Report of Geotechnical Study

Transcription

1 Report of Geotechnical Study Dumfries Mixed-Use Development Dumfries, Virginia F&R Project No. 74T-0009 Prepared For: Community Housing Partners 4915 Radford Avenue, Suite 300 Richmond, Virginia Prepared By: Froehling & Robertson, Inc Houser Drive Fredericksburg, Virginia May 20, 2015 Corporate HQ: 3015 Dumbarton Road Richmond, Virginia T F VIRGINIA NORTH CAROLINA SOUTH CAROLINA MARYLAND DISTRICT OF COLUMBIA A Minority-Owned Business

2 FROEHLING & ROBERTSON, INC. Engineering Stability Since Houser Drive Fredericksburg, Virginia T I F F&R Project No. 74T-0009 May 20, 2015 Mr. David Schultz VP of Development 4915 Radford Avenue, Suite 300 Richmond, Virginia Reference: Dumfries Mixed-Use Development Dumfries, Virginia Dear Mr. Schultz: The purpose of this report is to present the results of the subsurface exploration program and preliminary geotechnical engineering evaluation undertaken by Froehling & Robertson, Inc. (F&R) in connection with the proposed 30 acre development located in Dumfries, Virginia. Our services were performed in general accordance with F&R Proposal No dated April 2, 2015 as authorized by you. The attached report presents our understanding of the project, reviews our exploration procedures, describes existing site and general subsurface conditions, and presents our geotechnical evaluations and recommendations. We have enjoyed working with you on this project, and we are prepared to assist you with any additional geotechnical services, and the recommended quality assurance monitoring and testing services during construction. Please contact us if you have any questions regarding this report or if we may be of further service. Sincerely, FROEHLING & ROBERTSON, INC. Jolie A. Erickson, E.I.T. Staff Engineer Caleb M. Lange, P.E. Geotechnical Engineer Reviewed By: Donald J. Sipher, P.E. Senior Geotechnical Engineer Corporate HQ: 3015 Dumbarton Road Richmond, Virginia T F VIRGINIA NORTH CAROLINA SOUTH CAROLINA MARYLAND DISTRICT OF COLUMBIA A Minority-Owned Business

3 TABLE OF CONTENT SECTION PAGE EXECUTIVE SUMMARY PURPOSE & SCOPE OF SERVICES PROJECT INFORMATION SITE DESCRIPTION PROPOSED CONSTRUCTION EXPLORATION PROCEDURES SUBSURFACE EXPLORATION LABORATORY TESTING REGIONAL GEOLOGY & SUBSURFACE CONDITIONS REGIONAL GEOLOGY SUBSURFACE CONDITIONS General Surficial Soils Alluvial Soils Residual Soils Quantico Slate SUBSURFACE WATER LABORATORY TESTING RESULTS GEOTECHNICAL AND CONSTRUCTION RECOMMENDATIONS GENERAL SITE DEVELOPMENT STRUCTURAL FILL PLACEMENT AND COMPACTION TEMPORARY EXCAVATION RECOMMENDATIONS SEISMIC CONSIDERATIONS CONTINUATION OF SERVICES LIMITATIONS Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page - i -

4 APPENDICES APPENDIX I Site Vicinity Map (Drawing No. 1) Boring Location Plan (Drawing No. 2) APPENDIX II Key to Soil Classification Unified Soil Classification Chart Boring Logs APPENDIX III ASFE Document Important Information about Your Geotechnical Engineering Report Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page - ii -

5 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. It is anticipated that development of the site will include mixed-use buildings with commercial space on the first floor and residential apartments above. The 30 acre site was explored by 13 soil test borings performed between April 16, 2015 and April 28, Site subsurface conditions generally consist of surficial soils underlain by alluvial soils and residual soils. This initial investigation has identified soils that are suitable for conventional excavation and general site grading. These soils exist throughout most of the site. Because this is a preliminary investigation, building and pavement locations and loads are not yet known. Therefore, bearing and pavement design recommendations are not included in this report. However, SPT data suggests that bearing capacity may be between 2,000 psf and 4,000 psf. The presence of Quantico Slate was noted at low elevations on the north side of the site. This material should not be used in mass grading or as structural fill. Additionally, this material is known to have low ph values and can cause deterioration of utility lines, footings, and other elements. Due to the size of the site, the magnitude of grading, and the availability of suitable soil material, we expect that the site can be developed without requiring disturbance of the Quantico Slate formation. Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page - 1 -

