Lonesome Pine Regional Business & Technology Park


GEOTECHNICAL OVERVIEW

The Lonesome 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.

Marshall Miller 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 to bedrock.

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.

Schnabel Engineering 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.

Development 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 excavated material.

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.

Once building 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.

Following 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.