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Home My WebLink AboutGUARDIAN SELF STORAGE - FDP210018 - SUBMITTAL DOCUMENTS - ROUND 2 - STORMWATER-RELATED DOCUMENTS Terracon Consultants, Inc. 1901 Sharp Point Drive, Suite C Fort Collins, Colorado 80525 P [970] 484 0359 F [970] 484 0454 terracon.com September 22, 2021 ARCO Business Services, Inc. 4849 Greenville Avenue, Suite 1460 Dallas, Texas 75206 Attn: Mr. Ben Graber P: (469) 349-7909 E: bgraber@arcomurray.com Re: Engineering Calculations for Underdrain System Guardian Storage Northeast of South College Avenue and Fossil Creek Parkway Fort Collins, Colorado Terracon Project No. 20205090 Dear Mr. Graber: Terracon Consultants, Inc. (Terracon) has completed field slug-out testing and engineering analysis to determine the flow of groundwater into the underdrain system at the referenced project. Analysis was performed using the finite element software SEEP/W, which models groundwater flow in porous media. As part of our analysis, we reviewed the following information: ■ Guardian Storage Ft. Collins Utility Plan. ■ Supplemental Geotechnical Engineering Report Fossil Creek Self Storage Ft. Collins, CO, dated September 2, 2016, by Terracon. ■ Soil Foundation Investigation Addendum Guardian Storage Ft. Collins, CO, dated February 22, 2021, by CTL/Thompson Incorporated. ■ Surcharge Settlement Platform Layout Figure 1, by CTL/Thompson Incorporated. The project includes a drainage basin toward the southeastern side of the site, near the intersection of Fossil Creek Parkway and Snead Drive. The drainage basin includes a network of 4-inch diameter underdrain piping wrapped in a gravel blanket that drains into the adjacent storm sewer system, which has catch basins in the drainage basin to collect stormwater run-off. The total length of the underdrain pipe network is approximately 130 feet. The volume of the 4-inch underdrain pipe network, excluding the surrounding gravel blanket, is approximately 12 cubic feet or 90 gallons with a network flow capacity of 64 gallons per minute (gpm). Based on the available information, the drainage basin is nearest Boring TH-4 (performed by CTL/Thompson), which was used as the basis for our assumptions with respect to soil stratigraphy, groundwater elevation and selection of soil parameters for the analysis. The average proposed underdrain pipe invert elevation is +4938 feet, approximately one foot above groundwater elevation +4937 feet. Existing and proposed grade surrounding the drainage basin Engineering Calculations for Underdrain System Guardian Storage ■ Fort Collins, Colorado September 22, 2021 ■ Terracon Project No. 20205090 Responsive ■ Resourceful ■ Reliable 2 is assumed to be Elev. +4948 feet. The underdrain network is located within a comparatively soft layer of sandy lean clay (CL). Our services also included performed slug-out testing at two locations (Well 1 and Well 2 shown on the attached Exploration Plan) on the site within groundwater monitoring wells installed by others. The wells generally consisted of 1.5-inch, hand-slotted PVC pipe inserted into a borehole with no materials placed around the annulus of the pipe. These were the only wells we could find during our site visits. Information pertaining to each of these discovered well locations is summarized in the following table. Well No. Well Location Depth to Groundwater, ft. Approximate Elevation of Groundwater, ft. Approximate Depth to Bottom of Well, ft. 1 40.51492ºN / 105.07584ºW 10.3 4,940 27.3 2 40.51439ºN / 105.07563ºW 2.8 4,941 12.4 3 40.51424ºN / 105.07560ºW 3.05 4,940 12.45 4 40.51425ºN / 105.07550ºW 3.8 4,939 14.15 5 40.51463ºN / 105.07574ºW 9.35 4,938 12.15 Slug-out tests were conducted to collect site specific data to assist with estimating the coefficient of permeability of the site soils. The slug-out tests were conducted by removing several volumes of water from the wells with a bailer. Once the volume was removed, the depth to water was recorded versus time using a down-hole pressure transducer and data logger. Water level data recorded versus time was collected from each of the two test locations as the water level recovered within each well. The field data was analyzed using the Hvorslev Method. The coefficient of permeability was then calculated