Appendix_D_-_Transient_Geotechnical_Report_-2606xx.pdf

Sheetz – Mt. Aetna Road March 18, 2022 Triad Proposal No. 03-22-0246 P a g e 1

Report of Geotechnical Exploration Triad Project No. 07-25-0112

September 18, 2025 (Revised October 14, 2025)

Mattawoman WWTP Effluent Force Main Standpipe

5310 Hawthorne Road, LaPlata, Maryland

Prepared For:

Mr. Travis Junker

GHD

222 South Church Street, Suite 400 Charlotte, NC 28202

Direct: 240 206 0492 Mobile: 443 926 2279 Email: Madeline.Barbalace@ghd.com

Prepared By:

Triad Engineering Inc.

201 Davis Drive, Suite KK Sterling, VA 20164

Phone: 703.930.3401 Email: tgary@triadeng.com mailto:Madeline.Barbalace@ghd.com

September 18, 2025 (Revised October 14, 2025)

Mr. Travis Junker

GHD

222 South Church Street Suite 400 Charlotte, NC 28202

RE: Report of Geotechnical Exploration Mattawoman WWTP Effluent Force Main Standpipe

5310 Hawthorne Road LaPlata, Maryland GHD Project No.: 12571916

Triad Project No. 07-25-0112

Dear Mr. Junker:

In accordance with your request, we have conducted a geotechnical exploration for the proposed effluent force main standpipe and valve boxes to be constructed at the Mattawoman Waste Water Treatment Plant (WWTP) in the LaPlata area of Charles County, Maryland. The geotechnical scope of services was completed in substantial conformance with our revised proposal dated April 1, 2025 and authorized through issuance of the GHD Purchase Order No. 12571946. This report includes the results of the field exploration and laboratory testing, and it presents our recommendations related to the geotechnical design and construction elements of the project.

The exploration was performed to evaluate the subsurface conditions encountered at the site for the limited purposes of preparing design and construction recommendations for geotechnical aspects of the proposed effluent force main standpipe and valve boxes. It is emphasized that subsurface conditions may vary dramatically between the test borings, and Triad makes no representations as to subsurface conditions other than those encountered at the specific test locations.

This report has been prepared for the exclusive use of GHD for specific application to the design and construction of the proposed effluent force main standpipe and valve boxes to be constructed at the Mattawoman WWTP in LaPlata, Maryland. Triad’s responsibilities and liabilities are limited to our client and apply only to their use of our report for the purposes described above. To observe compliance with design concepts and specifications, and to facilitate design changes in the event that subsurface conditions differ from those anticipated prior to construction, it is recommended that Triad be retained to provide continuous engineering and testing services during the earthwork and foundation construction phases of the work.

Mattawoman WWTP Effluent Force Main Standpipe September 18, 2025 GHD Project No.: 12571916 Revised October 14, 2025 Triad Project No. 07-25-0112 Page 2

We appreciate the opportunity to assist you on this project and trust this report satisfies your needs at this time. Please feel free to contact us if you have any questions, or if we can provide further assistance.

Sincerely, TRIAD ENGINEERING, INC.

Tim Gary, P.E.

Senior Engineer

Mark E. Clippinger, P.E.

Geotechnical Practice Leader Professional Certification. I hereby certify that these documents were prepared or approved by me, and that I am a duly licensed professional engineer under the laws of the State of Maryland, License No.22106, Expiration Date: September 18, 2026.

TABLE OF CONTENTS

FOREWORD

SITE AND PROJECT DESCRIPTION

GEOLOGIC SETTING

General Geology Soil Mapping

FIELD EXPLORATION

SUBSURFACE CONDITIONS

LABORATORY TESTING

DISCUSSION

General Standpipe

DESIGN RECOMMENDATIONS

Valve Vaults – Shallow Spread Foundations Standpipe – Deep Foundation Systems

Drilled Shafts (Caissons) and Auger Cast Piles Driven Piles

Seismic Site Classification Slabs-on-Grade Lateral Earth Pressures Lateral Analysis of Deep Foundations

CONSTRUCTION RECOMMENDATIONS

Site Preparation Excavations Controlled Fill

Satisfactory Materials Placement and Compaction

Shallow Foundations Auger Cast Piles Drilled Shafts (Caissons) Driven Piles Slabs-on-Grade Utilities Construction Observations i

TABLE OF CONTENTS (continued)

APPENDIX A

Site Vicinity Map .................................................................................................... Figure No. A-1 Geologic Map ......................................................................................................... Figure No. A-2 USDA Soil Map ..................................................................................................... Figure No. A-3 Boring Location Plan .............................................................................................. Figure No. A-4

APPENDIX B

Key to Identification of Soil and Weathered Rock Samples .................................... Figure No. B-1 Logs of Borings ............................................................................................... SB-1 through SB-6

APPENDIX C

Laboratory Test Results ................................................................................................. 22 pages

APPENDIX D

Seismic Design Data ........................................................................................................ 4 pages ii

REPORT OF GEOTECHNICAL EXPLORATION

MATTAWOMAN WWTP EFFLUENT FORCE MAIN STANDPIPE

LAPLATA, MARYLAND

GHD PROJECT NO.: 12571916

TRIAD PROJECT NO. 07-25-0112

FOREWORD

This report has been prepared for the exclusive use of GHD for specific application to the design and construction of the proposed effluent force main standpipe and valve boxes to be constructed at the Mattawoman Waste Water Treatment Plant (WWTP) in the LaPlata area of Charles County, Maryland. The work has been performed in accordance with generally accepted geotechnical engineering practices. No other warranty, expressed or implied, is made.

