2._Geo_Report.pdf

Report of Subsurface Exploration

Maintain/Repair Airfield Storm Water System, Phase I Project Dobbins ARB Dobbins ARB, Georgia Geo-Hydro Project Number 170460.20

Prepared for POND June 14, 2017

1000 Cobb Place Boulevard, Suite 290 • Kennesaw, Georgia 30144 o: 770.4267100 • f: 770.426.5209 • www.geohydro.com

Mr. Mark Caspar, P.E. June 14, 2017

POND

4825 University Square, Suite 8 Huntsville, Alabama 35816

Report of Subsurface Exploration Maintain/Repair Airfield Storm Water System, Phase I Project

Dobbins ARB Dobbins ARB, Georgia

Geo-Hydro Project Number 170460.20 Dear Mr. Caspar:

Geo-Hydro Engineers, Inc. has completed the authorized subsurface exploration for the above referenced project. The scope of services for this project was outlined in our proposal number 19358.2 Revision 1 dated April 11, 2017.

PROJECT INFORMATION

As part of the Airfield Storm Water System Maintain/Repair project, a new storm line will be installed on the north side of the Dobbins ARB runway. The alignment will connect an existing headwall on the west side of Taxiway M to an existing drop inlet roughly 900 feet east. The new storm sewer line will be 36-inch diameter reinforced concrete pipe and will be located about 20 feet south and roughly parallel to the existing 30-inch corrugated metal pipe storm sewer line. We understand that the new line will be installed using cut-and-cover techniques and will involve excavation ranging from about 8 to 15 feet deep.

Rock outcroppings are visible at several locations on the west side of Taxiway M about 100 feet north of the planned alignment. The aerial photograph below shows the approximate proposed storm sewer alignment.

No rth

Airfield Storm Water System Maintain/Repair, Phase I Project • Dobbins ARB, Georgia Project Number 170460.20

June 14, 2017 | 2

EXPLORATORY PROCEDURES

Soil Test Borings The subsurface exploration included two hand auger borings performed at the approximate locations shown on Figure 1 included in the Appendix. The test borings were located in the field by Geo-Hydro by measuring angles and distances from existing site features. Ground surface elevations shown on the hand auger logs were interpolated from the topographic site plan provided to us and have been rounded to the nearest foot. In general, the locations and elevations of the test borings should be considered approximate.

Dynamic cone penetrometer testing (ASTM STP-399) was performed at select depths in the hand auger borings. Soil samples obtained from the drilling operation were examined and classified in general accordance with ASTM D2488 (Visual-Manual Procedure for Description of Soils). Soil classifications include the use of the Unified Soil Classification System described in ASTM D2487 (Classification of Soils for Engineering Purposes). The soil classifications also include our evaluation of the geologic origin of the soils. Evaluations of geologic origin are based on our experience and interpretation and may be subject to some degree of variation.

Descriptions of the soils encountered, groundwater conditions, dynamic cone penetrometer resistances, and other pertinent information are provided in the hand auger logs included in the Appendix.

Geophysical Exploration The geophysical exploration consisted of performing seismic refraction surveys along three traverses at the approximate locations shown on Figure 1 in the Appendix. The seismic refraction surveys were performed using a Geometrics ES-3000 engineering seismograph. Compression waves were generated at multiple locations along each traverse to provide redundancy of data and to allow the evaluation of dipping strata.

The traverses were located in the field by Geo-Hydro by measuring angles and distances from existing site features. The ground surface elevations shown on the refraction profiles were interpolated from the topographic site plan provided to us. In general, traverse locations and elevations should be considered approximate.

The purpose of performing the seismic refraction traverses was to evaluate the velocity with which a compression wave travels through subsurface materials. Since the compression wave velocity varies with material consistency, it can be an indicator of the excavation characteristics of subsurface materials.

In order to provide useful information, seismic refraction data requires analysis and interpretation. Field data consists of plots of the first arrival time of the compression wave versus distance between the sensing geophone and the point where the compression wave is generated. Abrupt changes in the slope of the plot generally indicate a change in the compression wave velocity through the subsurface material.

