B.08.03_MSF - USACE Final Geotechnical Report AUG2020.pdf

Prepared by:

U.S. Army Engineer District, Baltimore

Engineering Division Geotechnical Branch

August 2020

US Army Corps of Engineers Baltimore District August 2020

Maintenance and Supply Facility Training

(MSF)

Humphreys Engineer Center Support Activity (HECSA)

Fort Belvoir, Virginia

Final Geotechnical Report

FOR INFORMATION ONLY

TABLE OF CONTENTS

1. PROJECT LOCATION AND DESCRIPTION

2. GEOTECHNICAL SCOPE OF WORK

3. PRELIMINARY SUBSURFACE INVESTIGATIONS

a. Flat Plate Dilatometer Test (DMT)

b. Seismic Piezocone Penetration Tests (CPT)

4. SITE SPECIFIC SUBSURFACE INVESTIGATIONS

a. Standard Penetration Testing (SPT)

b. Test Pits

c. Infiltration Testing

d. Stabilized Seasonal High Water Table (SHWT) Borings

e. Flat Plate Dilatometer Tests (DMT)

f. Soil Resistivity Testing

5. LABORATORY SOIL TESTING

6. REGIONAL GEOLOGY

7. SITE SOIL STRATIGRAPHY

8. GROUNDWATER

9. DRAINAGE AND WATER CONTROL

10. TEMPORARY EXCAVATIONS

11. SATISFACTORY FILL and BACKFILL MATERIALS, MOISTURE CONTROL and

COMPACTION

12. STORMWATER DRY POND EMBANKMENT

a. General

b. Facility and Dam Type

c. Embankment Foundation and Abutments

d. Embankment

e. Outlet Structure Conduit

f. Emergency Spillway

g. Construction Inspection

h. Pond Liner

13. PAVEMENTS

a. Criteria

b. Traffic

c. Pavement Sections

d. Subgrade Properties and Requirements

e. Strength and Frost Design Parameters

f. Asphalt Materials

g. Concrete Strength, Joint Spacing, Doweling, & Reinforcing

h. Graded Aggregate Base Course

i. Rapid Drainage Material (RDM) Base

j. Subsurface Drainage (Pavement Subdrains)

k. Separation Geotextile

l. Mill and Overlay

14. INTERIOR SLABS NOT SUBJECTED TO VEHICULAR TRAFFIC

15. LATERAL EARTH PRESSURES

a. Below Grade Non-Yielding Walls

b. Cantilever Walls

16. FOUNDATIONS

a. Structural Loadings & Finished Floor Elevation

b. Foundation Recommendations

c. Field Verification of Allowable Bearing Capacity

17. SITE CLASS FOR SEISMIC CONSIDERATIONS

18. GENERAL LIMITATIONS

19. POINT OF CONTACT

APPENDIX A:

Location Map Site Dimensioning Plan

APPENDIX B:

Site Specific Exploration Plan SPT Boring Logs (SPT) Test Pit Logs (TP) Infiltration Test Logs (IT) Stabilized Seasonal High Water Table Logs (SP) Flat Plat Dilatometer (DMT) Results

APPENDIX C:

Preliminary Exploration Plan DMT Results CPT Results

APPENDIX D:

Soil Resistivity Test Results

APPENDIX E:

Laboratory Soil Test Results

APPENDIX F:

Dry Pond Details Selected Plates from Chapter 6 of the Fairfax County Public Facilities Manual

APPENDIX G:

Pavement Details

Maintenance and Supply Facility, Humphreys Engineer Center Support Activity (HECSA), Virginia –

Maintenance and Supply Facility (MSF), Humphreys Engineer Center Support Activity

(HECSA), Virginia August 2020

1. PROJECT LOCATION AND DESCRIPTION: The proposed project consists of the construction of a new 47,300 square foot Maintenance and Supply Facility located within the boundaries of the Humphreys Engineer Center Support Activity (HECSA) located in Fort Belvoir Virginia. Humphries Engineering Center Support Activity (HECSA) is within the 450+/- acre Research and Development tract along Telegraph Road in the extreme northern section of Fort Belvoir. Access to the site is by way of Leaf Road located off of Telegraph Road. Prior to 1970 the area was occupied by the Civil Defense Protective Structures Development Center; existing structures at that time consisted of several small Fallout Shelters and a large fenced area designated as a radiation level test facility. The ground elevations ranged from a high of approximately 100 feet msl along Telegraph Road to a low of approximately 20 feet msl at Drogue Creek. Since then, the facility has grown to its current size containing the Kingman Building completed in 1972;

the Coastal Engineering Research Center (CERC) completed in 1973; the Topographic R&D Building completed in 1974; the Casey Building completed in 1982, the Sensitive Compartmented Information Facility (SCI Facility; Addition to the Topographic R&D Building) completed in 1986, the HQ Operations & Training Facility completed in 2006, and other smaller shops and warehouses.

The proposed Maintenance and Supply Facility (MSF) is to be located in the wooded parcel of land on the western portion of the site just east of Telegraph Road. The existing ground elevation at the location of the proposed Maintenance and Supply Facility ranges from 91.6 feet on the north to 80.1 feet to the south. The proposed finished floor elevation of the facility is 90.0 feet. Appendix A contains a Location Map and Site Dimensioning Plan.

2. GEOTECHNICAL SCOPE OF WORK: The purpose of this report is to establish the subsurface conditions and provide geotechnical recommendations for the proposed MSF project. These geotechnical recommendations include those related to foundations, pavements, earthwork, groundwater, and excavations.

3. PRELIMINARY SUBSURFACE INVESTIGATIONS: Prior to the selection of the MSF project site, preliminary subsurface explorations were conducted at two locations, designated as Site 2 and Site 4, located approximately 500 and 1200 feet respectively southeast of the selected MSF project site to investigation the soil conditions in the northwest parcel of HECSA. The selected MSF project site was not selected until after these preliminary subsurface explorations were completed.