This report should not be used for estimation of construction quantities and/or costs, and contractors should conduct their own exploration of site conditions for these purposes. Please note that Triad is not responsible for any claims, damages or liability associated with any other party’s interpretation of the data or re-use of these data or engineering analyses without the express written authorization of Triad. Additionally, this report must be read in its entirety.

Individual sections of this report may cause the reader to draw incorrect conclusions if considered in isolation from each other.

The conclusions and recommendations contained in this report are based, in part, upon our field observations and data obtained from the borings performed at the site. The nature and extent of variations may not become evident until construction. If variations then appear evident, it may be necessary to re-evaluate the recommendations presented herein. Similarly, in the event that any changes in the nature, design, or location of the facilities are planned, the conclusions and recommendations contained herein shall not be considered valid unless the changes are reviewed, and the conclusions are modified or verified in writing by Triad. If we are not afforded the privilege of making this review, we will not assume responsibility for misinterpretation of our recommendations, as our recommendations are strictly limited to conditions represented to Triad at the time this report was issued.

SITE AND PROJECT DESCRIPTION

The project site is located within the northwestern end of the existing Mattawoman Waste Water Treatment Plant (WWTP) located at 5310 Hawthorne Road in LaPlata, Maryland. Mattawoman Creek traverses approximately 500 feet northeast of the proposed development location. A site vicinity map is presented as Figure No. A-1 in Appendix A.

We understand that GHD is assisting Charles County with a transient analysis on the WWTP effluent force main and construction of a recommended alternative to mitigate transient impacts on the system. The recommended alternative is to divert flow from the existing force main to a

Triad Project No. 07-25-0112 Page 2 standpipe with a 40-foot diameter and approximately 120-foot height. Influent and effluent valve vaults are planned to be constructed on each side of the proposed standpipe. Additional pipework and thrust blocks will also be constructed.

Preliminary plans indicate the base of the standpipe will be constructed near the current existing ground surface elevation. The valve vaults are anticipated to be supported approximately 15 feet below the existing ground surface. Depending on the final design configuration, the standpipe could have an effluent-filled dead load of approximately 10 to 16 kips/square-foot. Foundation types and depths will be determined based on the findings of this report.

Most of the standpipe, valve vaults and associated pipework will be situated within a currently paved area which is relatively flat with a typical surface elevation of approximately 17 feet above sea level. The ground beyond the pavement slopes downhill in all directions.

Based on the preliminary layout information provided, we anticipate that a combination of excavations and minor fills will be required to establish finished grades.

GEOLOGIC SETTING

General Geology

Referencing the Geologic Map of The Port Tobacco Quadrangle, Prince George’s and Charles County, Maryland published in 1984 by the Maryland Geological Survey, as viewed on-line at the United States Geological Survey (USGS) website, the site is clearly mapped within the Holocene-aged Alluvium of the Atlantic Coastal Plain physiographic province. The alluvium generally consists of heterogeneous interbedded sands, gravels, silts and clays with varying amounts of organics. These sedimentary soils were deposited in stream channels, floodplains, adjacent low areas and marshes during periods of fluctuating sea levels.

Referencing the Geologic Map of the Washington West 30' x 60' Quadrangle, Maryland, Virginia, and Washington D.C. published in 2017 by the Maryland Geological Survey, the project area mapped near the interface of the referenced previously referenced Alluvium and the upper Pleistocene-aged Lynnhaven and Poquoson Members (undivided) of the Tabb Formation. These members generally consist of sedimentary muddy sands and sandy silts which were deposited in low terraces of tidal creeks and bays near the Potomac River. An excerpt from the referenced 2017 geologic map is presented as Figure No. A-2 in Appendix A.

Soil Mapping

Referencing information obtained from the United States Department of Agriculture (USDA) Web Soil Survey, the entire the project area is mapped as Urban Land (UK). The nearest natural formation is mapped as the Potobac-Issue complex (Pu).

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The Potobac-Issue complex (Pu) (Hydraulic Soil Group B/D) soils generally consist of somewhat poorly-drained to poorly-drained, silt loam, loam, sandy loam, and gravelly coarse sand that have developed as alluvium in floodplains. These soils are rated as “very limited” for most site development categories because of flooding, ponding, depth to saturated zone and frost action.

These slightly acidic soils are rated as highly corrosive to concrete and moderately corrosive to steel.

The Urban Land soils (Hydraulic Soil Group D) have been reworked by site development and are covered with impermeable surfaces such as pavement. These soils may possess some of the parent soil attributes such as corrosivity. However, the majority of native characteristics associated with the original soil matrix, such as depths to saturated zones or hydraulic conductivity are not rated. A printout of the soil mapping of the project area is presented as Figure A-3 in Appendix A.