6 1.0 PURPOSE & SCOPE OF SERVICES The purpose of the subsurface exploration and geotechnical engineering evaluation was to explore the subsurface conditions, and provide preliminary evaluation of the site regarding the proposed development. F&R s scope of services included the following: Completion of thirteen total soil test borings; four drilled to a depth of 50 feet below existing site grades and nine drilled to a depth of 20 feet below existing site grades; Preparation of typed boring logs; Performing a geotechnical engineering evaluation of the subsurface conditions with regard to their suitability for the proposed construction; Preparation of this geotechnical report by professional engineers. Our scope of services did not include a survey of the boring location, quantity estimates, preparation of plans or specifications, infiltration testing, or the identification and evaluation of wetland or other environmental aspects of the project site. 2.0 PROJECT INFORMATION 2.1 Site Description The project property is located in Dumfries, Virginia. It is bound by Jefferson Davis Highway and Fraley Boulevard to the west, Graham Park Road to the north, and Old Triangle Road to the east (Drawing 1, Appendix I). The site is undeveloped, and slopes from higher elevations on the south side to lower elevations on the north side. Based on available data, site topography ranges from a high of approximately 154 feet to a low of approximately 68 feet. There is a small, possibly intermittent, stream flowing from south to north on the site. The site is wooded, except for a cleared area where power lines cross the site. We are also aware of a gas line which crosses the site. Site topography slopes generally from a high elevation at the south end to a low elevation at the north end, with some minor hills, valleys, and spurs. At least 20 feet of cut and fill is expected across the site; however cut and fill operations are expected to be limited by the above mentioned utilities and surrounding features. Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page - 2 -

7 2.2 Proposed Construction We understand that the proposed construction consists of mixed-use buildings with commercial space on the first floor and residential apartments above. This is a preliminary investigation, therefore, the locations of proposed buildings and paved areas are not yet known. Cut and fill operations are expected to be at least 20 feet. 3.1 Subsurface Exploration 3.0 EXPLORATION PROCEDURES The subsurface exploration program was performed on April 16-28, 2015, and consisted of thirteen soil test borings, designated B-1 through B-13. Borings B-1, B-3, B-4, and B-13 were drilled to a depth of 50 feet below existing site grades. The remaining nine borings were drilled to a depth of 20 feet below existing site grades. Boring locations were selected by Mr. David Schultz, of Community Housing Partners, and were staked in the field by F&R personnel by overlaying the provided site plan on aerial images. The overlay was used to space borings and gather coordinates. The coordinates were used in conjunction with a handheld GPS device to locate the borings in the field. The approximate locations of the borings are shown on the attached Boring Location Plan (Drawing No. 2, Appendix I). In consideration of the methods used in their determination, the boring locations shown on the attached Boring Location Plan should be considered approximate. Elevations noted on boring logs are taken from an image provided by the client and should also be considered approximate. The soil test borings were performed in accordance with generally accepted practice, using a truckmounted CME-55 rotary drill rig that is 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 is 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 or four consecutive 6-inch increments is recorded, and the blows of the second and third increments are 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 Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page - 3 -

8 correlations that are published in the technical literature are based on the N-value determined by the safety hammer method. This is commonly termed N 60 as the rope and cathead with a safety hammer delivers about 60 percent of the theoretical energy delivered by a 140-pound hammer freely 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 N 60 -values. The correction is made using the following equation: N 60 = N field x C E N field is the value recorded in the field and C E is the energy ratio for the hammer utilized in the field. The 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 Polytechnic Institute and State University, recommend that a correction factor (C E ) be used to covert N field values to N 60 values, when using an automatic hammer. The N- values reported on the Boring Logs included in this report are the actual, uncorrected, field derived values (N field ). It is recommended that corrected N 60 values be used for engineering analysis. We recommend that a correction factor (C E ) of 1.32 be used to convert N field values to N 60 values for the particular machine used for this investigation. Note that for boring B-13, a rope and cathead with a safety hammer was used, so the N field values are equal to the N 60 values for that boring. Prior to demobilization, the boreholes were backfilled with auger cuttings. Periodic observation of the backfilled borings should be performed, as the boring backfill could settle over time and result in subsidence of the ground at and around the boreholes. Upon completion of boring B-4, a hand-slotted piezometer was installed and the annular space was filled with sand. Representative portions of the split-spoon soil samples collected 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 D2488). 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. Copies of the boring logs are provided and classification procedures are further explained in Appendix II. Soil samples recovered from this project will be stored at F&R s office for a period of 60 days. The samples will be discarded after 60 days, unless prior notification is provided to us in writing. Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page - 4 -

