Murali M. Gadde, Ph.D., P.E.

Transcription

1 Murali M. Gadde, Ph.D., P.E.

2 Why Study Floor Stability? All major oal seams in the IL Basin have weak immediate floor materials Required at the permitting stage During mining Subsidene after mining 2

3 Central Issues How do we estimate the strength or load bearing apaity of the floor? What is the stress state or load on the floor? Does the stress exeed strength? Should we onnet stress and strength via a stability fator? Do we use different stability fators for shortterm vs. long term stability? If yes, how do we obtain those numbers? 3

4 How do we find answers? Several approahes Strength estimation an be done using omplex numerial modeling tools or simple soil mehanis based bearing apaity models or large-sale in-situ tests Load estimation an also be made using numerial models or simple equations like the popular tributary area method 4

5 Floor Strength - Multiple Fators In ontrast to the average stress estimation, determination of floor bearing apaity is lot more omplex and several variables need to be onsidered: geology of the underlay/laystone and rok units below nature of the floor revealed by laboratory properties In-situ plate bearing tests as a means to estimate the immediate floor ohesion Strength of main floor Simplified soil mehanis models Bak analysis against atual ases of floor instability and stability Advaned numerial models when the geo-mining onditions are omplex 5

6 Design Methods Vesi-Spek method is urrently popular AFSIL is based on large databases of laboratory and in-situ testing as well as ase histories 6

7 The urrent one-size-fits-all approah may not be adequate 7

8 Frequeny Frequeny 6 Seam moisture ontent Mean = 7.8% More Moisture Content, % Eastern Shelf Western Shelf Mean = 6.% More Moisture Content, % 8

9 6 seam Atterberg Limits Western Shelf Eastern Shelf

10 6 seam Atterberg Limits Western Shelf Eastern Shelf

11 Eastern and Western Shelf Atterberg Limit Data Loation Liquid Limit 6 Seam Average Values Plasti Limit Plastiity Index Eastern Shelf Western Shelf Seam Average Values Loation Liquid Limit Plasti Limit Plastiity Index Eastern Shelf Western Shelf

12 The two shelves of the Basin US 5 in IL an be used to delineate the two shelves 2

13 Spek s Equation Based on laboratory triaxial strength tests 23 tests on underlay done at 300 psi onfining pressure Data doesn t shown any signifiant differene between underlay and laystone 3

14 Original Spek s Data 4

15 Spek s Equation From the laboratory triaxial strength tests q = MC To ompute the weak bed ohesion, a orretion fator of 0.5 was reommended based on a handful of plate tests at two Zeigler mines around Deatur, IL Spek s equation does not give ohesion, stritly speaking Reommended a onstant 248 psi value for strong bed ohesion

16 In-situ Plate Test Database Largest database of plate tests 32 individual tests from 7 oal mines Colleted all the published data and personal ommuniations in addition to over 40 new tests at five mines

17 Floor Strength, psi Floor Strength, psi Floor Strength, psi Floor Strength, psi In-situ strength equations Test data Spek 2000 Test data Spek Moisture Content, % Moisture Content, % Test data SIUC-all SIUC-LL 2000 Test data SIUC-all SIUC - LL Moisture Content, % Moisture Content, % 7

18 Floor Strength, psi Floor Strength, psi Floor Strength, psi Gadde s Plate Strength equations a) q Gadde e Eastern MC q Gadde Western 0.MC 905 e b) Eastern Western Moisture Content, % Moisture Content, % ) All Moisture Content, % q Gadde 2240 e All 0.2MC 8

19 Uniaxial Compressive Strength, psi Gadde s Main Floor Unonfined Compressive Strength Equations for Claystone Uniaxial Compressive Strength, psi MC western e Moisture Content, % MC Eastern e Western shelf Moisture Content, % Eastern Shelf 9