These investigations were conducted between 13 and 19 August 2019 by the Baltimore and Savannah District U.S. Army Corps of Engineers Field Exploration Units. These explorations consisted of a total of four (4) Flat Plate Dilatometer tests in accordance with ASTM D 6635, and ten (10) Seismic Cone Penetration Tests (CPT) in accordance with ASTM D 5778. Three DMT’s and five CPT’s were conducted at Site 2 (designated as DMT 2 - * and CPT 2 - *) and one DMT and five CPT’s were conducted at Site 4 (designated as DMT 4-* and CPT 4 -*).

a. Flat Plate Dilatometer Tests (DMT): A total of five (5) Flat Plate Dilatometer Tests (DMT), four at Site 2 and one at Site 4, were performed by the Baltimore District U.S. Army Corps of Engineers Field Exploration accordance with ASTM D 6635. Locations of the DMT tests are illustrated on the Preliminary Exploration Plan contained in Appendix C and are designated as “DMT” (i.e.

DMT* - *).

The DMT was advanced utilizing the 20-ton tracked CPT rig provided and operated by the Savannah District U.S. Army Corps of Engineers Field Exploration Unit. In general the DMT consists of a stainless steel blade having a flat, circular expandable steel membrane mounted flush on one side. The DMT was hydraulically pushed into the ground and the test conducted at 8-inch intervals and advanced until a minimum of 10,000 kgf of thrust (refusal) was encountered resulting in test depths ranging from 25.9 to 59.0 feet below grade.

The expandable steel membrane was inflated and the pressures required to mobilize two pre-established strain levels are recorded. DMT results are contained in Appendix C and are presented graphically as “measured” parameters and “interpreted” parameters based upon published correlations. These interpreted parameters are based upon generally accepted correlations as defined in Report Number FHWA-PA-87-024+84-24, GUIDELINES FOR USING THE

CPT, CPTU AND MARCHETTI DMT FOR GEOTECHNICAL DESIGN,

VOLUME III.

b. Seismic Piezocone Penetration Tests (CPT): A total of ten (10) seismic piezocone penetration tests (CPT) were completed by the Savannah District U.S. Army Corps of Engineers Field Exploration Unit in accordance with ASTM D 5778.

Locations of the CPT tests are illustrated on the Preliminary Exploration Plan contained in Appendix C and are designated as “CPT” (i.e. CPT - *).

The CPT was advanced utilizing the 20-ton tracked CPT rig provided and operated by the Savannah District U.S. Army Corps of Engineers Field Exploration Unit. The CPT tests were planned to be conducted to a depth of 75 feet or until refusal, whichever occurred first. Refusal was encountered at every CPT location and ranged between 37 and 75 feet below grade. A 20-ton tracked CPT rig was utilized to hydraulically push the 10-ton cone. The CPT consists of a three-channel instrumented 1.5-inch diameter steel probe with a 60 degree apex conical tip hydraulically pushed into the ground that measures cone tip stress (qc), sleeve friction (fs), penetration pore water pressure (u), and shear wave velocity (Vs). These measurements are used to geotechnically characterize the site utilizing empirical correlations to estimate soil type, strength (undrained shear strength and friction angle), in-situ lateral earth pressures, and tangent modulus.

A single geophone (pseudo-interval downhole method) was utilized to measure the shear wave velocity at each one-meter rod break. CPT results are contained in Appendix C and are presented graphically as “measured” parameters and “interpreted” parameters based upon published correlations

4. SITE SPECIFIC SUBSURFACE EXPLORATIONS: Site specific subsurface explorations were conducted within the proposed project boundaries of the selected MSF building site between 6 and 22 April 2020 by the Baltimore District U.S. Army Corps of Engineers Field Exploration Unit and IN-SITU SOIL TESTING L.C. These explorations consisted of Standard Penetration testing, flat plate dilatometer tests, test pits, infiltration testing, and soil resistivity testing as discussed below.

a. Standard Penetration Testing (SPT): Standard Penetration Testing (SPT) was completed between 6 and 16 April 2020 by Baltimore District U.S. Army Corps of Engineers Field Exploration Unit. A total of five (5) Standard Penetration Test (SPT) borings were conducted in accordance with ASTMD 1586 to a depth of

51.5 feet. In general, sample intervals were obtained at 2.5 foot intervals until the hard Potomac clays or very dense Potomac sands were encountered at which time the sample intervals were increased to 5.0 feet. Disturbed soil samples collected from the borings were accomplished using the Standard Penetration Test Procedure (SPT) per ASTM D 1586, Standard Test Method for Penetration Test and Split-Barrel Sampling of Soils, using a 1-3/8-inch ID x 2-foot 8-inch long split spoon sampler. Sample spoons were advanced by a 140-pound hammer dropped 30 inches utilizing an automatic hammer. The number of blows required to advance the sample spoon in intervals of 6 inches (for a total 18 inch length sample) were recorded on the logs with the sum of the latter two blow counts (final 12 inches of penetration) referred to as the “Nfield” value. The SPT provides soil samples to evaluate the distribution of soil types and in-situ moisture contents and blow count data which indicates the relative density and consistency of the granular and cohesive soils, respectively. Borings were advanced between sampling utilizing 2 ¼ inch ID hollow stem augers. Unconfined compressive strength readings from a pocket penetrometer were taken on cohesive samples in the split spoon sampler. Locations of the SPT’s are illustrated on the Site Specific Exploration Plan contained in Appendix B and are designated as “SPT” (i.e. SPT

Standard Penetration Testing was conducted utilizing CME-45C Track mounted drill rig equipped with an automatic hammer. The automatic hammer on CME- 45C Track Drill Rig has been calibrated in accordance ASTM D4633-10, Standard Test Method for Energy Measurement for Dynamic Penetrometers, to determine the Energy Transfer Ratio (ETR) which is the ratio of the measured energy transferred to the drill rods to the theoretical potential energy. Based upon the ETR, a corresponding energy ratio correction factor (CE) is computed which can be multiplied to the measured field blow counts (Nfield) to obtain the “N60” value (N-value corrected to 60% of the maximum theoretical energy delivered by 140 pounds falling 30 inches). The Energy Transfer Ratio (ETR) and corresponding Energy Correction Factor (CE) for the CME-45C automatic hammer is 77.0 percent and 1.28 respectively.

