20036_Morehead Street Redevelopment_GeotechReport_2013.02.18

Correspondence:AMEC Environment & Infrastructure, Inc.2801 Yorkmont Road, Suite 100Charlotte, North Carolina 28208 Tel [phone]Fax[phone]
www.amec.com
February 18, 2013
Mr. Brian Kaiser
Director of Development and Construction
Tribute Properties, Inc.
P.O. Box 1229
Wilmington, North Carolina 28402
Subject:Report of Geotechnical Exploration
West Morehead Street Apartment Building
Charlotte, North CarolinaAMEC Project No. 6228-12-0086
Dear Mr. Kaiser:
As authorized by acceptance of our Proposal No. PROP13CHLTEC-004 dated January 28,
2013,by Tribute Properties, Inc.,AMEC Environment and Infrastructure, Inc. (AMEC) has
completed a subsurface exploration for the subject project. The purpose of the geotechnical
exploration was to develop information about the site and subsurface conditions and to provide
foundation recommendations for the proposed construction. This report describes the
geotechnical work performed and presents the results obtained, along with our geotechnical
recommendation for foundation design and site preparation.
AMEC previously performed a geotechnical exploration for the site, reported as MACTEC
Project No. 6228-05-3103 dated September 28, 2005. In addition,AMEC recently prepared and
reported under separate cover, a Phase I Environmental Site Assessment Update dated January
11, 2013.
Site and Project Information
Our understanding of the site and project information was obtained in a meeting with you in our
office on January 16, 2013, telephone conversations between our Mr. Mel Browning and Mr.
Don Woods, P.E., the project structural engineer,a building floor plan furnished by Mr. Charley
Report of Geotechnical Exploration February 18, 2013
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Watts of Watts Leaf Architects, PA., a previous Phase I Environmental Site Assessment
performed by AMEC and our previous preliminary geotechnical report.

The project site is bounded by McNinch Street on the west, South Cedar Street on the east,
West Morehead Street on the south and by adjoining property on the north. Two vacant
buildings and a concrete pad are present on the site. The ground surface on the property
ranges from approximately elevation 652 feet to 654 feet within the northwest portion of the
property, sloping up to approximately elevation 678 feet on the east portion of the site near the
intersection of West Morehead Street and South Cedar Streets. A relatively steep slope, some
18 to 20 feet high, is present within the northeast portion of the property. A buried stormwater
line is located along the north property boundary, which will have to be removed and a new
stormwater line constructed.

Gold mining activity occurred in the Charlotte area in the 1800’s, with several gold mines located
in the current downtown Charlotte area. Although there is no obvious site evidence of previous
mining activity on the project site, published information indicates that a vein trace of the St.
Catherine Mine ran in a north to northeasterly direction just east of the intersection of West
Morehead Street and South Cedar Street. Based on the literature concerning this mine, the
mining activity along this trace may have included both horizontal drifts and vertical shafts.

We understand that plans are to construct two buildings on the site that will occupy essentially
the entire site. Up to four levels of wood frame apartments will be constructed over a three level,
poured in place concrete parking deck. Up to 22 feet of cut below existing site grade is planned
within the highest northeast portion of the site. Little or no cut will be required over most other
areas of the site. If practical, we understand that a cut slope will be used to effect this cut, rather
than a tall retaining wall. Maximum column loads are anticipated to be about 450 kips. The
construction will also require some retaining walls of variable height and relatively lightly loaded,
load-bearing walls supporting loads of 3 klf or less.

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residual material with Standard Penetration Test (SPT) N-values in excess of 100 blows per foot
(bpf). Weathering is facilitated by fractures, joints and by the presence of less resistant rock
types. Consequently, the profile of the partially weathered rock and hard rock is quite irregular
and erratic, even over short horizontal distances. Also, it is not unusual to find lenses and
boulders of hard rock and zones of partially weathered rock within the soil mantle, well above the
general bedrock level.

Often, the upper soils along drainage features and in flood plain areas are water-deposited
(alluvial) materials that have been eroded and washed down from adjacent higher ground.
These alluvial soils are usually soft and compressible, having never been consolidated by
pressures in excess of their present overburden.