FIELD EXPLORATION

The exploration involved drilling six (6) soil test borings with a track-mounted Diedrich D50 turbo drill rig to generally evaluate the subsurface conditions at the sites. Boring SPT-1 was located near the center of the proposed standpipe. This boring extended to a depth of 22 feet below the ground surface (bgs) at which point a thin-walled Shelby Tube was lost down the hole. Boring SPT-1B was then offset approximately 2 feet from boring SPT-1 and continued to a depth of approximately 80 feet bgs. Borings SPT-2 through SPT-4 were located around the proposed standpipe perimeter and extended to planned depths of 30 feet bgs. Borings SPT-4 and SPT-5 were located within the proposed valve vault footprints and extended to planned depths of 40 feet bgs. A boring location plan is presented as Figure A-4 in Appendix A.

The drill rig borings included Standard Penetration Testing (SPT) and split barrel sampling (ASTM D 1586) at regular intervals to the planned termination depths. Correlations to the SPT N-value provide an indication of the soil bearing capacity and strength properties. At three (3) depths within boring SPT-1 or SPT-1B, thin-walled metal tubes (Shelby tubes) were pushed to collect relatively undisturbed samples of fine-grained soils (ASTM D1587). A geotechnical engineer from our office was present full time during the field exploration to log all recovered soil and to observe groundwater and rock conditions. Detailed descriptions of materials encountered in the borings are contained on the test boring logs in Appendix B. Figure No. B-1 contains descriptions of the classification system and terminology utilized. The recovered soil samples were transported to our laboratory for further testing.

SUBSURFACE CONDITIONS

The materials encountered in the test borings are generally described below. Stratification lines indicated on the logs represent the approximate boundaries between material types, and the actual transitions may be gradual or very abrupt.

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Surface Materials: The ground surface at the boring locations was covered with approximately 7 to 9 inches of bituminous asphalt overlying approximately 3 to 7 inches of base stone.

Existing Fill: Existing man-placed fill soils were encountered in each of the borings immediately below the surficial asphalt and gravel. These fill deposits extended to depths of approximately 5 to 9 feet below the ground surface (bgs). At boring SPT-4 “possible” fill materials extended to a depth of 13.5 feet bgs. The fill and possible fill materials generally consisted of sandy clay to sand with lesser percentages of gravel, silt and clay.

An SPT N-value of 19 was obtained in the fine-grained clays indicating a very stiff consistency. SPT N-values obtained within coarse-grained sandy soils ranged from 16 to 39 blows per foot indicating medium dense to dense relative densities.

Natural Soils (Alluvial/Fluviomarine deposits): Natural sedimentary soils were encountered below the surficial pavement and fill deposits. The soils ranged from interbedded clays with lesser fractions of sand and gravel to coarse sand and gravel with lesser fractions of silt and clay. SPT N-values within the fine-grained clays and silts ranged from 5 blows per foot (bpf) to 50 blows with 3 inches of penetration, indicating medium stiff to hard consistencies. SPT N-values obtained within coarse-grained sand sands and gravels ranged from 5 bpf to 50 blows with no penetration indicating loose to very dense relative densities.

Groundwater: Groundwater was encountered within the sand seams at depths of 10 to

23.5 feet below the ground surface during drilling. After a period of 24 hours subsequent to completion of drilling, groundwater was recorded at depths of 5.8 to 25.5 feet below the ground surface. The preliminary site plan indicates the 100 year flood plain is approximately 3 feet below the pavement surface. It is important to note that fluctuations in perched water and groundwater levels may occur due to variations in environmental conditions, surface drainage and other factors which may not have been evident at the time measurements were made and reported herein.

LABORATORY TESTING

Laboratory tests were performed to supplement the field classifications and establish geotechnical design criteria. All laboratory tests were completed in accordance with appropriate ASTM standard test methods. Detailed results of the laboratory tests are contained in Appendix C. A summary of the test results is presented as follows:

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TEST TYPE TEST RESULTS

Moisture Content 3.4 to 30.8% Atterberg Limits:

Liquid Limit Plasticity Index

33 to 43 (3 samples), NV (4 samples) 21 to 27 (3 samples), NP (4 samples)

Percent passing #200 sieve 6 to 69

USCS Soil Classifications CL (2 samples), SM (2 samples), SC, SM, SP-SM, and GP-GM

1-Dimensional Consolidation In-situ Dry Density (ɣ) Preconsolidation pressure (Pc) Compression Index (Cc) Initial Void Ratio (eo)

108.2 to 124.4

1.3 to 1.9 tsf

0.16 to 0.19

0.537 to 0.554

DISCUSSION

General

The subsurface information obtained from the field exploration, our past experience with similar projects and the noted design criteria were the basis for our assessment of the geotechnical issues currently existing at the site. Our geotechnical recommendations associated with the design and construction of the proposed effluent force main standpipe and valve boxes and structural fill are presented in the following sections of this report.

Provided that the recommendations presented in this report are strictly followed, the site soils encountered generally are conditionally suitable for support of the valve boxes with shallow spread footings or a slab-on-grade mat foundation. The laboratory testing indicated that the majority of the site soils will meet gradation and plasticity requirements for new fill construction.

However, it must be noted that lenses or pockets of moderately high plasticity, potentially expansive soils, are sometimes present within sedimentary soils. Although high plasticity soils were not specifically detected in the test borings, special consideration should be given to the possible isolated presence and treatment of these materials.

Standpipe

Soils encountered immediately below the existing pavement and to depths of 5 to 9 feet or more, generally consisted of existing fill deposits. The majority of sands and clays immediately below the fill deposits and to depths approximately 13.5 feet were characterized as medium dense or very stiff, respectively. Very dense soils generally were encountered at depth 20 feet or more.

