103

Benson, N.D., Haberfield C.M., Assessment of mass modulus of a weathered argillaceous rock. ISRM 2003–Technology roadmap for rock mechanics, South African Institute of Mining and Metallurgy, 2003.

Assessment of mass modulus of a weathered argillaceous rock

Neil D. Benson, Chris M. Haberfield

Golder Associates Pty Ltd, Melbourne, Australia

The assessment of overall modulus (or stiffness) of a fractured weak to moderately strong rock mass is a key parameter in the design of civil engineering works in these materials. It also provides the highest level of uncertainty due to scale effects, anisotropy and the influence of joint frequency and joint stiffness. Using the example of the Melbourne Mudstone, this paper compares approaches using empirical rules based on intact rock strength, laboratory testing, insitu field testing, semi-quantitative assessment techniques based on rock mass classification and back analyses. L'évaluation du module (ou rigidité) global d’une masse rocheuse faible et fissurée a modérément forte est un paramètre principal dans la conception des travaux du génie civil dans ces types de matériaux. Elle fournit également le niveau le plus élevé d'incertitude due aux effets d’échelle, à l'anisotropie et à l'influence de la fréquence et de la rigidité des joints rocheux. En utilisant l'exemple du schiste de Melbourne, cet article compare des approches empiriques basées sur la force intacte de La roche, les essais expérimentaux au laboratoire, les essais sur site, les techniques semi-quantitatives d'évaluation basées sur la classification de la masse rocheuse et les analyses en retour.

Die Schaetzung des allgemeinen Steifigkeit-Moduls des gebrochenen schwachen bis zu maessig starken Felsmassen ist eine Haupt- Annahme in dem Entwurf von Tiefbau-Projekten in diesen Materialien. Es verursacht den hoechsten Grad der Ungewissheit in Folge der Groessenordnung, Spaltorientierung und dem Einfluss der Haeufigkeit der Risse und Bruch-Steife. Unter Verwendung des Beispiels des Melbourner Tonsteine, vergleicht diese Darlegung die Anwendung der empirischen Regeln, die sich auf die Staerke des ungestoerten Fels Basieren mit Versuchen in der Material Pruefungsanstalt, Untersuchungen auf der Baustelle und halb-quantitative Schaetzungsverfahren, die sich auf die Felsmassen Klassifikation und fruehere Analysen stuetzen.

Introduction

The assessment of overall modulus (or stiffness) of a fractured weak to moderately strong rock mass is a key parameter in the design of civil engineering works in these materials. It also provides the highest level of uncertainty due to scale effects, anisotropy and the influence of joint frequency and joint stiffness. Rock mass properties are difficult to measure directly and their estimation has historically relied on subjective assessments of rock mass quality and measurements of intact values. Using the example of the Melbourne Mudstone, this paper compares approaches using empirical rules based on intact rock strength, laboratory testing, insitu field testing, semi-quantitative assessment techniques based on rock mass classification and back analyses. ‘Melbourne Mudstone’ is a generic name used to describe the open folded and faulted sedimentary deposits comprising predominantly siltstones with some fine grained sandstones and rare mudstones of Silurian and early Devonian age that underlie the Melbourne area. The weathering state, varying from extremely weathered to fresh, was first classified for engineering purposes by Bamford (1969) and Nielson (1970). The saturated water content of the siltstone provides a useful quantitative indicator of the engineering properties of the intact rock varying from around 20% (void ratio ≈ 0.54)

for extremely to highly weathered siltstone to less than 1% (void ratio ≈ 0.027) for fresh siltstone (Johnston and Chiu (1984)). Four major discontinuity sets are typically observed. Joints in the less weathered material tend to be clean and tight, planar to rough and undulating, although joints containing clay seams up to 100 mm thick are not uncommon. There are a number of significant faults in the formation where the siltstone is highly sheared and fractured. Minor faults, with sheared and slickensided zones, parasitic concertina type folding and contortion of bedding are common associated with the cores of the major anticlinal and synclinal fold structures. Late Devonian age intrusives are also common and associated with fault and fracture zones and along fold axes. The data presented in this paper has been collated from various sources and projects over a considerable period.