To analyze the seismic data it is necessary to make simplifying assumptions. These include the initial assumption that the subsurface profile is composed of 2 to 3 distinct layers with planar interfaces. Once this initial model is developed, a tomographic inversion is used to compare theoretical travel times created by sending theoretical sound waves through the model to actual travel times measured in the field. Multiple iterations of the tomographic inversion are performed until theoretical and measured travels times satisfactorily agree. Typically, 10 to 30 iterations are performed.

June 14, 2017 | 3

Seismic refraction is a good technique to obtain subsurface information, but it has certain limitations.

Seismic refraction surveys cannot detect softer layers beneath hard layers. Background noise and extremely irregular subsurface conditions may prevent collection of useful data. Seismic refraction surveys tend to provide indications of average subsurface conditions, and are generally not capable of detecting small discontinuities such as boulders or rock lenses. Lastly, the presence of groundwater may cause interpretation problems.

REGIONAL GEOLOGY

The project site is located in the Northern Piedmont Geologic Province of Georgia. Soils in this area have been formed by the in-place weathering of the underlying crystalline rock, which accounts for their classification as “residual” soils. Residual soils near the ground surface, which have experienced advanced weathering, frequently consist of red brown clayey silt (ML) or silty clay (CL). The thickness of this surficial clayey zone may range up to roughly 6 feet. For various reasons, such as erosion or local variation of mineralization, the upper clayey zone is not always present.

With increased depth, the soil becomes less weathered, coarser grained, and the structural character of the underlying parent rock becomes more evident. These residual soils are typically classified as sandy micaceous silt (ML) or silty micaceous sand (SM). With a further increase in depth, the soils eventually become quite hard and take on an increasing resemblance to the underlying parent rock. When these materials have a standard penetration resistance of l00 blows per foot or greater, they are referred to as partially weathered rock. The transition from soil to partially weathered rock is usually a gradual one, and may occur at a wide range of depths. Lenses or layers of partially weathered rock are not unusual in the soil profile.

Partially weathered rock represents the zone of transition between the soil and the indurated metamorphic rocks from which the soils are derived. The subsurface profile is, in fact, a history of the weathering process which the crystalline rock has undergone. The degree of weathering is most advanced at the ground surface, where fine grained soil may be present. And, the weathering process is in its early stages immediately above the surface of relatively sound rock, where partially weathered rock may be found.

The thickness of the zone of partially weathered rock and the depth to the rock surface have both been found to vary considerably over relatively short distances. The depth to the rock surface may frequently range from the ground surface to 80 feet or more. The thickness of partially weathered rock, which overlies the rock surface, may vary from only a few inches to as much as 40 feet or more.

Near-surface geologic conditions at the site have been modified by previous grading activities.

SOIL TEST BORING SUMMARY

Starting at the ground surface, hand auger borings H-1 and H-2 encountered about 6 and 2 inches of topsoil, respectively. Beneath the topsoil, both hand auger borings encountered residual soil typical of the Piedmont Region. The residual soils were classified as silty sand with varying amounts of rock fragments. Dynamic

June 14, 2017 | 4 cone penetrometer resistances recorded in the residual soils ranged from 17 to greater than 25 blows per increment.

Both borings encountered conditions causing auger refusal at depths ranging from about 9 inches to 7½ feet. Five attempts were made to advance boring H-2 and conditions causing auger refusal were encountered in the upper foot of the subsurface at all offset locations. It is important to note that hand equipment is limited in its ability to penetrate dense or rocky material. In general, hand auger refusal should not be correlated with dense material or the top-of-partially weathered rock or rock.

No groundwater was encountered in the hand auger borings. The borings were backfilled upon completion.

For more detailed descriptions of subsurface soil conditions, please refer to the hand auger logs included in the Appendix.

SEISMIC REFRACTION DATA SUMMARY

Based on local experience, we interpret compression wave velocities of less than 2,500 feet per second (fps) to be indicative of soil. We also interpret compression wave velocities over 5,000 fps to be indicative of mass rock. Compression wave velocities between 2,500 and 5,000 psf are interpreted as partially weathered rock.