b. Test Pits: Four (4) Test pits were excavated on 14 April 2020 by the Baltimore District U.S. Army Corps of Engineers Field Exploration Unit. Test Pits were excavated at four locations to depths of 4.0 feet below grade utilizing a backhoe to obtain bulk samples to conduct a remolded CBR test, modified Proctor test, and obtain natural moisture contents. Locations of the Test Pits are illustrated on the Site Specific Exploration Plan contained in Appendix B and are designated as “TP” (i.e. TP - *).

c. Infiltration Testing: Four (4) infiltration test holes were drilled on 21 and 22 April 2020 by the Baltimore District U.S. Army Corps of Engineers Field Exploration Unit within proposed SWM facilities to provide permeability data to support in obtaining SWM permits. The depths of the holes ranged from 7.5 to

9.0 feet below grade as directed by the AE. The intent was to conduct infiltration testing in accordance with the TESTING GUIDELINES FOR INFILTRATION FACILITIES, FAIRFAX COUNTY, VIRGINIA, July 2012. Unfortunately, groundwater was rose above the test elevation at all four locations and therefore testing was not completed. Locations of the infiltration tests borings are illustrated on the Site Specific Exploration Plan contained in Appendix B and are designated as “IT” (i.e. IT - *). Groundwater levels at each test location are illustrated on the logs in Appendix B and summarized below in paragraph GROUNDWATER. USDA textural classifications were conducted on representative samples from the IT holes and are provided in Appendix E.

d. Stabilized Seasonal High Water Table (SHWT) Borings: Four (4) SHWT borings were conducted by the Baltimore District U.S. Army Corps of Engineers Field Exploration Unit on 22 April to establish soil profiles and the stabilized seasonal high water table (SHWT) elevations. The depths of the holes ranged from 7.5 to

13.5 feet below grade as directed by the AE. Locations of the Seasonal High Water Table (SHWT) borings are illustrated on the Site Specific Exploration Plan contained in Appendix B and are designated as “SP” (i.e. SP - *). Groundwater levels at each test location are illustrated on the logs in Appendix B and summarized below in paragraph GROUNDWATER.

e. Flat Plate Dilatometer Testing: Three (3) Flat Plate Dilatometer Tests (DMT) were performed by IN-SITU SOIL TESTING L.C. on 7 April 2020 in accordance with ASTM D 6635. The DMT’s were conducted adjacent to SPT holes SPT1, 2, and 5. Locations of the DMT tests are illustrated on the Site Specific Exploration Plan contained in Appendix B and are designated as “SPT* DMT”. In general the DMT consists of a stainless steel blade having a flat, circular expandable steel membrane mounted flush on one side. The DMT was hydraulically pushed into the ground and the test conducted at 8-inch intervals. The intent was to advance the DMT to a test depth of 50 feet or until refusal was encountered, whichever occurred first. Refusal was encountered during all three tests at thrust values ranging between 13440 and 14100 kgf (refusal) resulting in test depths ranging from 26.6 to 36.7 feet below grade. The expandable steel membrane was inflated and the pressures required to mobilize two pre-established strain levels are recorded. DMT results are contained in Appendix B and are presented graphically as “measured” parameters and “interpreted” parameters based upon published correlations. These interpreted parameters are based upon generally accepted correlations as defined in Report Number FHWA-PA-87-024+84-24, GUIDELINES FOR USING THE CPT, CPTU AND MARCHETTI DMT FOR

GEOTECHNICAL DESIGN, VOLUME III

f. Soil Resistivity Testing: In-situ resistivity readings were taken in accordance with ASTM G57 utilizing an AEMC Model 4630 Digital Ground Resistance and Soil Resistivity Tester utilizing the 4-point measurement method. Testing was conducted adjacent to borings SPT-3 and 5. Resistivity readings were taken at probe spacing’s of five, ten, fifteen, twenty, and thirty feet at both locations. The Soil Resistivity Survey Report is contained in Appendix D. The average resistivity values (cm) at SPT-3 and SPT-5 were 5596 cm and 6189 cm respectively. Based upon UFC 3-570-02A CATHODIC PROTECTION Table 2- 1, indicates that the corrosion of buried metallic utilities in the soils are “moderate severe”.

5. LABORATORY SOIL TESTING: All laboratory soils testing was completed by the

Baltimore District Corps of Engineers Materials and Instrumentation Unit in accordance with the appropriate ASTM methods. All SPT, TP, IT, and SP jar soil samples were visually inspected and assigned a Unified Soil Classification System (USCS) classification by an experienced laboratory technician. Index properties of samples representative of soil stratification were determined by performing mechanical sieve analysis, Atterberg limits, and in-situ water contents. USDA textural classifications were conducted on samples obtained from the IT holes. One remolded California Bearing Ratio (CBR) tests and one modified Proctor test was completed to evaluate the ability to compact the in-situ subgrade soils based on their in-situ moisture contents relative to the target density, strength, and optimum moisture content. The results of these tests are included in Appendix E and reflected in the Final Boring Logs in Appendix B.

6. REGIONAL GEOLOGY: The site lies within the Coastal Plain province of Virginia.

Approximately 20 to 160 feet of normally consolidated soils overlie the highly preconsolidated Patapsco clay (also referred to as the Potomac clay). Rock is at a depth of several hundred feet. This Patapsco formation is part of the Potomac Group of the Cretaceous Age.

7. SITE SOIL STRATIGRAPHY: A general description of the soil conditions encountered during subsurface investigations conducted within the boundaries of the proposed projects are discussed below. These stratum designations do not imply that the materials encountered are continuous across the site. Actual soil conditions between SPT borings, TP’s, DMT, SP and IT locations may differ from these descriptions.

