Land SlrfunfenecfProceedings of the Fourth International Symposium on Land Subsidence, May 1991). IAHS Publ. no. 200,1991.

Grouting in Deep Flooded Mines

WILLIAM C. MORRISON United States Department of Interior Office of Surface Mining, Eastern Service Center, Pittsburgh, Pennsylvania, USA 15122

ABSTRACT A methodology developed to place a cement based grout in the Mary Lee coalbed in Graysville, Alabama. The coal seam is situate in excess of 700 feet below the ground surface. Several difficulties to be overcome were the disintegration of the grout mix from downhole flow of water and the separation of grout ingredients in long overland and downhole delivery systems.

BACKGROUND

In January, 1980, a subsidence event that affected approximately 14 homes, a shopping center, and a service station was reported in the City of Graysville, Alabama. In March, 1980, the Alabama Department of Industrial Relations reported the severity of the event to the Office of Surface Mining field office in Knoxville, Tennessee.

The area involved lies adjacent to U.S. Highway 78 in the city of Graysville, Jefferson County, Alabama and is located 8 miles north-west of Birmingham. It is located on the U.S. Geological Survey 7.5 minute Adamsville Quadrangle (Fig. 1). Beneath the affected subsidence area, there are two abandoned coal mines. The Bessie Mine is in the Pratt coalbed approximately 180 feet below the surface. The Flat Top Mine is in the Mary Lee coalbed, approximately 540 feet below the Pratt coalbed, or 720 feet below the surface.

The Office of Surface Mining (OSM) was requested to investigate the severity and extent of this event. OSM was also requested to make appropriate recommend­ations for mine subsidence stabilization.

In July, 1980, personnel from the OSM conducted an in depth investigation of the site and concluded that the affected area was larger than originally reported. The revised area embraced a total of 42 homes, two shopping centers, two service stations, and two churches (Morrison, et al, 1987).

EXPLORATORY DRILLING PROGRAM - Phase 1

The OSM team evaluated the need for mine stabilization in the project area and the various methods that could be employed. To make this evaluation, the condition of the mine in the project area had to be determined. Aside from the drilling conducted in 1981 around the strip shopping center, there was no information available concerning conditions at mine level. OSM initiated an exploratory drilling program to obtain the additional required information.

The exploratory drilling program consisted of drilling six borings (ATB-3, 4, 5, 6, 8 and 9) at the locations shown on Fig. 2. Staff engineers located the exploratory holes on the mine and topographic maps. They were located to provide information concerning the openness of the mine voids and the height of the coal pillars.

503

William C. Morrison 504

Scale In KS«

F I G . 1 L o c a t i o n m a p .

scat» in r»<?t ..,!:.'.'.'j;. , ":::i I:::5."::::; f;:;.1 :.~:l |~-.-:;:::! ::!. !:r.tilïQP^'

FIG. 2 Location plan: test core holes for 1983 grouting project.

505 Grouting in deep flooded mines

ANALYSIS OF DATA

The OSM team analyzed, the data from the coring to determine the condition of the mine and the amount of subsidence, if any, that occurred. The void height and amount of infilled material were ccnpared to the coal sections and mine heights given on the mine maps. This comparison showed that between 2 and 6 feet of reduction in mine height had occurred and there was roof fall material in the remaining void. Water level readings taken in the boreholes show that the mine was flooded and the phreatic surface in the mine is 70 to 80 feet above the mine roof.

CONCLUSIONS FROM INTERIM TEST CORING

It was apparent from the data collected that the mine in the Mary Lee coalbed was subsiding. The surface expression, as reflected in the damage to structures, indicated that not all the subsidence had migrated to the surface. The only exception to this was in the strip shopping center area. Also, the potential for more collapse at the mine level existed as open voids up to 5 feet in height were recorded. The presence of loose unconsolidated roof fall material in the mine made the building of point support columns difficult.

STABILIZATION METHOD AT MINE LEVEL

Effective stabilization of the project area required that future subsidence at the mine level be prevented from migrating to the ground surface. Subsidence that had already migrated up into the rock overburden, but had not yet expressed itself as surface movement must be kept from doing so.