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Based on the SPT values, the soils in the upper soil profile would appear to be suitable for 3,000 to 5,000 psf bearing capacity. However, limited calculations indicate that there likely will be significant ground settlement associated with the proposed standpipe loads if the structure is supported upon a shallow mat or ring foundation.

As such, we recommend that the standpipe structure be supported upon a conventional deep foundation system such as drilled piers (caissons), auger cast piles or driven piles. Unless specifically detailed in this report, the deep foundation design and construction should be performed in accordance with recommendations presented in the United States Department of Transportation Federal Highway Administration (FHWA) FHWA-NHI 18-024 Drilled Shafts Construction Procedures and Design Methods, FHWA-HIF-07-03 Design and Construction of Continuous Flight Auger Piles, and FHWA-NHI 16-009 Design and Construction of Driven Pile Foundations – Volume I and II. We recommend that the foundation system design be provided by a specialty foundation design-build contractor. All design plans and calculations should be submitted for review by Triad.

Settlement of the proposed piles should be analyzed after selection of final pile size, design capacity and spacing. Based on our experience with similar projects, depending on final design, we anticipate that the total and differential settlements can be limited to approximately 2 to 3 inches and 1 to 2 inches, respectively.

DESIGN RECOMMENDATIONS

Valve Vaults – Shallow Spread Foundations

Based on the encountered conditions, we believe that the proposed valve vaults can be supported on either conventional strip footing foundations or a base mat slab bearing either on approved in-situ materials and/or new controlled fill. Based on the anticipated bearing elevations and the conditions encountered during drilling, we recommend that a maximum allowable bearing pressure of 4,000 psf be utilized for design of the valve vaults. Minimum plan dimensions of eighteen (18) and twenty-four (24) inches should be adopted for continuous and isolated footings, respectively. All exterior footings should be placed at least 30 inches below the finished exterior ground elevations for frost protection.

We understand that the vault bases will likely extend approximately 15 feet below the existing ground surface. The vault bases must be extended to bear beneath any fat clay, elastic silt, or unstable uncontrolled fill deposits, if present. Where suitable bearing materials are not present the design bearing levels, we recommend that the foundation excavations be extended to suitable bearing materials (approved structural fill or approved natural ground), and these excavations can be backfilled to the planned bearing level with either select structural fill, lean mix concrete (fc’ = 1,500 psi) or flowable fill grout. If over-excavations are required, they should be stepped as appropriate to achieve level bearing surfaces on approved soils with suitable bearing capacity.

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Based on the assumed structural loads and results of the exploration, it is estimated that total settlements for isolated foundations bearing on new controlled fill or approved alluvial or fluviomarine soils will be one (1) inch or less. Differential settlements along continuous wall footings are not expected to exceed an angular distortion of 0.0015 inch/inch.

If extensive time will be required to place reinforcing steel, the contractor may elect to place a minimum 3-inch thick “mud mat” to protect the foundation bearing soils once approved. The “mud mat” may consist of flowable fill or lean mix concrete.

Standpipe - Deep Foundation Systems

As presented in the Discussion section of this report, we believe that use of a shallow foundation system to support the proposed standpipe would result in significant ground settlement. As such, we recommend that the standpipe be supported upon a conventional deep foundation system such as drilled piers (caissons), auger cast piles or driven piles.

Drilled Shafts (Caissons) and Auger Cast Piles

It is our opinion that drilled pier (caisson) and auger cast pile foundations should be designed to achieve capacity based on a combination of skin friction and end bearing.

Both the unit skin friction and unit end bearing in granular soils will be a function of the SPT N-values, the average overburden stresses and by the depth of the analyses.

Referring to the soil borings we recommend that subsurface soils within the tower area may be divided into three strata. We estimate that unit skin friction and end bearing will approximately vary with depth as indicated in the following table. Total friction and total end bearing capacities will be dependent upon the selected pile dimensions.

Soil Layer Depth (ft) Unit Skin Friction (ksf)

Unit End Bearing (ksf)

#1 0 to 20 0.75 - #2 20 to 50 2.0 30 #3 ≥50 4.0 18

Pile spacing should be uniform through the support area and at least 3 times the chosen pile diameter. Both individual and pile group interactions must be considered. The foundation system design should be provided by a specialty foundation design-build contractor. The final sizing and spacing of the drilled piles should be established and verified during the design process by a structural engineer working in conjunction with the geotechnical engineer.

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Studies have shown than significantly smaller settlement displacements are required to fully develop resistance from side-wall shear (side friction) compared to the displacements required to fully develop end bearing. For shafts with diameter D, displacements on the order of 1 to 2% x D may be sufficient to fully develop side-wall shear resistance.

However, displacements of 10 to 15% x D may be required for fully develop end bearing.

We strongly recommend that a qualified representative from our office be present during caisson or auger cast pile construction to monitor the drilling and verify that appropriate bearing conditions are present prior to concrete placement.

Driven Piles

It is our opinion that driven pile foundations should be designed to achieve capacity based on a combination of skin friction and end bearing. Both the unit skin friction and unit end bearing in granular soils will be a function of the SPT N-values, the average overburden stresses, the pile material and configuration, as well as the method of analyses and depth of the pile section in consideration. Depending on the depth of analysis, local unit skin friction may be on the order of 750 to 1,500 psf. Unit end bearing likely will be on the order of 3 to 20 tsf. Total friction and total end bearing capacities will be dependent upon the selected pile dimensions.

