stop. Alternating relay shall index on stopping of pump so that lag pump will start on next operation. If sump level continues to rise when lead pump is operating, override switch shall energize and start lag pump. Both lead and lag pump shall operate together until low level switch turns off both pumps. If level continues to rise when both pumps are operating, alarm switch shall energize and signal the alarm. If one pump should fail for any reason, the second pump shall operate on the override control and if level rises above override control alarm shall signal . All level switches shall be adjustable for level setting, from the surface. Payment for this item shall be on a Lump Sum basis and shall be full compensation for furnishing all labor materials, equipment, machinery, skills and tools necessary to fabricate and erect the specified septic tank with duplex pump system and all necessary appurtenances, as shown on the project drawings, and no further compensation will be made. 2. DRAINFIELD The Contractor shall furnish and install 556 L.F. of draintile in the area indicated on the project plan. The drainfield is to be constructed using the serial distribution method. The drainfield is to be divided into two equal sections, each section serving one of the pumps and each section containing 278 L.F. of draintile. A 1 1/4 inch pressure line shall connect each pump with one section of the drainfield, as indicated on the project drawing. Two standard distribution boxes shall be placed in the inlet line to the drainfield, as indicated. The portion of the existing twelve (12) inch storm drain pipe which lies within the drainfield area is to be replaced with a tight joint pipe. The type of pipe used for the tightline shall be one of the following: PVC Sewer Pipe with rubber gaskets or solvent weld joints, or corrugated galvanized metal pipe with metal pipe bands with gaskets made of three eights (3/8) inch thick by twelve (12) inches minimum width closed cell synthetic sponge rubber, grade SCE-43. The pipe is to be layed using twenty foot lengths, where applicable to limit the number of joints. Seventy (70) feet of pipe is to be installed. The drainfield shall be fenced where indicated on the plans to preclude vehicular traffic. The drainfield must be planted with a cover crop upon completion, following the recommendations of the County Extension Agent. Payment for this item shall be per Lump Sum and this Lump Sum payment shall be full compensation for all materials, equipment and labor required to complete the drainfield, as specified, and no further compensation will be made. NEW BELFAIR ELEMENTARY SCHOOL PHASE II SEPTIC SYSTEM DESIGN GENERAL CONCEPT AND DESIGN CRITERIA The septic -system will be composed of a 3,753 gallon septic tank with a 132 gallon dosing chamber and an absorption field consisting of a total of 556 linear feet of draintile. The septic tank will consist of two (2) compartments. The primary compartment is sized to hold 2,518 gallons and the secondary compartment is sized to hold 1 ,240 gallons. The dosing chamber will be equipped with two (2). three (3)-inch alterating siphons. Each siphon will dose one-half of the total drainfield area per cycle. DESIGN Septic Tank 7 classrooms Estimated 30 persons/classroom Classroom population = 210 persons Library population = 24 persons Total population = 234 persons Flow per person per day = 15 gallons/capita/day Total flow = 15 x 234 = 3510 g.p.d. Tank Capacity Required v = 1125 = .75 (3,510) = 3,758 gallons Primary Chamber size = 2518 gallons Secondary Chamber size = 1240 gallons Drainfield Perculation Rate = 4.4 min/inch Application Rate = 2.6 gal/sq.ft. (Table 11 , Manual Septic Tank Practic) Maximum Allowable Applica- tion Rate (Mason County) = 2.0 ga./sq.ft. Trench Area Required = 3510 gal ./2 gal./sq.ft. = 1755 sq.ft. Use Trench Width of 2 feet Trench Length Required = 1755 sq.ft./2 ft. = 878 L.F. In the area available for the drainfield, it is physically possible to install 556 Linear Feet of tile, using the serial distribution method. The trench length required can be reduced to 556 Linear Feet by the use of a deep absorption trench. The trench shall be constructed with the depth of gravel under the pipe being eighteen (18) inches. A standard distribution box will be installed in the inlet line to each section of the drainfield for inspection purposes. The drainfield is to be divided into two (2) sections (see project drawing). Pump Chamber Draintile Capacity = 556 L.F. x .087 sq. ft x 7.485 gal/C.F. = 362 gal . One half of the draintile shall be dosed per cycle with a volume of effluent equal to 70% of the tile capacity. Pump Chamber capacity = .70 x 362 gal/2 = 127 gal . The pump chamber shall contain two pumps, each pump dosing one-half of the drainfield on alternate cycles.