Attachment_2_-_Amendment_0001_-_Old_Rag_Parking_Lot_Geotech_Report_0001.pdf

Corporate HQ: 3015 Dumbarton Road Richmond, Virginia 23228 T 804.264.2701 F 804.264.1202 www.fandr.com

VIRGINIA • NORTH CAROLINA • SOUTH CAROLINA • MARYLAND • DISTRICT OF COLUMBIA

A Minority-Owned Business

Report of Geotechnical Study

Old Rag Parking Lot Shenandoah National Park Madison County, Virginia

F&R Project No. 72N-0139G

Prepared For:

GWWO, Inc./Architects 800 Wyman Park Drive, Suite 300

Baltimore, Maryland 21211

Prepared By:

Froehling & Robertson, Inc.

22923 Quicksilver Drive, Suite 111 Dulles, Virginia 20166-2013

May 31, 2012

GWWO, Inc./Architects Old Rag Parking Lot – Madison County, Virginia

F&R File No. 72N-0139G May 31, 2012 Page - i -

TABLE OF CONTENTS

SECTION PAGE

EXECUTIVE SUMMARY

1.0 PURPOSE & SCOPE OF SERVICES

2.0 PROJECT INFORMATION

2.1 SITE DESCRIPTION

2.2 PROPOSED CONSTRUCTION

3.0 EXPLORATION PROCEDURES

4.0 REGIONAL GEOLOGY & SUBSURFACE CONDITIONS

4.1 REGIONAL GEOLOGY

4.2 SUBSURFACE CONDITIONS

4.2.1 General

4.2.2 Surficial Materials

4.2.3 Fill Soils

4.2.4 Alluvium

4.2.5 Residual Soils

4.2.6 Soft Weathered Rock

4.2.7 Auger Refusal Materials

4.2.8 Bedrock Materials

4.3 SUBSURFACE WATER

4.4 LABORATORY TEST RESULTS

5.0 GEOTECHNICAL DESIGN RECOMMENDATIONS

5.1 GENERAL

5.2 FOUNDATION DESIGN

5.3 SETTLEMENT

5.4 GROUND FLOOR SLABS

5.5 PAVEMENT CONSTRUCTION

5.6 INFILTRATION

5.7 SEISMIC CONSIDERATIONS

6.0 GEOTECHNICAL CONSTRUCTION RECOMMENDATIONS

6.1 SITE PREPARATION

6.2 EXCAVATION CONDITIONS

6.3 FOUNDATION CONSTRUCTION

6.4 STRUCTURAL FILL PLACEMENT AND COMPACTION

6.5 SURFACE WATER/GROUNDWATER CONTROL

7.0 CONTINUATION OF SERVICES

8.0 LIMITATIONS

Page - ii -

APPENDICES

APPENDIX I

Site Vicinity Map (Drawing No. 1) Boring Location Plan (Drawing No. 2)

APPENDIX II

Key to Soil Classification Unified Soil Classification System

Boring Logs Subsurface Profile

APPENDIX III

Lab Test Results

APPENDIX IV

ASFE Document “Important Information about Your Geotechnical Engineering Report”

Page - 1 -

EXECUTIVE SUMMARY

This Executive Summary is provided as a brief overview of our geotechnical engineering evaluation for the project and is not intended to replace more detailed information contained elsewhere in this report. As an overview, this summary inherently omits details that could be very important to the proper application of the provided geotechnical design recommendations. This report should be read in its entirety prior to implementation into design and construction.

The site was explored by fifteen soil test borings performed between the dates of March

22, 2012 and April 20, 2012. Site subsurface conditions generally consisted of alluvial materials, deposited by the neighboring mountain runoff streams, and residual soils residing over relativity shallow bedrock.

The following Seismic Site Class Definition was established per Section 1613.5.2 of the

2009 International Building Code (IBC). Based on our experience in this area and the data from our testing and subsurface exploration, a Site Classification “C” should be used for further evaluations relative to earthquake load design.

For gravel pavement design purposes bulk samples were classified using the Unified Soil

Classification System, and soaked CBR values at 0.1 inches penetration were calculated.

We recommend a CBR value of 9.0 should be used for design; however the CBR value of the actual subgrade materials used should be verified prior to the construction of any pavements.

Boring D-3 is located within the footprint of the proposed Bio-Retention basin by the upper lot. We note that the empirical rate of infiltration from the soils collected in D-3 matches the minimum rate required by the DCR; however the result from the field test is significantly lower. Also, it should be understood subsurface water records were not available from boring D-3, but the neighboring boring D-4 encountered subsurface water at a depth of 2 feet below existing grade. Based on the foregoing, infiltration practices are not recommended at boring location D-3 at the elevation tested.

Page - 2 -

1.0 PURPOSE & SCOPE OF SERVICES

The purpose of the subsurface exploration and geotechnical engineering evaluation was to explore the subsurface conditions in the area of the proposed Old Rag Parking Lots and provide geotechnical engineering evaluation and construction recommendations that can be used during planning process of the proposed structures and site work.

F&R’s scope of services included the following:

Visited the site to observe existing surface conditions;

Coordinated utility clearance with Miss Utility;

Reviewed readily available geologic and subsurface information relative to the project site;

Completion of fifteen soil test borings to depths ranging from 2 feet to 18 feet below the existing ground surface;

Preparation of typed Boring Logs and development of a Subsurface Profile;

Performing a geotechnical engineering evaluation of the subsurface conditions with regard to their suitability for the proposed construction;

Provided recommendations for slab on grade design and construction;

Provided recommendations for parking lot design and construction;

Provided recommendations regarding the placement and compaction of fill materials required to achieve site subgrades, including an assessment of the suitability of the on-site soil for re-use as structural fill, and recommendations regarding rock excavation.