The foundation system design should be provided by a specialty foundation design-build contractor. The calculations should include a Wave Equation Analysis Program (WEAP) study for the proposed pile system and construction method. The final sizing and spacing of the driven piles should be established and verified during the design process by a structural engineer working in conjunction with the geotechnical engineer.

Pile spacing should be uniform through the support area and at least 3 times the chosen pile diameter. Both individual and pile group interactions must be considered. It should be noted that the much of upper sand and gravel stratum was characterized as dense to very dense, and some jetting or pre-drilling may be required in order for driven piles to achieve required depths. The final sizing and spacing of the driven piles should be established and verified during the design process by a structural engineer working in conjunction with the geotechnical engineer.

In order to verify the final capacities, we recommend performing a test pile program consisting of dynamically monitoring a minimum of 5% of the proposed production piles utilizing a Pile Driving Analyzer (PDA) system. Dynamic monitoring of the installed pile should be performed in conformance with ASTM D 4945. We strongly recommend that a qualified representative from our office be present during pile construction to monitor the construction and verify that pile capacities are achieved.

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Seismic Site Classification

The subsurface profile was evaluated and classified according to ASCE/SEI 7-22. This code establishes the criteria for project site evaluation and determination of several seismic design parameters. ASCE/SEI 7-22, Chapter 20 outlines the procedure for determination of the site classification based on the average shear wave velocity for materials to a depth of 100 feet. Table 20.2-1 includes ranges of estimated shear wave velocities for each class (type) of material, and those classes are determined as the result of soil type and in-situ consistencies and/or relative densities reflected by SPT testing in the borings. Based on the results of the test borings and our assumptions, the site has an average shear wave velocity of 1,200 feet per second (fps). Using this information along with knowledge of the site geologic setting, the seismic site class and additional seismic information is as follows:

SEISMIC PARAMETERS

Seismic Design Category A

Site Soil Class CD

Soil Profile Name Dense Sand or Very Stiff Soil

Site Modified Peak Ground Acceleration, PGAM 0.088

MCER ground motion (for 0.2 sec. period), Ss 0.16

MCER ground motion (for 1.0 sec. period), S1 0.043

Site-Modified Spectral Acceleration Value, SMS 0.19

Site-Modified Spectral Acceleration Value, SM1 0.076

Numeric Seismic Design Value at 0.2s SA, (SDS) 0.13

Numeric Seismic Design Value at 0.1s SA, (SD1) 0.051

TL 8

VS30 365

Based on results from the test borings, published regional geologic information and the probable maximum strength of earthquake, it is our opinion that liquefaction potential for the on-site soils during seismic activity is relatively low. Seismic parameters to be considered for structural design of the project are provided in Appendix D of this report.

Slabs-on-Grade

For the standpipe pile cap or other large concrete slabs supported on grade and bearing on new controlled fill and/or approved residual materials, we recommend that a modulus of subgrade reaction, "k," of 40 pci be adopted for analysis and design of the slab-on-grade. For smaller slabs

Triad Project No. 07-25-0112 Page 10 such as manhole bases constructed on new controlled fill and/or approved residual materials, we recommend that a modulus of subgrade reaction, "k," of 110 pci be adopted for analysis and design of the slab-on-grade. The slabs should be underlain by a minimum 4-inch thick layer of open-graded aggregate such as ASTM designation No. 57 stone. Slabs which will receive heavy or dynamic loading should be underlain by a minimum six (6) inch thick layer of well-graded, crushed aggregate such as VDOT 21B aggregate or an approved equivalent.

Lateral Earth Pressures

The current preliminary site plans do not identify any retaining walls. However, considering the elevation changes across the site, some retaining walls or below-grade building walls may be incorporated in the final topography. We recommend that the following lateral earth pressure coefficients be used to evaluate earth pressures for design of below-grade walls for the project.

It is noted that the coefficients are based on the angle of internal friction of the material, and these coefficients do not include a factor of safety. Accordingly, the designer should include a factor of safety of at least 1.5 in the structural design.

ACKFILL

MATERIAL

MOIST

UNIT

WEIGHT

(pcf)

FRICTION

ANGLE

(degrees)

LATERAL EARTH PRESSURE

COEFFICIENTS

(LEVEL BACK SLOPE)

Active (Ka)

At-Rest (Ko)

Passive (Kp)

Lean Clay or Silt 110 28 0.36 0.53 2.77

LOOSE (SPT N≤10)

Clayey or Silty Sand, Sands or More Granular

115 28 0.36 0.53 2.77

MEDIUM DENSE TO VERY DENSE

(SPT > 11) Clayey or Silty Sand, Sands or More Granular 125 34 0.28 0.44 3.54

MD #57 gravel 110 38 0.24 0.38 4.20 Well-graded crushed aggregate

(GAB or CR-6) 145 40 0.21 0.36 4.60

Any surcharge loads anticipated at the surface should be multiplied by 0.5 and superimposed as a uniform horizontal pressure on the recommended design lateral loading.

Based on IBC Table 1806.2, the coefficient of friction (µ) which can be used for determination of sliding resistance on the base of foundation elements can be computed as 0.25 times the total dead load on the foundation.

