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Powell River Project
Reclamation Guidelines for Surface-Mined Land in Southwest Virginia

Stabilizing Reclaimed Mines to Support Buildings and Development

Authors: Carl E. Zipper, Extension Specialist, Department of Crop and Soil Environmental Sciences, and Associate Director, Powell River Project, Virginia Tech; and Steven Winter, Professional Engineer, Schnabel Engineering Associates, Blacksburg, Virginia

Publication Number 460-130, Posted July 1997

Land use constraints hinder economic activity in the Virginia coalfield region; lack of flat land hinders infrastructure construction, industrial recruitment, and business development. Reclaimed coal mines are widely discussed as potential development sites, but modern reclamation rarely prepares mined areas for building-support purposes. This publication describes mine-stabilization procedures that can be employed in developing reclaimed mine areas for building construction.

This publication outlines general concepts and guidelines. Parties seeking to construct buildings on any mined area should engage the services of a professional engineer.

Background: Land Settlement and Structural Distortion

Rigid or semi-rigid structures employing large volumes of concrete and structures covering large lateral areas are particularly sensitive to distortion and damage due to differential settlement.

It is especially important to construct lands that will minimize differential settlement if an industrial use is intended for a reclaimed site. Industrial facilities typically employ concrete floors covering large areas. High-value manufacturing and processing operations typically require high levels of mechanical precision; a shifting or cracking concrete floor will not be compatible with such activities. Nonreinforced masonry structures will be especially sensitive to settlement.

Mine Reclamation Procedures to Minimize Differential Settlement

Spoil Depth

The reason for maintaining a uniform spoil depth under building locations is that differential settlement is a condition that should be avoided. All spoils will settle, regardless of placement procedures; depth of spoil will be a major factor that influences amount of settlement. If spoil depths under a building site are even and uniform, ground settlement is more likely to be even and uniform. Locating a building site where spoils are of minimum practical depth will reduce the costs of spoil-placement and site-stabilization procedures if employed.

If only a portion of the reclaimed site will consist of shallow spoil over a solid bench area, it is important to survey prior to spoil placement so the area suitable for building construction can be located precisely after mining and reclamation are complete.

Spoil Placement

The following are factors that should be considered in developing a mine reclamation plan for a site intended to support a high-value building:

A well-defined spoil "pad" should be carefully constructed under the planned location of the building and its immediate surroundings using controlled-placement procedures. The pad should extend beyond the building's actual perimeter by at least 10 feet in all directions. General guidelines for construction of the spoil pad follow, but the actual procedure should be designed by a professional engineer who has access to accurate data describing material properties and settlement characteristics of available mine spoils. At the conclusion of pad construction, the area should be surveyed so the pad's exact location can be identified after reclamation is complete.

The ideal thickness of each lift will depend upon a variety of factors, including physical factors such as the spoil's engineering characteristics and the type of equipment available for spoil compaction. Other factors will include total depth of spoils that will underlie the building, the length of time likely to elapse between spoil placement and building construction, and the building's design.

A lift thickness of about 1 - 2 feet will be sufficient to minimize settlement in most situations, if that lift is thoroughly compacted. Achieving this 1-to-2 foot lift thickness will not be economically feasible on most mines. A 2-to-4 foot thickness, although not ideal, will achieve a reasonable level of stability if compacted uniformly and if spoil composition is controlled. Mine spoils with a composition dominated by hard sandstone spoils and rock fragments will compact more effectively at 3-to-4 foot thicknesses than spoils with composition dominated by shales and clays. If operational problems prevent spoil placement in lifts that are 4 feet or less in thickness, use of controlled lifts of minimum practical thickness (even if greater than 4 feet) and controlled composition would be a superior practice to random dumping.

Ideally, spoils within each lift should be of a single rock type, although this may not be practical on some sites. If possible, use of rock fragments larger than 1 foot in diameter should be avoided during construction of the building-support pad. Use of a hard, competent material (such as durable-rock sandstone) is preferred to use of rocks that will slake in water, such as most siltstones and shales.

From a pure geotechnical standpoint, the best procedure to minimize settlement would be to utilize a screening procedure so as to rigorously control rock fragment size while building the support pad. This will not be a practical option on most operating coal surface mines.

An alternative practice -- one that is not ideal from a geotechnical standpoint, but is less costly -- will be to control spoil fragment size, as well as rock type, of the building-support pad by exercising control over spoil movement procedures. Under this practice, spoil excavation and movement procedures would employ one or more alternate dumpsites that do not underlie future building areas. If the spoil is being moved by truck, the truck operator can be instructed to dump loads containing materials that are not suitable for building support in an alternate dumpsite location, while suitable materials are placed in the building-support fill.

The loader operator will be the person on most jobsites who has the best view of the material in each truckload. The loader operator can be instructed to direct spoil movement by communicating with the truck operators. Given a small number of dumpsite locations, hand signals (viewed by the truck operator through the rear view mirror), horn signals, or other means can be used by the loader operator to direct spoil movement.