Perform infiltration tests and conduct USDA classifications for recommendations regarding infiltration practices;

Provided a seismic site class definition. The seismic site class definition was assigned based on the test boring Standard Penetration Test data and correlations provided in the 2009

IBC;

Preparation of this geotechnical report by professional engineers.

Our scope of services did not include a survey of the boring locations, quantity estimates, preparation of plans or specifications, or the identification and evaluation of wetland or other environmental aspects of the project site.

Page - 3 -

2.0 PROJECT INFORMATION

2.1 Site Description

The project site is located at the base of Old Rag Mountain, just south of VA Route 600 in

Madison County, Virginia (See Site Location Plan, Drawing No. 1, Appendix I). The site is currently occupied by relatively light forest overgrowth consisting of thin trees and shrubs that is divided by mountain stream channels and an existing gravel road running northeast. A mapped wetland currently bisects the site. This wetland runs from the southwest corner of the site to the northeast corner where it intersects the existing roadway. The wetland channel is covered with large boulders. These boulders were also encountered scattered throughout the project site. A second wetland is located to the south of the project site.

An archeological area has also been identified on site. This area is located on the southern portion of the site and is occupied by a small wooden structure. Topographically the area gradually slopes uphill away from the existing paved road; in general existing grades within the anticipated project area slope from El 945 in the southwest to approximately El 900 in the northeast.

2.2 Proposed Construction

Project information was provided in an email and our correspondence with Mr. Bryan Fisher, AIA of GWWO, Inc./Architects, including our review of the “Schematic Design Development of

Alternatives & Class C Construction Cost Estimate”, dated April, 2007. It is understood that two gravel parking lots and a connector trail containing footbridges are planned for the project site.

Parking lots will accommodate up to 169 vehicles and 4 busses.

The lower lot will serve as primary parking for the Old Rag Mountain trail. This lower lot will contain a stormwater BMP on the northern edge of the parking area. The location of this BMP was not finalized at the time of our field exploration. Future expansion of the lower lot will also include a new Contact Station; this station will serve as the main toilet facility for the parking areas. Structural loads were not provided, however, based on experience with similar projects we anticipate column loads less than 50 kips and wall loads less than 6 kips per linear foot.

The upper lot will serve as an overflow parking lot and provide primary parking for larger vehicles including mobile campers and busses. A stormwater BMP is planned at the northeast corner of the upper lot.

At the time of this exploration the type of BMP had not been finalized. This field exploration includes some infiltration testing to evaluate the use of infiltration on site. Based on the provided preliminary layout and the existing topography, we anticipate that cuts and fills will be less than 5 feet to establish planned subgrades.

Page - 4 -

3.0 EXPLORATION PROCEDURES

3.1 Subsurface Exploration

The exploration program was performed between the dates of March 22, 2012 and April 20, 2012, and consisted of fifteen soil test borings designated A-1 through A-4 (proposed pedestrian bridge abutment locations), B-1 (proposed contact station location), D-1 through D-

4 (stormwater detention/infiltration) and P-1 through P-6 (parking lots). The soil test borings were drilled until auger refusal materials were encountered. The locations of the borings are shown on the attached Boring Location Plan (Drawing No. 2). The planned boring locations were determined and staked in the field by F&R by measuring from existing site features such as topography, roads, edges of pavement, etc. Surface elevations at the boring locations were estimated to the nearest foot from the topography indicated on the provided site plans. In consideration of the methods used in their determination, the test boring locations shown on the attached boring location plan and the elevations shown on the boring logs should be considered approximate.

The soil test borings, except for A-3 which was hand augered, were performed in accordance with generally accepted practice using an ATV-mounted CME-55 rotary drill rig equipped with an automatic hammer. Hollow-stem augers were advanced to pre-selected depths, the center plug was removed, and representative soil samples were recovered with a standard split-spoon sampler (1 3/8 in. ID, 2 in. OD) in general accordance with ASTM D 1586, the Standard

Penetration Test. For these tests, a weight of 140 pounds was freely dropped from a height of

30 inches to drive the split-spoon sampler into the soil. The number of blows required to drive the split-spoon sampler three consecutive 6-inch increments was recorded, and the blows of the last two increments were summed to obtain the Standard Penetration Resistance (N-value).

The N-value provides a general indication of in-situ soil conditions and has been correlated with certain engineering properties of soils.

An automatic hammer was used to perform the Standard Penetration Test (SPT) on this project.

Research has shown that the Standard Penetration Resistance (N-value) determined by an automatic hammer is different than the N-value determined by the safety hammer method.

Most correlations that are published in the technical literature are based on the N-value determined by the safety hammer method. This is commonly termed N60 as the rope and cathead with a safety hammer delivers about 60 percent of the theoretical energy delivered by a 140-pound hammer falling 30 inches. Several researchers have proposed correction factors for the use of hammers other than the safety hammer to correct the values to be equivalent to the safety hammer SPT N60-values. The correction is made using the following equation:

N60 = Nfield x CE

Page - 5 -

Nfield is the value recorded in the field and CE is the drill rod energy ratio for the hammer utilized in the field. When using an automatic hammer, it is recommended that a correction factor (CE) of 1.3 be utilized to covert Nfield values to N60 values in accordance with guidelines provided in the Performance and Use of the Standard Penetration Test in Geotechnical Engineering Practice manual published by the Center for Geotechnical Practice and Research at the Virginia

Polytechnical Institute and State University. The N-values reported on the Boring Logs and

Subsurface Profiles included in this report are the actual, uncorrected, field derived values

(Nfield). It is recommended that corrected N60 values be used for engineering analysis.