It is emphasized that the designer must carefully consider the rigidity of the retaining system in order to select either an active or at-rest condition. In order to use an active earth pressure, the wall(s) must undergo a slight amount of rotation which will result in some translation at the top of

Triad Project No. 07-25-0112 Page 11 the wall. Typically, for well-compacted granular backfill, the translation may be on the order of

0.001 to 0.002H, where H is the height of the wall. Similarly, for compacted cohesive soils, the translation may be on the order of 0.01 to 0.02H. If the retaining system is extremely rigid and/or upper levels are constructed over the wall to provide additional horizontal resistance, an at-rest pressure will be more appropriate.

The lateral pressure values recommended above are based on adequate drainage behind the walls without build-up of hydrostatic pressures. Consequently, a permanent backwall drainage system should be constructed along exterior retaining walls or below grade walls. The permanent backwall drainage should include a 4-inch diameter Schedule 40 PVC or HDPE perforated pipe surrounded by an 18-inch wide zone of free draining gravel such as ASTM Size No. 57 and separated from the general site backfill by a non-woven geofabric, such as TC Mirafi 140-N or an approved equal. Backwall drains should be sloped such that water will flow by gravity to a sump or an appropriate drain and day-light beyond the structures.

Lateral Analysis of Deep Foundations

Resistance to lateral loads can be achieved by passive soil pressures exerted on the vertical piles and the passive soil pressures of the proposed pile cap at each group. A full analysis of the lateral capacity should be evaluated by the structural engineer during the design phase of the tower foundations. In order to aid in this evaluation, the following parameters are provided. The given parameters are from the LPILE Reference Manual (unless otherwise noted), and may be used for lateral analysis and design of the proposed foundations:

Material Type

Material Modeled

As

Unit Weight1, γ (pci)

Undrained Cohesion, c (psi)

Strain Factor ε50

Friction Angle Φ’

(degrees) k (pci)

Unconfined Compressive

Strength (psi)

Clays Soft Clay (Matlock) 0.0637 1.736 0.02 0 50 1.736

Clays Medium Clay (Reese) 0.0695 3.472 0.01 0 100 3.472

Sands2 (Loose)

Sand (Reese) 0.0666 0 0 28 20 N/A

Sands3

(Dense) Sand

(Reese) 0.0724 0 0 34 225 N/A

1 Table provides average recommended moist unit weights. Unit weight of water must be subtracted to obtain effective unit weight (γ’) below water table.

2 Loose submerged sand (below the water table), such as for 30 to 65 feet in the standpipe borings.

3 Dense sand and gravel above the water table, such as upper 20 feet of standpipe area borings.

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CONSTRUCTION RECOMMENDATIONS

Site Preparation

Initial site clearing and grubbing should include removal of pavement, forest cover and any other deleterious materials within the structures and any controlled fill sites and extending five (5) feet beyond their perimeters. After removal of the unsuitable materials, the subgrade soils should be heavily proof-rolled with approved construction equipment to locate isolated soft spots or areas of excessive "pumping" which are too wet to accommodate compacted fill or building construction.

These areas should be scarified, air-dried to sufficient moisture content and re-compacted prior to fill placement, mechanically stabilized, or removed to the level of stable soils. The exposed subgrade should be examined and tested by on-site geotechnical engineering personnel prior to placement of compacted fill.

Excavations

In general, the sedimentary site soils can be excavated with conventional earth moving equipment such as backhoes and tracked loaders. Some very dense desiccated soils and deeper gravels were identified, however, neither cemented soils nor hard rock was encountered in the borings.

It should be noted that although not identified in any of the borings, area geology indicates that very dense materials such as cobbles or cemented sands could be present in isolated discrete pockets.

Groundwater was encountered at depths relatively shallow depths and flood plains are mapped only a few feet below the existing ground surface within the work area. Additionally, groundwater elevations may fluctuate seasonally and with precipitation events. It is likely that groundwater will be encountered during vault excavations, some pipe excavations and during deep foundation construction. Therefore, the contractor should be prepared to implement, if necessary, temporary de-watering measures in these areas during construction. These measures can include sloping the cut areas to appropriate sump pit(s) and pumping accumulated surface runoff from precipitation. All cut areas should be sealed at the end of each day, to the extent which construction practicality will permit, to help prevent infiltration of precipitation and subsequent unsuitable soil conditions.

All utility trenches should be sloped and/or supported in accordance with current Occupational Safety and Health Administration (O.S.H.A.) requirements. Site soils generally should be considered O.S.H.A. Type C with regard to trench and excavation support. Trenches below structure and pavement areas should be backfilled in accordance with the Controlled Fill section of this report.

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Controlled Fill

Satisfactory Soils

Based on the results of the field exploration, the majority of the sedimentary on-site soils can selectively be used for structural fill provided that requirements of this section are strictly maintained. Materials proposed as new fill within areas of structural support must meet the criteria outlined below. In accordance with Charles County ordinances, fill areas are divided into the following categories:

Type I: Fills within this category include load-bearing fills for the support of buildings, walls, and other structures, the function thereof which would be especially impaired by settlement.

Type II: Fills within this category included load-bearing fills for the support of roadways, pavements, utility lines and structures which would not be especially imparted by moderate settlement.

Type III: Fills within this category included common fills in green space, landscaping or other non-load-bearing usage.