Subsurface Conditions

The subsurface conditions encountered by the borings are described below and depicted
graphically on the attached Subsurface Profiles. The proposed design grades at each boring
location based on our current understanding of the project, are also shown on these profiles.

Soils

Borings B-1 and B-3 encountered surficial covering of gravel, approximately 8 inches thick.
Borings B-2, B-7, B-9, B-11, B-12 and B-13 typically encountered asphalt (approximately 1 to 2
inches) underlain by gravel (approximately 6 to 8 inches). Borings B-4, B-5, B-6, B-8, and B-10
encountered a surficial covering of topsoil with grass roots, approximately 1 inch thick.

Below the asphalt, gravel and topsoil layer, the borings typically encountered man-made fill to
depths ranging from 7 to 21 feet. No fill was encountered in boring B-5. The fill material
consisted of clayey silty sand, silty sand, sandy clayey silt and sandy silt. The fill contained rock
fragments, metal, brick, asphalt and concrete pieces. The fill material in B-6 from 5.5 to 8 feet
had an odor. SPT N-values in the sampled fill ranged from 2 to 27 bpf. The fill material
encountered in B-8 from a depth of 5.5 feet to boring refusal and termination at 8 feet and in B-
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13 from 8 to 12 feet had higher SPT N-values of 50 bpf to 50/3 inches. These higher N-values
are is likely due to the hard deleterious materials present in the fill.

Alluvial soils were encountered below the fill in borings B-1 and B-2 and below the topsoil in B-5.
The alluvial soils consisted of soft to firm sandy clayey silts, clayey sandy silts and sandy silts
and very loose to loose clayey silty sands and silty sands. The standard penetration resistances
ranged from 3 to 8 blows per foot.

Below the alluvium in B-1, B-2 and B-5, and below the fill elsewhere, residual soils resulting from
the in-place weathering of parent rock material was encountered by all the borings except B-8,
which encountered auger refusal below the fill materials. The sampled residual soils typically
consisted of firm sandy clayey silts and loose to very dense silty sands. SPT N-values in the
sampled residuum ranged from 8 to 89 bpf.

Residual soils hard enough to be designated partially weathered rock for engineering purposes
(N≥ 50 = 6”) were encountered at depths ranging from 7 to 34 feet in borings B-1, B-2, B-3, B-4,
B-5, B-6, B-9, B-10, B-11 and B-13. The sampled partially weathered rock typically consisted of
silty sand with rock fragments.

Mechanical auger refusal was encountered at 23.2 ft in B-1, 22.2 ft in B-2, 13.7 ft in B-3, 8.1 ft in
B-4, 14.2 ft in B-5, 8.3 ft in B-8, and 30 ft in B-13. Refusal may result from boulders, lenses,
ledges or layers of relatively hard rock underlain by partially weathered rock or residual soil;
refusal may also represent the surface of relatively continuous bedrock. Core drilling procedures
are required to penetrate refusal materials and determine their character and continuity.

Rotary drill refusal was encountered at 13.7 feet in B-3. The refusal material in B-3 was cored
from 13.7 to 19.7 feet, with recovered rock consisting of very strong, slightly decomposed,
granite. Rock recovery was 88 percent and RQD was 62 percent.

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Groundwater

Groundwater was encountered at depths ranging from 7 to 29.5 feet in borings B-1 through B-7
and B-10 at the time of drilling. Stabilized groundwater was measured at a depth of 12 ft in B-1,
11 ft in B-2, 11 ft in B-3, 5 ft in B-5, 29 ft in B-6, 21.3 ft in B-7and 27 ft in B-10. After 24 hours,
borings B-4, B-8, B-11, B-12, and B-13 were caved and dry at depths of 5.6, 7, 18.5, 25 and 27
feet, respectively. Caved depths can occur due to stabilized groundwater or to soil fall-in
occurring when the drilling tools are removed from the boreholes.

Groundwater levels may fluctuate several feet with seasonal and rainfall variations and with
changes in the water level in adjacent drainage features. Normally, the highest groundwater
levels occur in late winter and spring and the lowest levels occur in late summer and fall.