In some soils it is not always practical to drive a split-spoon sampler the full three consecutive

6-inch increments. Whenever more than 50 blows are required to drive the sampler over a 6-inch increment, or the sampler is observed not to penetrate after 10 blows, the condition is called split-spoon refusal. Split-spoon refusal conditions may occur because of obstructions or because the earth materials being tested are very dense or very hard. When split-spoon refusal occurs, often little or no sample is recovered. The SPT N-value for split-spoon refusal conditions is typically estimated as > 100 blows per foot (bpf). Where the sampler is observed not to penetrate after 10 blows, the N-value is reported as 10/0. Otherwise, the depth of penetration after 50 blows is reported in inches, i.e. 50/5, 50/2, etc.

The test borings were advanced through the soil overburden by soil drilling procedures until auger refusal material was encountered. Rock coring was performed at borings A-1, A-2, A-4 B-

1, and D-4. Rock coring was accomplished in general accordance with ASTM D 2113 using a 2-inch nominal inside diameter diamond-impregnated drill attached to the end of a double-tube core barrel. Rock core specimens were measured for recovery immediately upon retrieval, placed in core boxes for protection, labeled and transported to the laboratory for further evaluation by our professional staff. In the laboratory, the rock core specimens were measured for Percent Recovery and Rock Quality Designation (RQD) by a member of our professional staff. Percent Recovery is the ratio of the recovered core length to the length of rock drilled, expressed by a percentage. RQD is the ratio of the cumulative length of all pieces of rock greater than or equal to four (4) inches to the total amount drilled, expressed as a percent of the total amount drilled. The RQD value is related to the soundness and quality of the rock mass and has been correlated with engineering properties of rock. Qualitative descriptions of the rock cored were also developed and are included on the boring logs.

Upon completion of the test borings D-1 and D-3, an offset test boring was conducted per each of the initial test borings were drilled to depths of of 3.2 and 3.4 feet below the existing ground surface for infiltration testing at locations D-1 and D-3 respectively. A solid stand pipe was installed in each of the offset test borings and 24 inches of water was placed in the pipes to allow the soil to pre-soak. Twenty-four hours after initial soaking of the soils, the water level was returned to 24 inches and allowed to infiltrate into the soil. Water levels were then

Page - 6 -recorded every hour for four hours. After four hours of testing, the pipes were removed from the ground, and the holes were backfilled with auger cuttings.

Subsurface water level readings were taken in each of the borings during the drilling process.

Upon completion of drilling, the boreholes were backfilled with auger cuttings (soil). Periodic observation of the boreholes should be performed to monitor subsidence at the ground surface, as the borehole backfill could settle over time.

Representative portions of the split-spoon soil samples obtained throughout the exploration program were placed in glass jars and transported to our laboratory. In the laboratory, the soil samples were evaluated by a member of our engineering staff in general accordance with techniques outlined in the visual-manual identification procedure (ASTM D 2488). The soil descriptions and classifications discussed in this report and shown on the attached Boring Logs are based on visual observation and should be considered approximate. A copy of the boring logs are provided and classification procedures are further explained in Appendix B.

Split-spoon soil samples recovered on this project will be stored at F&R’s office for a period of

60 days. After 60 days, the samples will be discarded unless prior notification is provided to us in writing.

3.2 Laboratory Testing

Representative soil samples were subjected to Water Content (ASTM D 2216), Sieve Analysis

(ASTM D 442), and Atterberg Limits (ASTM D 4318) testing to substantiate the visual classifications and assist with the estimation of the soils’ pertinent engineering properties.

Additional soils testing conducted on bulk samples included the California Baring Ratio (ASTM D

1883) and Stand Proctor (ASTM D 698). The results are shown in Section 4.4.

4.0 REGIONAL GEOLOGY & SUBSURFACE CONDITIONS

4.1 Regional Geology

The project site is located in an upland area of the Blue Ridge Physiographic Province, an area underlain by ancient metamorphic and igneous rocks. Information obtained from the Geologic

Map of Virginia (1993) indicates that this area is underlain by the Old Rag Granite Formation, of the Blue Ridge Basement Complex of middle Proterozoic age. The Old Rag Granite is described as a leucocratic, coarse grained, foliated, alkali feldspar-blue quartz granite, containing accessory biotite, muscovite, garnet, magnetite, and zircon. The virgin soils encountered in this area are the residual product of in-place chemical and mechanical weathering of the parent

Page - 7 -bedrock formation that underlies the site. These materials consist of sandy silts near the surface where soil weathering is more advanced, underlain by sandy gravel.

The boundary between soil and rock is often times not sharply defined. The transitional term

“Hard or Soft Weathered Rock” is normally found overlying the parent bedrock. For engineering purposes, SWR is described as broken and partially weathered rock with Standard

Penetration Resistance N-values between 50 blows per 6 inches and 50 blows per inch. HWR is described as broken and partially weathered rock with N values in excess of 50 blows per inch.

Weathering is facilitated by fractures, joints and the presence of less resistant rock types.

