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Geotechnical Engineering

 

Interstate Technical Group on Abandoned Underground Mines
Fourth Biennial Abandoned Underground Mine Workshop

Lessons Learned from Pressure Grouting an Open Shaft

Abstract

Michael L. Bourland, P.E.
Iowa Department of Agriculture and Land Stewardship
Division of Soil Conservation
Wallace State Office Building
Des Moines, Iowa 50319
(515) 242-6130

Co-author: Len Meier
Office of Surface Mining
Mid Continent Regional Coordinating Center
501 Belle Street
Alton, Illinois, 62002
(618) 463-6460

Introduction

A shaft from an abandoned mine was discovered by a pastor of a church during the month of April 2000, in Indianola, Iowa. The shaft appeared in the middle of a concrete road that provided access to the church. The concrete pavement broke as he was driving over it. Fortunately, no one was hurt. The road was subsequently closed and access to the church was obtained by driving around the shaft on the grass and into the gravel parking lot of the church. The Pastor spent some time trying to determine who owned the land where the shaft was located. It was finally determined that the property was owned by Aldi, an adjacent discount grocery store, even though the road only provided access to the church. The manager of Aldi contacted a local geotechnical consulting firm who contacted the Iowa Abandoned Mine Land (AML) Program.

The Iowa Geological Survey was contacted to determine if any mine records existed for the area and the possible thickness and elevation of a coal seam. No mining records were available for this area. However, well logs from several miles away, indicated one coal seam about 1.5 feet thick was present between elevations 850 and 870 feet. It was estimated that the surface elevation of the subsidence feature was about 910 feet based on old topographic plans made available from design of the Aldi store.

The initial site visit was made on May 18, 2000. The surface dimensions of the hole measured 27 feet by 33.5 feet and appeared to be about 15 to 20 feet deep. Timbers were observed on the north and east sides of the hole. Timbers may have also been present on the other sides, but concrete slabs and soil covered these areas. Two large concrete slabs from the road lay in the bottom of the hole, obscuring the true depth. A water service line to the church was also present along the west side of the hole and had a small crimp in it, but was not severely damaged. Telephone and fiber optic lines extended along the east and west sides of the surface feature. These lines were not affected by the subsidence, but had to be avoided during implementation of the repairs. See Figure 1 for a general site plan.

The Iowa AML Program contacted the Office of Surface Mining (OSM), Mid Continent Regional Coordinating Center and recommended that the site be declared an AML emergency due to the eminent safety hazard it presented to the church members and shoppers at the nearby Aldi and Wal-Mart stores. Based on information provided by the AML Program, OSM authorized immediate exploratory drilling to determine the nature and extent of the problem. The authors worked together to conduct geotechnical investigations, develop the remedial design and manage the construction contract for closure and Stabilization of the problem.

Geotechnical Investigations

A local geotechnical firm was contracted to perform exploratory drilling. A vertical boring (Boring E-1) was drilled using flight augers and was located about 10 feet from the edge on the northwest side of the hole (Figure 2). The augers were pulled from the hole every 10 feet to classify the materials and log the hole. This boring encountered fill and native clays to a depth of about 18 feet underlain by clayey sand and sand to a depth of about 28 feet. The overburden materials were underlain by sedimentary rock typical of the area that consisted of interbedded shale, limestone, and siltstone to the boring termination depth of about 75 feet. The limestone was difficult to drill through with flight augers and the drill crew did not have any more augers available to extend the boring deeper at that time. No coal seam or void was found in the boring to this depth. The groundwater level was observed at a depth of about 11 feet during drilling.

We decided that angle borings into the shaft were needed to provide additional information that would be used to complete the remedial design. The drill rig of the local geotechnical firm required an additional part before it was able to perform angle drilling. On May 30, two borings were drilled into the shaft from about the same location as Boring E-1. Borings E-2 and E-3 were completed as auger probes in a similar manner as the original vertical boring. However, when a void was encountered the augers were not pulled until after it had been fully penetrated.

Boring E-2 was drilled at an angle of about 30 degrees (60 degrees from the surface) into the suspected shaft. We were able to push the augers or drill with little resistance from auger lengths of about 33 feet (28 feet depth) to about 48 feet (41 feet depth). Hard drilling continued to an auger length of about 52 feet (45 feet depth). Based on the auger cutting observed during auger removal, it appeared that the portion of the shaft from depths of 28 to 41 feet was filled with loose clay materials.

