Should the piller fail, we know that the slab will not bridge a major collapse, we only expect that it will settle in such a way that we have a little warning so that we can get the road closed before accidents happen.

You didn't say anything about how much cover you had over the mine, or the size and placement of the pillers. So it's hard to make any meaningful comments.

In Maryland, geologists and geotechs have input to the foundation design, but the bridge designer is the lead and is the one who must be satisfied. Our designers are so very conservative that no amount of study would satisfy them that a mine would never pose a threat. We would probably end up grouting an area of the mine for at least an area bounded by 1/2:1 slopes around the foundation. The shallower the cover, the flatter this slope would be.

The approach pavement would get a reinforced concrete slab under it.

I can get you the contact at VPI if you want it. Also, the consulting firm of GAI in Pittsburgh, PA has done some good work like this for us in the past.

-David-

If an abandoned underground mine, including mine openings (horizontal, sloping or vertical) is beneath one of our roadways and no surface subsidance has occurred in the area, the ODOT Abandoned Underground Mine Inventory and Risk Assessment (AUMIRA) process evaluates the site in two manners. First, the site is evaluated with regard to the risks associated with the mine openings (shafts, slopes, drifts) and then the site is separately evaluated regarding the risks associated with the mine workings. Detailed site evaluation criteria for the ODOT AUMIRA Mine Openings Site Group and the High Risk Site Group are provided in Section 6 of our AUMIRA manual. There is a link to our manual on this website. The ODOT angle of draw default value provided in the AUMIRA manual is 35 degrees extending from the mine workings upward to the surface. The AUMIRA process assumes there is some level of risk associated with any abandoned underground mine beneath a roadway.

Some of the design considerations we study at a mine shaft location would include 1) the joints and their orientation within the limestone 2) how much soil overlies the shaft 3) how much bedrock overlies the shaft 4) what is the shear strength and compressive strength of the rock 5) where is the ground water table (high v. low).

ODOT has performed one emergency mine remediation project in 2001 which involved the stabilization of two vertical mine shafts beneath U.S.30 in Stark County, Ohio. The ODOT designation for this project work was STA-30-0.65. The project involved the stabilization of a main haulage shaft directly beneath the eastbound lanes and also the stabilization of an air shaft adjacent to the inside shoulder of the westbound lanes. The shaft depths per a 1905 mine inspector's records (and later confirmrd by our drilling) were 257 feet. Shafts were filled with random dumped backfill during original construction and later maintenance.

During our 2001 emergency mine remediation project, the two shafts were angle drilled (utilizing concentric drilling) and stabilized through the placement of flyash grout. A combined total of 2414 CY of grout were placed in the two shafts. Total project costs were approximately $1.8 million. E-mail me if you have questions.

Rick Ruegsegger Special Projects Coordinator Design Resource Section ODOT, Office of Geotechnical Engineering (614) 275 - 1395 / Rick.Ruegsegger@dot.state.oh.us FAX: (614) 887-4086

The bridge footings should be placed far enough to avoid the failure mechanism that can occur from the shaft walls. The trick is to identify the failure mechanism that is likely from the walls: e.g. individual block ravelling or lamina progressively sliding out, uniaxial compressive failure of rock columns, general rock mass movement along a stepped shear failure plane. The appropriate bearing capacity canculations should be included in the shear failure surface calculations and on the peripheral block force conditions.

Proximal pillar failures can sufficently destress the rock mass so that shat walls can be affected

Ideally a numerical modelling code which considers the footings a represenative blocky rock mass should be performed to get a good sense of rock mass displacements; this can include joint infillings, groundwater effects, rock anchors, etc. and the effects of nearby openings.

We normally carry out these analyses, with numerical modelling and limit equilibrium means.

With a high RQD with no weak zones for the site mentioned, it would appear that joint failure control would be the anticipated failure mode.

The classic historical expectation that the rock mass will cave into the shaft and leave behind a caving front which reach back significantly away from the shaft, along a caving front is not normally in effect, especially in this case.

** Marc Betournay's comments on failure mechanisms are also to be taken seriously. Shafts represent a true 3-D problem, and numerical methods are the easiest approach for determining relative failure conditions and safe proximities of footing loads. In the U.S., ITASCA provides 3-D jointed ground block analyses with UDEC. Agapito and Assoc., Colorado, have 3- D capabilities. And several of the universities have 3-D jointed rock models with Finite-Element codes (e.g. PLAXIS). I am not aware of empirical design methods specifically for blocky ground.

** "Angle of draw" commonly refers to bulk overburden failure behavior, and is generally associated with full-extraction mining or mine collapse over a large area. There are different types of draw angles depending on the degree of areal collapse vs. cover depth. For the problem at hand, there may be an angle of draw associated with overburden soils and residual soil masses, equivalent to friction angles times a safety factor. For the rock in the shaft, I agree with Marc, there is probably not an angle of draw per se - implying some type of cave front - but a defined failure mechanism (joint sliding, bedding translation, sidewall failures, etc.).

** Failure at the mine level is another issue altogether, and has little or nothing to do with surface construction. Earthwork or structure loads are inconsequential at 200 ft of cover. If mine subsidence is a concern, and very well could be if the mine is cycled with water (drained/filled), then a study on pillar stability needs to be undertaken. For 8-12 ft mining heights (is that correct?) with 20-ft wide pillars (w/h = 1.7-2.5) in strong ground with very little cover, pillar failure is unlikely - even with the wide spans given. For worst case (60 ft openings, 12 ft height, 200 ft cover, 20 ft pillar, assumed UCS of 5,000 psi - probably very conservative) the FOS is around 3.0 for pillar stability.

** Water is an issue. All calcs should consider maximum water heights. If water levels are approaching the collar of the shaft, sealing is not worth the consideration. Ground improvement between the shaft and footing is certainly an approach, and can be based on the findings of the recommended rock mass investigations. If water is not an issue, shaft sealing is possible to forestall near-surface failures. The shafts in the area are probably quite small in diameter, so plug construction is not problematic.