Consequently, the profile of the SWR or HWR is often quite irregular, even over very short horizontal distances. Also, it is not unusual to find lenses, layers, or zones of less resistant SWR and more resistant HWR, and boulders of hard rock and within the soil mantle well above the general bedrock level.

4.2 Subsurface Conditions

4.2.1 General

The subsurface conditions discussed in the following paragraphs and those shown on the attached Boring Logs represent an estimate of the subsurface conditions based on interpretation of the boring data using normally accepted geotechnical engineering judgments.

The transitions between different soil strata are usually less distinct than those shown on the boring logs. Sometimes the relatively small sample obtained in the field is insufficient to definitively describe the origin of the subsurface material. In these cases, we qualify our origin descriptions with “possible” before the word describing the material’s origin (i.e. possible fill, etc.). Although individual soil test borings are representative of the subsurface conditions at the boring locations on the dates shown, they are not necessarily indicative of subsurface conditions at other locations or at other times. Data from the specific soil test borings is shown on the attached Boring Logs in Appendix II.

Subsurface Profiles have been prepared from the boring data to graphically illustrate the subsurface conditions encountered at the site. The Subsurface Profiles can be found after the boring logs in Appendix II. Strata breaks designated on the Boring Logs and Subsurface Profile represent approximate boundaries between soil types. The transition from one soil type to another may be gradual or occur between soil samples. This section of the report provides a general discussion of subsurface conditions encountered within areas of proposed construction at the project site.

Below the existing ground surface, the borings generally encountered surficial organics, possible fill, alluvial soils, residual soils, soft weathered rock, and auger refusal materials. These materials are generally discussed in the following paragraphs.

Page - 8 -

4.2.2 Surficial Materials

Surficial organic soils were encountered in each test boring and extended to depths of 2 to 6 inches. Surficial organic soil is typically a dark-colored soil material containing roots, fibrous matter, and/or other organic components, and is generally unsuitable for engineering purposes. F&R has not performed any laboratory testing to determine the organic content or other horticultural properties of the observed surficial organic soil materials. Therefore, the term surficial organic soil is not intended to indicate a suitability for landscaping and/or other purposes. The surficial organic soil depths provided in this report are based on driller observations and should be considered approximate. We note that the transition from surficial organic soil to underlying materials may be gradual, and therefore the observation and measurement of surficial organic soil depths is subjective. Actual surficial organic soil depths should be expected to vary.

4.2.3 Fill Soils

Fill may be any material that has been transported and deposited by man. The majority of the project site appears to be undisturbed land. Soils considered Possible Fill material were encountered below the surficial materials in test borings D-2, P-2, and P-4, and extended to a depth of 3.5 feet below existing grades. Fill materials were described as Sandy SILT (ML), and

Sandy Lean CLAY (CL) with varying amounts of gravel and organic material. The sampled fill materials were dark brown and brown, in color, with water contents visually characterized as moist to very moist. The Standard Penetration Test values (N-Values) in the fill materials ranged from 6 bpf to 8 bpf.

4.2.4 Alluvium

Alluvial deposited materials were sampled from borings A-1 through A-4, and D-1. The alluvium deposited during periodic flooding events were brown, dark brown, black, and gray in color, and soils visually classified as Sandy SILT (ML), silty SAND (SM), and silty GRAVEL (GM), with varying amounts of organic material. Water contents were visually characterized as very moist to wet. SPT N-values in the sampled alluvium material ranged from 3 bpf to 45 bpf.

4.2.5 Residual Soils

Residual Soils, formed by the in-place weathering of the parent rock, were encountered under the fill soils and/or alluvium, and extended until encountering soft weathered rock, or auger refusal. These residual soils were generally described as sandy SILT (ML), silty SAND (SM), and silty GRAVEL (GM). The sampled residual soils were dark brown, light brown, reddish brown, orangish brown, brown, and gray in color, with water contents visually characterized as moist to very moist. The Standard Penetration Test values (N-Values) in the fill materials ranged from

2 bpf to 64 bpf.

Page - 9 -

4.2.6 Soft Weathered Rock

Soft weathered rock (SWR) is a transitional material between soil and rock which contains the relic structure of the rock with very hard consistencies or very dense densities. SWR materials were encountered below the residual soils in borings A-1, B-1, D-1, D-2, D-4 at depths ranging from 2.5 and 8.5 feet below existing grades. When sampled, the SWR was generally described as silty SAND (SM) and silty GRAVEL. Sampled SWR materials were dark brown, reddish brown, and brown, moisture contents visually characterized as moist to wet. SPT values in SWR ranged from 50/6 to 50/1.

4.2.7 Auger Refusal Materials

Auger refusal occurs when materials are encountered that cannot be penetrated by the soil auger and is normally indicative of a very hard or very dense material, such as boulders, rock lenses, rock pinnacles, or the upper surface of rock. Auger refusal was encountered in each drilled test boring at a depths ranging from 3.5 feet to 12 feet below existing grades. We note boring A-3 was hand augered to a depth 2 feet below existing grade but not considered to have reached CME 55 auger refusal material. Auger refusal conditions with a CME 55 do not necessarily indicate conditions impenetrable to other equipment. Auger refusal conditions will likely very in unexplored areas of the site.

Boring No.