Boring E-3 was performed at an angle of about 20 degrees (70 degrees from the surface) into the suspected shaft. This boring extended through the overburden and bedrock before encountering the shaft at an auger length of about 52.5 feet (49.3 feet depth). The augers were pushed or drilled with little resistance to an auger length of about 89.5 feet (84.1 feet depth). The boring was continued to an auger length of about 93 feet (87.4 feet depth). The auger cuttings in the portion of the shaft included pieces of wood, brick, and coal, along with some sand. The augers were not very full indicating the lower portion of the shaft might be open or have very loose fill. The auger cuttings of the materials drilled the last several feet consisted of shale that appeared to be natural. The shale had maroon and gray coloration and relatively low moisture contents. It was thought at that time the augers may have followed along the side of the shaft and extended past the floor of the mine.

Remedial Design Development

Exploratory drilling confirmed the sinkhole was caused by failure of an inadequately sealed abandoned coal mine shaft. Based on this information, OSM declared the problem an AML emergency. We assumed the shaft extended to a depth of at least 85 feet and contained areas with substantial voids and loose fill. We considered three options for filling and stabilizing the shaft: excavation and backfilling, a poured concrete cap, and compaction grouting. The first two options were ruled out. The depth of the shaft and weakness of surrounding strata negated the possibility of digging out the loose material and backfilling with crushed stone. We ruled out construction of a reinforced concrete cap because the site is located on a slope and we were concerned that long term erosion around the cap would compromise its integrity. Since the area above the shaft includes a road providing access to a church and is adjacent to substantial commercial development, we decided the most positive means to achieve long term shaft closure was to utilize a compaction grouting program.

The general approach for development of the contract assumed that the contractor would drill and case a series of angle holes into the shaft. Grout would be injected in ten foot vertical stages beginning at the bottom and ending 20 feet below the surface. Grouting in each stage would be terminated when one of the following conditions was met:

  • Maximum injection pressure 1000 pounds per square inch (psi) is reached;
  • Ground movement in excess of 0.5 inches is detected;
  • More than 10 cubic feet of grout/ foot of injection hole in any 10-foot stage at a back pressure of 100 psi or greater; and
  • More than 1000 cubic feet of grout is injected per foot of a 10-foot stage.

After stabilizing the shaft, muck would be removed and the surface opening backfilled with gravel. Additional vertical holes would then be drilled into the shaft and grouted. Exploratory holes would be drilled after completion of grouting to evaluate the effectiveness of compaction grouting and further drilling and grouting would be performed if needed.

The technical portion of the specifications was adapted from information obtained from OSM projects in Pennsylvania and Kentucky. Some of the specific requirements included:

  • Drill rig capable of drilling at an angles from 10 to 40 degrees, either pneumatic, hydraulic, percussion, or rotary;
  • Steel casing for grouting, advanced with drill rig to avoid redrilling;
  • Positive displacement, piston-type grout pump capable of pressures up to 1000 psi and controllable rates from 0.5 to 4 cfm;
  • Ground movement measurements during grouting;
  • Gout mix design by contractor for both low slump (1 inch) and high slump (4 inch), with minimum requirement of 1 part cement to 4 parts sand, and 400 psi compressive strength;
  • Definitions of stage completion of grouting as described previously in this paper;
  • Backfilling procedures for the surface feature;
  • Site restoration;
  • Test drilling with sampling after completion to determine effectiveness.

The bid form included unit prices for drilling injection holes (including casing), placement of both low slump and high slump grout, gravel and soil backfill placement, and test drilling. Lump sum bid items included mobilization, demobilization, and site restoration. We decided to have separate bid items and quantities for the two grout types. We planned to use the low slump grout to compact the fill materials within the shaft and to use the higher slump material for larger voids that were encountered.

Contract documents were developed and mailed to potential contractors on June 2, 2000. A mandatory pre-bid meeting was set for June 12, 2000, and contractors were required to submit their bids the same day at a set time after the pre-bid meeting. The selected contractor was to mobilize within three days of receiving notice to proceed and complete the work within 60 days.

A total of four contractors attended the pre-bid meeting and all of them submitted bids. The low bid was accepted and a contract was awarded on June 16, 2000.

