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HomeMy WebLinkAbout15.1_Engineering Report LetterProfessional Consulting Engineers and Land Surveyors Equal Opportunity Employer | www.hyaliteeng.com Engineering Report: for 705 S Church Ave. Bozeman, Montana August, 2025 Prepared For: Landern West LLC Prepared By: Hyalite Engineers, PLLC Brett Megaard, PE 705 S CHURCH AVE ENGINEERING REPORT AUGUST 2025 1. Project Information The property is approximately 1.4 acres located at 705 South Church Ave in Bozeman in the southwest quarter of the southeast quarter of Section 7, Township 2S, Range 6E in Gallatin County, Montana. The property is within the City of Bozeman city limits. Currently the property has an existing house which is serviced by a drainfield (permit no. 05109) and a city water service. The property has a 6” clay sewer main that runs along South Church Street to the east and across Bozeman Creek to the west an 8” clay sewer main and manhole that runs on the western edge of the property. The owner is seeking approval to connect a townhouse with nine (9) bedrooms total to the city sewer system. Currently the sanitary sewer main that runs along South Church does not have the capacity to service the proposed residence. However, the existing sewer main and manhole on the western edge of the property as mentioned above has capacity. The proposed townhouse will have 2 units, the first unit will have 4-bedroom and the second unit will have 5- bedrooms. The proposed connection to the city’s sewer system will consist of 2 septic tanks outside the townhouse that will discharge to two separate E-one pump stations (one for each unit). Once the effluent reaches the pump station it will be pumped to an existing 8” clay sewer main approximately 274 feet away, this will also require that the transmission line from the effluent pump to be bored and sleeved underneath Bozeman Creek. Each unit will have a separate transmission line. Two 4” inserta tee are proposed to be inserted into the existing 8” clay main. Extending from each inserta tee will be approximately 21 feet of 4” SDR 26 sewer service line that will connect to each 1.5” transmission line. This connection to the western sewer main would serve as a temporary option until the S Church City sewer main is upgraded to accommodate the flow. This private service connection is to be privately maintained. 2. Water Supply and Wells The existing house on site is connected to city water and this service is proposed to be abandoned at the main per City of Bozeman Standards. The proposed new townhouse will also be connected to city water with two water services. There is an abandoned well on the property however it is no longer in use. The closest neighboring well is to the south and is more than 100’ from the property boundary. No other wells are known to exist within 100’ of the subject property as the neighbors to the North & South are both currently on City water & sewer. 3. Wastewater Design Flow The proposed building will contain 2-units. Unit A will be 4-bedrooms (350 GPD) and unit B will be 5-bedrooms (400 GPD). The wastewater design flow for the proposed 2-unit residence is 750 GPD (350 GPD + 400 GPD). Calculations were based on MDEQ Circular 4, Chapter 3 Regulations. Maximum Day Flow Peaking factor = 2.3 (per City of Bozeman Design Standards) 750 gpd (2.3) = 1,725 gpd Max day flow = 1,725 gpd / 1,440 = 1.20 gpm Peak Hour Demand Peaking factor = 3.0 (per City of Bozeman Design Standards) 750 gpd (3.0) = 2,250 gpd Peak hour flow = 2,250 gpd 2,250 gpd / 1,440 = 1.56 gpm Design Peak Instantaneous Flow 705 S CHURCH AVE ENGINEERING REPORT AUGUST 2025 Peaking Factor = 10 750 gpd (10) = 7,500 gpd Peak instantaneous flow = 7,500 gpd 7,500 gpd / 1,440 = 5.2 gpm 4. Septic Tanks There is currently one septic tank that serves the existing residence which will be removed. The proposed residence will have a new 4,000-gallon septic tank and E-one grinder pump station. The sizing of the septic tank was based on MDEQ Circular 4, Chapter 5 for 2 units (A & B) with four (4) and five (5) bedrooms. The proposed tanks are to be reinforced concrete, non-traffic rated. The E- one pump stations will have one (1) submersible grinder pump that will pump effluent to the existing sewer main. The septic tank is to be manufactured by Three Fork Precast and shall be watertight. All tanks located in high groundwater areas shall have a water leakage test performed and concrete additive for water proofing (or approved equal). A vent is to be connected to the pump station with a carbon filter. The septic tanks and pump stations are both located outside of the 100-year floodplain of Bozeman Creek. Buoyancy calculations were checked to make sure the weight of the septic tanks, pump stations and soil were more than the upward force due to buoyancy. The groundwater depth can be as high as 4.5-6 feet during the spring months. The buoyancy calculations are below: Pump station Volume of liquid displaced = 22x3.14*7.5’ (depth) = 94.25 cubic feet Force of Buoyancy = 62.4 lbf/cubic foot x 94.25 cubic feet = 5881 lbs Weight of Pump station = Concrete collar around pump station base 12” thick = 