The above descriptions provide a general summary of the subsurface conditions encountered.
The attached Test Boring Records contain detailed information recorded at each boring location.
These Test Boring Records represent our interpretation of the field logs based on engineering
examination of the field samples. The lines designating the interfaces between various data
represent approximate boundaries and the transition between strata may be gradual.

Geotechnical Evaluation and Recommendations

Based on the available information, the results of the soil test borings, our engineering evaluation
and our past experience with similar type soils and similar projects, we offer the following
recommendations regarding site preparation and foundation options for the proposed structures.

We considered several types of foundation systems for the project – including spread foundations,
geopiers, and drilled piers (caissons). Neither the existing variable fill or the alluvial soils present
below the fill within the lower portions of the site is suitable to provide direct support for spread
foundations. Due to the large amount and depths of the unsuitable fill material, complete
undercutting is not a practical option. Our evaluation regarding the various foundation systems are
discussed below.
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Spread Foundations

In our opinion, spread foundations could be used to support the buildings where suitable residual
soil conditions are present. The spread foundations would bear directly in residual soils where such
soils are encountered near design grades, and on geopiers where existing variable fill soils and
alluvium overly the residual materials. Geopiers are discussed in the following section of this report.

Based on the boring data and our past experience with similar soils, the residual soils present at or
at shallow depth below design grade at elevation 655 feet in borings B-6, B-10, and B-13 could
provide direct shallow foundation support for moderately loaded structures, as planned at this site.
(In this higher northeastern portion of the site, the planned lower floor level will require removal of
the existing fill).

The boring data indicates that an allowable soil bearing pressure of 4,000 psf would be available for
foundations bearing on non-plastic undisturbed residual soils with a standard penetration resistance
of 9 bpf or better encountered at design grade in the above borings. Higher soil bearing pressures
ranging up to 8 ksf in very dense soils are available at design grade in some borings. However,
due to the variable presence of the harder soil and partially weathered rock conditions, we
recommend use of a maximum general soil bearing pressure of 4,000 psf in the residual materials.
For foundations bearing in structural fill constructed of suitable on-site soils, compacted to 95
percent of the standard Proctor maximum dry density and placed on a properly prepared residual
soil surface, an allowable soil bearing pressure of 3,000 psf would be available. A higher allowable
soil bearing pressure of 4,000 psf could be used in structural fill compacted to 98 percent of this
criteria.

Foundations should not bear in existing variable fill soils such as encountered in most of the borings
or in shallow alluvial soils such as encountered in B-5. Shallow foundation support would also not
be available in lower site areas where only minimal depths of new site fill will be placed to obtain
design lower floor level over existing variable fill and alluvium, such as at B-1 through B-4 and at B-
7 and B-8.

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We recommend that masonry walls, but not the supporting footings, be provided with periodically
spaced suitable construction joints, in order to accommodate some possible differential settlement
and thermal stress. Individual column footings should bear entirely in firm or better residual soil or
compacted fill over their entire bearing area.

We recommend that the minimum widths for individual column and continuous wall footings be 24
and 18 inches, respectively. The minimum widths are considered advisable to provide a margin of
safety against a local or punching shear failure of the foundation soils. Footings should bear at
least 18 inches below final exterior grade and finished floor elevation to provide frost protection (for
exterior footings) and protective embedment.

If localized rock is encountered at footing bearing level over a portion of individual column or wall
footings, adjacent to soil bearing conditions, the rock should be excavated down at least 1 ft before
footing bearing and be replaced with crusher run gravel compacted to 100 percent of standard
Proctor maximum dry density. This is to provide a “cushion” over the rock and help avoid the
potential for locally overstressing the foundation concrete at this point.

In order to verify that the soils encountered in footing excavations are similar to those encountered
in the soil test borings, we recommend that foundation excavations be examined and checked with
a dynamic hand penetrometer by an experienced engineering technician working under the direct
supervision of the geotechnical engineer.