9 3.2 Laboratory Testing Our service included laboratory testing to aid in identifying and evaluating soils that may affect building design or construction. In accordance with our proposal, the following tests were performed on representative soil samples collected during our subsurface exploration. o 2 ph tests o 5 Sieve Analysis (ASTM D1140) o 5 Atterberg Limits (ASTM D4318) o 5 Natural Water Content (ASTM D2216) The laboratory test results are shown in Section Regional Geology 4.0 REGIONAL GEOLOGY & SUBSURFACE CONDITIONS Based on the Geologic Map of Virginia (1993), the project site lies within the Potomac Formation near the western edge of the Coastal Plain Physiographic Province of Virginia. This site is also underlain by the Quantico formation, also known as Quantico slate. These soil deposits are briefly described below. The soils within the project site are mapped as a Cretaceous-aged (66 to 145 million year old) Potomac Formation. Potomac Formation sediments were deposited on subsiding crystalline rocks by freshwater rivers meandering along the inner margin of the Coastal Plain. The deposits are characterized by cross bedding, channels filled with clays or sands, rapid pinching and thickening of beds, and lateral and vertical size grading. The silts and clays of the Potomac Formation are typically highly plastic and are commonly called marine clay. These soils are noteworthy in that shallow slope failures are common due to slickensides, the weathering properties, and groundwater characteristics. These characteristics include gray sand, mottled red or reddishbrown sandy clay and silt, and thin-bedded to laminated carbonaceous clay and silt. The Potomac Formation is characterized by interlayered and unconsolidated sand, silt, and clay. Our findings coincide with these characteristics. The alluvial deposits within this site are underlain by the Ordovician-aged (443 to 488 million year old) Quantico Formation. The Quantico Formation consists of generally dark gray to black very fine-grained slate and porphyroblastic schist. The slates and schist in the area are typically graphitic and pyritic. The porphyroblasts may include staurloite, kyanite, garnet, biotite, and Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page - 5 -

10 muscovite. The soils resulting from in-situ weathering of these rocks, without significant transportation, are called residual soils. The residual soil profile generally grades downward gradually from fine-grained highly plastic soils near the ground surface to granular soils consisting of silty fine sands at greater depth. A transitional zone of partially weathered rock of varying thickness occurs between the residuum and the underlying bedrock. The depth of the soil profile is continually altered over geologic time by gradual weathering at the soil/rock interface, and more rapidly by erosion of surficial soils. Weathering of the parent bedrock is generally more rapid near fracture zones. Therefore, the bedrock surface may be highly irregular. 4.2 Subsurface Conditions 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. 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 are shown on the attached boring logs in Appendix II. Below the existing ground surface, the borings generally encountered alluvial soils. materials are generally discussed in the following paragraphs. These Surficial Soils Surficial soil was encountered in all borings and extended to depths ranging between 1 and 2 inches, according to driller observations. According to our experience, we estimate that the surficial soil probably actually extends to at least 6 inches. Surficial 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 soil materials. Therefore, the term surficial soil is used instead of topsoil to indicate that the soil has not been evaluated for its suitability for landscaping and/or other purposes. The surficial soil depths provided in this report are based on driller observations and should be considered approximate. We note that the transition from surficial soil to underlying materials may be gradual, and therefore the observation and measurement of surficial soil depths is subjective. Actual surficial soil depths should be expected to vary across the site, especially near trees where root balls exist. Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page - 6 -