using the Hvorslev equation: K = r2[ln(L/r)] 2LT0 Where: K = Coefficient of permeability r = Radius of the well L = Length of the well T0 = Time intercept where the head ratio (h/h0) equals 0.37 T0 is termed the “basic time lag” and is intended to account for transients in the well that do not correspond to those in the adjacent formation, such as borehole radius. The coefficient of permeability calculated for each of the well locations ranged from about 2 to 4 x 10-7 m/s. GROUNDWATER MONITORING WELL LOCATION PLAN Guardian Self Storage Underdrain System ■ Fort Collins, CO September 22, 2021 ■ Terracon Project No. 20205090 DIAGRAM IS FOR GENERAL LOCATION ONLY, AND IS NOT INTENDED FOR CONSTRUCTION PURPOSES AERIAL PHOTOGRAPHY PROVIDED BY MICROSOFT BING MAPS Project: Guardian Self Storage Underdrain System Project No.: 20205090 PAGE: 1 OF 8 DRAINAGE BASIN UNDERDRAIN ANALYSIS Soil Design Parameters Soil Permeability:k 10 7-m s 3.281 10 7-´ft s×=:=Ky/Kx Ratio = 0.2 EPRI manual on estimating soil properties for foundation design Soil Constraint Modulus: ν 0.3:=EPRI Table 5-1 average N value, use N 3:= average shear wave velocity from different correlations (Ref: Guidelines for Estimation of Shear Wave Velocity Profile)vs 131 m s:= γ 105pcf:=g 32.174 ft s2×=assume unit weight Gmax γ g vs 2× 6.028 105´psf×=:= C:\Users\lduan\OneDrive - Terracon Consultants Inc\Documents\Work\Aug\Guardian sotrage underdrain\Guardian storage Project: Guardian Self Storage Underdrain System Project No.: 20205090 PAGE: 2 OF 8 E 0.5 Gmax× 2× 1 ν+()× 7.837 105´psf×=:= M E 1 ν-()× 1 ν+() 1 2ν-()1.055 103´ksf×=:=EPRI (5-6)use M 1200ksf:=1 M 8.333 10 7-´1 psf×= Soil Porosity:ns 0.6:=(mixed soil sandy or silty clay in NAVEFAC 7.1 Table 6) C:\Users\lduan\OneDrive - Terracon Consultants Inc\Documents\Work\Aug\Guardian sotrage underdrain\Guardian storage Project: Guardian Self Storage Underdrain System Project No.: 20205090 PAGE: 3 OF 8 Groundwater Design Considerations GWT Elevation:GWT 4939ft:=(assuming 2' rise above normal elevation 4937' as shown in boring TH-4) Maximum Groundwater level between subdrainage trench network:DewaterEL 4938ft:= Subdrainage System Geometry Considerations Maximum Invert Elevation of Subdrainage:INV 4938ft:= Average spacing between trenches:B 25ft:= Pipe Diameter:Dia 4in:= Minimum Pipe Slope (ft/ft)S 0.0075:= Manning's Number (Corrugated Heavy Duty HDPE):n 0.015:= Estimated Trench Flow Considering Darcy's equation Expected flow on trench per linear foot:Q ft k GWT DewaterEL-() B× GWT DewaterEL-()× 1.312 10 8-´ft3 sec ft××=:= C:\Users\lduan\OneDrive - Terracon Consultants Inc\Documents\Work\Aug\Guardian sotrage underdrain\Guardian storage Project: Guardian Self Storage Underdrain System Project No.: 20205090 PAGE: 4 OF 8 Considering finite element analyses with SEEP/W C:\Users\lduan\OneDrive - Terracon Consultants Inc\Documents\Work\Aug\Guardian sotrage underdrain\Guardian storage Project: Guardian Self Storage Underdrain System Project No.: 20205090 PAGE: 5 OF 8 C:\Users\lduan\OneDrive - Terracon Consultants Inc\Documents\Work\Aug\Guardian sotrage underdrain\Guardian storage Project: Guardian Self Storage Underdrain System Project No.: 20205090 PAGE: 6 OF 8 C:\Users\lduan\OneDrive - Terracon Consultants Inc\Documents\Work\Aug\Guardian sotrage underdrain\Guardian storage Project: Guardian Self Storage Underdrain System Project No.: 20205090 PAGE: 7 OF 8 use Q ft_SEEP_ex 1.8 10 7-× ft3 sec ft×:= Maximum Pipe Discharge Rate Considering:Dia 4 in×=Pipe Pipe Wetted Section Area (full):Afull π Dia2× 4 12.566 in2×=:= Pipe Wetted Section Perimeter (full):Pw_full π Dia× 12.566 in×=:= Hydraulic Radius (full):Rfull Afull Pw_full 0.083 ft×=:= Manning Equation Coefficient (Imperial Units):K 1.486 ft3 sec:= Q 4in_max K Afull ft2× Rfull ft æçè ö÷ø 2 3 × S 1 2 n 64.111 gpm×=:=(maximum capacity of 4" pipe) C:\Users\lduan\OneDrive - Terracon Consultants Inc\Documents\Work\Aug\Guardian sotrage underdrain\Guardian storage Project: Guardian Self Storage Underdrain System Project No.: 20205090 PAGE: 8 OF 8 Pipe Network Analysis Total flow (seepage rate from sandy clay into the pipe network) QT_ex Qft_SEEP_ex 130×ft 0.011 gpm×=:= C:\Users\lduan\OneDrive - Terracon Consultants Inc\Documents\Work\Aug\Guardian sotrage underdrain\Guardian storage