Exposure to the environment may weaken the soils at the footing bearing level if the foundation
excavations remain open for long periods of time. Therefore, we recommend that all footing
excavations be extended to final grade and the footings constructed as soon as possible to
minimize the potential damage to bearing soils. The foundation bearing area should be level or
suitably benched and be free of loose soil, ponded water and debris. Foundation concrete should
not be placed on soils that have been disturbed by seepage. If the bearing soils are softened by
surface water intrusion or exposure, the softened soils must be removed from the foundation
excavation bottom immediately prior to placement of concrete. If the excavation must remain open
overnight or if rainfall becomes imminent while the bearing soils are exposed, we recommend that a
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2 to 4-inch thick “mud-mat” of “lean” (2000 psi) concrete be placed on the bearing soils before the
placement of reinforcing steel.

Geopiers

The patented Geopier Intermediate Foundation System is a grid of aggregate piers used in
conjunction with spread footings. The system is made up of densely compacted aggregate piers
that somewhat resemble piles of caissons when arranged in a group under a spread footing
foundation (thus resembling a pile cap). The piers are construction by forming a hole in the soils by
drilling a 30 to 36-inch diameter hole. The soil at the bottom of the hole is pre-stressed and
densified using a large tamper. Then, well-graded aggregate base stone is placed in the hole in 18-
inch lifts, each compacted using a high energy tamping system until the hole is filled. The
aggregate piers can be installed to depths in the range of up to about 30 feet. At this site, based on
the borings, the required depth of geopiers to extend through the existing fill and alluvium would
likely range from about 10 to 25 ft.

With geopiers, spread foundations are designed using a higher soil bearing pressure than normal
for the unimproved soil. At this site, the geopiers would primarily be needed in the areas of existing
fill as found in borings B-1 through B-4, B-7, B-9, B-11 and B-12 and also in B-5, where alluvial soils
are present. (As discussed previously, spread foundations without geopiers could be used where
suitable residual soils occur at shallow depth below proposed design grades.) In our opinion, an
allowable bearing pressure of 4 to 5 ksf could potentially be achieved in the fill soils with the
geopiers. Since the system is patented, the design and construction of the system is performed by
the specialty contractor.

Dewatering may be required for some of the Geopier system installations at this site, depending
upon whether the augered holes will remain open during geopiers installation. Geopiers can be
installed below the groundwater table by using a temporary steel liner, as needed. The geopier
installations should be continuously monitored and documented by an experienced engineering
technician. Geopiers can also be used to provide lateral uplift and resistance for foundations
through appropriate design measures. Considering the range in subsurface conditions encountered
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by the borings, at least one and preferably two static load tests (ASTM D 1143) would be required
to verify the design geopier modulus for compression loading.

The use of geopiers along the north side of the planned building footprint should consider the
presence of the new stormwater line to be installed and the possible future need to excavate and
repair this line. Thus, geopiers located near this buried line may need to utilize concrete piers down
to the invert elevation of this pipe or to use cement treated aggregate to avoid the potential for the
integrity of the geopiers being compromised due to nearby excavation for stormwater line repair.

Caissons (Drilled Piers)

From a technical standpoint, drilled straight-sided shaft foundations (also called caissons or piers)
would be a possible alternative to the previously discussed spread foundation/geopier foundation
scheme. Caissons would transfer loads through the fill material and residual soils to the partially
weathered rock through skin friction and end bearing in the partially weathered rock materials. An
allowable end bearing pressure for drilled caissons bearing in continuous partially weathered rock
would be 30 ksf (partially weathered rock with N>50 blows per 6 inches). At the borings, this
allowable end bearing pressure is potentially available at depths ranging from about 10 to 25 feet
below proposed site grades. Caissons would have to be extended through any partially weathered
rock that is interlayered with soil layers. An allowable skin friction of at least 3.5 ksf may be used
for caissons extended several feet into continuous partially weathered rock.

Drilled piers are not compatible with spread foundations due to differential settlement
considerations. A major disadvantage of drilled piers at this site is that they would have to be
installed over the entire building footprints, even in areas where spread foundation support could
otherwise be used in competent residual soils. Thus, in our opinion, drilled piers would not be a
practical foundation option for this project.

