Pine Regional Business & Technology Park
Pine Regional Business & Technology Park site is located within
the area of a reclaimed surface mine near the Lonesome Pine Airport
in Wise County, Virginia. Prior to mining, the local topography
was characterized by steep-sided hills and narrow drainage valleys.
One such valley dominated the central portion of the site. Upon
completion of mining operations, the site was reclaimed by filling
the mined areas and the valley up to existing grades. According
to information in previous reports, mining and reclamation at
this site occurred prior to 1986. In the 13 years since, the site
has been primarily used as pastureland for livestock.
and Associates performed a preliminary geotechnical engineering
study of the project site in 1994 for the Wise County Industrial
Development Authority. Their investigation of the subsurface conditions
consisted of a seismic refraction survey and four test borings.
The seismic refraction survey indicated that from 50 feet to over
150 feet of heterogeneous mine spoil fill covers the site. According
to the survey results, depths to bedrock are more uniform in the
western half of the site, typically varying from 120 to 150 feet.
In a majority of the eastern half of the site, bedrock depths
are more erratic. The authors attributed this variability to past
mining in this vicinity which apparently resulted in numerous
highwalls at varying depths. None of the test borings drilled
for the preliminary study extended completely through the fill
In 1997, American
Geotech, Inc. performed a geotechnical engineering study at this
site, also for the Wise County Industrial Development Authority.
Their investigation of the subsurface conditions consisted of
18 test borings across a potential building footprint. These test
borings indicated that a heterogeneous mine spoil fill underlies
the site. None of the test borings for this study extended through
the fill to bedrock.
Associates personnel observed the excavation of seven test pits
at this site on December 4, 1997. These test pits extended to
depths of 7 to 13.5 feet below the ground surface. The pits contained
a heterogeneous mixture of soil, shale rock fragments and sandstone
rock fragments in the excavations.
The mine spoil
fill covering the site consists of a random mixture of soil and
rock fragments ranging in size from gravel to boulders. Soil contained
within the fill matrix appears to consist primarily of silty sands
and low plasticity silts and clays. Rocks within the fill consist
of a relatively uniform mixture of shale, sandstone and siltstone.
The density and consistency of fill soils are also variable, though
some improvement with depth can be inferred from most of the seismic
refraction data. No groundwater was encountered in any of the
borings to the depths explored.
It has been
shown that uncontrolled fills have the potential to settle significant
amounts beneath their own weight, with settlement continuing for
long periods after placement. Total and differential settlement
potential of uncontrolled fills is even greater when new loads
are added. Factors affecting the magnitude and duration of total
and differential settlements include placement procedures, material
composition, depth of fill, age of fill, groundwater levels, rate
of surface water infiltration and loading conditions. Some of
these factors are more difficult to quantify than others. Overall,
estimating uncontrolled fill settlement is very imprecise. However,
empirical correlations indicate that an uncontrolled fill could
settle between 1 and 3 percent of its thickness under its own
weight for a period of 20 to 30 years after placement. At this
site, that could translate into total long-term settlements of
up to 4 feet. While a majority of these settlements would be expected
to occur during placement and in the two to three years immediately
following placement, significant settlements could continue for
30 or more years. Assuming that fill placement was completed just
prior to 1986, it is estimated that an additional 8 inches of
settlement are possible in areas where fill depths are about 150
feet. As aforementioned, additional fill settlements would be
expected upon the addition of new loads.
of reclaimed mine sites for building construction is common in
this part of southwest Virginia. However, the abundance of large
rock blocks typically encountered within mine spoil fills and
the uncontrolled nature in which mine spoils were placed complicate
the site development and building construction processes. The
existing heterogeneous mine spoil fill will require special consideration
during the site grading phase of the project. Segregation and/or
disposal of large rock blocks within the excavated mine spoil
will probably be required prior to its reuse as compacted structural
fill. The new compacted fill will have to be defined in different
classes depending on its depth and proximity to the proposed development.
Due to the potential for damaging total and differential settlements
of the existing uncontrolled fills, some form of ground improvement
is usually required prior to foundation or building construction.
Based on the available data, it appears that the mine spoil covering
the site is not suitable for direct foundation support and ground
improvement will be required prior to development.
The type and
extent of ground improvement required will depend on the amount
of site grading needed to prepare the site, the type of construction,
magnitude of loads and the locations of structures and pavements.
The most economical means of ground improvement at this site will
be either dynamic compaction, surcharge preloading or a combination
of both. Dynamic compaction is a reliable method for densification
of low plasticity soils and rock fills to depths of up to 20 or
25 feet. The dynamic compaction process effectively creates a
20 to 25 foot thick raft of improved material which is suitable
for support of lightly to moderately loaded shallow foundations.
Based on previous experience, with dynamic compaction on sites
similar to this, it is possible to achieve an allowable bearing
pressure of the improved fill on the order of 2500 psf to 3000
psf. Unimproved materials below this raft will still be susceptible
to settlements beneath their own weight and beneath the weight
of new fill at the surface. Surcharge preloading is expected to
be an effective means for inducing and accelerating settlements
of these deeper, unimproved soils. A surcharge height of 20 to
25 feet, left in place for a period of four to six months, may
be required in building areas. Lesser heights of surcharge will
probably be required in pavement areas. In areas where significant
cuts will be required to grade the site, it may be possible to
reduce surcharge heights due to the past loading imparted by the
In the Marshall
Miller and Associates preliminary report, some areas of the site
were termed less favorable for development than others based on
the seismic survey results. These less favorable areas were those
where, according to the survey, the depths to bedrock and the
consistency or density of mine spoil fill are highly variable.
Areas where depths to bedrock, and thus thickness of fill, change
abruptly are unfavorable for building construction due to the
increased potential for damaging differential settlements. However,
presuming that the amount of organic matter in themine spoil is
minimal, the existing variability in the consistency or density
of the mine spoil is not considered a limiting factor in development.
The purpose of ground improvement is to minimize or eliminate
this variability, thereby producing a more competent and consistent
load bearing mass.
and pavement locations and grades have been established, an additional
site exploration and final geotechnical engineering study should
be performed at this site. The site exploration should consist
of additional test borings and an additional seismic refraction
survey with seismic lines parallel and perpendicular to building
column lines. The final geotechnical study should provide specific
recommendations regarding site preparation, earthwork, ground
improvement, foundation design and construction considerations.
As part of the final study, it is felt that it will be very important
to reevaluate the mining history at this site with a particular
emphasis on the completion date of reclamation activities. The
age of an uncontrolled fill is a critical factor in evaluating
its potential settlement and can significantly impact the type
and extent of ground improvement recommended.
design and prior to the advertisement of bids, a geotechnical
engineer should be retained to assist in the preparation of specifications
for the ground improvement and earthwork phases of the project.
Comprehensive monitoring and testing during earthwork, ground
improvement and building construction will also be very important.
With regards to post-improvement evaluation, it is anticipated
that a seismic refraction survey performed along the same lines
as the pre-construction survey will be the most reliable means
for evaluating the effectiveness of the ground improvement program.