Existing Elevation

SWR Depth (feet)

SWR

Elevation

Refusal Depth (feet)

Auger Refusal Elevation

A-1 932 8.5 923.5 9.5 922.5

A-2 938 -- -- 7.8 930.2

A-4 943 -- -- 9.5 933.5

B-1 944 2.5 547.5 3.5 940.5

D-1 930 8.3 921.7 9.0 921.0

D-2 905 3.5 901.5 5.5 899.5

D-3 919 -- -- 7.8 911.2

D-4 922 5.0 917 6.5 915.5

P-1 943 -- -- 11 932.0

P-2 934 -- -- 10.5 923.5

P-3 925 -- -- 8.0 917.0

P-4 902 -- -- 12 890.0

P-5 931 -- -- 7.0 924.0

P-6 928 -- -- 4.6 923.4

Page - 10 -

4.2.8 Bedrock Materials

Rock coring was performed in borings A-1, A-2, A-4, B-1 and D-4. The bedrock materials were classified as light gray, moderately weathered, very closely fractured GRANITE. A summary of the recovery, RQD, and Rock Mass Rating values for each rock core run is indicated in the following table:

Boring No.

Compression Strength (psi)

Depth (ft) Recovery

RQD

Rock Mass Rating Description

A-1 6,547 9.5-14.5 30 12 19 Very Poor Rock

A-2 18,225 7.8-12.8 25 22 24 Poor Rock

A-4* - 9.5-13 52 10 - Very Poor Rock

A-4 1,795 13-18 50 15 14 Very Poor Rock

B-1* - 3.5-8.5 37 0 - Very Poor Rock

D-1* - 6.5-11.5 10 0 - Very Poor Rock

* Cores did not provide a sample that was suitable for unconfined compressive strength testing.

4.3 Subsurface Water

The test borings were monitored during and after drilling operations to obtain short-term subsurface water information. Subsurface water was encountered during drilling in test borings

A-1, A-2, A-4, and D-4, at depths ranging from 0.5 to 2 feet. Subsurface water was not recorded in the other borings after auger removal due to cave-in. It should be noted that the location of the subsurface water table could vary by several feet because of seasonal fluctuations in precipitation, evaporation, surface water runoff, local topography, and other factors not immediately apparent at the time of this exploration. Normally, the highest subsurface water levels occur in the late winter and spring and lowest levels occur in the late summer and fall.

4.4 Laboratory Test Results

As discussed in Section 3.2, laboratory testing was performed on a representative soil samples collected during our subsurface exploration. The results from the laboratory testing of recovered split-spoon samples are included in the following table. A complete summary of laboratory testing data, including bulk sample CBR results, is providing in Appendix II.

Boring No.

Sample Depth (Feet)

Natural Water Content (%)

Liquid Limit/ Plasticity Index

% Passing No. 200 Sieve

USCS

Class.

USDA

Class.

A-1 2.5-4 20.2 38/13 40.6 SM --

A-2 2.5-4 16.7 27/5 39.5 SM --

A-4 2.5-4 16.5 NP 23.7 SM --

Page - 11 -

Boring No.

Sample Depth (Feet)

Natural Water Content (%)

Liquid Limit/ Plasticity Index

% Passing No. 200 Sieve

USCS

Class.

USDA

Class.

D-1 3.5-5 17.0 -- 26.7 SM

Sandy loam

D-3 0-1.5 19.6 40/14 63.9 SM --

D-3 5-6.5 18.5 49.5 SM Loam

D-4 2.5-4 25.5 48/5 51.4 SM --

P-1 3.5-5 17.9 35/9 39.9 SM --

P-2 0-1.5 19.9 42/19 53.0 CL --

P-6 2.5-4 18.3 38/3 36.6 SM --

5.0 GEOTECHNICAL DESIGN RECOMMENDATIONS

5.1 General

The following evaluations and recommendations are based on our observations at the site, interpretation of the field obtained during this exploration, and our experience with similar subsurface conditions and projects. Soil penetration data has been used to evaluate relative consistency and compressibility of the underlying soil stratum using established correlations.

Subsurface conditions in unexplored locations may vary from those encountered. If the structure locations, loadings, or elevations are changed, we should be notified and requested to confirm and, if necessary, re-evaluate our recommendations.

Determination of an appropriate foundation system for a given structure is dependent on the proposed structural loads, soil conditions, and construction constraints such as proximity to other structures, etc. The subsurface exploration aids the geotechnical engineer in determining the soil stratum appropriate for structural support. This determination includes considerations with regard to both allowable bearing capacity and compressibility of the soil strata. In addition, since the method of construction greatly affects the soils intended for structural support, consideration must be given to the implementation of suitable methods of site preparation, fill compaction, and other aspects of construction, where applicable.

Page - 12 -

5.2 Foundation Design

We envision that the proposed bridge abutments and Contact Station foundations can be supported on a shallow foundation system bearing on approved alluvium and/or firm residual soils. We recommend that foundation be designed for a net allowable bearing pressure not to exceed 3,000 pounds per square feet (psf).

To reduce the possibility of localized shear failures, column and wall footings should be a minimum of 3 feet and 2 feet wide, respectively. We recommend that all exterior footings be placed a minimum of 2 feet below finished exterior grades to protect against the effects of frost.

5.3 Settlement

Based on the boring data, anticipated structural loads and grading, published literature, and assuming that undercuts are implemented as recommended, we estimate total settlements of less than 1 inch with differential settlement of up to one-half the estimated total settlement.

The magnitude of differential settlements will be influenced by the variation in excavation requirements across the building footprint, the distribution of loads, and the variability of underlying soils.

Our settlement analysis was performed on the basis of structural and grading assumptions discussed in the project information section of this report. Actual settlements experienced by the structure and the time required for these soils to settle will be influenced by undetected variations in subsurface conditions, actual structural loads, final grading plans, and the quality of fill placement (if needed) and foundation construction.