Topsoil/Root Mat: The existing site is located in a wooded area with approximately 1.5 feet of low plastic clays (CL) intermixed with sand, root mat, and topsoil materials. This layer is soft and shall be removed within pavement and building areas.

STRATUM A: Stratum A extends to depths ranging between 6 to 34 feet below grade with corresponding elevations of 85.5 and 53.3 feet respectively. These materials consist primarily of medium to dense granular (cohesionless) materials (SP, SM, SC SP-SM, SP-SC) with isolated lenses of stiff low plastic clays (CL).

Constrained (Tangent) modulus values obtained from flat plate dilatometer tests (DMT’s) typically range between 2000 and 5000 tsf with triaxial friction angles ranging between 37 and 40 degrees.

STRATUM B: Stratum B, encountered below Stratum A consists of hard, high plastic, fissured (slickensided) clay (CH). The formation is known locally as the Patapsco (Potomac) formation and the clay is known locally as the Patapsco (Potomac) clay and is part of the Potomac Group of the Cretaceous Age.

Typically, this clay is fissured (slickensided); however, it is only troublesome when unconfined, such as on cut slopes. It is also highly preconsolidated and when undisturbed it has very low compressibility characteristics. When confined, as is the case for supporting the proposed foundations, the soil is an excellent bearing layer and is used to support numerous structures in the Washington D.C.

area. Average pocket penetrometer values (qu) conducted on split spoon samples typically exceed 5.0 tsf. Layers of dense to very dense clayey sands are interbedded within the Patapsco clay. Constrained (Tangent) modulus values obtained from flat plate dilatometer tests (DMT’s) typically range between 2000 and 5000 tsf with undrained shear strength values exceeding 6000 psf. SPT borings were terminated in this material at 51.5 feet. DMT tests refused in this material at depths ranging between 26.6 to 36.7 feet below grade.

8. GROUNDWATER: Groundwater readings are an indication of the groundwater levels at the time, date, and conditions as indicated on the boring logs. Fluctuations in the groundwater levels will occur due to variations in season, precipitation, temperature, construction activities, site development, change in surface grades, and other factors not evident at the time measurements were taken. Groundwater depths and corresponding elevations are illustrated on the boring logs in Appendix B and summarized below. In general the long term groundwater level varied from elevation 77.1 to 87.3 feet.

Groundwater Depth/Elevations

Location Top of Hole Elevation

Encountered Completion of Drilling

24 hours after Drilling

4/20/2020

SPT-1 91.32 11.6/79.7 10.6/80.7 10.4/80.9 8.2/83.1

SPT-2 84.39 8.4/76.0 8.6/75.8 7.0/77.39 4.9/79.5

SPT-3 85.88 7.1/78.8 5.6/80.3 3.5/82.4 3.4/82.5

SPT-4 91.62 23.2/68.4 19.6/72.0 4.3/87.3 4.3/87.3

SPT-5 87.34 7.3/80.0 7.8/79.5 5.5/81.8 3.2/84.1

SP-1 82.39 NE 6.9/75.5 3.5/78.9 NT

SP-2 83.48 11.0/72.5 6.1/77.4 5.2/78.3 NT

SP-3 81.65 4.7/77.0 Hole collapsed Hole collapsed NT SP-4 80.12 NE Hole collapsed Hole collapsed NT

IT-1 82.97 NE NE 6.1/76.9 NT

IT-2 82.29 7.3/75.4 3.7/78.6 3.2/79.1 NT

IT-3 83.02 NE 5.4/77.6 5.4/77.6 NT

IT-4 83.64 NE 6.5/77.1 6.5/77.1 NT

SPT – Standard Penetration Test Boring SP – Soil Profile – Stabilized Seasonal High Water Table (SHWT) Boring IT – Infiltration Test Boring NE = Water Not Encountered.

NT = Water Level Not Taken

9. DRAINAGE AND WATER CONTROL: Proper drainage and protection measures shall be employed during the progress of the work to prevent soils from being rendered unsuitable for subgrade, fill or backfill. These measures include such things as proper grading, sealing soils with a roller prior to rain events, proper scheduling of earthwork activities (i.e. in summer), etc. Appropriate measures shall be employed as necessary to maintain dewatered excavations.

10. TEMPORARY EXCAVATIONS: Temporary excavations more than 4 feet high shall be shored or cutback no steeper than to a 2H on 1V slope. Temporary excavations adjacent to structures and roadways shall be cut back on a 2H to 1V slope with the top of the excavation beginning 10 feet away from the outside edge of the footing or pavement edge. Temporary excavations that remain exposed for more than 30 days shall be stabilized to prevent surface sloughing (i.e. vegetated or covered with plastic to reduce infiltration). Where site restraints make cutting back the excavation impractical, the excavation shall be shored using a soldier beam and wood lagging system or other equivalent temporary excavation support system. Temporary excavation support systems are typically designed by a specialty contractor and then reviewed by the geotechnical engineer provided by Baltimore District’s Geotechnical Branch. Excavations will be completed in both granular and cohesive soils. Groundwater flow into the excavation resulting from precipitation or perched water tables may result in sloughing of the excavated slopes. Any groundwater flowing into excavations shall be intercepted with sumps and pumps to ensure that the foundation soils in the excavation are not adversely affected. Heavy equipment and stockpiles shall not be located near the top of excavations.

11. SATISFACTORY FILL and BACKFILL MATERIALS, MOISTURE CONTROL and COMPACTION: : Satisfactory materials for fill, embankment, and backfill shall comprise of any materials classified by ASTM D 2487 as GW, GM, GC, SW, SM, SP, SC, or combinations thereof, properly worked by the Contractor to obtain the specified compaction while maintaining the moisture content as specified hereinafter. CL and ML materials may be used in overlot areas only. The Contractor should, however, be aware of the possible construction difficulties and the very close moisture control required in the proper field placement and compaction of CL and ML materials or combinations thereof. Fill and backfill materials shall not contain organics, compressible materials or trash/debris. The maximum particle size shall be limited to 3 inches in diameter.