11 4.2.3 Alluvial Soils Alluvial soils were encountered beneath the surficial soils and extended to the termination depths for most borings, with the exception of B-1, B-3, and B-13. These soils were generally described as Fat and Lean CLAY (CH and CL), Clayey SAND (SC), and Silty SAND (SM). The field N-values for finegrained soils (CLAYs and SILTs) range from 2 to 48 bpf and indicate that these soils are soft to hard in consistency. The field N-values in the coarse-grained soils (SANDs) range from 3 to 58 bpf and indicate that these soils are very loose to very dense in relative density Residual Soils Residual soils were encountered beneath the surficial and alluvial soils in borings B-1 and B-13, and extended to 21 feet in boring B-1 and 17 feet in boring B-13. These soils were generally described as Fat CLAY (CH) and SILT (ML). The field N-values for these soils range from 15 to over 100 bpf and indicate that these soils are very stiff to hard in consistency Quantico Slate Quantico slate was encountered in borings B-1, B-3, and B-13 and extended to the termination depths for those borings. This material is generally described as SILT (ML). The field N-values for this material are over 100 bpf and indicate that the slate is hard in consistency. Quantico slate is known to be corrosive, and it is known to have a high level of acid sulfate potential. 4.3 Subsurface Water The test borings were monitored during drilling operations to obtain subsurface water information. In borings B-3, B-4, and B-5, groundwater was encountered. In B-4, a piezometer was also installed, and the water level was measured after 11 days. Cave-in depths were also noted. Cave-in can be indicative of groundwater levels which can cause sidewall instability in the borehole, particularly in sandy soils. The subsurface water data obtained during our subsurface exploration has been summarized in the following table: Boring Depth of Boring (ft) Approximate Elevation (ft) Subsurface Water Depth During Drilling (ft) Cave-in Depth (ft) Piezometer Water Depth (ft) B B B B B Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page - 7 -

12 Boring Depth of Boring (ft) Approximate Elevation (ft) Subsurface Water Depth During Drilling (ft) Cave-in Depth (ft) Piezometer Water Depth (ft) B B B B B B B B 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 Testing Results As discussed in Section 3.2, laboratory testing was performed on soil samples collected during our subsurface exploration. The results from the laboratory testing are summarized in the table below. Results listed as pending will be reported in an addendum issued as soon as the results are available. Boring No. Sample Depth (Feet) LL/PI (a) % Passing #200 Sieve (b) % Natural Water Content % Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page ph USCS (c) Class. B SC* B CL* B / CH* B SC* B / SC* B SC* B SC*

13 Boring No. Sample Depth (Feet) LL/PI (a) % Passing #200 Sieve (b) % Natural Water Content % ph USCS (c) Class. B SM* B NP SM* B SM* B SC* B SC* B / CH* B SM* B SM* B NP SM* B SM* (a) Liquid Limit and Plasticity Index from Atterberg Limits test (ASTM D 4318) (b) Percentage of fines (silt and/or clay) from #200 Sieve Wash (ASTM D 1140) (c) Unified Soil Classification System *visually evaluated 5.0 GEOTECHNICAL AND DEVELOPMENT CONSIDERATIONS 5.1 General The following evaluations and recommendations are based on our observations at the site, interpretation of the field data obtained during this exploration, and our experience with similar subsurface conditions and projects. Subsurface conditions in unexplored locations may vary from those encountered. 5.2 Site Development Based on our findings, we anticipate that mass grading of the site can be accomplished using conventional excavation and earth moving methods. From what we understand of the grading plan, Quantico slate will likely not be encountered during excavation. However, if slate or other hard or dense soils (soils with standard penetration resistances of 30 or more blows per foot) are encountered during excavations or general site work, the material can 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. Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page - 9 -