OSM engineers believed that a stable mine could be obtained if saturation grouting could produce large zones or areas of support, similar to the unmined panel pillars. They also believed that no structure in the project area should be more than 150 horizontal feet from support at the mine level. Also, the support at the mine level should extend laterally so that the assumed 15° angle of draw is taken into consideration. The material injected into the mine should be designed to permeate the roof fall material and travel a substantial distance from the hole (150 +/- feet) prior to setting up. The flow of down hole water had to"be contained to prevent washing of the material. Engineers decided to place water stops at the project perimeter to reduce the flow of water at the mine level through the project area. The conceptual stabilization program at mine level is shown as it was envisioned on Fig. 3.

STABILIZATION OF OVERBURDEN

The stabilization of the top 350 feet of overburden to prevent latent movement from expressing itself at the surface required pressure grouting. The presence of unmined zones at the mine level results in high shear stress levels at the base of the Pratt coalbed and for a distance of 150 feet below this coal. It is possible to eliminate latent movement by strengthening this zone (Craft, et al, 1987). Also, the overburden above the Pratt coalbed can be strengthened in the same manner. It was therefore important on this site to grout the upper 350 feet of overburden. The presence of open vertical fracturés in this section, as recorded from the core drilling, reduced the ability of the rock mass to resist movement.

William C. Morrison 506

FIG. 3 Conceptual stabilization program, 1983 project.

DRILLING AND GROUTING-PHASE II

In August 1983, OSM sent out specifications for bidding on a drilling and grouting program in the Graysville Area. Since there were specific requirements to be net in the preparation of the bids, there was a limited tine frame for bid preparation.

The contractor had to design a rapid set grout for water containment barriers. The grout had to attain an initial set in thirty (30) minutes with a compressive strength of 1000 psi in twenty-four (24) hours. The specifications also defined the compressive strength for the saturation grout to be used in the mine as pillar and overburden support. This grout had to attain a compressive strength of 1500 psi in twenty-eight (28) days.

The contractor started drilling operations in December 1983. The first set of holes drilled were the water barrier holes in the south end of the project area. The engineers selected these holes to be drilled first because of the dip in the mine. The contractor drilled fourteen (14) holes for water barriers in the Mary Lee coalbed. Seven (7) holes were drilled in the south end of the project and seven (7) in the north end of the project (Fig. 4 and Table 1)

The rapid-set grout for the containment barriers was composed of cement, sand, lime kiln dust and calcium chloride. The proportions of the various ingredients were:

507 Grouting in deep flooded mines

Cement Sand Line Kiln Dust Calcium Chloride Flake Water

769 pounds 576 pounds 618 pounds 16 pounds 427 pounds

This mix worked very well in all holes except S-10 which took a total of 2662 tons of grout. The contractor was directed to increase the amount of calcium chloride to further accelerate the set tine.

FIG. 4 Water barrier grout holes.

TABLE X Water barrier grout holes and grout quantities.

Hole Depth Void Grout No. Ft. Ft. Tons

Hole Depth Void Grout No. Ft. Ft. Tons

Hole Depth Void Grout No. Ft. Ft. Tons

SI S2 S3 S4 S5

699 699 695 689 687

4 5 8 8 9

132 286 484 66

1067

S6 S7 S8 S9 S10

700 705 706 684 682

9 2 B* B* B*

242 77 264 1221 2662

Sll S12 S13 S14

762 663 655 700

B* 2.5 2 B*

77 588 44 121

B* - Broken zone, no recorded open void

William C. Morrison 508

These water barrier plugs controlled water flow in the mine to permit the saturation grouting. By creating a quiescent pool in the mine, or reducing the water flow, the grout would stay in place. Also, the grout would not separate as it had in previous efforts.

In February 1984, the contractor started drilling the saturation grout holes. Drilling started at the north end of the project area and proceeded in a south-easterly direction toward the intersection of Route 78 and Main Street. Originally, sixteen (16) holes were to be drilled for saturation grouting. (See Fig. 5) .

The original saturation grout was composed of cement, flyash, sand and water. The grout mix was composed of:

Cement 257 pounds Fly Ash 184 pounds Sand 1559 pounds Water 43 gallons

The grout was pumped about 300 feet overland then 700 feet down into the mine. While testing the grout mix by pumping it through the delivery system, it was determined that the sand in the grout was separating and settling to the bottom of the horizontal grout line. This settlement caused a bridging or clogging of the grout line. The contractor, in consultation with the engineer altered the mix to the following:

FIG. 5 Control saturation grout holes.

509 Grouting in deep flooded mines

Cement 300 pounds Fly Ash 200 pounds

Sand 1500 pounds

Bentonite 6 pounds Water 54.7 gallons

Water reducing agent 75 ounces

This produced a cohesive, pumpable grout mix that would not separate as it was being purtped through the system and into the mine. The contractor grouted the original design holes, B-l through B-16, with the modified grout mix (Fig. 5 and Table 2)..

