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					Stabilization of Shade
Landfill Deep Mine
Workings
Mine voids and overburden fractures are grouted.

            aste Management wished to       present throughout the area.                 slump (non-flowable) grout to create a

 W          permit an expansion for
            Shade Landfill, located in
            Somerset County, PA. The
            expansion area encompasses
                                               To permit the expansion, Civil &
                                            Environmental Consultants, Inc (CEC,
                                            www.cecinc.com) evaluated the poten-
                                            tial impact mine subsidence could have
                                                                                         curtain, while production holes were to
                                                                                         be filled with flowable grout. The plan
                                                                                         also required corehole drilling to con-
                                                                                         firm the adequacy of stabilization. The
about 118 acres, of which 70 acres over-    on the performance of a liner system.        Pennsylvania        Department        of
lie abandoned deep mine workings of         Evaluations showed the area remained         Environmental Protection (DEP)
the Upper Kittanning coal mine.             susceptible to subsidence, with resulting    approved an expansion permit provid-
Design-phase investigations indicated       surface strain magnitudes incompatible       ing Shade Landfill with 31 million cu yd
mine workings were present within 20        with geosynthetic liner materials. To        of additional airspace, contingent upon
to 120 feet beneath proposed excavation     allow for landfill expansion, CEC devel-     stabilizing the mine workings.
grades. While much of the mine roof         oped a mine stabilization plan designed
had collapsed and mine workings were        to stabilize mine voids and overburden       First Stabilization Increment
gob-filled (That part of a mine from        fractures. The plan required drilling bar-       Construction of the first stabilization
which the mineral has been partially or     rier holes around the stabilization area     increment, encompassing 8.3 acres,
wholly removed and now filled with the      perimeter, with production holes drilled     occurred in 2007. CEC was retained to
waste left in old workings.) voids aver-    within the barrier hole perimeter.           prepare construction documents and
aging two feet in thickness were still      Barrier holes were to be filled with high    provide construction quality assurance.
                                                                                         Howard Concrete Pumping Company,
                                                                                         Inc. (Pittsburgh) was contracted to con-
                                                                                         struct the mine stabilization increment.
                                                                                         Based on spacing requirements of the
                                                                                         plan, CEC laid out 195 barrier holes
                                                                                         and 218 production holes. Howard sub-
                                                                                         sequently air-rotary drilled each hole to
                                                                                         a depth slightly lower than the mine
                                                                                         floor. CEC’s technician logged each
                                                                                         hole, and CEC’s management used the
                                                                                         information to assess mine conditions
                                                                                         and adjust drilling as appropriate. In
                                                                                         total, 24,800 ft of drilling was complet-
                                                                                         ed, with conditions ranging from intact
                                                                                         coal (50 percent of boreholes) to gob-
                                                                                         filled zones (40 percent of boreholes), to
                                                                                         voids (ten percent of boreholes).
                                                                                             Grout, consisting of fly ash, sand,
                                                                                         cement, and water, was mixed on-site.
                                                                                         Grout consistency was adjusted to pro-
                                                                                         vide high slump barrier grout or flow-
                                                                                         able production grout. A concrete pump
Panoramic view of grouting operations at Shade Landfill in Pennsylvania.                 truck was used to deliver grout to the

20   ■   GOVERNMENT ENGINEERING   ■   MAY–JUNE 2008                                                          www.govengr.com
                                                            minimal takes to 675 cu      These observations provided indirect evi-
                                                            yd     per     borehole.     dence grout had infilled gob, fractures,
                                                            Generally, grout vol-        and voids. Direct evidence was provided
                                                            umes       corresponded      in core samples that contained grout
                                                            with borehole condi-         ranging from thin lenses between frac-
                                                            tions. Large grout vol-      tures, conglomerates of grout mixed
                                                            umes were generally          within gob rubble, or columns of void-
                                                            placed in boreholes          filling grout.
                                                            with voids, followed by          Based on observations, testing, and
                                                            smaller volumes in           sampling, CEC concluded the deep mine
                                                            boreholes containing         stabilization mitigated subsidence that
                                                            mine gob, and minimal        could have potentially impacted the
                                                            grout volumes were           proper functioning of the overlying liner
                                                            placed in boreholes          system. Total project cost, including engi-
                                                            with intact coal.            neering, materials, drilling, grouting, and
Grout column between intact rock layers.                       Confirmation coring       coring, was slightly more than $100,000
                                                            was conducted through-       per acre. This cost represents about two
bottom of each borehole using a tremie     out the stabilization area to evaluate the    percent of the value of the airspace creat-
hose. CEC’s technician logged grout        adequacy of grouting. Eleven coreholes        ed by being permitted to expand over the
placement, verified grouting was con-      were drilled throughout the stabilization     deep mine workings.                      GE
ducted in accordance with the plan, pre-   area. Observations during coring indicat-
                                                                                         The preceding was excerpted from the fall
pared test cylinders, and conducted        ed there were no drill rod drops, decreas-
                                                                                         2007 issue of Elements, published by Civil &
slump flow cone tests. A total of 11,600   es in drill stem down pressure, excessive     Environmental Consultants, Inc.
cu yd of grout was placed, ranging from    drill stem chatter, or loss of drill water.   (www.cecinc.com).




www.govengr.com                                                              GOVERNMENT ENGINEERING      ■   MAY–JUNE 2008      ■   21

				
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