Construction

The General Contractor (GC) arrived on Monday morning June 19, with only a truck and utility trailer. He worked on getting the trailer set up, placing surface monitoring points, and breaking up the concrete slabs that had fallen in the subsided feature.

The drilling was subcontracted to a local drilling company that brought a SR-20 rig that is normally used to install deep water wells. The drill rig used air pressures of up to 200 pounds per square inch with flows of 700 cubic feet per minute to remove the cuttings from the hole. The rig also had relatively high torque and downward thrust capacities. The drill rig used a hammer (percussion) bit for most of the drilling, but had to use a rotary bit towards the end of the project because the hammer bit was damaged. The drill stem consisted of 3-inch diameter steel casing with lengths of 20 feet. If there were no problems encountered while drilling, the rig could drill a 100-foot deep hole, even in bedrock, in less than an hour.

The casing used for grouting consisted of 3-inch diameter, galvanized steel pipe that were 20 feet in length. The drill stem casings were removed after reaching the desired depth and often loose materials would collapse in the just completed borehole. The grout casing was typically set using the large SR-20 drill rig that used air pressure to remove the caved in material. Either a rubber-tired backhoe or the SR-20 drill rig was used to retrieve the grout casing and raise it during grouting. The casing was reused throughout the duration of the project.

The drilling subcontractor was delayed and did not arrive until the afternoon of June 20. The GC wanted to do a boring to determine the general conditions of the site. Boring G-1 was drilled vertically to a depth of 102 feet near the first exploration hole on the northwest side of the shaft. No coal seam or open void was found to the boring termination depth. However, very soft drilling was found at several intervals ranging from 8 to 36 feet in depth. This lead the GC to believe that a shallow void may be present that sloped downward to the north, generally following surface contours. Boring G-2, also a vertical hole, was drilled more than 20 feet north of the surface opening to a depth of 72 feet. A thin coal seam was found at a depth of 62 feet and a soft zone was found between 21 and 30 feet. Several times when drilling these borings, the air return around the annulus of the drill casing became clogged or restricted. Air was then forced out into the mine shaft surface opening, and to the ground surface at several other locations as shown on Figure 2.

A typical rubber-tired, backhoe with an extendable arm was used to excavate the top of the shaft to try to determine the extent of the shaft dimensions and if possible, initiate a collapse. The backhoe worked from the edge of the street and was able to reach down about 20 feet. Numerous timbers were removed from the northwest portion of the surface opening revealing that the shaft was located near this area. No further collapse of the materials was initiated.

The GC assumed that the soft zones encountered outside the main shaft could potentially be voids because of the ease of drilling, the loss of air return, and the airflow to the surface at several locations. If this were true, it would mean that the mine included a shallow drift portion that sloped to the north at a depth of around 20 feet. Since we did not have any information to the contrary, we authorized one vertical boring about 40 feet north of the hole to determine if a horizontal tunnel was located at such a shallow depth. Boring G-3 was drilled vertically and encountered minimal drilling resistance between depths of about 25 and 30 feet. See Figure 3 for the general locations of the first three borings.

The GC attempted to grout Borings G-1 and G-2 on June 22. Grout was brought to the site using concrete trucks from a local ready-mix plant. The grout placement was subcontracted to a local concrete pumping company. The grout pump mobilized to the site was a boom type concrete pumping truck typically used for placing concrete at higher elevations and into the interior of structures. The GC said he had difficulty finding a pump that could achieve a pressure of 1000 psi and had to use this type of pump to meet this requirement.

The GC initially tried to pump the stiffer grout (1-inch slump) through the long line on the boom that included a reducer from 4-inches in diameter to 3-inches in diameter near the connection to the casing. No grout passed through the reduction. We believe that the grout mix used did not have enough cement to be pumped at a 1-inch slump using this arrangement. Water was added to the grout delivered to the site to increase the slump to about 4 inches, the higher slump grout allowed in the contract. This higher slump grout was able to pumped and less than one cubic yard went into Boring G-3 before grout came up around the annulus of the casing. The GC then attempted to place grout in Boring G-1. A small volume of grout was pumped to this hole and when the casing was pulled, we observed that no grout had passed through the end of the casing into the suspected void.