The refraction profiles indicate that the upper 15 feet of the subsurface profile consists of a combination of soil and partially weathered rock. In general, the rock profile indicated by the refraction profiles is within 20 to 25 feet of the ground surface along profiles B-B’, C-C’, and D-D’. Profile A-A’ indicates the presence of rock about 15 feet below the ground surface. Seismic refraction profiles included in the Appendix represent each traverse of seismic refraction performed. The profiles indicate the average compression wave velocity for each layer identified.

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EVALUATIONS AND RECOMMENDATIONS

The following evaluations and recommendations are based on the information available on the proposed construction, the data obtained from the test borings and geophysical exploration, and our experience with soils and subsurface conditions similar to those encountered at this site. Because of the test borings and refraction traverses represent a very small statistical sampling of subsurface conditions, it is possible that conditions different from those indicated by the test borings could be encountered during construction.

Excavation Characteristics

Based on the results of the test borings, excavation of partially weathered rock and dense soil will be necessary to install the new storm sewer line. Excavation of partially weathered rock typically requires large equipment capable or ripping. Additionally, ripping partially weathered rock from trench excavations is often impractical because of the leverage necessary to successfully remove partially weathered rock. The use of impact hammers may be necessary to remove partially weathered rock from trench excavations.

Additionally, the refraction profiles indicate that rock is present at depths ranging from about 15 to 25 feet below the ground surface. Excavation for the proposed storm sewer line is expected to involve excavation up to 15 feet deep. It is important to note that the refraction profiles indicate average conditions along the profile and the actual rock surface will most likely be encountered above or below the indicated elevation at any given point along the profile. Based on the results of the refraction profiles, some rock will be encountered during excavation for the sewer line.

For construction bidding and field verification purposes it is common to provide a verifiable definition of rock in the project specifications. The following is a typical definition of trench rock:

• Trench Rock: Material occupying an original volume of at least one-half cubic yard which cannot be excavated with a hydraulic excavator having a minimum flywheel power rating of 123 kW (165 hp);

such as a Caterpillar 322C L, John Deere 230C LC, or a Komatsu PC220LC-7; equipped with a short tip radius bucket not wider than 42 inches.

Earth Slopes

Temporary construction slopes should be designed in strict compliance with OSHA regulations. The hand auger borings indicate that soils along Taxiway M are Type B as defined in 29 CFR 1926.650 (1994 Edition). In general, we recommend that temporary construction slopes be no steeper than 1H:1V for excavation depths of 20 feet or less. However, temporary excavation slopes in fill materials or in materials below the groundwater level should have a gradient no steeper than 1.5H:1V. Temporary construction slopes should be closely observed on a daily basis by the contractor’s “competent person” for signs of mass movement: tension cracks near the crest, bulging at the toe of the slope, etc. The responsibility for excavation safety and stability of temporary slopes should lie solely with the contractor.

We recommend that extreme caution be observed in trench excavations. Several cases of loss of life due to trench collapses in Georgia point out the lack of attention given to excavation safety on some projects.

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We recommend that applicable local and federal regulations regarding temporary slopes, and shoring and bracing of trench excavations be closely followed.

Temporary Excavation Bracing

If at a given location a sloped excavation is not feasible, temporary excavation bracing will be required.

The most appropriate type of excavation bracing will be dictated by subsurface conditions at the specific excavation or trench location. Typically, the contractor will design and implement temporary excavation bracing as part of means and methods. Temporary excavation support systems submitted by the contractor should be reviewed by POND and Geo-Hydro.

Construction Dewatering

Groundwater was not encountered in the hand auger borings. However, water was observed flowing into the drop inlet at the east end of the line during our initial meeting on site. Also, localized areas of perched water may be encountered during installation of the sewer. If necessary, dewatering should be performed to maintain the groundwater level approximately 2 to 3 feet below the lowest prevailing excavation depth.

In most cases we expect that direct pumping from the excavation will provide satisfactory temporary construction dewatering. However, the actual dewatering approach will be dictated by conditions at the time of excavation. Sand layers or other more permeable soil layers may significantly increase the amount of water inflow into open excavations.