If mining economics prevent placement of material in compacted lifts of controlled thickness, the stability of the postmining landform will be increased if the composition of spoils placed in areas of potential building construction is controlled. Use of hard-sandstone spoils and exclusion of siltstones and shales in areas of potential building construction will enhance the postmining stability of those areas, even if spoil is not placed in compacted lifts. If this practice is followed, it will be important to document the location of those lands constructed using hard sandstone spoils through a survey or other means.

Special compaction equipment, such as a vibratory roller, may be employed to achieve a thorough and uniform compaction of each lift. Such practice is preferred from a geotechnical standpoint. However, use of specialized compaction equipment will be cost prohibitive for most coal-mining operations.

An alternative method of achieving spoil compaction will be to use loaded haul trucks. As loaded trucks drive over previously placed spoil to reach the dumpsite, significant compaction will occur. The challenge to the mine foreman will be to design a spoil placement sequence that will cause compaction to occur as a result of routine spoil movement. Truck drivers can be instructed to vary the location of each route so as to achieve more-or-less uniform wheel coverage over the entire compaction area. If this procedure is employed, it will be important to maintain consistent lift thicknesses. Generally, the weight of a track dozer will not be sufficient to compact spoil to a degree necessary to achieve a stabilized building support.

Mine spoils cannot be compacted effectively if they are extremely dry or extremely wet. If a high-value building is to be constructed on the reclaimed site, and the goal is to reduce postreclamation settlement to an absolute minimum, spoil placement procedures should be designed with an engineer's input so as to assure spoil-moisture contents that are optimal for compaction. The optimum moisture content for compaction of most mine-spoils containing large quantities of soil-sized materials will range from 5 to 15 percent.

On most operating surface mines, use of procedures to exert firm control over spoil moisture contents during compaction will be cost-prohibitive. On eastern U.S. mines, spoils in the field will rarely become too dry for effective compaction; however, they can become too wet during damp weather. The effect of compaction is to press the mineral particles close together, reducing pore volumes and opportunity for further consolidation. When spoils are extremely moist, water occupying the pores between small mineral particles effectively limits the ability of the particles to be forced together. Therefore, it is advisable to avoid placement and compaction of building-support pads during wet weather whenever possible.

A buildup of water in the fill can cause substantial settlement -- even if it occurs many years after fill placement. Surface drainage, building placement, the orientation and pitch of subsurface compaction planes, and the placement of rock underdrains are factors which can help to prevent a buildup of water in the fill. For example, construction of a building-support pad using lifts that have slight crowns would be a superior practice to construction of lift surfaces which drain inward. Large durable-rock fragments can be placed at the base of those spoils that surround the building-support pads so as to assure adequate drainage.

Summary

When land is constructed using the above procedures, precautions remain in order. After mine-fill construction is complete, some time should be allowed to pass prior to building construction. Survey points should be established to monitor postmining settlement; only when settlement has slowed to acceptable levels should construction begin. Settlement will be most severe during the first year following spoil placement.

Thorough documentation of procedures employed to construct the building-support pad will aid future building and site design. Periodic visits of a geotechnical engineer to the site during construction will be the best way to provide that documentation. If the engineer who designs postmining site-usage does not have precise knowledge of the procedures utilized to construct the reclaimed landform, the resulting building/site design is likely to be more conservative (and more costly) than necessary. If the engineer is able to come on site periodically to observe controlled spoil-placement procedures and obtain spoil samples, site-usage design can accommodate those spoil placement procedures that did occur.

Building on Sites Prepared Using Conventional Spoil Placement

Several options are available for building on settlement-prone reclaimed mines. These are described below.

Allow the Site to Stabilize

Depth of settlement after the first year was calculated using the formula

where TS represents total settlement, H represents the fill height, and T₁ and T₂ represent time after mining expressed in years (Krebs, 1987). A is a settlement factor which varies with spoil composition. Figure 4 was constructed using a settlement factor of 1.0 for loose siltstone, and 0.2 for compacted sandstone. The sandstone settlement factor represents excellent compaction of material placed in 1 to 2 foot lifts. Figure 4 shows that a 100-foot depth of loose siltstone spoil would be likely to settle about 1 foot between the second and tenth years after mining, and another foot in the 90 years after that. The compacted sandstone fill would settle about one-fifth of these amounts.

These figures represent settlement of non-saturated spoils. If spoils become saturated, due to a high water table or a concentration of surface water on the site, more rapid settlement can be expected during and immediately following saturation.

While Figure 4 represents total settlement, differential settlement is what damages buildings. If the spoil in the building-support fill is relatively uniform in physical characteristics and degree of compaction, settlement differentials are likely to be only a fraction of the total settlement represented in Figure 4.

If an expensive building design is to be employed, placement of survey stakes and measurement of actual settlement through at least one winter season prior to building would be desirable.

If settlement of the mine-fill surface is continuing, removal of surface spoils to the depth of a design surcharge (see below) will expose spoils that have been stabilized by the weight of overlying materials and may be suitable for building, depending on the nature of the underlying material and the age of fill.