Several changes were discussed with the GC including the grout mix and grout pump. The GC located another concrete pump company that was able to pump from the hopper to a side discharge that bypassed the boom hose and included a reducer from 4 inches in diameter to 3 inches in diameter right at the pump. The new concrete pumping company suggested changing the grout mix to add a lot more cement to increase its pumpability, even if the higher slump grout was used. The GC resisted this change because of the additional cost. The contract required the GC to supply the grout mix that would meet the specifications at his unit bid price. We agreed to allow the 4-inch slump grout to be used as the initial grout placed into the grout holes to see how the operation would perform, with the understanding that we could require a change to the 1-inch grout when conditions warranted. The original grout mix design for the 4-inch slump is provided below.

Grout Mix Design
80 lbs. Cement
300 lbs. Flyash
2913 lbs. Sand
253 lbs. (30.3 gallons) Water
3 oz. Water Reducer per 100# cement

We needed to have assurance that no mining void was present at a depth of around 20 feet below the church parking lot, as was possibly indicated by Borings G-2 and G-3. Since Boring G-3 took less than one cubic yard of grout, we believed that no void was present at this location. Borings G-2 and G-3 indicated the possible presence of sizable voids, but were drilled with the well drilling rig using high volumes of air. Additional auger borings were authorized to determine the presence or absence of voids at this shallow depth. The drilling subcontractor supplied a typical rotary drill rig, CME 550, to bore test holes S-1 through S-4 in the locations of the apparent shallow voids. A continuous sampler was used between depths of about 9 to 25 feet. No voids were found. Very sandy soils or sand seams were present where the softer zones of drilling had been encountered with the large rig. We concluded that the method of drilling with the large rig and high air volumes was unable to provide the sensitivity we needed to delineate between friable materials and voids. The locations of the sample holes are shown on Figure 2.

Additional borings were placed with the large SR-20 rig at angles into the shaft. These borings indicated substantial voids and/or soft, loose materials, similar to the original exploratory borings. However, the borings also encountered stiffer clay and sand materials within the shaft. It was common to have connection with the air used during drilling between the other grout holes, the mineshaft, and the other surface outlets. Based upon the existence of sizable voids and clay materials with the density that appeared similar to grout, we determined that the use of 1-inch slump for true compaction grouting was unnecessary. Therefore, we allowed the GC to pressure grout all voids using the 4-inch slump grout already contained in the contract.

The second time the GC grouted was much more successful. Nine-hundred and seventy-two (972) cubic feet of 4-inch slump grout were placed in Boring G-6, which was at an angle into the shaft with about 100 feet of casing in the hole. Backpressure during pumping was observed, so it appeared that the higher slump was staying within the shaft and not flowing out into the mine. Several cubic feet were placed in other borings that had been completed. Delays between trucks tended to stop the grout takes in most of the other borings. Boring G-2, north of the subsidence feature, was also grouted at this time. The surrounding gravel started to heave after placing several cubic feet, further verifying that no void was present at the shallow depth.

Over the weekend of June 25, the GC filled in the shaft surface feature with gravel to make drilling safer. Additional angle drilling occurred around the perimeter of the shaft. After several series of drilling and grouting at angles into the shaft, the GC shifted to drilling and grouting vertical holes directly over the mine. Due to the method of drilling, auger cuttings and drilling resistance were used to log the borings. Layers of little to no resistance were regularly encountered, even after several series of grouting sequences. Based on previous experience, we knew that these layers could potentially be sand or soft materials, not necessarily a void.

Several of the grouting sequences performed directly over the mine heaved the gravel that had been placed at the surface opening. This heaving provided an indication that the grout was filling the voids and sealing the shaft. Several times the GC cleaned the area assuming they were finished with grouting and that the next boring would show that he had completely filled the voids in the shaft. However, the next boring would show that soft zones or potential voids were still present. Additional holes would be drilled and another series of grouting would be completed. The drilling and grouting would disturb the area and clean up would have to be completed again.

Excluding the first grouting attempt, a total of two grouting sequences including three to four angle holes were completed placing a total of 2119.5 cubic feet of grout into the mine shaft. After the surface hole was filled, three additional grouting sequences were completed using 1 to four vertical holes each time with a total placement of 1998 cubic feet of grout into the mine shaft. Refer to Figures 3 for grout hole locations.