TABLE 2 Saturation grout holes.

Hole No.

B 1 B 2 B 3

B 4 B 5

Depth

Ft.

703 705 700 700 703

Void Ft.

7 B* 2 B* 4

Grout Cu.Yds.

1160 1411

51 7

456

Hole No.

B 6 B 7

B 8 B 9

B10

Depth

Ft.

711 728 694 722 695

Void

Ft.

B* 3 3 6 3

Grout Cu.Yds.

372 797 19 718 385

Hole No.

Bll B12 B13 B14 B15 B16

Depth

Ft.

711 689 710 689 722

689

Void Ft.

B* 3 B* 5 4 2

Grout Cu.Yds.

734 473 20

1011

4968 509

B*= Broken Zone, no recorded open void

When the initial sixteen saturation grout holes had been grouted to refusal, the contractor then grouted the test holes drilled in 1983 (Table 3 and Fxg.2).

OSM analyzed the amount of grout placed into the mine in the original sixteen holes. By doing a plot on a project map, it was determined there were areas that received very little if any additional support in the mine due to very small grout takes. Therefore, to provide the desired results, additional holes were required. The engineers located six additional holes to intercept the areas that needed more grout (Table 4 and Fig. 6).

While the contractor drilled and pumped grout into the mine through the six "AB" holes, another crew drilled the pressure grout holes. These holes were 350 feet deep, or approximately 150 feet below the Bessie Mine in the Pratt coalbed. A total of 124 holes were drilled for this purpose.

There was always the possibility of drilling into, or through a mine void in the Bessie Mine (Pratt coalbed). This possibility was in the area north of Main

TABLE 3 Exploratory holes, 1983.

Hole No.

ATB 1 ATB 2 ATB 3

ATB 4 ATB 5

Depth

Ft.

689 722 709 715 721

Void Ft.

2 -0-5 B* 5

Grout Cu.Yds.

972 D.P. 1040

19 25

Hole Depth

No. Ft.

ATB 6 700 ATB 7 680 ATB 8 716 ATB 9 714

Void Ft.

1 -0-B* B*

Grout Cu.Yds.

85 D.P. 745 7

D.P.- Drilled into Pillar

B*« Broken Zone, no recorded open void

William C. Morrison 510

TABLE 4 Additional grout holes, 1984.

Hole No.

Depth Ft.

Void Grout Ft. Cu.Yds.

Hole No.

Depth Ft.

Void Grout Ft. Cu.Yds.

AB 17 AB 18 AB 19 AB 19A

689 710 728 728

2 2 B* 7

653 1037 22

6247

AB 20 AB 21 AB 22

714 700 703

6 2 7

1779

720 2070

B*= Broken Zone, no recorded open void

Street and at the extreme eastern end of the project site. To overcome this eventuality, grouted gravel columns were planned where the drill hole penetrated mine voids. The drilling was advanced through this column to the lower bed, the Mary Lee. This concept is more fully described in "Recent Developments In Grouting For Deep Mines" (Ackenheil and Dougherty, 1970).

As the drill hole advanced, the return drilling fluid was closely monitored as it carried the drill cuttings to the surface. When, and if, the drill broke through the mine roof in the Pratt coalbed, the hole was then advanced into rock in the mine floor. The hole was then reamed and cased through the overburden with 10-inch I.D. pipe and a 2-inch I.D. steel slurry grout injection pipe was installed into the hole into the floor of the mine. Gravel up to 3/4 inch in size

Scale In Feet

FIG. 6 Additional controlled saturation grout holes.

511 Grouting in deep flooded mines

was then loaded down the hole around the injection pipe. The amount of gravel per hole was based on a chart that was developed in the Ackenheil-Dougherty paper defining volume of gravel versus void height (See Figure 7). With the gravel in place, grout slurry was pumped through the injection pipe into the gravel. The slurry was composed of one (1) part cement, four (4) parts flyash and four (4) parts water. The slurried gravel column was then allowed to set for a period of forty-eight (48) hours, then redrilled with the original size bit to completion at the 350 feet depth.

