Supplemental Subsoil Investigation and Seepage Analyses, Proposed

Consulting Engineers in the Earth Sciences, Geotechnology, Hydrogeology and Construction Materials Testing

September 30, 2015

June Engineering Consultants, Inc. 132 West Plant Street Suite 200 Winter Garden, Florida 34 777

Attention: Mr. Jeff Sedloff, P.E.

Subject: Supplemental Subsoil Investigation and Seepage Analyses, Proposed Dry Bottom Retention Areas, Imperial Oaks Subdivision, Hillsborough County, Florida (PN 14-E0001.74B)

Dear Mr. Sedloff:

As requested, Yovaish Engineering Services, LLC has completed a supplemental subsoil investigation and seepage analyses for the proposed dry bottom retention areas referenced above. The following report presents the results of our analyses and recommendations for pond underdrains.

In addition, this report was prepared in response to the following comments from Hillsborough County Staff and the South West Florida Water Management District Staff, respectively, in their review and approval of the construction plans:

7. The end-of-boring elevation of boring PB-7, within the Pond SWM-2 is depicted at 38.0 feet. The max bottom elevation of Pond SWM-2 has been revised to 37.0'. Please limit the proposed maximum bottom elevation of Pond SWM-2 to elevation 39.0', NAVD 88, or provide additional boring information to support proposed maximum elevation.

20. Please provide the recovery calculations for the dry retention ponds in the form of a mounding analysis with unsaturated infiltration excluded. Please demonstrate that they can recover the treatment volume within 72 hours. Please account for permeability rates, depth to aquitard, seasonal high groundwater elevations, etc. and provide the pertinent geotechnical tests, borings and reports. Please revise the design if necessary to accomplish this. {Rule 62-330.301 (l)(e), F.A.C.]

l.OBACKGROUND

The subsoil conditions within the subject ponds were previously investigated by Y ovaish Engineering Services, LLC. The results of our evaluations were presented in our report to you entitled, "Subsurface Soil and Groundwater Level Investigation, Proposed Roadway and Retention Areas, Imperial Oaks Subdivision, Hillsborough County, Florida (PN 14-E0001. 74)", dated March 20, 2014.

953 Sunshine Lane • Altamonte Springs, FL 32714 Phone: 407-774-9383 • Fax: 407-478-8978 Seepage Analyses - Proposed Dry Bottom Ponds, Imperial Oaks PN 14-EOOOI. 74B Page2

The pertinent borings locations from our previous report referenced above are depicted on the Location Plan on Figure 1, attached. As additional input to our evaluations, we performed supplemental borings (SB-1 through SB-4) within the pond areas (see Figures 1 and 2).

The results of the soil investigations outlined above and the stormwater runoff information (dated April10, 2014 and revised June 29, 2015) prepared by June Engineering Consultants, Inc. form the basis for our evaluations.

2.0 SUBSOIL AND GROUNDWATER LEVEL CONDITIONS

2.1 Soil Conditions

As a result of former mining operations, the subsoil conditions encountered by the soil borings were dissimilar to the soil conditions presented by the USDA soil conservation units. Within the majority of the pond limits, the subsoil conditions encountered generally consisted of gray colored slightly silty to silty fine sand with occasional small roots, topsoil (Strata 1 and 1A) to depths varying from approximately 1.0 to 1.5 feet. Underlying the surficial layer, the subsoil conditions encountered generally consisted of alternating layers oflight colored fine sands grading to darker colored slightly silty to silty fine sands (becoming more silty with increasing depth) to the boring termination depths of approximately 10.0 to 25.0 feet.

As an exception, the subsoils encountered within the borrow limits generally consisted of clayey sand and/or sandy clay (Strata 6, 8, 8A and 9) to the boring termination depths.

A more detailed representation of the soil stratigraphy encountered is presented as Soil Profiles on Figure 2, attached. The borings are plotted on an elevation basis as interpreted from survey information provided to us.

2.2 Groundwater Table Conditions

2.2.1 Measured Groundwater Levels: The depth to the shallow groundwater table was measured in the open bore holes at the time the borings were performed and recorded again at least 24 hours later, by installing piezometers. The groundwater table was encountered at depths varying from approximately 2.0 to 12.5 feet below the existing ground surface. The measured groundwater levels re indicated adjacent to the Soil Profiles on Figures 2.

2.2.2 Normal Seasonal High Groundwater Levels: To evaluate the normal seasonal groundwater fluctuations on the site, a review was made of the following:

• U.S.D.A. - S.C.S. Soil Survey maps for the site • the soil stratigraphy encountered during this study • wetland limits and corresponding estimated normal high elevations • the antecedent rainfall conditions prior to the time the groundwater levels were measured Seepage Analyses - Proposed Dry Bottom Ponds, Imperial Oaks PN 14-EOOOJ. 74B Page3

Based on the information discussed above, it is our opinion that the seasonal high groundwater will occur at approximately 1.5 to 2.0 feet above the measured levels. In addition, the normal seasonal high groundwater levels are estimated to temporarily "perch" above the low permeable stratum (Strata 6 through 10), following periods of heavy and frequent rainfall. A more detailed representation of the estimated normal seasonal high groundwater levels (apparent and "perched") are presented adjacent to the Soil Profiles on Figure 2.