Use a Building Design that Can Tolerate Settlement

Another strategy would be to use a rigid structure that is capable of withstanding settlement of underlying ground. For relatively small buildings, a prestressed concrete slab can be designed to "float" on the mine spoil surface, bridging whatever voids may develop and remaining rigid despite any differential settlement of underlying materials. Larger buildings can also make use of prestressed slabs, but only with greater design difficulty and expense. Nonreinforced masonry is poor in ability to withstand differential settlement without fracturing. Concrete block is generally a poor choice for a construction material on settlement-prone ground.

A third possible strategy would be to set piers or pilings through the mine spoil. If the piers are strong enough to support the building and long enough to reach undisturbed ground, they should provide a stable support despite any settling of the mine spoils that underlie the building. Generally speaking, drilling piling locations will be preferable to driving the pilings, especially in a stony mine spoil. Pilings will be an expensive way to support buildings on all but the thinnest mine spoils - where it may make greater economic sense to excavate and replace underlying spoil than to place the building on pilings.

Employ Procedures to Stabilize the Building Site

Depending on spoil thickness, it may be possible to excavate spoil from beneath the building's location; the spoil could then be replaced using controlled placement and compaction procedures.

Another option would be to compact the inground spoil using mechanical methods. Several procedures are available to achieve this goal. A procedure called dynamic compaction employs a heavy weight that is dropped from a specified height to compact the uppermost spoil layer in a repetitive grid-like pattern. When prescribed, dynamic compaction is typically intended to achieve a tightly-compacted "zone" of spoil constituting the upper 15 to 30 feet of the fill. This tightly compacted zone is expected to remain rigid despite any settling of the underlying noncompacted spoil, thus acting in a fashion similar to the prestressed concrete slab described above.

Another method for achieving compaction and stabilization of the building-support fill is called surcharge preloading. This procedure is carried out by loading the building-support spoil with soil or mine spoil material. The weight exerted by this mass (typically, more than the planned building) will accelerate consolidation of underlying spoil. Settlement plates are placed on the spoil surface prior to fill placement; these are horizontal metal plates attached to vertical riser pipes that extend upward through the surface material, allowing the rate of spoil settlement to be monitored with surveying instruments. When the downward movement of the settlement plates slows to an acceptable rate, the surcharge load is removed and the building site is prepared for construction.

On sites that are more than 2 years old, a less costly alternative to surcharge may be available. Removal of the upper 10 to 20 feet of spoil will expose materials that have been compacted by overlying spoils over time. In some cases, this will be less costly than applying a surcharge because the spoils that need to be moved must only be handled one time. Of course, this procedure will lower the ground surface, requiring a disposal location for the excavated spoils.

Case Study: Fill Stabilization of the Red Onion Maximum-Security Prison Site

The 16-acre site is a former surface mine that was reclaimed in the late 1980s. Controlled spoil placement and compaction procedures were not employed during reclamation because the mining company did not expect the site to be used for building construction. The result was a relatively flat landform that was perfectly stable, from an environmental standpoint, but was lacking the high level of structural stability required to support the prison's precast masonry building units. Depths of spoil under the proposed building location ranged from 30 to 50 feet. A site investigation and geotechnical engineering study conducted by Schnabel Engineering Associates in 1994 determined that the site's subsurface provided insufficient support to allow the prison to be constructed effectively.

In 1994, funds were allocated by the Virginia General Assembly to construct the prison on a portion of 300 acres of land donated to the state by Pittston Coal Company. The funding included the amount needed to stabilize the site to support the prison, based on an estimate prepared by Schnabel. Stabilization consisted of a combination of dynamic compaction and surcharge procedures. Both procedures were applied because of the maximum security prison project's sensitivity. In most cases, either one or the other of these two procedures would have been adequate.

Dynamic Compaction

Several different 15-ton weights were used in the high-energy compaction procedures. Where the ground was relatively soft (where uncompacted soil-like materials were present at the surface), a weight exerting 900 pounds-per-square-foot (psf) contact pressure was used; this weight had a 32 square-foot footprint area. Two 1500 psf (20 square-foot) 15-ton weights were used in areas of rocky or compacted fill.

Surcharge

On some portions of the site, it was necessary to remove more than 15 feet of existing spoil prior to construction so as to reach final grades. No surcharge was applied in these areas.

Costs

Conclusion and Summary

Lands reclaimed using conventional practices will often remain subject to rates of postmining settlement that are considered unacceptable by most land developers for many building types -- even years after mining. In time, settlement of these lands will essentially cease -- but in most cases, many years will be required. It is possible to employ procedures that will accelerate stabilization of settlement-prone mined lands. However, site-stabilization procedures do require a considerable expense.

This bulletin contains only general background and guidelines. Both site stabilization and spoil placement procedures to minimize postconstruction settlement should be designed in consultation with a professional engineer.

Acknowledgments

References

Krebs, R., and C. Zipper. 1989. Foundations for Housing on Reclaimed Mined Lands. 12 pages.

Corresponding Author: Carl E. Zipper, Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg, Virginia 24061-0411. Phone: 540-231-9782. FAX: 540-231-3431. Email: czip@vt.edu