After the hole was drilled to the required depth, the grout slurry was pressure injected into the overburden. A balloon packer was set at a depth of two-hundred-twenty-five (225) feet, or into the first competent rock below the Pratt coalbed. Pressure was applied to the grout in the amount of one (1) psi for every vertical foot of overburden thickness including the gravity head of the slurry. This overburden thickness was measured between the Pratt coalbed and the ground surface. After the initial grout had set, the contractor retrieved the grout injection system to an elevation approximately twenty-five (25) feet above the Pratt coalbed to allow this section to be gravity grouted. This grout set for a minimum of sixteen (16) hours. The contractor then grouted the remainder of the hole by gravity in one (1) lift to the top of rock below the soil line.

During the pressure grouting, the contractor drilled five (5) holes for testing purposes. The holes were drilled and cored in two stages. The contractor first drilled the hole to within 10 to 15 feet of the roof of the mine in the Mary Lee coalbed with the pneumatic hammer. He retrieved the drill, cased the hole with NQ coring casing, then advanced the hole using a N2 wire-line core drill into the bottom rock of the mine. OSM sent the retrieved core to a laboratory for compressive strength tests and pétrographie analyses.

Table 5 compares the break strengths of the grout before the grout was injected into the mine with the grout cores taken from the mine.

NDMDGRAPH

6

5

u.4 z z a. D 3 b_ D

HE

IGH

T

ro

1

/ /

/ /

/

/ /

/ À

y y

10 20 30 40 50 60 VDLUME OF CDNE (Cubic Ya rds )

70

FIG. 7 Volume of gravel as function of void height (after Ackenheil and Dougherty, 1970).

William C. Morrison 512

TABLE 5 Compressive break strengths of grout.

HOLE GROUT ON GROUT FROM HOLE GROUT ON GROUT FROM NO. SURFACE CORE SAMPLE NO. SURFACE CORE SAMPLE

B 3 1055 psi 3963 psi B 5 1699 psi 1705 psi B 14 457 psi 1760 psi B 15 3752 psi 4785 psi B 10 2433 psi 5022 psi

TABLE 6 Comparison of grout mixes by pétrographie

analysis.

HOLE

NO.

S 3

B 14

B 10

B 5

B 15

Cement

Sand Muscovite

Cement Fly Ash

Sand Muscovite

Cement Fly Ash

Sand Muscovite

Cement Fly ash

Sand

Muscovite

Cement Fly Ash

Sand Muscovite

MIXER PROPORTION

25.00%

75.00%

12.80% 9.20%

77.90%

12.85%

9.20% 77.95%

15.00% 10.00% 75.00%

15.00%

10.00% 75.00%

IABORATORY PROPORTION

50.00% 48.70 1.30%

43.20% 9.0 %

47.70% 0.10%

54.30%

14.80% 30.80% 0.10

51.80% 9.00%

38.70%

0.50%

60.30%

12.70%

26.20% 0.80%

Table 6 compares the composition of the grout mixes: first as they were prepared in the mixer for injection into the mine and second, the same mixes after they were retrieved from the mine as core samples.

The major conclusion determined from Table 5 is that the grout had a better chance to harden and set in a very controlled condition in the mine. This table, along with Table 6 show there was a definite reconstitution of the grout mix. This was accomplished during the pumping operation. Even with the bentonite and super-plastizer added, some of the larger sand grains separated from the grout mix.

From the beginning of this project through the pump testing on the ground, there was a need for a bonding agent such as bentonite to assure a cohesive grout mix. Without a bonding agent a grout pumped such a distance overland and to such a depth to a mine will separate. In effect, the larger grains of sand would not stay in suspension in the grout mix. According to the laboratory tests, specif­ically the pétrographie analysis, a decided redistribution of grout mix constit-

513 Grouting in deep flooded mines

uents occurred. With t h i s r ed i s t r ibu t ion of ingredients , the grout strengthened and densif ied t o become a ranch higher compressive s t rength grout .

REFERENCES

Ackenheil,A.C, Dougherty, M.T. (1970) Recent Developments in Grouting in Deep Mines, Jour, of Soil Hech. and Foundations, A.S.C.E.

Craft, J.L., Crandall, T.M. (1987) Mine Configuration And Its Relationship To Surface Subsidence, OSMRE File Report, Eastern Field Operations, Pittsburgh, PA, 22 pps., Presented at the 1987 Annual Meeting, Association of Engineering Geologists.

Morrison, W.C., Elder, C.H., Craft, J.L. (1987) Subsidence Abatement Project Report, Graysville, Jefferson County, Alabama; OSMRE File Report, Eastern Field Operations, Pittsburgh, PA.