Empirical Methods Empirical relationships to assess rock mass modulus based on intact rock strength (uniaxial compressive strength, qu) have been proposed by a number of authors (eg Deere, 1968; Hobbs;1974) . Chiu (1981) presented the correlations for Melbourne Mudstone between intact rock strength (qu) and intact rock

104 ISRM 2003 – TECHNOLOGY ROADMAP FOR ROCK MECHANICS

0.01

0.1

1

10

100

1000

0.0 5.0 10.0 15.0 20.0 2 5.0

W ater Content (%)

Se

ca

nt

Yo

un

gs

Mo

du

lus

(G

Pa

)

10

100

1000

0.1 1 10 100

Uniaxial Compressive Strength, qu (MPa)

E/q

u

0

1

10

100

1000

0 5 10 15 20

W ater Content (%)

qu (

MP

a)

modulus (Ei) (measured at 50% of peak load) with

saturated water content for Melbourne Mudstone. Reasonable correlations between qu and water content

(Figure 1) and secant Young’s modulus and water content (Figure 2) exist but correlation between intact rock strength and Ei/qu is poor (Figure 3).

Figure 1: Correlation between intact uniaxial compressive strength and saturated water content. Figure 2: Correlation between intact secant Young’s modulus and saturated water content. Figure 3: Variation of E/qu with qu for intact rock. Correlations to extrapolate from moduli measured in the laboratory on intact samples to insitu values have been proposed using RQD and a modulus reduction ratio (Bieniawksi 1978). This approach has, however, generally

been found to be unsatisfactory in practice. Given that the properties and performances of rock fractures under different stress regimes influence the relationship between modulus of intact rock and the rock mass, the limitations of correlations with RQD are acutely obvious from the narrow range over which RQD operates (see Figure 4). Figure 4: Limited range covered by RQD (after Palmstrom, 2000) Modulus assessment using rock mass classification systems have had some success. For example, Rock Mass Quality (Q) system (Barton et al, 1974), Rock Mass Rating (RMR) (Bieniawski, 1978; Serafim and Peraira, 1983). However, these systems have limitations for the Melbourne Mudstone which is a typically weak rock mass with an intact rock strength generally in the range of 1 MPa to 25 MPa (but can be as high as 80 MPa in fresh rock) and typical Em values in the range of 0.1 GPa to 4 GPa (but

reach 10 GPa in massive slightly weathered to fresh siltstone). The Q and RMR correlations are typically more robust for rock environments where Em is greater than

about 10 GPa. Hoek’s Geological Strength Index (GSI) (Hoek et al., 1995; Hoek and Brown, 1997; Marinos and Hoek, 2000) which has a higher ‘geological’ content in assessment of lithology, structure and condition of discontinuities has been found by the authors to have more practical applications in the lower strength rock mass of the Melbourne Mudstone. Hoek et al (1995) proposed a variation on Serafim and Peraira’s 1983 amendment to calculating rock mass modulus from RMR.

−

= 40

10

10.100

GSI

cmE

σ

Sonmez and Ulusay (1999) suggested integrating volumetric joint count Jv and Surface Condition Ratio

(SCR) (from Bieniawski’s RMR Geomechanics classification) into the assessment of GSI. This has been found to provide reasonable results when assessing the mass modulus of Melbourne Mudstone. It is considered to be effective because the local correlations between weathering grade, saturated moisture content and unconfined compressive strength are well developed and the weathering grade scheme is based on, a general, homogeneous rock material degradation process. The depth of highly and moderately weathered Melbourne Mudstone can extend to 30 m before slightly weathered or better rock is encountered. Typical ranges of GSI values for the various weathering grades and water contents of

1

11 1010 10 1000.1 0.1 1000 10 000 100 000

10100

0 10030 60 90

VOLUMETRICJOINT COUNT

BLOCKVOLUME

RQD

1

50 20 5 2 0.5 0.2 joints/m3

dm3 m3cm3

105

0.01

0.1

1

10

100

0 2 4 6 8 10 12 14 16 18 20

W ater Content, w (%)

Yo

un

g's

Mo

du

lus

, E

pl (

GP

a)

50qu

200qu

400 qu

100qu

0.01

0.1

1

10

100

0 2 4 6 8 10 12 14 1 6 18 20

W ater Content, w (%)

Initi

al Y

ou

ng

's M

od

ulu

s,

Ep

m (

GP

a)

0 - 4 joints/metre

5 - 10 joints/metre

11 - 20 jo ints/metre

> 20 joints/metre

100qu

400qu

50qu

200qu

Melbourne Mudstone are listed in Table 1 along with the range of modular ratios Em/qu assessed using Equation 1.