5.4 Ground Floor Slabs

Ground floor slabs for the Contact Station may be designed as a slab-on-grade supported by approved residual soils, or newly placed controlled fill. Any loose/soft or otherwise unsuitable materials should be remediated as judged necessary by the Geotechnical Engineer. We note that soft residual soils were encountered in boring B-1 to a depth of 2.5 feet below existing grades; some undercutting and replacement of soft soils may be needed for adequate slab support.

We recommend that the slab-on-grade be underlain by 4-inches of well-compacted granular materials, which should conform to an open graded aggregate (such as VDOT No. 57 Stone).

This granular material provides a capillary break between the subgrade and slab-on-grade;

while also providing a uniform bearing surface. A vapor retarder should be used beneath ground floor slabs that will be covered by tile, wood, carpet, impermeable floor coatings, and/or if other moisture-sensitive equipment or materials will be in contact with the floor.

However, the use of vapor retarders may result in excessive curling of floor slabs during curing.

Page - 13 -

We refer the floor slab designer to ACI 302.1R-96, Sections 4.1.5 and 11.11, for further discussion on vapor retarders, curling, and the means to minimize concrete shrinkage and curling.

Proper jointing of the ground floor slab is also essential to minimize cracking. ACI suggests that unreinforced, plain concrete slabs may be jointed at spacings of 24 to 36 times the slab thickness, up to a maximum spacing of 18 feet. Floor slab construction should incorporate isolation joints along bearing walls and around column locations to allow minor movements to occur without damage. Utility or other construction excavations in the prepared floor subgrade should be backfilled to a controlled fill criteria to provide uniform floor support.

Structural analyses and design of floor slab foundation may require the use of a vertical modulus of subgrade reaction (k). We note that typical practice for slab-on-grade and pavement design is to provide a “k” value based on published correlation with soil types and

California Bearing Ratio (CBR) test values. Such correlations are based on empirical data from plate load tests. The plate load test sufficiently models typical floor and wheel loads that exert stresses on the order of 3 to 5 feet. Based on published correlations, we estimate that a design modulus of subgrade reaction (k) = 150 pci is appropriate for floor slab design calculations, provided that the recommended 4-inch subbase is utilized.

5.5 Pavement Construction

We understand that gravel pavements are planned for the upper and lower parking lots. We expect that both areas will be subject to automobile and bus traffic, although detailed traffic loading was not provided. Therefore, two gravel pavement sections have been designed based on different loading conditions: standard-duty traffic (automobile traffic only) and medium-duty traffic (bus traffic).

CBR testing was completed on a bulk samples taken from borings P-1, P-2, P-4, P-5, P-6, D-1 and

D-2 at depths of 0 to 5 feet below the existing ground surface. Bulk samples from borings P-2 and P-4, and borings D-1 and D-2 were mixed in the laboratory to make composite samples.

The samples were classified using the Unified Soil Classification System, and soaked CBR values at 0.1 inches penetration are provided in the following table. The CBR value of the actual subgrade materials used should be verified prior to the construction of any pavements.

Boring USCS Maximum Optimum Natural Soacked Resiliency

No. Classification Dry Density Moisture Moisture CBR Factor

P-1 SM 109.8 pcf 17.3% 21.6% 22.9 2.5

P-2/P-4 SM 129.4 pcf 9.5% 9.3% 20.5 2.5

P-5 SM 114.9 pcf 14.5% 19.8% 4.0 2.5

P-6 ML 99.1 pcf 23.0% 19.8% 8.1 2.0

D-1/D-2 SM 110.7 pcf 16.4% 20.9% 17.5 2.5

Page - 14 -

In accordance with VDOT requirements, the design CBR value should be taken as two-thirds of the average CBR value. Therefore, a CBR value of 9.0 should be used for design.

The following design values were used for our analysis:

Standard duty traffic loading 10,000 equivalent single axle loads (ESAL)

Medium duty traffic loading 100,000 ESAL

Design life 20 years

Reliability 85 %

Variance 0.45

Initial serviceability 4.2

Terminal serviceability 2.0

Our design analysis was based on methodology from the American Association of State

Highway and Transportation Officials’ (AASHTO) Guide of Design of Pavement Structures, 1993.

Based on the assumptions and methodologies presented above, we recommend the following pavement sections:

Gravel Pavement Section (Covered Storage Building)

Pavement Section Standard Medium

Layer VDOT Specification

(SUPERPAVE)

Thickness

(inches)

Thickness

(inches)

Subbase

Course

Type I Crushed Aggregate

(No. 21A or No. 21B)

10.0 10.0*

* This requires that a layer of biaxial geogrid reinforcement (such as Tensar BX 1200 or approved alternative) be placed at the mid-depth of the layer.

Our recommendations are based on pavements being supported on soils similar to the soils we tested. Fill materials for parking lot construction should be placed in accordance with the controlled fill and pavement subgrade recommendations contained in this report. In addition, all pavement subgrades should be evaluated by a geotechnical engineer prior to basestone placement. If excessive subgrade movement is observed, appropriate improvements such as undercutting and/or in-place stabilization will be required at that time.

The aggregate subbase course should be placed, compacted, and tested in accordance with the requirements of Section 309 of Virginia Department of Transportation Road and Bridge

Specifications, 2007 (VDOT Specifications).

Page - 15 -

5.6 Infiltration

Typically, in Virginia, the infiltration potential of the soils is evaluated using the Virginia

Department of Conservation and Recreation (DCR) Standard General Infiltration Practices.