Satisfactory fill materials shall be placed in loose lifts not exceeding 8 inches. Lift thickness shall be limited to 6 inches where hand operated compaction equipment is used.

To achieve the desired long-term soil strength characteristics, all fill and backfill materials within the building footprint and beneath pavements shall be compacted at moisture contents within plus or minus 2 percent of optimum moisture to 90 percent and 95 percent of laboratory maximum density in accordance with ASTM D 1557, Procedure C for cohesive and cohesionless soils respectively. In grassed overlot areas, higher moisture contents are allowed as long as 85 percent and 90 percent compaction in accordance with ASTM D 1557, Procedure C for cohesive and cohesionless soils respectively is achieved. Compaction adjacent to walls shall be limited to hand tamping operations within a lateral distance equal to the height of the wall.

12. STORMWATER DRY POND EMBANKMENT

a. General: To satisfy future storm water demand and the latest Fairfax County requirements for storm drainage, a dry detention reservoir is to be constructed upon the site. The pond will consist of an excavation of existing material varying from 1.5 feet along the east to a maximum of 5.0 feet in the center. A new 4.0 foot high embankment will be constructed along the western portion of the pond at a crest elevation of 81.0 feet and corresponding toe elevation of 77.0 feet. The crest width of the embankment will be 16.0 feet with 3 horizontal to 1 vertical side slopes resulting in an embankment foundation width of approximately 40 feet. A 100 foot emergency spillway will be constructed within the embankment at elevation 79.50 feet. Reference Dry Pond Details in Appendix G. The new embankment will consist of a Modified Homogeneous Dam with an 18 inch diameter reinforced concrete pipe outlet structure constructed through the embankment designed and constructed in accordance with the requirements of chapter 6 of the Fairfax County Public Facilities Manual. Selected plates from the Fairfax County Public Facilities Manual are contained in Appendix G. All paragraph and plate references below refer to the 2011 Fairfax County Public Facilities Manual Chapter 6 Storm Drainage.

b. Facility and Dam Type: The subgrade soils will consist of predominately sandy materials is therefore considered pervious. Groundwater, likely perched, may be encountered as high as elevation 77.0. The reservoir height will be 4.0 feet and consist of a dry pool. Therefore, per paragraphs 6-1605.1B and plate 64-6, the reservoir shall be classified as a D Type Reservoir and the embankment constructed as a “Homogeneous Dam” with a cutoff/inspection trench per paragraph 6-1605.1C and plate 65-6.

c. Embankment Foundation and Abutments:

i. Foundation Dewatering: As illustrated above in paragraphs

GROUNDWATER, the presence of perched groundwater is anticipated.

Measures shall be taken to collect, control and divert this water and to dry out the subgrade prior to subgrade preparation for the embankment and outlet structure. The foundation shall be dewatered to ensure that the surficial 6 inches of the subgrade to be compacted does not pump under the weight of compaction equipment as indicated in paragraph 6- 1605.3C(4)(c).

ii. Clearing, Grubbing and Stripping: The foundation and abutment area shall be cleared, grubbed and stripped of all vegetation, topsoil and/or organic soil to the limits of 15 feet beyond the contact with the abutments and 10 feet beyond the toe of the slope of the dam embankment in accordance with paragraph 6-1605.3A.

iii. Foundation Subgrade and Abutment Inspection and Cutoff Trench: Based upon nearby boring logs, the foundation subgrade soils may consist of permeable sand materials. Therefore, although the base of the embankment is approximately 40 feet wide and the height only 4.0 feet, a cutoff /inspection trench is recommended per paragraphs 6-1605.3C(1)(b)

Page and 1605.3C(2). The cutoff trench will likely not extend to an impervious stratum and is therefore considered an inspection trench that will extend the drainage path beneath the embankment foundation. The width of the cutoff trench should be 10 feet in width, 4 feet in depth with maximum 1H:1V side slopes per paragraph 6-1605.3C(4)a. The cutoff trench materials shall consist of impermeable, CL, CH, SC materials compacted at a moisture content within plus or minus 2 percent of optimum moisture to 95 percent of laboratory maximum density in accordance with ASTM D 698 as indicated in paragraph 6-1605.3C(4)(b) ; these materials are expected to be available onsite generated from excavations. The subgrade of the foundation and abutment contact shall be inspected to verify the materials consist of impermeable SC, SM, ML, CL, or CH materials.

Materials not meeting these requirements shall be removed and replaced with impermeable SC, CL, or CH materials to the depths and limits directed by the Contracting Officer.

iv. Subgrade Preparation: The subgrade shall be proof rolled in the presence of the Contracting Officer. Proof rolling shall consist of the application of at least five coverages with a fully loaded 10-ton dump truck or with compaction equipment approved by the Contracting Officer. Areas shown to be unstable or no uniform by proof rolling shall be either recompacted;

removed, dried, and recompacted; or removed and replaced with satisfactory impermeable backfill material, as directed. Proof rolling shall be done immediately prior to placement of the embankment. The top 6-inches of subgrade shall then be compacted at a moisture contents within plus or minus 2 percent of optimum moisture to 95 percent of laboratory maximum density in accordance with ASTM D 698 as indicated in paragraph 6-1605.3C(4)(b).

d. Embankment:

i. Embankment Geometry:

1. Crest Width: The width of the crest of the dam will be a minimum of 12 feet to meet the requirements of paragraph 6-1605.6A (2) (a).

The top surface shall be an access road section consisting of 6.0 inches of 21A Base per Table II-9 of Section 208 Subbase and Aggregate Base Material of the Virginia DOT Standard Specifications for Roads and Bridges, 2007, placed upon a separation geotextile meeting the requirements of Class 1 Survivability in accordance with AASHTO M 288-06. The geotextile shall have MARV hydraulic properties meeting the requirements of AASHTO M 288-06 geotextile criteria for

Page stabilization (i.e. minimum permittivity of 0.05 sec -1 and an apparent opening size (AOS) equal or less than the No. 40 sieve).