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Lateral Earth Pressures

Below ground walls must be capable of resisting the lateral earth pressures that will be imposed on
them. No triaxial shear testing was performed on soil samples to assist in the determination of
lateral earth pressure coefficients for design of such walls. Based on testing of reasonably similar
soils on other projects, the following earth pressure coefficients are recommended. Walls which will
be prevented from rotating should be designed to resist the "at-rest" lateral earth pressure. The at-
rest coefficient to be used in design will depend upon the type of backfill used. If non-plastic
(PI<10) residual soils without clay are used for backfill behind walls, we recommend that an at-rest
coefficient (Ko) of 0.6 be used. If more granular material such as compacted clean washed sand is
used as backfill, a lower at-rest coefficient of 0.45 could be used. In order for this coefficient to be
used, the soil wedge within an angle of 45 degrees from the base of the wall to about 2 ft below the
exterior grade should be excavated and replaced with compacted clean washed sand. In no case
should clayey soils be used as backfill behind retaining walls.

Walls such as exterior retaining walls which are permitted to rotate at the top may be designed to
resist “active” lateral earth pressure. Typically, a top rotation of about 1 inch per 10 ft height of wall
is sufficient to develop active pressure conditions in soils similar to those encountered at the site.
We recommend that an active earth pressure coefficient (Ka) of 0.4 be used for design of such walls
if on-site non-plastic (PI<10) residual soils without clay are used for backfill. If a properly
compacted clean washed sand is used as backfill behind the wall within the active failure zone, a
lower active earth pressure coefficient of 0.30 can be used.

The compacted mass unit weight of the backfill soil (which we estimate could reasonably be
assumed as 115 pcf) should be used with the above earth pressure coefficients to calculate lateral
earth pressures. Lateral pressure arising from surcharge loading, earthquake loading, and
groundwater, should be added to the above soil earth pressures to determine the total lateral
pressures which the walls must resist. In addition, transient loads imposed on the walls by
construction equipment during backfilling should be taken into consideration during design and
construction. Excessively heavy grading equipment (that could impose temporary excessive
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pressures or long term excessive residual pressures against the constructed walls) should not be
allowed within about 5 ft (horizontally) of the walls.

In order to minimize potential for hydrostatic pressure, we recommend that a suitable pre-
engineered drainage system be installed behind the retaining walls, with a conduit at the base of
the wall to convey water collected by the wall drain to a lower site elevation, away from the building.

A coefficient of 0.35 could be reasonably assumed for evaluating ultimate frictional resistance to
sliding at the foundation-soil contact. A passive earth pressure coefficient of 2.5 could be
reasonably assumed for evaluating ultimate lateral resistance of the soil against the side of the
foundation where this is a permissible condition. This passive earth pressure should be divided by
a safety factor of at least 2 to limit the amount of lateral deformation required to mobilize the
passive resistance.

Grade Slab

The grade slab may be soil supported in accordance with the recommendations in this report.
For slab design, a modulus of subgrade reaction of 130 pci may be used for proofrollable non-
plastic residual soils or properly compacted fill. Unless a turned down slab is used, the grade
slab should be jointed around columns and along footing supported walls so that the slab and
foundations can settle differentially without damage. Joints containing dowels or keys may be
used in the slab to permit movement between parts of the slab without cracking or sharp vertical
displacements. We recommend that a suitable vapor barrier be placed below the slabs to
minimize potential for soil moisture transmission through the slab. We also recommend that at
least 4 inches and preferably 6 inches of well-compacted crusher run gravel be placed on the
subgrade soils, below the grade slab.

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Excavation

The fill and residual soils encountered by the borings down to the proposed design grades
should be readily excavatable with conventional excavation equipment such as dozers and
backhoes. Refusal conditions were encountered as shallow at about 8 ft in B-4 and B-8 and
could possibly be locally shallower in some site areas. Difficult excavation could be locally
required locally if shallow refusal conditions are encountered during excavations for utility lines,
elevator pits, etc. below the building lower design level.

Site Preparation and Grading

All existing topsoil, vegetation, disturbed soils and surface soils containing organic matter or other
deleterious materials should be stripped from within the proposed building and paved areas. All
existing concrete or asphalt pavement, slab footings, buried stormwater lines, underground storage
tanks, etc. should be removed and replaced with new structural fill. After stripping and rough
excavation grading, we recommend that areas to provide support for the foundations, floor slab,
structural fill and any pavements be carefully inspected for soft surficial soils and proofrolled with a
25 to 35-ton, four-wheeled, rubber-tired roller or similar approved equipment. The proofroller
should make at least four passes over each location, with the last two passes perpendicular to the
first two. Any areas which wave, rut or deflect excessively and continue to do so after several
passes of the proofroller should be undercut to firmer soils. The undercut areas should be
backfilled in thin lifts with suitable compacted fill materials. The proofrolling and undercutting
operations should be carefully monitored by an experienced engineering technician working under
the direct supervision of the geotechnical engineer.

