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Home My WebLink AboutTAILWIND S12719Onsite File Prepared for: LAND LLC Prepared By: ENGINEERING,LLC P.O. Box 111989 Anchorage, Alaska 99511 (907) 561-6537 ••a, •m REV October 2023 Prepared By: Grant Mathews, P.E. -00'-oF A "'A TH CE 135444 2 PR 0 *F E'SS Project Location The project site consists of an existing 2 -acre parcel with a single-family home in South Anchorage, at the northwest intersection of Flyway Ave and Gander St. Surrounding properties include a mix of residential single-family housing. The property is bordered by Kempton Hills to the south with public water and sewer utilities. The parcels to the east, west, and north are 1 -acre plus lots with on-site water and wastewater systems. Existing Conditions & Land Cover The site contains an existing single-family home, garage, and driveway but is otherwise well vegetated with mature trees and brush. Topographically, the site drains in a southwesterly direction. Roughly 30 feet of relief occurs from the northeast corner to the southwest. The average slope across the parcel is 10-15%. Proposed Development Plan The proposed development intends to subdivide and create two residential lots from the 2 -acre parcel. One lot will retain the existing single-family home, well, and septic system that serves it. Each lot is roughly 1 -acre. Existing grades are less than or equal to 30% across the site, and the proposed lot sizes exceed the 40,000 SF minimum lot size required to support onsite systems. Testing Process Triad Engineering, LLC performed septic testing onsite on July 22"d, 2023. Weather conditions were generally sunny with temperatures in the mid -60's. Land LLC mobilized a Kubota L45 backhoe to the site and provided an operator for the geotechnical exploration. Access for the excavator was such that minimal to no clearing was required for testing. The test hole was advanced to a depth of roughly 10 feet below ground surface (BGS). Upon completion of the test hole, a 4 -in diameter perforated pipe was placed within the excavation and backfilled with the stockpiled soils. A bench was excavated adjacent to each test hole at an appropriate elevation so that percolation testing could be completed. A roughly eight -inch -deep hole was hand dug into undisturbed soils using a shovel and approximately two inches of 3/8" pea gravel was placed at the bottom of the hole. A four -inch diameter perforated PVC pipe was placed vertically into the test hole and the annulus backfilled with pea gravel. Water was then carefully poured into the pipe to a level approximately six inches above the bottom of the hole. Measurements were then taken using a standard tape measure. The process was repeated for each test hole as necessary to meet Title 15 requirements. The initial reading in all test holes exceeded %" drop, so pre-soaking was not required. Groundwater was not encountered during the investigation, but surficial weeps were encountered. Soils Soils encountered during the investigation were visually classified in general accordance with the Unified Soil Classification System (USCS). The general subsurface profile encountered was sand, transitioning to silt. A test hole was dug on Lot 2, but no hole was dug on Lot 1 since it has an existing functioning septic system with an approved COSA completed in 2023 by Garness Engineering. Bedrock Bedrock was not encountered during the investigation and is not anticipated to negatively impact future development of the lots. Percolation Test Results Below is a summary of the percolation test results by lot and the subsequent reserve areas required. See the attached site plan for test hole locations on each lot. Table 1 - Septic Testing Results Summary Lot Perc Rate Depth of TH Groundwater Groundwater Reserve Area( 017-462-05 (min/in) 68 ft (Initial) Monitoring (Oct'23) 0.00 2 <1 10' NGWE NGWE 10,000 As mentioned, no test hole was dug on Lot 1. The lot has an existing functioning septic system and adequate space to install a replacement