The 21A shall be placed in a single 6-inch lift compacted at 95 percent maximum dry density at optimum moisture in accordance with ASTM D 1557. The surface shall be sloped a minimum of

2.0 percent to provide positive surface runoff. Reference plate 70- 6.

2. Slopes: It is not anticipated that the pond will be subjected to a rapid drawdown condition (6 inches or more per day). The embankment will be a Modified Homogeneous Dam constructed of clayey sand (SC) or lean clay (CL) and fat clay (CH) materials.

Therefore, the slopes shall be no steeper than 2.5 horizontal to 1 vertical which will satisfy the requirements of a Homogeneous Dam per plate 69-6.

3. Dam Height and Base Width: The finished grade of the interior of the pond and the existing grade at the downstream toe is elevation

77.0 feet. The dam crest elevation is 81.0 feet resulting in a maximum dam height of 4.0 feet. The base width of the dam will therefore be 36.0 feet for a 2.5H to 1V embankment slope and 40.0 feet for a 3H to 1V embankment slope.

ii. Satisfactory Embankment Materials and Compaction Requirements: The pond embankment shall be constructed of Select Earth Fill. Select Earth Fill materials shall be well graded having a maximum particle size not exceeding 2/3 of the placement lift thickness and not less than 20% by weight passing a Standard No. 200 sieve. The select earth fill shall consist of materials meeting the above requirements and classified in ASTM D 2487 as clays (CL and CH), silty or sandy clays (CL-ML, CL); silts (ML), or clayey sands (SC). Silty Sands (SM) with not less than 25% by weight passing a Standard No. 200 sieve may be used ONLY on the upstream half of the embankment away from the drainage blanket and any filters.

Uniform silts or sands, soft organic soils, frozen materials, or other soils deemed unsuitable shall not be utilized in the Select Earth Fill. Uniform silts or sands are defined as silts and sands with a uniformity coefficient (Cu=D60/D10) which is less than 6.

Select Earth shall be placed and spread in layers not exceeding 8 inches in uncompacted thickness. Lift thickness shall be limited to 4 inches where hand operated compaction equipment is used. The maximum stones and rock fragments permitted in any lift shall not exceed 2/3 the uncompacted

Page lift thickness measured by the greatest dimension. Select Earth shall be compacted at moisture contents within plus or minus 2 percent of optimum moisture to 95 percent of laboratory maximum density in accordance with AASHTO T 99 or ASTM D 698 (Standard Proctor). Any layer that becomes smooth under compaction or construction traffic shall be scarified to a depth of 2-inches to allow adequate bond between lifts.

Reference paragraph 6-1605.6F (2).

To ensure proper compaction and contact at the abutments, select earth fill shall be benched into the abutments every 8-inch lift maintaining a 2.0 horizontal to 1.0 foot vertical slope along the abutment. Compaction at the abutments shall be achieved by rolling perpendicular to the abutment while the remaining embankment shall be compacted with the roller parallel to the embankment. Final benches at the abutments shall be benched just prior to placement of the embankment materials.

e. Outlet Structure Conduit:

i. Backfill around Outlet Conduit: Due to the difficulties in obtaining proper compaction around anti-seep collars, anti-seep collars shall not be used (reference paragraph 6-1605.4). The portion of the outlet pipe extending from the outlet control structure to within 1/3 of the distance of the base width of the embankment measured from the downstream toe shall be placed upon a concrete cradle that extends up to the level of the spring line of the pipe and to a minimum lateral distance and depth below the invert of the pipe of 8.0 inches as illustrated by plate 67-6 and paragraph 6- 1605.5A (1). The concrete shall have a minimum 28 day compressive strength of 4000 psi and the sides sloped at 1H to 10V for proper compaction adjacent to the concrete cradle. If the conduit is to be laid in a trench, the sidewalls of the excavation may be used as the cradle form.

The material above the concrete cradle shall consist of Select Earth Fill as defined above in paragraph Satisfactory Embankment Materials and Compaction Requirements.

The final portion of the outlet pipe located beneath the final 1/3 of the embankment shall be placed upon a minimum of 8-inches of bedding of VDOT #78 stone meeting the quality and grading requirements of the Virginia Department of Transportation Road and Bridge Specification, Section 203, Table II-3 Sizes of Open-Graded Coarse Aggregates. The VDOT #78 stone shall extend up to the level of the spring line and 8.0 inches laterally around the pipe. Fine Drainage Fill material meeting quality and grading requirements of the Virginia Department of Transportation Road and Bridge Specification, Section 202, Table II-1

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Fine Aggregate Grading A, shall then be placed from the spring line of the pipe to a height of 12-inches above the crown of the pipe and 8 inches beyond each side of the pipe. IN NO CASE SHALL THE VDOT #78

STONE BE IN DIRECT CONTACT WITH SELECT EARTH FILL. A

MINIMUM OF 12 INCHES OF FINE DRAINAGE FILL MATERIAL

SHALL BE BETWEEN THE VDOT #78 STONE AND THE SELECT

EARTH FILL. VDOT #78 Stone meets the filter requirements for a Fine Drainage Fill Base and Fine Drainage Fill meets the filter requirements for Select Earth Fill base per paragraphs 6-1605.4A. Gradation requirements and Filter calculations are contained in Appendix F.

ii. Placement and Compaction Requirements around Conduits: Fill shall be placed as defined in paragraph 6-1605.5A (7). Fine Drainage Fill and VDOT #78 Stone shall be placed in lifts not exceeding 8-inches and com-pacted by not less than four (4) complete coverages of a 10 ton (minimum) smooth drum static roller. In confined areas, lift thicknesses shall be reduced to 4-inches and the materials shall be compacted with at least four

(4) complete coverage with a vibratory plate tamper and as many additional coverages as necessary to achieve the same density obtained with heavy compaction equipment. Select Earth fill shall be compacted as defined above in paragraph Satisfactory Embankment Materials and Compaction Requirements. Fill and backfill adjacent to conduits/structures should be placed such that lifts are at the same level on both sides of the conduit and shall be ramped away at from the conduit on a 6H:1V slope to improve the quality of compaction.

iii. Conduit Type and Joints: The pipe shall be a reinforced concrete pipe meeting the requirements of paragraphs 6-1605.5A (2) and 6-1607.1B (4).