Ground Water Control

Based on the proposed design grades and the measured groundwater and caved depths, the
borings do not indicate the need for permanent underslab drainage. However, the contractor
should be prepared to promptly remove any surface water, perched water, or ground water from
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the construction area. This has been done effectively on past jobs by means of gravity ditches
and pumping from filtered sumps.

Engineered Fill

Fill used for raising site grade or for replacement of material that is undercut should be uniformly
compacted in thin lifts to at least 95 percent of the standard Proctor maximum dry density
(ASTM D 698). In addition, at least the upper 18 inches of subgrade fill beneath pavements and
floor slabs and 24 inches below pavements subject to truck traffic should be compacted to 100
percent of the same specification.

We have not performed any laboratory or compaction testing. Laboratory compaction testing
will be required to verify the suitability of proposed soils for use as structural fill. Existing site fill
that contains organics or debris would not be suitable for use as structural fill. In general, soil
containing more than 1 to 2 percent (by weight) fibrous organic materials or having a Plasticity
Index (PI) greater than 30 (with less than 15 being preferable) should not be used for fill. We
recommend that structural fill used for this project have a maximum dry density of at least 90 pcf.

Before filling operations begin, representative samples of the proposed fill material should be
collected and tested to determine the compaction and classification characteristics. The maximum
dry density and optimum moisture content should be determined. Once compaction begins, a
sufficient number of density tests should be performed by an experienced engineering technician
working under the direct supervision of the geotechnical engineer to measure the degree of
compaction being obtained.

The edge of the structural fill should extend horizontally beyond the outside edge of the building
foundations at least 10 ft or a distance equivalent to the height of fill to be placed, whichever is
greater, before sloping. The outer edge of fill should be at least 5 ft beyond paved areas. We
have not performed any laboratory triaxial shear tests for slope stability calculations, but our
experience suggests that permanent cut and fill slopes placed on a suitable foundation should be
constructed at 2:1 (horizontal to vertical) and 2.5:1, respectively, or flatter. Fill slopes should be
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adequately compacted. Cut and fill slope surfaces should be protected from erosion by grassing
or other means. Permanent slopes of 3:1 or flatter may be desirable for mowing.

The surface of compacted subgrade soils can deteriorate and lose its support capabilities when
exposed to environmental changes and construction activity. Deterioration can occur in the form
of freezing, formation of erosion gullies, extreme drying, exposure for a long period of time or
rutting by construction traffic. We recommend that the surfaces of floor slab and pavement
subgrades that have deteriorated or softened be proofrolled, scarified and recompacted (and
additional fill placed, if necessary) immediately prior to construction of the floor slab or pavement.
Additionally, any excavations through the subgrade soils (such as utility trenches) should be
properly backfilled in compacted lifts. Recompaction of subgrade surfaces and compaction of
backfill should be checked with a sufficient number of density tests to determine if adequate
compaction is being achieved.

Seismic Site Class

Based on the results of the borings and the currently proposed site grades, the site classifies as
seismic site class D by the average N method of IBC 2009, incorporated in the 2012 N.C.
Building Code. However, when the final building grades have been determined, the seismic site
class can be re-evaluated. It may be possible that the two buildings, if structurally separate,
could utilize different seismic site classes for design. Determination of the shear wave velocity
to a depth of 100 feet below the lower site design grades by a ReMi field study may be
considered to explore the possibility of a higher seismic site class for the project.

Qualification of Report

Our evaluation of foundation support conditions has been based on our understanding of the site
and project information and the data obtained in our exploration. The general subsurface
conditions utilized in our foundation evaluation have been based on interpolation of subsurface
data between the borings. In evaluating the boring data, we have examined previous
correlations between penetration resistances and foundation bearing pressures observed in soil
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