According to DCR requirements, infiltration is only permissible where infiltration rates exceed

0.52 inches per hour, and groundwater or an impermeable layer is not located within 2 to 4 feet of the planned bottom of the facility. The infiltration rate can be determined from field testing and estimated empirically from USDA soil classification. Infiltration is generally not acceptable in fill soils.

Infiltration tests were conducted at locations offset from borings D-1 and D-3 at the depths of

3.2 and 3.4 ft below existing grades, respectively. The results of the infiltration test are included in the table below:

Location Infiltration Test

Result (inches/hour)

USDA Soil

Classification

Empirical Rate

(inches/hour)

D-1 9.0 Sandy Loam 1.02

D-3 0.02 Loam 0.52

Boring D-3 is located within the footprint of the proposed Bio-Retention basin by the upper lot.

We note that the empirical rate of infiltration from the soils collected in D-3 matches the minimum rate required by the DCR; however the result from the field test is significantly lower.

Also, it should be understood subsurface water records were not available from boring D-3, but the neighboring boring D-4 encountered subsurface water at a depth of 2 feet below existing grade. Based on the foregoing, infiltration practices are not recommended at boring location

D-3 at the elevation tested.

5.7 Seismic Considerations

The following Seismic Site Class Definition was established per Section 1613.5.2 of the 2009

International Building Code (IBC). Our scope of services did not include a seismic conditions survey to determine site-specific shear wave velocity information. IBC 2009 provides a methodology for interpretation of Standard Penetration Test resistance values (N-values) to determine a Site Class

Definition. This method requires averaging N-values over the top 100 feet of the subsurface profile. Based on our experience in this area and the data from our testing and subsurface exploration and in general accordance with Section 1613.5.2 of the 2009 IBC, a Site Classification

“C” should be used for further evaluations relative to earthquake load design.

Page - 16 -

6.0 GEOTECHNICAL CONSTRUCTION RECOMMENDATIONS

6.1 Site Preparation

Before proceeding with construction, existing structures, utilities, asphalt, concrete and crushed stone, mulch and other deleterious non-soil materials (if any) should be stripped or removed from the proposed construction area. Soils containing a high organic content were encountered near the surface in borings A-1, A-2, D-1, D-3, P-2, P-4, P-5, and P-6. Attention should be given to these areas to ensure all unsuitable material is removed prior to continuing with construction. During the site preparation operations, positive surface drainage should be maintained to prevent the accumulation of water.

After stripping, areas intended to support ground floor slabs or new fill should be carefully evaluated by a geotechnical engineer. At that time the engineer may require proofrolling of the subgrades with a 20 to 30-ton loaded truck or other pneumatic-tired vehicle of similar size and weight. Proofrolling should be performed during a time of good weather and not while the site is wet, frozen, or severely desiccated. The purpose of the proofrolling is to locate soft, weak, or excessively wet soils present at the time of construction.

The proofrolling observation is an opportunity for the geotechnical engineer to locate inconsistencies intermediate of our boring locations and evaluate the stability of the existing subgrade materials. Any unsuitable materials observed during the evaluation and proofrolling operations should be undercut and replaced with compacted fill or stabilized in-place. Low consistency near-surface soils, within the proposed parking area and Contact Station, that will likely require undercutting were encountered in borings B-1, P-1, P-3, P-4, P-5, and P-6. The possible need for, and extent of, undercutting and/or in-place stabilization required can best be determined by the geotechnical engineer at the time of construction. Once the site has been properly prepared, at-grade construction may proceed.

6.2 Excavation Conditions

Soft weathered rock was encountered in borings A-1, B-1, D-1, D-2 and D-4 at depths ranging from 2.5 feet to 8.5 feet below existing grades and extended to auger refusal materials. Auger refusal material was encountered at each boring location, except for A-3 (hand augered), at depths ranging from 3.5 feet to 12 feet below existing grades. Auger refusal materials were encountered below anticipated excavation levels, however, F&R notes that the profile of the bedrock surface will be highly irregular, and that bedrock could be encountered at higher elevations between test boring locations. It should be noted that rock outcrops were observed within the area of proposed construction.

Page - 17 -

In mass excavations for general sitework, hard or dense soils (soils with standard penetration resistances of 30 or more blows per foot) can usually be removed by ripping with a single-tooth ripper attached to a large crawler tractor or by breaking it out with a tracked excavator or large front-end loader. However, we note that while ripping and/or breaking out with large tracked equipment might be possible, it may be time prohibitive for deep mass excavations. Blasting is not recommended due to the concern for maintaining the integrity of the surrounding slopes and the relatively limited amount of rock excavation which is expected. In confined excavations such as foundations, utility trenches, etc., removal of partially weathered rock typically requires use of large backhoes, or pneumatic spades. If ripping or hoe ramming does not effectively remove the rock, expansive grout may be considered. The gradation of the material removed by ripping or blasting is typically erratic, making it unsuitable for use as structural fill.

The definition of rock can be a source of conflict during construction. The following definitions have been incorporated into specifications on other projects and are provided for your general guidance:

GENERAL EXCAVATION:

Rip Rock - Any material that cannot be removed by scrapers, loaders, pans, dozers, or graders; and requires the use of a single-tooth ripper mounted on a crawler tractor having a minimum draw bar pull rated at not less than 56,000 pounds.