Pipes Joints shall be watertight with rubber gaskets set in groove per paragraph 6-1605.5A (3).

f. Emergency Spillway: Riprap Emergency Spillway shall be designed in accordance with paragraphs 6-1604.3. A combined Principal and Emergency Spillway should be considered per paragraph 6-1604.6.

g. Construction Inspection: Testing shall be the responsibility of the Contractor and shall be performed at no additional cost to the Government. Testing shall be performed by an approved commercial testing laboratory or by the Contractor subject to approval. Approval of testing facilities shall be based on requirements indicated in SECTION 01 45 00.00 40, QUALITY CONTROL. Moisture-density relations shall be determined in accordance with ASTM D 698 or ASTM D 1557, Procedure C or AASHTO T 180-01, Method D as specified in the latter paragraphs. Field in-place density shall be determined in accordance with ASTM

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D 1556, ASTM D 2167 or ASTM D 2922. When ASTM D 2922 is used, the calibration curves shall be checked and adjusted using only the sand cone method as described in ASTM D 1556. ASTM D 2922 results in a wet unit weight of soil and when using this method ASTM D 3017 shall be used to determine the moisture content of the soil. The calibration curves furnished with the moisture gauges shall also be checked along with density calibration checks as described in ASTM D 3017; the calibration checks of both the density and moisture gauges shall be made at the beginning of a job on each different type of material encountered and at intervals as directed by the Contracting Officer. ASTM D 2937. When test results indicate, as determined by the Contracting Officer, that compaction is not as specified, the material shall be removed, replaced and recompacted to meet specification requirements. Tests on recompacted areas shall be performed to determine conformance with specification requirements.

Sieve analyses shall be performed in accordance with the latest ASTM C 117, ASTM C 127, ASTM C 128, ASTM C 136 and ASTM D 422; sieves shall conform to the latest ASTM E 11; and liquid limit and plasticity index determinations shall be performed in accordance with ASTM D 4318. Copies of these test results shall be furnished to the Contracting Officer. Inspections and test results shall be certified by a registered professional geotechnical engineer.

These certifications shall state that the tests and observations were performed by or under the direct supervision of the engineer and that the results are representative of the materials or conditions being certified by the tests. The following number of tests, if performed at the appropriate time, will be the minimum acceptable for each type operation. Locations of tests shall be at the direction of the Government.

i. Sieve Analysis: One sieve analysis and one liquid limit and plasticity index determination per 3000 square yards or fraction thereof of subgrade and per 1000 cubic yards or fraction thereof of fill and backfill.

Additional gradation, liquid limit, plasticity index determinations, and moisture-density curves shall be required if there are any changes in gradation or particle shape or when any change occurs in the material which may affect the permeability, optimum moisture content or maximum laboratory density.

ii. In-Place Densities and Moisture Contents: One test per 1000 square yard, or fraction thereof, of each lift of fill or backfill with at least one test occurring every other lift.

iii. Optimum Moisture and Laboratory Maximum Density: Tests shall be made for each type material or source of material including borrow material to determine the optimum moisture and laboratory maximum density values. One representative test shall be done for each type of

Page subgrade material (embankment, fill and cut areas); per 3000 cubic yards of fill and backfill or fraction thereof, or when any change in material occurs which may affect the optimum moisture content or laboratory maximum density.

h. Pond Liner: Geotechnically, a pond liner is nor required since there are no environmental concerns and the pond is not constructed upon Karst geology.

13. PAVEMENTS

a. Criteria: Pavements were designed to meet both strength and frost protection requirements for a 25-year service life utilizing PCASE VERSION 2.09 computer software (http://www.pcase.com.). PCASE Version 2.09 software incorporates the latest Department of the Army criteria, UFC 3-250-01Nov 16, PAVEMENT DESIGN FOR ROADS AND PARKING AREAS, for the design of the pavements.

b. Traffic: : The Access Road was designed to accommodate 500 trips of 5-axle trucks, 104 trash trucks, and 14,600 passenger cars per year resulting in 12,500 passes of 5-axle trucks, 2,600 passes of trash trucks, and 365,000 passenger cars per the 25 year life span of the pavement. The exterior concrete loading dock was designed to accommodate 25,000 passes of 5-axle trucks per the 25 year life span.

Parking areas were designed to accommodate only POV vehicles. Interior concrete pavements were designed to accommodate 10 passes per day of a 25kip axle load forklift resulting in 91,250 passes per the 25 year service life span.

c. Pavement Sections: The proposed pavement sections illustrated below do not consider construction traffic. Construction traffic may result in failure of the subgrade and pavement section. Asphalt surface courses shall not be placed until pavement areas will no longer be subjected to construction traffic. Failed subgrades and base courses shall be repaired prior to placing asphalt. Pavement details are provided in Appendix G.

