Rodc Mechanics as a Guide for Efficient Utilization of Natural Resources, Khair fed) Q 1939Bakema. Rotterdam. ISBN 90 6191 871 5

Rock classification for portal design

G.K.Rogers & C.Haycocks Department ofMining and Minerals Engineering, Virginia Polytechnic and State Universit)! Blacksburg, Va., USA

ABSTRACT: The portal, which is the near-horizontal, surface point of entry to an underground excavation, can often be an exceedingly difficult area in terms of ground control. Surface and subsurface failures at portals, as discerned during a study involving over 300 case histories, are unfortunately, common. To aid in the engineering evaluation of portals, the Geomechanics Classification System was appended with discontinuity orientation assessment values and support/excavation guidelines. Also, a design model utilizing RMR predicted rock loads in conjunction with half-dome theory is proposed for the most common type of portal failure. Other pertinent comments and recommendations relating to portal stability are included.

1. INTRODUCTION

The portal zone (Figure 1) is a frequently overlooked and often difficult area in terms of ground control. Failures of rock and support during the 'turning-under process' re&q&arly occur in high angle approach cuts and the initial subsurfikce portion of the portal. These problems are exacerbated by the wsatherad, anisotropic, and stress-relieved nature of most near-sureace rock masses.

To facilitate portal design for aithar mining or civil applications, the Geomechanics (RMR) Classification System was appended to aid in the evaluation of the external and internal stability requirements. These modifications are based on a portal database utilizing over 300 case studies (Rogers & Haycocks, 1988), comments from industry designers/oontractors, and critical field observations. Furthermore, a design model, based on half-dome theory and the RMR rock load concept, is suggested for the most common type of portal failure. The resulting integration of rock slope and subsurface engineering is necessary for a safe but efficient, long- term mortal deeinn.

2. CLASSIFICATION

Since the initial introduction of the Geomechanics (e.g. Rock Mass Rating or RMR) Classification System (Bieniawski, 1973), the concept has been expanded For the engineering evaluation of a variety of

PORTAL AXIS PERPENOlCULAR TO S U P

'ACE

VIEW ORIENTATION

PORTAL ENTRY

Figure 1. External and Internal Views of a Portal in a 45O~ock Slope (Rogers & Haycocks, 1989).

specialized areas ranging from dam foundations to rock slopes (Bieniawski, 1984). Current research involving the stability of portals in rock slopes (Rogers & Haycocks; 1988, 1989) acknowledged a need for the rapid evaluation of proposed, existing, and abandoned portals in terms of stability assessment and support requirements.

To accomplish this task, information pertinent to portals was gleaned from an extensive literature review, industry contacts, and field investigations. Support system designs, excavation methodologies, and back analyses of portal failures from this database formed the basis for upgrading existing classification system support/excavation guidelines. In particular, it is interesting to note that even though the existing support guidelines from various empirical methods (e.g. RMR, Q, Modified Terzaghi, Corps of Eng., etc.) are considered conservative, data from portal failures indicate that more conservative measures are routinely warranted, especially within approximately 1 to 5+ diameters inby and outby the portal interface (Barton, et al, 1974; Rose, 1982; U.S. Army Corps of Eng., 1978, 1980).

A discontinuity orientation adjustment table (Table 1) was established, via a back-analysis of both active and abandoned portals in conjunction with a correlation of possible failure types for certain ranges of diplstructure orientations, to aid in the evaluation of the external rock slopes, especially those which form the approach cut. Note that even high quality rock masses with an unfavorable dip assessment will require support or slope re-design to mitigate the assessment adjustments. Indeed, this is the specific purpose of the orientation adjustment, to delineate zones in the approach cut or overall surrounding slope which necessitate excavation design modifications or require additional support. The final support and slope design should be based upon standard rock slope stability analysis techniques and any unique project restraints.

Next, as to the internal stability assessment, portal (for portals i 30 ft. diameter) supportlexcavation guidelines (Table 2) are given for the various RMR classes. These guidelines are slightly more conservative than the existing RMR or Q System support recommendations. This additional degree of support is deemed necessary in order to thwart the commonly encountered portal failures which occur in the highly variable and often adverse conditions confronted in near-surface rock masses. Figure 2 illustrates typical rock boltlanchor installations at portals.