20 Computation of Bearing Capaity The urrent approah used in the Basin is to treat the floor underneath a pillar as a twolayer material and use a soil mehanis based bearing apaity model developed by Vesi o details known Has been used for over two deades

21 Vesi s Model Weaker bed, Stronger bed 2, 2 q floor m m 2 s; 6.7 for 0 and for a square pillar

22 Verifiation of Vesi s Model Modeling showed: Vesi s model underestimates the floor bearing apaity The effet of B/H, C 2 /C ratios is not as simple as portrayed by Vesi s model Up to B/H ratio of about 0, the error is not signifiant. At higher B/H values, the underestimation an be substantial

23 If plate test data is available 2 m floor m q 6.7 q plate 2 2 floorucs main 2

24 Vesi-Spek Model 2 m ( m) floor q

25 Vesi-Gadde Model Gadde m Gadde Vesi q 2 Gadde m MC e MC e

26 The Best Judge Comparison against atual ases of unstable and stable floor ases is the best way to judge a model s validity Despite the long history, no redible bak analysis of Vesi-Spek approah was done in the past

27 Case Histories Several stable and unstable floor ases olleted from published and unpublished soures 7 long-term stable and unstable ases Only ases where the ritial input data was available from a reasonably nearby test holes were seleted

28 Vesi-Spek egative stability fators While FSF=.5 explains about 90% of unstable floors, 84% of all pillars had negative FSF. 57% of stable floors had FSF below.5.

29 Vesi-Gadde o negative stability fators FSF=.5 explains 83% of unstable floors. 86% of stable floors had FSF greater than.5.

30 Design Floor Stability Fators

31 Stability Fators Vesi-Gadde performs better than Vesi-Spek as seen by the omparison against ase histories Beause of the way Spek s equation was derived, it predits negative and physially meaningless strength at high moistures suh as those seen in several Western shelf mines Based on the ase history analysis, for long-term stability, a floor stability fator of.5 is reommended with Vesi-Gadde method For short-term stability, a stability fator around.0 is suffiient for room-and-pillar mining. For longwall appliations, given the very short term and loalized effets of abutment loading, stable gateroad floor might be seen even with SF values below.0. At this point, no analysis of longwall gateroads is available

32 Stability Fators Beause of the several ultra low and negative stability fators, it is hard to find a meaningful disriminating floor stability fator for Vesi-Spek. Experiene, however, indiates stability fators in the range of.3 to.5 might work with Vesi-Spek The maximum depth of unstable ases in the database was 250ft. Therefore, the reommended stability fators are stritly valid only up to this depth. However, a few stable ases had depths between 250ft and 300ft. Hene, the.5 stability fator may be used in this range of depth as well. At depths in exess of 300ft, perhaps suh high stability fator may not be neessary. At this point, a tentative reommendation is made to use stability fators between.3 and.5 for depths greater than 300ft.

33 Stability Fators If enough loal history is available at a mine, then the design stability fators should be based on the sitespeifi bak analysis. Loal experiene always supersedes the general basin wide databases While the.5 stability fator is adequate for almost all normal situations, for high sensitivity surfae strutures (e.g. hospitals, shopping malls, hurhes, histori buildings et), onsideration may be given to use a stability fator of 2.0 with Vesi-Gadde. Perhaps even better alternative is to use numerial modeling based methods to study the effet of all possible variables on floor stability and its potential for induing surfae subsidene Efforts are being made to ollet more ase histories to refine the reommended stability fators

34 Conlusions Evidene shows the two-shelf model is more appropriate for floor stability analysis The largest plate-test database shows the onservative nature of Spek s equation Case history data shows Vesi-Gadde method provides the best performane AFSIL methodology inorporates two-shelf division of the Basin and appropriate equations to ompute immediate and main floor strengths for use with Vesi s bearing apaity equation When the geo-mining onditions are not amenable to use simple soil mehanis based approahes, numerial modeling is reommended othing supersedes loal experiene 34