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HomeMy WebLinkAbout20130 July 8, 2013 Mr. Ralph Maloon, PE RIM Engineering Co, Inc. 150 North Main Street Mansfield, MA 02048 RE: Rolling Fields Subdivision Drainage Dear Ralph, Attached please find a copy of Mr. Peter Fletcher's Site Visit/Soil Investigations memorandum of July 1, 2013 for Rolling Fields subdivision. You will recall that Mr. Fletcher's services were retained by the Town as a result of concerns about the viability of the Rolling Fields drainage design as it relates to groundwater elevation. Mr. Fletcher's letter, while not recommending any specific drainage improvements, does repeat our concerns about the actual elevation of the groundwater table on the subject site and how best to determine same. Mr. Fletcher points out in his review that the groundwater elevation test hole log shown on your plans varies by as much as 4.59 feet around the site. As you know, groundwater table elevations on a site consisting of sand and gravel tend to be found at a similar elevation. To further complicate matters, this former gravel pit site has been significantly disturbed such that using soil mottling may not be an accurate indicator of seasonal high groundwater, as stated in Mr. Fletcher's memo. In response, Mr. Fletcher has indicated that monitoring wells placed throughout the site prior to the wet season may be one method of ascertaining high groundwater elevation. Thus far, we have experienced a close working relationship with RIM Engineering and Walsh Development on this site. We hope this relationship continues through the resolution of this drainage concern. At this point, the Town requires a definitive seasonal high groundwater table elevation be established because the proposed design and supporting calculations are not what we have observed on the site. If there is an alternative drainage design, additional or definitive information or proposed elevation changes, please bring them forward so we can resolve the drainage/groundwater elevation issues. Without resolution of this key issue, which appears likely to recur, the road must remain unpaved. In addition, we will be unable to establish a surety amount if we do not know the cost of the drainage solution. If you have any questions or would like to discuss this matter in further detail, please feel free to contact me. Sincerely, '14Icu �1 (i -� d�'c4 Shaun P. Burke, AICP Director of Planning and Development Attachment Cc: Tom Walsh, Boro Sand and Gravel William Ross, Town Manager Lee Azinheira, DPW Director Richard C. Alves, Jr., P.E., Town Engineer Mike Ahern, Subdivision Inspector Scott Leite, Health Agent Peter Fletcher SPB/jd Site Visit/ Soil Investigations Date: July 1, 2013 Project: Rolling Fields Village subdivision Requested by: Richard Alves Jr., Town Engineer/Assistant DPW Director for Mansfield, Massachusetts Compiled by: Peter C. Fletcher, Certified Professional soil Scientist On Thursday, June 27, 2013; 1 met with staff of the Mansfield Engineering Department, the landowners, and their consultant. Followingthe meeting I participated during a site walk and conducted on -site soil investigations. Atthis time, three test pits were dug using an excavator. It should be understood that I am a Certified Professional Soil Scientist and my area of expertise is interpreting soil conditions for various land uses. My area of expertise does not include the technical aspects of stormwater management. I have included a copy of my resume with this report. With this understanding, my comments are stated below. 1. Interpreting the soil conditions at the Site for depth to seasonal high groundwater elevation: This is an altered/disturbed site. In the past it was a gravel pit and, judging by the depths of cuts in adjacent areas, ten or more feet of earthen material may have been removed from the Site. In discussion with Town officials familiar with this Site, in many areas the gravel had been excavated down to the depth of the underlying groundwater. At that time, the prior landowner was instructed by the Town to raise the land surface, and earthen fill material was then placed at the Site. In addition, an existing subdivision that abuts the Site to the north has a drainage easement that discharges stormwater in close proximity to the property line. The current landowners have since re -graded the Site and added additional fill material. All of these alterations at this Site have the potential to effect the elevation of underlying groundwater system and alter the range of seasonal fluctuations. At this Site, the soil features (redoximorphic) that formed when this was a natural landscape, prior to any site alterations, are not reliable indicators for interpreting the depth to the current seasonal high groundwater elevation. At sites like this, my recommendation is to install monitoring wells within representative areas of the site and record the groundwater fluctuations at a frequent interval during a typical wet season of the year. 2. Depth to soil features (redoximorphic) that indicate periods of soil saturation and reduction: On the Topographic Plan for Rolling Fields Village dated June 23, 2012; the depth to redoximorphic features documented within deep holes varied in elevation by 4.59 feet. This is unusual for sites located within areas of sands and gravely. In areas of coarse sediments, groundwater can flow easily through the large pore spaces between the sand and gravel. Typically water tables on these landscapes occur at a similar depth throughout the area. A possible explanation for the differing depths to redoximorphic features at this Site is that the sands and gravels are underlain by finer textured sediments that restrict groundwater flow. In conversation with the Landowner's consultant and Town officials familiar with the Site, many of the deep holes had fine textured sediments (silts and/or clay) underlying sands and/or gravel. 3. Standing water within Common Open Space B: At the time these investigations were conducted, there was ponded water within the low areas of Common Open Space B. A water pump with an intake hose extending to the ponded water was not operating at the time these investigations were conducted. Comments by those familiar with the Site, confirmed that the water level had been significantly higher following recent rains and standing water has remained in the area for an extended period of time. A test pit was dug within this area. There was a surface layer of extremely gravelly fill material (the landowner's consultant commented that this was tailings from a screening plant), underlain by stratified in situ sands that became finer (higher silt content) with depth. The material at the bottom of the pit was a loamy, very fine sand. Water flowed into the test pit from all sides at different depths, indicating that the soil was saturated throughout. This was interpreted as groundwater and not a surface phenomenon. 4. Proposed 8 foot Wide Grass Swale: At the time these investigations were conducted, the soil within the bottom of the proposed wide grass swale was saturated and there were small areas of ponded water. If the purpose of this proposed swale is to recharge stormwater into the underlying groundwater system, additional study is recommended to determine the depth to groundwater within this area. 5. Sediment observed in stormwater recharge field: Sediment was observed (inspection portal) in one of the constructed stormwater recharge fields at the time these investigations were conducted.