7.06 x 145 lb/cubic foot =1,024 lb Total =1,024 lb Assuming groundwater at 2 feet. Soil Bearing Weight on pump station = 56.55 x 110lb/cubic ft (soil weight) – 21.2 x 62.4lb/cubic ft (water weight) = 4,898 lb Total weight = 5,922 lbs Septic tank Volume of liquid displaced = 8.17’x16.42’x6.58’ (depth) = 883 cubic feet Force of Buoyancy = 62.4 lbf/cubic foot x 883 cubic feet = 5,508 lbs Weight of Pump station = (2 walls) = 15.58’x0.42’x5.74’x145 lb/cubic foot x 2 (walls) = 10,893 lb (2 walls) = 7.33’x0.42’x5.74’x145 lb/cubic foot x 2 (walls) = 5,125 lb (bottom) = 16.42’x0.42’x8.17’x145 lb/cubic foot = 8,170 lb (lid) = 16.42’x0.42’x8.17’x145 lb/cubic foot = 8,170 lb Total = 32,358 lb Assuming groundwater at 2 feet. Soil Bearing Weight on pump station = 8.17’ x 16.42’ x 3.5’ x 110lb/cubic ft (soil weight) – 8.17’ x 16.42’ x 1.5’ x 62.4lb/cubic ft (water weight) = 39,092 lb Total weight =71,450 lbs 705 S CHURCH AVE ENGINEERING REPORT AUGUST 2025 The unit weight of the concrete and soil bearing for both the pump station and the septic tank is more than the upward force due to buoyancy, so the pump station will not try to float. 5. Manhole Location and Force Main As mentioned above the owner is proposing to connect to the existing sewer main that is on the far west end of the property. Currently there are 8” vitrified clay mains that come in and out of a manhole that is located on the property. The existing gravity sewer mains have been sized to meet the minimum standard by the City of Bozeman, as an 8” sewer main at the minimum of 0.4% slope, can handle approximately 314 gpm. With an average daily flow of 0.52 gpm produced from the residence this is a very minimal impact to the current system. The peak instantaneous flow calculated above has minimal impact on the City’s system with a gpm of 5.2. It is proposed that the force lines will be 1.5” diameter DR 9 HDPE from the pump station to the existing sewer main. This force line connection to the city will require it to cross Bozeman Creek. The force lines will be directionally drilled and sleeved as close to perpendicular to Bozeman Creek as allowed in order to tie into the existing gravity sewer system. The proposed 8” HDPE DR 7 sleeve will be bored underneath the creek and will have both force lines inside the sleeve. The proposed bore and sleeve will be located outside of the floodplain of Bozeman Creek. The design of the force line was based on MDEQ Circular 2 Chapter 49. Once the effluent has left the force/transmission line it will gravity flow to the City of Bozeman’s wastewater treatment facility. To maintain a 2 ft/s minimum velocity per DEQ Circular 2 Chapter 49 a design flow of 10.4 gpm is needed to achieve the 2 ft/s. The proposed design flow of the E-one grinder pump is to be 14.3 gpm. Using a pump curve for E-one’s SPD grinder pump and Orenco Pump Select program to find total head loss and the design flow. Leakage testing will be completed on all portions of the sewer force line in accordance with the same testing procedures for water mains and in accordance with the MPWSS. In addition to testing the force main is proposed to have antiphon, shut off valve and check valve as it leaves the pump station. The proposed bore and sleeve under Bozeman Creek is to be 6.5’ from the bottom of the creek to maintain adequate cover for forced mains. BMP’s will be used where necessary, however adequate separation will be included for the directional drilling in order to ensure no siltation or erosion occurs near or in the stream/streambank. 6. System Specifications The Orenco Systems Pump-select software was used to determine the pipes, flow rate, and total dynamic head of the proposed systems. The transport pipes to the existing sewer main is to be 1.5 in dia. DR 9 HDPE. The transport pipe is to be approximately 274 feet to the proposed sewer service line that connects to the existing sewer main. An E-one grinder pump was determined to be adequate for the system. The proposed pump station will have 1 (one) E-one grinder pump each pump station. The alarm system is located on the control panel which is to be mounted on the proposed building. The dose volume for the proposed townhouse residence supplied by E-one and is to be 31 gallons per cycle, which is greater than the volume of the transport line. The E-one pump station holds 7.75 gal. per inch. The distance between the “off float” and the “on” float is to be 4 inches, allowing 705 S CHURCH AVE ENGINEERING REPORT AUGUST 2025 for a true dose volume of 31 gal. According to E-one this pump is designed to run frequently hence the low dose volume. The reserve volume required by MDEQ is 25% of the daily flow rate of unit A is 350 gpd and unit B is 400 gpd. For unit A this is equal to 87.5 gal. or 11.3 inches and for unit B this is equal to 100 gal. or 12.9 inches. A E-one grinder pump requires at least 13” of submersion, based on this criterion the low-water redundant off switch is to be positioned at 13”. The floats monitoring the dose pump switches are to be set at the following elevations according to E-one design parameters (inches from the bottom of the tank): Hi-water alarm - 25” “On” - 17” “Off” - 13”