14 Before proceeding with site development, and after clearing and grubbing, surficial soils and other deleterious non-soil materials (if any) should be stripped or removed from the proposed construction area. Attention should be given to these areas to ensure all unsuitable material is removed prior to continuing with construction. During the stripping operations, positive surface drainage should be maintained to prevent the accumulation of water. After stripping, areas intended to support new fill should be carefully evaluated by a geotechnical engineer. This can be accomplished by proofrolling the subgrade with a 20- to 30-ton loaded truck, or other pneumatic-tired vehicle of similar size and weight, under the observation of the geotechnical engineer. Proofrolling should be performed during a time of good weather and not while the site is wet, frozen, or severely desiccated. The proofrolling observation is an opportunity for the geotechnical engineer to locate inconsistencies intermediate of our boring locations in the existing subgrade. Any unsuitable materials observed during the evaluation and proofrolling operations should be undercut and replaced with structural fill or stabilized in-place. See Section 5.4 of this report for further details. The actual need for, and extent of, undercutting and/or in-place stabilization required can best be determined in the field by the geotechnical engineer at the time of grading. 5.3 Structural Fill Placement and Compaction Controlled structural fill may be constructed using either on-site soils or off-site borrow having a classification of CL, ML, SC, SM, or better, as defined by the Unified Soil Classification System. Quantico Slate, typically identified by its dark grey color, should not be used as structural fill. It should be noted that any import material that is intended for use as structural fill should be tested to verify that it meets these criteria prior to its import or use. Other materials may be suitable for use as general fill materials that are to be placed in landscaped areas, and should be individually evaluated by the geotechnical engineer. Controlled structural fill should be free of boulders, organic matter, debris, or other deleterious materials and should have a maximum particle size no greater than 3 inches. For general site grading, we recommend that structural fill be compacted to at least 95 percent of the Standard Proctor (ASTM D698 or AASHTO T99) maximum dry density and that the moisture content be maintained within three percentage points of the optimum moisture content, as determined from the Standard Proctor density test. Fill materials should be placed in horizontal lifts with maximum height of eight inches loose measure. During fill operations, positive surface drainage should be maintained to prevent the accumulation of water. Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page

15 Generally, we do not anticipate significant problems controlling moistures within approved fill during periods of dry weather, and due to the nature of the on-site soils, moisture may need to be added to achieve satisfactory compaction. Also, depending on the weather conditions, moisture control may be difficult during winter months or extended periods of rain. We recommend that the contractor have equipment on site during earthwork for both drying and wetting of fill soils. Attempts to work the soils when wet can be expected to result in deterioration of otherwise suitable soil conditions or of previously placed and properly compacted fill. If construction traffic or weather has disturbed the subgrade, the upper eight inches of soils intended for structural support should be scarified and re-compacted. Each lift of fill should be tested to confirm that the recommended degree of compaction is attained. In confined areas, a greater frequency may be required. 5.4 Temporary Excavation Recommendations Mass excavations and other excavations required for construction of this project must be performed in accordance with the United States Department of Labor, Occupational Safety and Health Administration (OSHA) guidelines (29 CFR 1926, Subpart P, Excavations) or other applicable jurisdictional codes for permissible temporary side-slope ratios and/or shoring requirements. The OSHA guidelines require daily inspections of excavations, adjacent areas and protective systems by a competent person for evidence of situations that could result in cave-ins, indications of failure of a protective system, or other hazardous conditions. All excavated soils, equipment, building supplies, etc., should be placed away from the edges of the excavation at a distance equaling or exceeding the depth of the excavation. F&R cautions that the actual excavation slopes will need to be evaluated frequently each day by the competent person and flatter slopes or the use of shoring may be required to maintain a safe excavation depending upon excavation specific circumstances. The contractor is responsible for providing the competent person and all aspects of site excavation safety. F&R can evaluate specific excavation slope situations if we are informed and requested by the owner, designer, or contractor s competent person. 5.5 Seismic Considerations The following Seismic Site Class Definition was established per Section 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. We note that the soil test borings for this project were extended to a maximum depth of Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page

16 50 feet below the existing site grades. We extrapolated from the data in the four borings which were extended to 50 feet below existing site grades to calculate the seismic site classification. The available subsurface data from our exploration indicates corrected N-values of 3 to 132 bpf. Based on our experience in this area, and the data from our testing and subsurface exploration and in general accordance with Section of the 2009 IBC, a Site Classification C should be used for further evaluations relative to earthquake load design. We note that the above provided Site Classification is based on information available at the time this report was written. Final site grading may alter the average density of the site soils such that a lower seismic site class is warranted. Once the final layout and development plan is available, the seismic site class should be re-evaluated. Should this classification be so onerous to the project cost that further study is warranted, we can perform deeper subsurface exploratory soil test borings and/or a site-specific geo-physical survey to attain sufficient detail to refine the project s Seismic Site Class Definition. This additional testing would be beyond the currently authorized scope of services for this project. 6.0 CONTINUATION OF SERVICES We recommend that we be given the opportunity to perform a construction-specific geotechnical study once building and pavement plans and loads are known. A construction-specific geotechnical study will include specific foundation, bearing, and settlement recommendations, as well as any needed pavement design information. After a construction-specific geotechnical study is completed, we recommend that we be allowed 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. As the Geotechnical Engineer of Record, F&R 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 Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page