3.0 ENGINEERING EVALUATIONS

3.1 Pond Descriptions

The stormwater management system for the proposed site improvements will include three dry bottom ponds constructed throughout the site. The approximate location and configuration of each pond is depicted on the Location Plan on Figure 1. Based upon information provided by June Engineering Consultants, Inc., the dry bottom retention ponds will be designed with the respective elevations and retention volumes outlined in Table 1.

Table 1. Summary of Dry Bottom Retention Area Design Configurations. Provided Outfall Recommended Bottom Weir Retention '"' Flow-Line Minimum Description Elevation Elevation (ft) (ft) Volume Elevation Underdrain (ac- feet) (ft) Length (ft) PondSWM-2A +45.0 +46.45 2.59 +43.5 1280 PondSWM-2B +45.0 +46.96 3.09 +43.5 1223 Pond SWM-3A +45.0 +46.62 2.145 +42.5 1086

3.2 Pond Grading and Filling

In consideration of the required cutting and filling discussed above, we recommend that filling and grading within the pond areas be performed as follows:

• The pond areas shall be stripped to remove the existing vegetation and topsoil layer (where present)

• Fill within 3.0 feet below the pond bottom elevations (where required to attain fmal grades) should comprise clean fine sands, with the following engineering properties:

-minimum permeability equal to 15 feet per day, when compacted to minimum density equivalent to 92 percent of the soil's Modified Proctor Density Value (ASTM D-1557) Seepage Analyses - Proposed Dry Bottom Ponds, Imperial Oaks PN 14-EOOOI. 74B Page4

-maximum fmes content (percent passing the U.S. No. 200 sieve) of 3 percent. The fine sands comprising Strata 2 through 4 (see attached Soil Profiles) should meet these engineering requirements.

• low permeable soils may be encountered near the proposed final grade within portions of the pond bottom areas. We recommend that these limits be further explored at the time of construction and by performing test pits. Where the low permeable soils are encountered within 1.5 feet of the pond bottom grades, the low permeable soil should be undercut and removed to a depth of 2 feet below the affected areas. The backfill soils should comprise permeable fine sands as specified above.

• In order to maintain infiltration capacity, the soils below the retention areas shall not be over compacted/ densified by the construction equipment. Where over compacted, the affected pond bottom area(s) should be scarified using a root rake and/or similar equipment (to loosen the soils to a minimum depth of 2 feet). Thereafter, the pond area may be re-graded using light weight rubber tire and/or low contact pressure trac-mounted equipment and the permeability of the loosened soils retested.

3.3 Underdrain Analyses

Based upon our evaluation of the proposed pond grades and subsoil conditions encountered, the dry bottom ponds will require underdrain filtration to provide adequate retention volume recovery. The recommended underdrain locations and respective lengths are included on the Location Plan on Figure 1, attached. A typical cross-section, depicting the underdrain geometry and materials is presented on Figure 3, attached.

Given the underdrain lengths and layouts described above, Darcy's law was used to determine the respective minimum underdrain widths necessary to recover the required retention volumes in less than 1.5 days (in order to provide a factor of safety of 2.0 for the underdrains systems). These calculations are presented in Attachment A. Based on the results of our calculations, the under drain widths are as follows:

Pond SMW-2A Minimum Underdrain Width...... 18 inches Pond SMW-2B Minimum Underdrain Width ...... 20 inches Pond SMW-3A Minimum Underdrain Width ...... 18 inches

The proposed underdrain systems as designed herein provide a maximum recovery time of 1.5 days for the required retention volumes. Seepage Analyses - Proposed Dry Bottom Ponds, Imperial Oaks PN 14-EOOOJ. 74B PageS

4.0 CLOSURE

We sincerely appreciate the opportunity to be of service on this portion of your site evaluations and trust that the information presented herein is complete and sufficient for your needs. If you have any questions regarding the contents of this report or if we may be of further assistance, please do not hesitate to contact the undersigned.

Sincerely,

Attachments:· Figtrres 1 through 3 Attachment A

Attachment A Calculations to Determine Minimum Underdrain Width Required

The exfiltration capacity of the underdrain system may be estimated using Darcy's law:

q=kxixa

a= area of under drain (length x width) ( in square feet)

I= hydraulic gradient ((pond bottom elevation-( outfall flowline+ allowance of0.5 foot head losses in underdrain pipe)) /(length of filter media 2 ft))

k = permeability of filter sand (in ft/day)

q =required daily flow rate (in cubic feet per day) to underdrain system equal to (retention volume)/(allowable recovery 1.5 days)

Using the proposed pond bottom elevations; respective outfall flow-line elevations; and recommended underdrain layouts presented on Figure 1, the corresponding minimum required underdrain widths for each pond may be determined as outlined below:

The minimum underdrain width= q/(k xI x L)

Recommended Minimum Pond q (ft/day) k (ft/d ay) i ( ft/ft) Length (ft) Minim urn Width width (ft) (in) SWM-2A 75213 60 0.5 1280 1.31 18 SWM-2B 89733 60 0.5 1223 1.63 20 SW M-3A 62291 60 0.5 1086 1.2 7 18

. Based upon the above, the recommended minimum underdrain widths and respective lengths presented on Figure 1 are sufficient to provide retention volume recovery in less than 1.5 days.