drainfield. The septic tank was replaced in 2023 as part of the recent COSA approval. Soils are anticipated to closely match those encountered during the investigation of Lot 2 based on the topography, vegetation, and review of neighboring historical test hole data. Additionally, the existing seepage pit has functioned for over 50 years indicating that the soils on site are well draining. A reserve area of 16,000 square feet is shown on the grading plan for Lot 1; if needed, there is adequate space on the lot for installing a new system in the future. Groundwater During the initial investigation, no groundwater was encountered. Groundwater monitoring in October 2023 confirmed no groundwater in the monitoring tube. Water Availability The MCA's COSA database was reviewed to assess water availability in the immediate vicinity of the proposed subdivision. The properties surrounding this parcel are largely undeveloped. However, the five closest properties with published data were found to have flow rates of 4-20 gpm. A summary of those rates are as follows: Parcel ID Legal Description Well Depth Static Water Production Nitrate Levels (mg/L) 017-462-05 Tailwind, Lot 1 68 ft 26 ft 6 gpm 0.00 017-462-29 Landing Zone, Lot 2 154 ft 40 ft 10 gpm 2.18 017-462-28 Landing Zone, Lot 1 164 ft 38 ft 80 gpm 0.00 017-462-16 Drake, Blk 4, Lot 4 Unk 13 ft 4 gpm 2.81 017-462-15 Drake, Blk 3, Lot 1 82 ft 24 ft 4 gpm 6.36 Based on the available data, well production is not a concern for the proposed lots and no negative impacts to the well flows of adjacent properties is anticipated with the development of this subdivision. All nitrate readings in the immediate vicinity are less than 7 mg/L and most readings were less than 5 mg/L. The lots directly south of the subject property are served by public water and sewer. A screenshot of the MOA Nitrate Values Map is shown on the following page. Conclusion In summary, perc test results indicate that the proposed vacant lot contains soils capable of supporting an on-site septic system. The existing house has an approved COSA from this year, with plenty of space to install a new system at a later date if the existing drainfield requires replacement. Well -draining soils were encountered for Lot 2 and groundwater was not encountered during the month of October, when groundwater is typically at a seasonal high. Review of available well log information indicates that water is abundant in the area with an average production around 5-10 GPM. Thank you for your time and consideration in this matter. If you require additional information, please call 907-720-6128 or email me at grantmathews@triadak.com. Sincerely, Triad Engineering, LLC 1. J Grant Mathews, P.E. Attachments: Grading Plan Perc Test Results Municipality of Anchorage Development Services Department On -Site Water and Wastewater Section 4700 Elmore St. P.O. Box 196650 Anchorage, AK 99519-6650 www.muni.org/onsite (907)343-7904 Soils Log - Percolation Test Performed For: Land LLC Legal Description: Tailwind Subdivision, Lot 2 TH-1 1- 2- 3- 4- 5- 6- 7- 8- 9- 10- 11- 12- 13- 14- 15- 16- 17- 18- 19- 20 - Organics, root mat Silty sand, brown, SM Sand w/ trace silt gray -brown, SP Silt w/ sand, tan, dense ML FA *; 4,9 TH a"{{aJ�iyr • • • 4;A ;W. MATHEWS �' % CE 135444 c�`PlF . • 0�'�� 11\'OPRQFESSIONP�� Date Performed: 7.22.23 Township, Range, Section: Site Plan e t e WAS GROUND WATER ENCOUNTERED? No S BOH IF YES, AT WHAT DEPTH? N/A L NGWE Depth to Water After P Monitoring? NGWE E Date: 10.12.23 Reading Date Gross Time Net Time Depth to Water Net Drop F'LKUULA I IUN KA I E � I (minutes/inch) PERC HOLE DIAMETER 6" TEST RUN BETWEEN 2 FT AND 3 FT COMMENTS Added approx 2 gal of water in less than one min, all water absorbed in less than 4 min. Added approx 2 gal of water in less than one min, all water absorbed in less than 5 min. PERFORMED BY: Triad Engineering, LLC I Grant Mathews CERTIFY THAT THIS TEST WAS PERFORMED IN ACCORDANCE WITH ALLSTATE AND MUNICIPAL GUIDELINES IN EFFECT ON THIS DATE. DATE: 10.12.23