17 during construction will serve to validate the subsurface conditions and recommendations presented in this report. We recommend that F&R be employed to monitor the earthwork and foundation construction, and to report that the recommendations contained in this report and the subsequent specifications are completed in a satisfactory manner. Our involvement on the project will aid in the proper implementation of the recommendations discussed herein. To reiterate, the following is a recommended scope of services: Review of project plans and construction specifications to verify that the recommendations presented in this report have been properly interpreted and implemented; Observe and perform testing during earthwork to document that subsurface conditions encountered during construction are consistent with those anticipated in this report; Observe subgrade preparation including any proofrolling operations, undercutting of soft/loose unsuitable soils, installation of drainage materials, geotextiles and fill placement; Observe all foundation excavations and footing bearing grades for compliance with the geotechnical recommendations. These services are not included in our current scope of services and can be rendered for an additional cost. 7.0 LIMITATIONS This preliminary report has been prepared for the exclusive use of Community Housing Partners, or their agent, for specific application to the proposed project, in accordance with generally accepted soil and foundation engineering practices. No other warranty, express or implied, is made. Our evaluations and recommendations are based on design information furnished to us, the data obtained from the previously described subsurface exploration program, and generally accepted geotechnical engineering practice. The evaluations and recommendations do not reflect variations in subsurface conditions which could exist intermediate of the boring locations or in unexplored areas of the site. Should such variations become apparent during construction, it will be necessary to re-evaluate our recommendations based upon on-site observations of the conditions. There are important limitations to this and all geotechnical studies. Some of these limitations are discussed in the information prepared by ASFE, which is included in Appendix III. We ask that you please review this ASFE information. Regardless of the thoroughness of a subsurface exploration, there is the possibility that conditions between borings will differ from those at the boring locations, that conditions are not as Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page

18 anticipated by the designers, or that the construction process has altered the soil conditions. Therefore, experienced geotechnical engineers should evaluate earthwork and foundation construction to verify that the conditions anticipated in design actually exist. Otherwise, we assume no responsibility for construction compliance with the design concepts, specifications, or recommendations. In the event that changes are made in the design or location of the proposed structure, the recommendations presented in the report shall not be considered valid unless the changes are reviewed by our firm and conclusions of this report modified and/or verified in writing. If this report is copied or transmitted to a third party, it must be copied or transmitted in its entirety, including text, attachments, and enclosures. Interpretations based on only a part of this report may not be valid. Community Housing Partners Dumfries Mixed-Use Development F&R File No. 74T-0009 May 20, 2015 Page

19 APPENDIX I

20 Froehling & Robertson, Inc. SITE VICINITY MAP Drawing No. 1 Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Multi-Use Development City/State: Dumfries, Virginia Source: Google Maps Scale: As Shown Date: May 2015 Approximate Site Location

21 Froehling & Robertson, Inc. BORING LOCATION PLAN Drawing No. 2 Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Multi-Use Development City/State: Dumfries, Virginia Source: Google Earth Scale: As Shown Date: May 2015 B-13 B-1 B-2 B-3 B-4 B-5 B-6 B-7 B-8 B-10 B-9 B-11 B-12 - Approximate Boring Locations