The amount of temporary dewatering actually required during construction is related not only to the prevailing weather conditions, but also the contractor’s sequencing of construction activities. Construction specifications should include performance guidelines for temporary dewatering. Performance guidelines allow the contractor to select the actual means and methods of construction dewatering. The following sample specification1 could be used as a guide for development of actual specifications.

Control of groundwater shall be accomplished in a manner that will preserve the strength of the foundation soils, will not cause instability of the excavation slopes, and will not result in damage to existing structures. Where necessary to these purposes, the water level shall be lowered in advance of excavation, utilizing trenches, sumps, wells, well points, or similar methods. The water level, as measured in piezometers, shall be maintained a minimum of 3 feet below the prevailing excavation level. Open pumping from sumps and ditches, if it results in boils, loss of soil fines, softening of the ground, or instability of slopes, will not be permitted.

Wells and well points shall be installed with suitable screens and filters so that continuous pumping of soil fines does not occur. The discharge shall be arranged to facilitate collection of samples by the Engineer.

We recommend that pipe bedding be used where groundwater is encountered. This will provide a level, stable base for pipe installation. We recommend #78 crushed stone meeting Georgia DOT specifications for gradation as pipe bedding.

1 The sample specification was adapted from Construction Dewatering - A Guide to Theory and Practice, John Wiley and Sons, and is not intended for direct use as a construction specification without modifications to reflect specific project conditions.

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Structural Fill Placement

We anticipate that the overburden soils (fill and residuum) can be reused as structural fill to backfill the pipe trench. Materials selected for use as structural fill should be free of organic matter, waste construction debris, and other deleterious materials. In general, the material should not contain rocks having diameters over 4 inches. It is our opinion that the following soils represented by their USCS group symbols will typically be suitable for use as structural fill and are commonly found in abundance in the Piedmont region:

(CL), (SM), and (ML). The following soil types are typically suitable but are not abundant in the Piedmont region: (SW), (SP), (SC), (SP-SM), and (SP-SC). The following soil types are considered unsuitable:

(MH), (CH), (OL), (OH), and (Pt).

Laboratory Proctor compaction tests should be performed on representative samples of proposed fill materials to provide data necessary to determine acceptability and for quality control. The moisture content of suitable borrow soils should generally be no more than 3 percentage points above or below their optimum moisture contents at the time of compaction. Tighter moisture limits may be necessary with certain soils.

Suitable fill material should be placed in thin lifts. Lift thickness depends on type of compaction equipment;

but in general lifts of 8 inches loose measurement are recommended. The soil should be compacted by heavy compaction equipment such as a self-propelled sheepsfoot roller. Within confined areas, such as around the pipe or manhole structures, we recommend the use of “wacker packers” or “Rammax” compactors to achieve the specified compaction. Loose lift thicknesses of 4 to 6 inches are recommended in small area fills.

In general, we recommend that structural fill be compacted to at least 95 percent of the standard Proctor maximum dry density (ASTM D698). Following Georgia DOT guidelines, the upper 12 inches of pavement subgrade soils should be compacted to at least 100 percent of the standard Proctor maximum dry density.

Geo-Hydro should perform density tests during fill placement.

Based on the results of test borings and our observations, the existing fill materials appear to be suitable for reuse as structural fill. However, it is possible that some excavated fill may not be suitable for reuse.

Geo-Hydro should observe the excavation of existing fill materials to evaluate their suitability for reuse.

Soft, unstable fill soils free of deleterious materials may be reusable after routine moisture adjustment.

Highly organic soils and debris-laden soils will not be suitable for reuse.

The residual soils at the project site appear suitable for reuse as structural fill material. Routine adjustment of moisture content will be necessary to allow proper placement and compaction.

Pipe Support

Based on the results of the refraction profiles and our observations, it is likely that conditions varying from typical residual soil to partially weathered rock or rock will be exposed at bearing elevation for the sewer line. In order to limit potential differential settlement and stress concentrations at the interface of dissimilar bearing materials, soft soils should be removed and pipe bedding consisting of crushed stone

June 14, 2017 | 8 should be placed as necessary. This approach will provide a stable and relatively level working surface during installation of pipe sections.