Blast Rock - Any material which cannot be excavated with a single-tooth ripper mounted on a crawler tractor having a minimum draw bar pull rated at not less than 56,000 pounds (Caterpillar D-8K or equivalent) or by a

Caterpillar 977 front-end loader or equivalent; and occupying an original volume of at least one (1) cubic yard.

TRENCH EXCAVATION:

Blast Rock - Any material which cannot be excavated with a backhoe having a bucket curling force rated at not less than 25,700 pounds (Caterpillar

Model 225 or equivalent), and occupying an original volume of at least one-half (1/2) cubic yard.

Page - 18 -

6.3 Foundation Construction

All foundation subgrades should be observed, evaluated, and verified for the design bearing pressure by the geotechnical engineer after excavation and prior to reinforcement steel placement or setting of any structure. If low consistency soils are encountered during foundation construction, localized undercutting and/or in-place stabilization of foundation subgrades will be required. The actual need for, and extent of, undercutting should be based on field observations made by the geotechnical engineer at the time of construction.

Excavations for footings should be made in such a way as to provide bearing surfaces that are firm and free of loose, soft, wet, or otherwise disturbed soils. Foundation concrete should not be placed on frozen or saturated subgrades. If such materials are allowed to remain below foundations, settlements will increase. Foundation excavations should be concreted as soon as practical after they are excavated. If an excavation is left open for an extended period, a thin mat of lean concrete should be placed over the bottom to minimize damage to the bearing surface from weather or construction activities. Water should not be allowed to pond in any excavation.

6.4 Structural Fill Placement and Compaction

Fill materials may consist of the non-organic on-site soils, or an off-site borrow having a classification of CL or more granular, as defined by the Unified Soil Classification System.

Controlled structural fill should have a maximum particle size of 4 inches and should be free of organics or other deleterious materials.

Based on our visual classifications and the laboratory testing, we anticipate that the on-site soils should serve satisfactorily as fill provided that the moisture contents can be maintained within acceptable limits. The on-site soils are considered moisture sensitive and may be difficult to work with when they are wet of the optimum moisture content. The laboratory test results indicate that the moisture content of some of the soil samples were well above their anticipated optimum moisture content. Therefore, some drying of the on-site soils should be anticipated.

Predicated on our experience with similar projects, the best time for construction of the structural fills and compacted subgrades would be during the warmer, drier months of the year, such as from late April through early October. During this time frame, any borrow materials that are delivered wet of optimum can usually be dried to near optimum levels with relatively little effort. If grading is performed during the colder, wetter months of the year, such as late

October through early April, and structural fill materials are determined to be wet of optimum, then difficult drying can be expected and an alternative source of borrow may be necessary.

Page - 19 -

The fill should be placed in horizontal lifts, 8 inches in loose thickness, and compacted to at least 95 percent of the maximum dry density as determined by the Standard Proctor compaction test (ASTM D 698). Where portable compaction equipment is used, such as utility trenches, the lift thickness may need to be reduced to 4 inches to achieve the required degree of compaction. Excessively wet or dry soils should not be used as fill materials without proper drying or wetting. A moisture content range of plus or minus 3 percentage points from the optimum moisture of the fill material is recommended. We recommend that the contractor have equipment on site during earthwork for both drying and wetting of fill soils.

Where construction traffic or weather has disturbed the subgrade, the upper 8 inches of soils intended for structural support should be scarified and re-compacted. Field density tests to determine the degree of compaction should be performed at the discretion of the geotechnical engineer.

6.5 Surface Water/Groundwater Control

Subsurface water for the purposes of this report is defined as water encountered below the existing ground surface. F&R anticipates that some dewatering will be necessary during construction, particularly in the area of the existing stream beds. This can be accomplished by utilizing positive drainage and localized sumps and pumps. In addition, the contractor should be prepared to dewater should water levels vary from those encountered during the drilling program. Fluctuations in subsurface water levels and soil moisture can be anticipated with changes in precipitation, runoff, and season.

All work on permanent structures shall be carried out in areas free from water. The Contractor should construct and maintain all temporary dikes, levees, cofferdams, drainage channels, and stream diversions necessary to protect the areas to be occupied by the permanent works. The contractor should also furnish, install, operate, and maintain all necessary pumping and other equipment required for removal of water from various parts of the work and for maintaining the excavations, foundation, and other parts of the work free from water as required or directed by the engineer for constructing each part of the work. After having served their purpose, all temporary protective works should be removed or leveled and graded to the extent required to prevent obstruction in any degree whatsoever of the flow of water to any spillways or outlet works. The removal of water from the required excavation and/or foundations should be accomplished in a manner and to the extent that will maintain stability of the excavated slopes and bottom required excavations and will allow satisfactory performance of all construction operations. During the placing and compacting of material in required excavations, the water level at the locations being refilled should be maintained below the bottom of the excavation.

Page - 20 -

7.0 CONTINUATION OF SERVICES

We recommend that we be given the opportunity to review the foundation plan, grading plan, and project specifications when construction documents approach completion. This review evaluates whether the recommendations and comments provided herein have been understood and properly implemented. We also recommend that Froehling & Robertson, Inc.

be retained for professional and construction materials testing services during construction of the project. Our continued involvement on the project helps provide continuity for proper implementation of the recommendations discussed herein.

The Geotechnical Engineer of Record should be retained to monitor and test earthwork activities, and subgrade preparations for foundations, excavations and floor slabs. It should be noted that the actual soil conditions at the various subgrade levels and footing bearing grades will vary across this site and thus the presence of the Geotechnical Engineer and/or his representative during construction will serve to validate the subsurface conditions and recommendations presented in this report.