22 APPENDIX II

23 KEY TO SOIL CLASSIFICATION Correlation of Penetration Resistance with Relative Density and Consistency Sands and Gravels Silts and Clays No. of Relative No. of Relative Blows, N Density Blows, N Density 0-4 Very loose 0-2 Very soft 5-10 Loose 3-4 Soft Medium dense 5-8 Firm Dense 9-15 Stiff Over 50 Very dense Very stiff Hard Over 50 Very hard Particle Size Identification Boulders: Cobbles: Gravel: Sand: Silt and Clay: (Unified Classification System) Diameter exceeds 12-in. (300-mm) 3-in. (75-mm) to 12-in. (300-mm) diameter Coarse - ¾-in. (19-mm) to 3 in. (75-mm) diameter Fine - No. 4 (4.75-mm) sieve to ¾-in. (19-mm) diameter Coarse No. 10 (2.0-mm) to No. 4 (4.76 mm) sieve Medium No. 40 (0.425-mm) to No. 10 (2.0-mm) sieve Fine - No. 200 (0.075-mm) to No. 40 (0.425-mm) sieve Less than No. 200 (0.075-mm) sieve Modifiers The modifiers provide our estimate of the amount of silt, clay or sand size particles in the soil sample. Approximate Content Modifiers Field Moisture Description 5%: Trace Dry Absence of moisture, dusty, dry to touch 5 to 10%: Few Moist Damp but no visible water 15 to 25%: Little Wet Visible free water, usually soil is 30 to 45%: Some below water table 50 to 100% Mostly

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25 R Froehling & Robertson, Inc. BORING LOG Boring: B-1 (1 of 3) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation Depth Description of Materials (Classification) 2 inches of surficial soil ALLUVIAL: soft, yellowish brown, lean CLAY with sand - moist Elevation: 70 ± Total Depth: 50.0' Location: See Boring Location Plan (CL) medium-dense, strong brown, clayey SAND - moist (SC) very stiff, reddish brown, sandy lean CLAY - moist (CL) * Sample Blows Sample Depth N-Value (feet) (blows/ft) Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/23/15 Driller: C. Sequist Remarks No groundwater was encountered during drilling RESIDUAL: very stiff, red and gray mottled, fat CLAY - moist (CH) hard, red with gray, SILT with fine sand - moist (ML) BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/ hard, dark red with bluish gray, SILT - moist (ML) hard, greenish gray, weathered rock sampled as SILT with sand and weathered rock (Quantico *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value

26 R Froehling & Robertson, Inc. BORING LOG Boring: B-1 (2 of 3) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation Depth Description of Materials (Classification) slate) - moist Elevation: 70 ± Total Depth: 50.0' Location: See Boring Location Plan (ML) * Sample Blows Sample Depth N-Value (feet) (blows/ft) Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/23/15 Driller: C. Sequist Remarks 50/ / / BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/ hard, bluish gray, weathered ROCK sampled as SILT with sand and weathered ROCK (Quantico slate) - moist (ML) 32-50/ *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value. 50/

27 R Froehling & Robertson, Inc. BORING LOG Boring: B-1 (3 of 3) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation Depth Description of Materials (Classification) Elevation: 70 ± Total Depth: 50.0' Location: See Boring Location Plan * Sample Blows Sample Depth N-Value (feet) (blows/ft) Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/23/15 Driller: C. Sequist Remarks Cave-in depth 44.2' 50-50/ Boring terminated at 50 feet. Boring backfilled upon completion BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/15 *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value.

28 R Froehling & Robertson, Inc. BORING LOG Boring: B-2 (1 of 1) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation 89.9 Depth 0.1 Elevation: 90 ± Total Depth: 20.0' Location: See Boring Location Plan Description of Materials (Classification) 1 inch of surficial soil ALLUVIAL: loose to medium-dense, yellowish brown, clayey SAND - moist (SC) * Sample Blows Sample Depth N-Value (feet) (blows/ft) Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/23/15 Driller: C. Sequist Remarks No groundwater was encountered during drilling very stiff, yellowish brown with gray mottled, sandy lean CLAY - moist (CL) very stiff, yellowish brown with gray mottled, lean CLAY - moist (CL) RESIDUAL: very stiff, gray and strong brown and pale red mottled with red foliation, lean CLAY - moist (CL) BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/ very stiff, bluish gray mottled with dark reddish brown, lean CLAY - moist (CL) Boring terminated at 20 feet. Boring backfilled upon completion Cave-in depth 17.1' *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value.