The most common type of portal failure (e.8. > 36+ X of acknowledged portal failures), that of 'Crown Face Overbreak' (Figure 2), may be modelled for design purposes with the assumption of a parabolic half-dome. This half-dome, bounded by the crown face and crown, has been observed to extend approximately 0.5D (D = diameter) inby the portal interface into the portal entry and is defined by (Figure 3):

(after Zhou, 1988)

where: d - maximum height of half-dome, W I width of half-dome (e.g. portal diameter),

O(- pillar loading angle = angle of draw.

Table 1. Asseesment of Discontinuity Orientation Upon Rock Slope Stability for Portal Approach Cuts (Rogers & Haycocks, 1989).

NOTES: This table is based on experience and case studies involving field assessment of rock slopes adjacent to portals with near vertical face-a Discontinuity cimrecteristics (e. g. wv-s,nf2ing, continuity, etc.) anU face angle8 lvwer than dip ralubs may nec~~sitatm assusmant Ipodificatiom. M r s 3 e p with a significant unreinforcd in sihr wwity(s) (e.g. abandonad mine, cam, ate.) a minimum &uction of one RHR rook class (a. - 20 pU.) is mi trrd until additioml ineorma%oa baaolact. a v a i l a r m the actual relevance of &he cavity to shpe st~biLi . Future modification8 are expected for this tantayive table.

Furthermore, research indicates that the height of the parabolic half-dome, d, at the portal interface, is approximately equal to the RMR based rock load height, ht, given by:

( 2 ) ht - [(I00 - RMR)/lOO]b (Unal, 1983) where: ht - height of rock load,

RMR - rock mass rating, b - tunnel (e.g portal) diameter.

Hence,

( 3 )

Table 2. Portal ( 30' ~ia.' ) ~xcavation/Support Guidelines for the Geomechanics Classification System.

1 e . E % ~ A D. Full lath - -~

mad..

2. Omd Rxk nmI lder vlaia. 1 ParllMar 2 - b i n . m c r m 6 Li&tcmm l+D RR: SO - 61 - Z+ D -. MI b~ltm/am~mm e I+ D crm tra 2 in m in I-. f . . ~

f a a tap Miq la. Pattern blt. imnlcute; rib.: rib. @ 3 - 6 ft. ad baach. Cranr 0.5 - l + D 1- - w/prt i . l beach. sprt . p s s d e & - s l r .

wlmcsb cn c m . la*. Pads. cmnr f r a . d r i b . .

3. Fair bd Ruo uder vlmia. 2+ R r h t e r 4 - 6 i n . m c m 6 Hsdimtobwrg RR. 60 - 41

in. on uppr slap z: lZi2 k",",'3is P 0.5 imr /w te r r i b . steel sets E 2 - L m t i d . hm - 1+ D l m n sDaced @ ft. center6 v/hrll rmods. c m , c& tam,

- -

d inxrlcuter rib.

Ruo \rider vlmin. 2+ Perimeter D-. Tm bltm/urbot. @ 1.5

w-hall r cmn, - fa-, rppsr slap 6 i u m l a ~ t e r rib. 'Mditiol~l 81- aehon n

Invs* w . p c i - 1 .

6 - 12+ in. m c m . c m facs.

slop. 2+ 6'0 f r a . RillhTd coacntairrnrt slab. miaLolead

auu F 2 ft. erntstr wlrul1 Steel a Dn-ut

5. very poor -2 nmn ~ d e r wlmih 2 ~aiimtar 8 - 2 b + k r . m m a r RR: c 20 - k D-. ~bo wu~rrbon e 2 - -. - t-. -XL=II typ

ad t a d o r 3+ D m. Pattern cram W . 6 Z3. drifts. bolt. oa tnmsaa E -/ortar rib.. 2 % 5 6 Pilot -1 I+ D 1QP - a t - Ol in. m - 2- IODOlt..

slap. 2+ i&-m rra. R s i l l h T d mamtm ian+t

Huy steel-imu m 2 ft. OSOM. wlml .ta?l m

1 I m u f f i c i e dat. 5x 1s avsr 30 ft. diemter. 2 y-iq .s - r Z b w r ra c u . 3 Dr 11 ad b l a t tnclmiw a 8 tha mat d~ usd -1 oxcavatirn slathod..