We recommend that project plans require at least 6 inches of #78 crushed stone meeting Georgia DOT specifications as bedding for the pipe. This approach should result in satisfactory removal of the upper portion of loose soils, where present, and would establish a relatively uniform bearing surface.

Subsurface conditions will vary, and we recommend that a qualified geotechnical engineer be present during preparation of bearing surfaces for the pipeline.

We appreciate the opportunity to serve as your geotechnical consultant for this project. If you have any questions concerning this report or any of our services, please call us.

Sincerely, GEO-HYDRO ENGINEERS, INC.

A. Marty Peninger, P.E. Brian K. Ingram, P.E.

Senior Geotechnical Engineer Senior Geotechnical Engineer mpeninger@geohydro.com bingram@geohydro.com

AMP/BKI/170460.20 - Airfield Stormwater System Phase I Project - Geotechnical Report

APPENDIX

HA-2

HA-1

A'

A

B

B'

C

C'

D

D'

Approximate Scale: 1"=150'

Maintain/Repair Stormwater System, Phase I Project

Dobbins ARB, Georgia

Geo-Hydro Project Number 170460.00

Figure 1: Boring Location Plan

75 150 300 450

LEGEND: Refraction Traverse

Hand Auger Location

>25

>25

Topsoil (Approximately 6 inches) Red-brown silty fine to coarse sand (SM) (RESIDUUM)

Hand Auger Refusal at 7.5 feet

G R

A P

H

IC

L O

G

D E

P T

H (f ee t)

L A

B T

E S

T S

D R

Y U

N

IT

W T pc f)

Elevation(ft): 1064

Penetration resistance was evaluated in accordance with ASTM STP-399.

The penetration resistance is the number of blows of a hammer weighing 15 lbs. falling 20 inches to drive a 1.5 inch diameter cone 1.75 inches.

MATERIAL DESCRIPTION M

O

IS

T

U R

E C

O N

T E

N T

P E

N

R E

S

IS

T

Equipment: Hand Auger and Penetrometer

Date Performed: 5/31/17 Logged by: KD, MP

S A

M

PL

E

HAND AUGER LOG HA-1

Maintain/Repair Airfield Stormwater System, Phase I Project Dobbins ARB, Georgia

170460.20

H A

N D

A U

G E

R H

A N

D A

U G

E R

B O

R

IN

G S

.G P

J L

O G

A G

N N

N

.G D

T

/1 4/

Topsoil (Approximately 2 inches) Gray-brown silty fine to coarse sand (SM) with rock fragments

(RESIDUUM)

Hand Auger Refusal at 9 inches

Boring was offset 4 times and equipment refusal was encountered before 9 inches on each offset.

G R

A P

H

IC

L O

G

D E

P T

H (f ee t)

L A

B T

E S

T S

D R

Y U

N

IT

W T pc f)

Elevation(ft): 1063

MATERIAL DESCRIPTION M

O

IS

T

U R

E C

O N

T E

N T

P E

N

R E

S

IS

T

Equipment: Hand Auger and Penetrometer

Date Performed: 5/31/17 Logged by: KD, MP

S A

M

PL

E

HAND AUGER LOG HA-2

SEISMIC REFRACTION PROFILES

Maintain/Repair Stormwater System, Phase I Project

This profile is an approximate representation based on interpretation of seismic refraction surveys. The interface between materials of different velocity is intended to represent an idealized subsurface profile suitable for planning purposes only, and should not be considered to be a precise description of subsurface conditions. Elevations should be considered approximate

Line A-A’

South North velocity is intended to represent an idealized subsurface profile suitable for planning purposes only, and should not be considered to be a precise description of subsurface conditions. Elevations should be considered approximate

Line B-B’

West East velocity is intended to represent an idealized subsurface profile suitable for planning purposes only, and should not be considered to be a precise description of subsurface conditions. Elevations should be considered approximate

Line C-C’ velocity is intended to represent an idealized subsurface profile suitable for planning purposes only, and should not be considered to be a precise description of subsurface conditions. Elevations should be considered approximate

Line D-D’

GEO-HYDRO ENGINEERS, INC.