29 R Froehling & Robertson, Inc. BORING LOG Boring: B-3 (1 of 3) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation: 108 ± Total Depth: 50.0' Location: See Boring Location Plan Description of Materials * Sample Sample Elevation Depth Depth N-Value (Classification) Blows (feet) (blows/ft) inch of surficial soil ALLUVIAL: very loose to medium-dense, yellow 3 brown and greenish gray mottled, clayey SAND - moist (SC) 10 Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/23/15 Driller: C. Sequist Remarks BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/15 *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value

30 R Froehling & Robertson, Inc. BORING LOG Boring: B-3 (2 of 3) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation Depth Description of Materials (Classification) Elevation: 108 ± Total Depth: 50.0' Location: See Boring Location Plan * Sample Blows Sample Depth N-Value (feet) (blows/ft) Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/23/15 Driller: C. Sequist Remarks RESIDUAL: hard, bluish gray, weathered rock sampled as SILT with sand and weathered rock (Quantico slate) - moist (ML) 30.0 Water Level 32.0' BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/ / *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value. 50/

31 R Froehling & Robertson, Inc. BORING LOG Boring: B-3 (3 of 3) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation Depth Description of Materials (Classification) Elevation: 108 ± Total Depth: 50.0' Location: See Boring Location Plan * Sample Blows Sample Depth N-Value (feet) (blows/ft) Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/23/15 Driller: C. Sequist Remarks Cave-in depth 46.2' 50/ Boring terminated at 50 feet. Boring backfilled upon completion BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/15 *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value.

32 R Froehling & Robertson, Inc. BORING LOG Boring: B-4 (1 of 3) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation Depth Elevation: 140 ± Total Depth: 50.0' Location: See Boring Location Plan Description of Materials (Classification) 1 inch of surficial soil ALLUVIAL: stiff, strong brown, lean CLAY with sand - moist (CL) medium-dense, strong brown, clayey SAND - moist (SC) medium-dense to dense, dark yellowish brown, clayey SAND - moist (SC) * Sample Blows Sample Depth N-Value (feet) (blows/ft) Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/27/15 Driller: C. Sequist Remarks medium-dense, pale brown, clayey SAND - moist (SC) BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/ very stiff, light brownish gray, lean CLAY - moist (CL) *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value

33 R Froehling & Robertson, Inc. BORING LOG Boring: B-4 (2 of 3) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation Depth 22.0 Elevation: 140 ± Total Depth: 50.0' Location: See Boring Location Plan Description of Materials (Classification) dense, strong brown and gray mottled, clayey SAND with gravel - moist (SC) * Sample Blows Sample Depth N-Value (feet) (blows/ft) Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/27/15 Driller: C. Sequist Remarks dense, light yellowish brown, clayey SAND - wet (SC) Water level 30.0' very dense, yellowish red, clayey SAND with gravel - moist (SC) BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/ hard, dark yellowish brown and gray mottled, lean CLAY - moist (CL) *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value

34 R Froehling & Robertson, Inc. BORING LOG Boring: B-4 (3 of 3) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation Depth Description of Materials (Classification) Elevation: 140 ± Total Depth: 50.0' Location: See Boring Location Plan * Sample Blows Sample Depth N-Value (feet) (blows/ft) Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/27/15 Driller: C. Sequist Remarks hard, strong brown, sandy lean CLAY - moist (CL) Boring terminated at 50 feet. Boring backfilled upon completion BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/15 *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value.

35 R Froehling & Robertson, Inc. BORING LOG Boring: B-5 (1 of 1) Project No: 74T-0009 Client: Community Housing Partners Project: Dumfries Mixed-Use Development City/State: Dumfries, Virginia Elevation Depth Elevation: 110 ± Total Depth: 20.0' Location: See Boring Location Plan Description of Materials (Classification) 2 inches of surficial soil ALLUVIAL: medium-dense, strong brown, clayey SAND - moist (SC) loose to medium-dense, light greenish gray, silty SAND - moist (SM) * Sample Blows Sample Depth N-Value (feet) (blows/ft) Drilling Method: HSA 3-1/4" ID Hammer Type: Automatic Date Drilled: 4/24/15 Driller: C. Sequist Remarks Water level 10' medium-dense to dense, strong brown and light greenish gray, clayey SAND with gravel - moist (SC) Cave-in depth 15.4' BORING_LOG 74T-0009.GPJ F&R.GDT 5/18/ Boring terminated at 20 feet. Boring backfilled upon completion *Number of blows required for a 140 lb hammer dropping 30" to drive 2" O.D., 1.375" I.D. sampler a total of 18 inches in three 6" increments. The sum of the second and third increments of penetration is termed the standard penetration resistance, N-Value