HEAVY DUTY BITUMINOUS PAVEMENT SECTION (Access Roads)

1.5 inches Bituminous Concrete Surface Course (9.5mm) Tack Coat

2.5 inches Bituminous Base Course (19mm)

4.0 inches Graded Aggregate Base Course

6.0 inches Rapid Drainage Material (RDM) Base Course Separation Geotextile

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LIGHT DUTY BITUMINOUS PAVEMENT SECTION (POV Parking Stalls)

1.5 inches Bituminous Concrete Surface Course (9.5mm) Tack Coat

2.5 inches Bituminous Base Course (19mm)

4.0 inches Graded Aggregate Base Course

4.0 inches Rapid Drainage Material (RDM) Base Course

EXTERIOR HEAVY DUTY CONCRETE PAVEMENT SECTION (Loading docks)

7.0 inches Concrete (5000 psi compressive strength @28 days) with 4 X 4 W 4 W 4 placed 2.75 inches below concrete surface

8.0 inches Rapid Drainage Material (RDM) Base Course

INTERIOR CONCRETE SLABS SECTION (Subjected to forklift traffic)

7.0 inches Concrete (5000 psi compressive strength @28 days) with 4 X 4 W 4 W 4 placed 2.75 inches below concrete surface 10 mil Polyethylene Sheeting Vapor Barrier

5.5 inches Capillary Water Barrier

d. Subgrade Properties and Requirements: The cohesive nature of the surficial soils makes them susceptible to strength loss under adverse moisture conditions. Based upon a comparison between the optimum moisture content obtained from the results of remolded CBR test (ASTM D 1883) and the in-situ natural moisture contents, it is anticipated that unstable subgrade conditions may be encountered throughout the site. These unstable subgrade soils will result in excessive pumping and loss of strength when subjected to vehicular traffic. Based upon the permeability characteristics of these subgrade soils, isolated high in-situ moisture contents, and the potential for winter and spring construction operations, isolated areas of the surficial site soils may be too wet to dry back to within a satisfactory range within a reasonable time frame. Therefore, to provide trafficability to the site and to obtain the required moisture control and compaction requirements beneath pavements and within the building footprints, it should be anticipated that isolated areas of the subgrade soils will have to be stabilized as directed by the Contracting Officer. Where possible, construction of the pavements should be scheduled for dryer seasons when moisture conditions are more favorable.

The subgrade beneath interior slabs and exterior paved areas shall be proof rolled in the presence of the Contracting Officer to identify isolated areas of soft, wet and unstable subgrade soils. Proof rolling shall consist of the application of at least three coverage’s with a loaded (approximately 10 cubic yard dump truck) dump truck weighing a minimum of 10 tons. Soft or loose areas identified

Page beneath interior slabs shall be overexcavated and replaced with materials as defined in paragraph SATISFACTORY FILL AND COMPACTION REQUIREMENTS. Areas beneath proposed pavements where substantial pumping of the subgrade is observed and the in-situ moisture contents are 6 percent or greater above optimum moisture shall be stabilized as directed by the Contracting officer. Subgrade stabilization is not required in areas where pumping is not observed when subjected to a 10-ton proof roller or where soil moisture contents are less than 6 percent above optimum moisture. These areas where subgrade stabilization is not required shall be dried back to within plus 2 percent of optimum, plowed, disked, or otherwise broken up, thoroughly mixed, and compacted to at least 90 percent laboratory maximum density for cohesive materials or 95 percent laboratory maximum density for cohesionless materials.

Stabilization will generally consist of the overexcavation of unstable in-situ subgrade materials to a depth of 12 inches beneath the existing grades and extending laterally 5 feet beyond the edge of the unstable area. Replace the 12-inch excavation with Graded Aggregate Base Course. To bridge the wet subgrade soils, the 12-inch lift of Dense Graded Aggregate shall be placed between 90 and 95 percent maximum dry density at optimum moisture in accordance with ASTM D 1557. Vibratory compaction shall be used cautiously to minimize the “wicking up” of moisture from the wet subgrade soils into the stabilization layer therefore jeopardizing the stability of the stabilization (bridging) layer. Prior to placement of the Graded Aggregate Base Course, a soil stabilization geotextile shall be placed directly upon the subgrade. The stabilization geotextile shall be a woven or non-woven geotextile except that heatbonded non-woven and slit film woven geotextile shall NOT be used. The geotextile shall meet the requirements of Class 1 Survivability in accordance with AASHTO M 288. The geotextile shall have MARV hydraulic properties meeting the requirements of AASHTO M 288 geotextile criteria for stabilization (i.e. minimum permittivity of 0.05 sec -1 and an apparent opening size (AOS) equal or less than the No. 40 sieve).

e. Strength and Frost Design Parameters: A California Bearing Ratio (CBR) of 3 was used for the design of the bituminous pavements and a modulus of subgrade reaction (k-value) of 150 pci was used for the design of the interior and exterior concrete slab and pavements. The CBR value is based upon the results of the remolded CBR test. The k-value is based upon an empirical correlation between the in-situ moisture contents and soil type and correlations with the CBR value.

The subgrade is classified as an F-3/F-4 material for frost considerations

f. Asphalt Materials: Asphalt paving shall consist of a Virginia DOT approved “Superpave” mixture. The mix design shall use a PG 64-22 binder, a nominal maximum aggregate size of 9.5 mm for the surface and 19.0 mm for the base and a Design ESAL frequency of 0.3 million to 3 million.

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g. Concrete Strength, Joint Spacing, Doweling, and Reinforcing: Concrete pavements shall have a minimum compressive strength of 5000 psi at 28 days.

All longitudinal and transverse control joints of the reinforced concrete shall be spaced at a maximum distance of 30.0 feet to create square to near square panels.

All joints in concrete pavements shall be doweled. Dowels shall consist of 0.75 inch diameter smooth bars 16 inches in length with a minimum spacing of 12 inches. All dowels shall be placed at a depth midway into the slab. One end of the dowel shall be painted and oiled to prevent bonding with the concrete.

Dowels shall not be placed within thickened edge expansion joints.

Reinforcement is required in all exterior concrete pavements as well as interior pavements subjected to forklift traffic and shall consist of 4 X 4 W 4 W 4 placed

2.75 inches below the surface of the concrete.

h. Graded Aggregate Base Course: Graded Aggregate Base shall meet the requirements for 21A Base in Table II-9 of Section 208 Subbase and Aggregate Base Material of the Virginia DOT Standard Specifications for Roads and Bridges, 2016. Crushed Recycled Concrete shall not be permitted.

i. Rapid Drainage Material (RDM) Base Course: The RDM layer shall be incorporated into the final pavement sections as illustrated in paragraph Subsurface Drainage (Pavement Subdrains).