All Type I and Type II fill shall be free of any debris, waste, or frozen materials and they should contain less than two (2) percent vegetation-organic materials by weight. Any isolated limbs, roots or other similar organics that are greater than approximately 2 inches in diameter shall be removed from the fill areas. Soils classified as MH, CH, OL, OH, or Pt based on the Unified Soil Classification System are not suitable for use as structural fill. Additionally, Type I fill materials shall have a liquid limit less than 40 and a plasticity index less than 15. Type II fill materials shall have a liquid limit less than 45 and a plasticity index of less than 20.

Materials placed within the top five (5) feet of any structure areas, controlled fill slopes, and the top one (1) foot of pavement subgrade shall be free of gravel or cobbles larger than four (4) inches in any dimension.

All proposed fill materials shall be approved by a geotechnical engineer prior to placement as controlled fill. Representative samples of proposed fill material shall be submitted by the contractor one week prior to placement of that material to allow time for completion of the necessary laboratory tests.

Placement and Compaction

Before compaction, each layer should be moistened or aerated as necessary to obtain the required compaction. Each layer should be compacted to the required percentage of maximum dry density. Fill should not be placed on surfaces that are muddy or frozen or have not been approved by testing and/or proof-rolling. Free water should be prevented from appearing on the surface during or subsequent to compaction operations.

Triad Project No. 07-25-0112 Page 14

Soil material which is removed because it is too wet to permit proper compaction can be spread and allowed to dry. Drying can be facilitated by discing or harrowing until the moisture content is reduced to an acceptable level. When the soil is too dry, water should be applied uniformly to the subgrade surface or to the layer to be compacted.

All fill material compacted by heavy compaction equipment should be placed in maximum 8-inch loose lifts. All fill material compacted by hand-operated tampers or light compaction equipment should be placed in maximum 4-inch loose lifts.

Unless more stringent code requirements apply, we recommend that fill materials be compacted to the following minimum standards:

Type I: Fills for the support of buildings, walls, and other structures must be compacted to at least 95% of the Modified Proctor maximum dry density.

Type II: Fills for the support of pavements and utility lines be compacted to at least 92% of the Modified Proctor maximum dry density. We recommend that fills within the upper 12 inches of roadway or pavement subgrade be compacted to at least 97% of the Modified Proctor maximum dry density.

Type III: Fills in non-load bearing green space should be compacted to at least 85% of the Modified Proctor maximum dry density.

The moisture content of the soils should be at or within two (2) percentage points of the optimum moisture content.

Shallow Foundations

We anticipate that conventional earth excavation equipment such as a backhoe or trackhoe can be utilized to excavate the natural sedimentary soils and controlled fill for shallow or mat foundation construction. We recommend that any loose materials present at the bottom of footing excavations because of excavation work be re-compacted or completely removed by hand to reduce differential settlements.

Foundation concrete should be placed the same day that excavations are completed to reduce the potential for softening due to precipitation and/or runoff. In areas where backfill adjacent to wall construction has not been placed prior to precipitation events, any ponded water that accumulates in these areas should be pumped out immediately to help prevent softening and deterioration of the surrounding soils. In addition, all rough grades around the structures should be sloped away from the structures both during and after construction such that water from precipitation does not build up or pond adjacent to the perimeters.

Triad Project No. 07-25-0112 Page 15

Any underground utilities which are located below or adjacent to new foundations should be backfilled with approved, well-graded crushed aggregate, lean mix concrete or flowable fill grout to grades which are at or above the design bearing levels. In addition, minimal thicknesses of bedding stone should be utilized beneath the utility lines to help prevent significant accumulation of water from precipitation developing within the utility trench area.

Auger Cast Piles

We recommend that all auger cast piles be monitored by the geotechnical engineer or his representative during installation in order to verify drilling procedures (including auger withdrawal rates), grout pumping rates, steel type and installation and grout takes so that grout quantities can be verified. A Pile Installation Recorder (PIR) should be provided on the installation drill to monitor auger cast pile installation.

Auger withdrawal should be monitored such that “necking” of the pile cannot occur. The withdrawal rate should be such that the grout take is no less than 115% of the theoretical grout take. If grout take is less than 115%, the pile should be rejected and re-drilled. A minimum 10-foot grout column should also be maintained during auger removal to prevent caving. If any auger displacement and/or grout return is encountered during auger withdrawal, the auger should be re-advanced to a depth of no less than 5 feet below the area of questionable grout take and re-grouted. Grout should be placed to the existing ground surface and reinforcing steel should be inserted immediately into the freshly placed grout. Freshly placed grout should be tested in accordance with ASTM C 31, C 109 and C 469 at intervals determined by the owner.

According to Federal Highway Administration (FHWA) Geotechnical Engineering Circular No. 8, traditional static load testing and/or integrity testing is only applied to large scale projects, and this does not typically apply to this project type. It should be the determination of the owner or installer if these tests are warranted.

Drilled Shafts (Caissons)

The caisson contractor should be capable of removing groundwater from the shaft. Temporary casing will be needed during the drilling operations to support the in-situ soils and to facilitate dewatering the shaft.

After the caisson has been drilled, the caisson bottom should be prepared to receive concrete.

This will require cleaning the hole with the drilling equipment. In order to facilitate smooth placement of concrete, we recommend that the concrete slump range between 5 and 7 inches for the drilled caissons, provided that a suitable mix design is developed to provide the necessary strength at the appropriate water-to-cement ratio.