Since the majority of portal 'Crown Face Overbreak' failures occur in the 'Fair' to 'Very Poor8 rock classes (e.g. RMR of 60 - 01, the half-dome height, d, will range from 0.4 to 1 times the portal diameter. Thus, with the portal diameter and a RMR rating, a support mystem can be designed to reinforce this critical portal half-dome area, thereby averting this particular portal failure mode.

ROWN FACE FAILUR

CROWN FACE

PORTAL ENTRY

l NVERT RIGHT INNER R I B J

Figure 2. Crown Face Overbreak Failure (Rogers 6 Haycocks, 1988).

PORTAL HALF-CCME

PORTAL ENTRY

PORTAL HALF-DOME CROWN FACE

VOLUME - 0.2S)r(W/2)2d -f-

Figure 3. Portal Half-Dome Model.

3. CONCLUSIONS

The recommended analysis sequence, a combination of empirical and analytical methods, for portals in rock slopes is (Rogers 6 Haycocks, 1988) :

a) Geologic Characterization of Site, b) Overall Slope Stability, C) Slope Stability in the Immediate Portal Area, and d) Underground Excavation/Cavity Stability.

Joint orientation adjustment assessments for high angle portal slopes have been suggested for use with existing rating values for slopes in the RMR system (Rogers & Haycocks, 1989).

Support/excavation guidelines are suggested for the five rock mass classes in the RMR System. Empirical design methods should always be cross-checked with other empirical methods such as the Q System which has an internal adjustment for portals (e.g. Jn x 2).

The most common type of portal failure, that of 'Crown Face Overbreak', may be modelled for support design purposes utilizing parabolic half-dome theory in conjunction with the RMR predicted rock load height. Since the portal typically only composes a small percentage of the

total project in terms of excavation length, construction time, and cost (e.g. typically projected at 3 - 6%), a conservative approach using multiple support systems is highly recommended.

ACKNOWLEDGEMENTS

This paper is based on work supported by the Mining and Mineral Resources Research Institute. Opinions, findings, conclusions, and recommendations herein presented are those of the authors and do not necessarily reflect the views of the sponsor. Caee study information/identification is restricted.

The authors would especially like to thank the staff members of all the cooperating mining, quarrying, and tunnelling companies, as well as the various state and federal agencies who have made many valuable contributions to this ongoing research project.

REFERENCES

Barton, N., Lien, R. and Lunde, J. 1974. nEngineering Classification of Rock M a s ~ e s for the Design of Tunnel Support. RockMechanfcs. Vof. 6, NO. 4. pp. 183-236.

Bieniawski, Z.T. 1973. "Engineering Classification of Jointed Rock Masses." Transactions. South A f r i c a n n of C i v U M. Vol. 15, NO. 12. pp. 335-344.

Bieniawski, Z.T. 1984. Rock.Mechanice Dee* in Mininn andTunnelinn. A.A. Balkema. Boston. 272 pp.

Pratt, H.R., Stephenson, D.E., Zandt, G., Bouchon, M., and Hustrulid, W.A. 1979. "Earthquake Damage to Underground Facilities." Chapter 2. -. SME-AIME. New York, New York. pp. 19-51.

Rogers, G.K. and Haycocks, C. 1988. 'Portal Stability in Rock." ence on in w. West

Virginia University. August 3-5. Morgantown, WV. 8 pp.

Rogers, G.K. and Haycocks, C. 1989. "The Stability of Rock Slopes with In Situ Cavities." 1989 -ence on wand May 22-26. Lexington, Kentucky. 6 pp.

Rose, D. 1982. "Revising Terzaghi's Tunnel Rock Load Coeficients." Chapter 95. Ptoceedinns 23rd Sp- of m. SMB- AIME. New York, NY. pp. 953-960.

Unal, E. 1983. I)esiPn Guid-of -ds fpy Coal. Ph.D. Thesis, The Pennsylvania State University.

U.S. Army Corps of Engineers. 1978. Tunnels in in. EM 1110-2-2901. 1982 Revision. Office of the Chief of Engineers. Washington, D.C.

U.S. Army Corps of Engineers. 1980. Rock Relnfo.rcemant. 1110-1- 2907. Engineering and Design. 15 February. Office of the Chief of Engineers. Washington, D.C.

Zhou, Yingxin. 1988. Desinninn for Uooer Seam S-litv in Multi- -. Ph.D. Dissertation. Department of Mining and Minerals Engineering. Virginia Polytechnic and State University. Blacksburg, VA. May. 202 pp.