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HomeMy WebLinkAboutSW3210801_Soils/Geotechnical Report_20210804July 28, 2021 NorthPoint Development, LLC 4085 Montgomery Road, Suite 310 Cincinnati, OH 45212 Attn: Mr. Michael Johnston P: (614) 546-9850 E: mjohnston@northpointkc.com Irerracon GeoReport Re: Supplemental Geotechnical Consulting Services — Seasonal High Water Table 1-85 Commerce Center 410 Webb Road Salisbury, North Carolina Terracon Project No. 71215021 Dear Mr. Johnston: Terracon Consultants, Inc. (Terracon) has completed a seasonal high water table exploration for the five planned stormwater ponds at the proposed 1-85 Commerce Center project located in Salisbury, North Carolina. This study was performed in general accordance with our Supplement to Agreement for Services, Reference No. P71215021, dated July 12, 2021. This letter report presents the findings of the subsurface exploration. Five (5) hand auger borings, SHWT-1 through SHWT-5, were performed to depths of approximately 58 to 120 inches below the existing ground surface within the proposed stormwater pond areas. The boring locations were located using the 1-85 Commerce Center — Existing Conditions" plan sheet, EX1.1, dated 7/1/2021 provided by Brad Smith of Thomas & Hutton. The ground surface at each hand auger boring was estimated based on the provided topographic plan. A Site Location Plan and Boring Location Plan are included in the attachments. The soils encountered were field classified according to the Unified Soil Classification System as clays with varying amounts of sand (CL), silts with varying amounts of sand (ML), and sands with varying amounts of silt (SM). Groundwater was encountered in boring SHWT-3 at a depth of approximately 8.5 feet at the end of the day prior to backfilling. The remaining borings were backfilled immediately after completion of hand auger activities. As such, groundwater was not encountered in the remaining borings in the relatively short amount of time they were left open. Saprolite was also encountered within three of our hand auger borings (SHWT-1, SHWT-2 and SHWT-3) at depths of approximately 2.5 to 5 feet. Saprolite is a clay -rich material left over from the weathering of igneous and metamorphic rocks. As a result of its formation, saprolite often has quartz veins and color banding that are relics of the parent materials, rendering the identification of hydric soil components within saprolite unreliable. Its composition leads to low permeability and can act as an aquitard, which can result in a perched water table. Supplemental Geotechnical Consulting Services — Seasonal High Water Table lrerracon 1-85 Commerce Center Salisbury, North Carolina July 28, 2021 Terracon Project No. 71215021 GeoReport The soil samples in the borings were observed for in situ moisture content as well as redoximorphic features of the soil profile to estimate the seasonal high water table levels at each boring location. Redoximorphic features (a gray or bluish -gray colored soil matrix) and mottles are formed by the process of reduction, translocation, and/or oxidation of iron and manganese oxides as the water table fluctuates. A soil layer exhibiting redoximorphic features can be representative of the seasonal high water table level. The following table summarizes the results of our hand auger borings: Estimated Approximate Approximate Approximate Boring Seasonal High Elevation of Boring Depth Boring Water Table Observed Depth of No. Elevation Saprolite (feet) Elevation - Groundwater - (feet) (feet) (feet) (feet) NE 5 SHWT-1 10 774 NE 4 SHWT-2 10 773 NE NE 2.5 SHWT-3 10 762 758.7 753.5 3.5 SHWT-4 4.83 762 759 NE NE NE NE SHWT-5 5.25 766 762.7 Below existing ground surface. Elevations referenced from Existing Conditions plan sheet EX1.1 dated 7/1/2021 provided by Thomas & Hutton. Saprolite can sometimes affect the ability to accurately determine seasonal high water table, due to the relic characteristics of the parent material and the potential for perched water due to low permeability. NE = Not Encountered Soil science provides a powerful tool for interpreting the estimated average seasonal high water table level throughout the year, even during dry seasons when water tables may not be present. In many situations, trained personnel without a formal soil science education can make reliable interpretations. However, in areas of unique and complex soil conditions or disturbed sites careful consideration of the soil's morphology, the kind of parent materials, the position of the soil in the landscape, and topography need to be assessed by a qualified soil scientist before a determination can be made. Field determinations of water table depth are typically made through the identification of redoximorphic features in the soil profile. Monitoring wells and piezometers are used to determine actual water table depths, which are then used for calibrating field indicators of a high water table. Groundwater level fluctuations occur due to seasonal variations in the amount of rainfall, runoff and other factors not evident at the time the borings were performed. Therefore, observed groundwater levels during construction or at other times in the life of the structure may be higher or lower than the levels indicated above. The possibility of groundwater level fluctuations above Responsive r Resourceful L r<eliable 2 Supplemental Geotechnical Consulting Services — Seasonal High Water Table ��jt`f`aCtN1 1-85 Commerce Center Salisbury, North Carolina July 28, 2021 Terracon Project No. 71215021 GeoReport or below the estimated limits should be considered when developing the design and construction plans for the project. We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report, or if we may be of further service, please contact us. Sincerely, Terracon Consultants, Inc. liam O. Frazier, P.G. Project Geologist Attachments: Site Location Plan Exploration Plan Christopher R. Briggs, P.E. 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Lita kery... to THg : wh � �. err ', � • R , .'„R � �Pp� � fWj:� .�®262i IvficrosaftCorporafi6W V AWAMmaging 0� 202� om'fam EXPLORATION PLAN 1-85 Commerce Center Salisbury, Rowan County, North Carolina July 28, 2021 Terracon Project No. 71215021 1 rerracon GeoReport Approximate SHWT Boring Location — 1 I� �ji% II7' Ir 1 � -. .. - ■w• rwa I ... Tow Ie I ru�w. If f� l N *OAr .. 7�. r-- u A OWL,TfdA MR. rHp >bRC t • .i _ _ _ - t DIAGRAM IS FOR GENERAL LOCATION ONLY, AND IS NOT INTENDED FOR CONSTRUCTION PURPOSES PLAN PROVIDED BY THOMAS & HUTTON