After the last grouting sequence took a relatively small amount of grout resulting in heaving of the gravel backfill, we determined that exploratory borings using rotary drilling should be performed to determine if any voids remained. The same drilling rig, CME 550, with a continuous sampler was used. We assumed that the exploratory borings would extend to at least 70 feet into the shaft. Borings S-5 and S-6 were drilled, but could only penetrate to a depth of about 30 feet because of squeezing clay. (See Figure 2.) Excessive cuttings were brought to the surface and the drill rig engine would die out when trying to drill below this depth. No voids were found within this thirty-foot depth and the last grout hole, Boring G-17, that was completed with the larger SR-20 drill rig did not encounter soft materials below a depth of 45 feet. Therefore, we felt confident that the shaft had been filled and that the soil in the upper portions of the shaft were under pressure as evidenced by the squeezing clay. Cleanup and restoration were completed on July 7, 2000.

Project Summary

In all, 17 grout holes were drilled for a total of 1,261 linear feet. A total of 4,117.5 cubic feet of grout were pumped into the shaft. The original contract bid quantities included 600 linear feet of drilling, 7000 cubic feet of low slump grout, and 2000 cubic feet of high slump grout.

This emergency project completed permanent Stabilization of an abandoned, partially filled mine shaft using pressure grouting methods. Project investigation, design and construction were completed in only 51 days. The site is considered permanently stabilized with no concern about long term erosion that may have lingered if the concrete cap option had been chosen. Reclamation work disturbed only minimal surface area and allowed for continuous access of the church facility during construction. Because the shaft was filled and stabilized in place, there was no need to dispose of the unknown materials that were originally placed inside the shaft.

Contractor Claims

After the job was completed and the GC had demobilized to the site, a claim to adjust the drilling and grouting costs was made. The GC stated that additional borings and setups were required that could not be anticipated and the grout quantity was much less than the contract amount. At the time of this writing, the claims remained unresolved.

Lessons Learned

Several lessons can be learned as we reflect back on the project. These lessons are summarized and discussed below.

  1. More information is better and worthwhile.

    It is important to get as much information up front as needed or is available before the contract is developed. If more exploratory borings had been performed, both vertical and angle, we would have been able to avoid having the GC drill north of the shaft and the vertical borings closer to the shaft. Some of the reasons for not doing more exploratory borings were the length of time it took to drill through the hard limestone layers and the schedule of the geotechnical consultant. However, by not having the additional exploratory borings, we ended up using the construction borings as exploratory borings. The construction borings were more expensive and less useful for characterizing the mine.

    Additional exploratory drilling would have also allowed us to better characterize the amount of void space and the type of fill materials within the shaft. We changed the plan from compaction grouting to standard pressure grouting after realizing the shaft contained sizable voids interlaced with thick muds. These conditions did not seem appropriate for compaction grouting with the stiff grout. Better understanding of the conditions would have allowed us to tailor the specifications more closely, and possibly get lower bids for the work.

  2. Separate out the drilling and the casing costs.

    In some of the borings, the casing was only placed in the upper portion, while extending the full length in other borings. The Contractor received payment for the same amount for each foot of boring drilled, whether it was cased or not.

  3. Be familiar with the equipment.

    The equipment requirements in the contract were fairly general and allowed the contractor to use somewhat unconventional equipment for this project. Only a partial list of equipment was made available prior to starting and we were not familiar with an SR-20 drill rig. It is important to be familiar with the equipment and ask for appropriate specifications about the equipment if it is not familiar to you.

  4. Recognize that specification requirements can limit equipment selection.

    The pumping requirements of pressures of 1000 psi limited the selection of equipment for this site. Sometimes it is desired to limit the type of equipment that can be used to obtain the desired result. At other times, it can add unnecessary cost to the project.

  5. Document changes in the field.

    It is very important to discuss and document changes as they occur in the field. In our case, we expect that good documentation will minimize the cost of contractor claims.

  6. Limit the amount and types of work that the General Contractor can subcontract.

    While this contractor attempted to perform the job well and completed the work as required, he had limited control over the subcontractors and their equipment. This made his costs more unpredictable and made it more difficult for us to work within a fixed unit price contract. In the future, we would probably write the contract to require the contractor to perform the production drilling and grout pumping, limiting subcontracting to grout supply and specialty test drilling.

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Updated: 04/07/2011

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