The caisson bottom must be clean of debris and have less than 2 inches of standing water prior to placement of concrete. If groundwater in the caisson becomes problematic, the concrete may

Triad Project No. 07-25-0112 Page 16 need to be placed with a tremie tube, placing the concrete near the bottom and forcing the water out of the caisson hole. The caissons should be concreted immediately and should not be left open. We do not recommend down hole inspections of the bearing material at this site. However, we do recommend that a representative from Triad be on site during the caisson construction to verify the bearing conditions in the bottom of each caisson.

Driven Piles

It should be noted that the much of dense to very dense soils were encountered are varying depths within the soil borings. Some pre-drilling may be required in order for driven piles to achieve design depths.

During pile installation, the geotechnical engineer or his representative should be on-site to document all production piles. Installation records should be completed for each installed production pile.

A Wave Equation Analysis of Piles (WEAP) was not performed on the proposed driven piles.

The wave equation analysis includes a soil-hammer interaction model in order to determine an acceptable hammer for driving the piles. We recommend that a final WEAP analysis be performed by the installer in order to select an acceptable hammer capable of driving the selected pile to the ultimate capacity at an acceptable blow count without reaching refusal or overstressing the piles.

Slabs-on-Grade

Prior to placement of crushed stone for the floor slab, the subgrade soil within the limits of the structure should be proof-rolled to detect any soft, wet "pumping" areas. Any unsuitable areas should be either scarified, aerated to an approved moisture content, and re-compacted or undercut and replaced with controlled fill. The subgrade should be properly sloped to allow water from precipitation to drain from the stone prior to slab placement. Water should not be allowed to pond within the stone prior to placement of concrete. We recommend that the sequence and timing of floor construction be coordinated such that slab concrete will be placed within a very short time, i.e., within a few days or less, after placement and compaction of the aggregate base course.

Utilities

Locations and invert elevations for proposed utilities were not available at the time of report preparation. In general, we anticipate that conventional excavation equipment such as a backhoe or trackhoe can be used for utility excavations in the natural sedimentary soils and controlled fill.

Selection and placement of trench backfill materials shall be in accordance with previous sections of this report.

Triad Project No. 07-25-0112 Page 17

Construction Observations

We recommend that the geotechnical engineering firm of record, Triad, be retained to observe the construction activities to verify that the field conditions are consistent with the findings of our exploration. Construction observation services should be provided on a full-time and/or intermittent basis, as required, to:

• observe removal of all organics, topsoil, unsuitable existing fill, and any other deleterious materials and observe proof-rolling of original subgrade material prior to initial fill placement.

• observe and test controlled fill construction. Field density tests should be performed in accordance with ASTM D 6938 (nuclear method). A minimum of three field density tests should be performed for each lift of fill placed or at a frequency determined to be sufficient by the testing agency based on the amount of fill being placed to confirm the required soil compaction.

• examine all shallow foundation bearing levels and slab subgrade, foundation depths, and reinforcing steel size, amount, and placement for the proposed structure. The inspection should be conducted by a professional engineer or qualified representative working under the direct supervision of the professional engineer from our office. All foundation bearing levels should be tested immediately prior to placing reinforcing steel and concrete to confirm that the required bearing support is available.

• observe drilling and placement of selected deep foundation system to confirm compliance with our construction recommendations.

• test fresh structural concrete placed for the project to verify compliance with approved submittals for mix design, slump, air entrainment and compressive strength, as applicable.

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MATTAWOMAN WASTEWATER TREATMENT PLANT

SITE LOCATION PLAN

LA PLATA, MARYLAND

TRIAD PROJECT NO: 07-25-0112 FIGURE NO.: A-1

DRAWN BY: CHECKED BY:

EMO

DATE:

09/11/2025

SCALE:

1"=2000'

TMG

SITE LOCATION

SOURCE: USGS 7.5

PORT TOBACCO (MD) 2023;

Topographic Maps

Base Map: Geologic Map of the Washington West 30' x 60' Quadrangle, Maryland, Virginia, and Washington D.C. (2017)

MATTAWOMAN WASTEWATER TREATMENT PLANT

GEOLOGIC MAP

LA PLATA, MARYLAND

TRIAD PROJECT NO: 07-25-0112 FIGURE NO.: A-2

DRAWN BY: CHECKED BY:

EMO

DATE:

09/11/2025

SCALE:

1"=2000'

TMG

SITE LOCATION

SOURCE:

U.S. GEOLOGIC SURVEY

www.triadeng.com ap Unit Symbol Map Unit Name Percent Slopes

Corrosion of Concrete

Drainage Class Hydrologic Soil Group

UK Urban Land N/A N/A N/A D

Pu Potobac-Issue complex

0 to 2 High risk Poorly Drained B/D

SOURCE:

USDA

Web Soil Survey

CHECKED

BY: TMG

MATTAWOMAN WASTEWATER TREATMENT PLANT

LA PLATA, MARYLAND

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SCALE:

AS SHOWN TRIAD PROJECT NO. 07-25-0112

DRAWN

BY: EMO

DATE:

09-11-2025

USDA SOIL MAP

FIGURE NO.: A-3

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BASEMAPPROVIDEDBY:

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PROJECTNO:

07-25-0112

DRAWNBY:

EMO

SCALE:

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DATE:

09-11-2025

CHECKEDBY:

TMG

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