Table 1: Typical Parameters – Melbourne Mudstone Weathering Grade Saturated

Moisture Content GSI

range Em/qu range

Highly 8% - 12% 25-38 100 - 500 Moderately 4%-8% 30-45 100 - 500

Slightly 2%-4% 35-55 100 - 400 Slightly to Fresh 1%-3% 40-60 50 - 400

Fault/shear* zones 15-25 * Values are subject to weathering grade and intact rock strength of host rock

The values compare well with GSI estimates for siltstone and siltstone/sandstone combination proposed by Marinos and Hoek 2000.

Plate Load Tests Plate load tests (up to 750 mm diameter) in the Melbourne Mudstone are relatively limited and mostly documented in a number of references from the 1970’s and early 1980’s (Moore, 1977; McKenzie, 1977; Baxter and Bennet, 1981). A summary of the Young’s Modulus assessed from plate load tests after several load/unload cycles is presented in Figure 5. As indicated in Figure 5, typical ratios of modulus to uniaxial strength range from 100 qu to 400 qu.

Figure 5: Correlation between Young’s modulus from plate load tests and saturated water content.

Insitu Pressuremeter Tests Considerable practical and research effort was undertaken in the 1980’s on the use of the pressuremeter to establish engineering properties of the Melbourne Mudstone. Johnston (1992) concluded that the response of the pressuremeter in these materials reflected drained conditions and that the Young’s Modulus for Melbourne Mudstone could be reasonably derived from pressuremeter tests. Figure 6 shows values of Young’s modulus derived from the initial loading response of the pressuremeter plotted against water content. The results are also identified with respect to joints per metre from the core recovered from the pressuremeter test section. Of note is the significant scatter which does not appear to show any distinct dependence on joint spacing and the wider scatter at low moisture contents (slightly weathered to fresh)

compared to higher moisture contents. This is considered to be a function of the greater stiffness contrast between intact weathered rock and the joints (walls/infill material). Figure 6: Correlation between initial Young’s modulus from pressuremeter tests and saturated water content. The moduli assessed from pressuremeter tests in the Melbourne Mudstone would appear to be influenced by the location of the displacement gauges in the probe relative to discontinuities/seams in the rock and to the characteristics of these (eg. open or tight, clay filled, spacing, orientation and so on) (Haberfield 1987). This is clearly illustrated in Figure 6. Meaningful assessment of Young’s moduli from pressuremeter tests therefore need to consider not only the output from the test but also the characteristics of the rock at the test location. Initial Young’s moduli assessed from the pressuremeter test have been used successfully in many large foundation projects around Melbourne involving bored piles and shallow foundations. As shown in Figure 7, the Young’s modulus assessed from unload/reload cycles is significantly greater than the initial modulus (note maximum measurable value of 12 GPa). Local experience indicates that unload/reload moduli appear to be less dependent on joint characteristics and have been found to give reasonable estimates of modulus (as assessed from back calculation of field response) for unloading situations such as deformations due to excavation and tunnelling.

Pile Load Tests Pile load tests can also be used to assess rock modulus (Williams,1980; Williams and Ervin,1980). Values of Young’s modulus obtained from static load tests are plotted against water content in Figure 8. The closed symbols in Figure 8 have been assessed from initial loading, while the open symbols from an unload/reload cycle. The reload stiffness for any particular test was generally stiffer than on initial loading, however both values appear to fall within the overall scatter of results as also observed in the plate loading and pressuremeter tests. Other than one very low value obtained from a pile in extremely fractured fresh rock, most of the data suggests that a relationship of E = 200 qu to 300 qu would appear reasonable.

106 ISRM 2003 – TECHNOLOGY ROADMAP FOR ROCK MECHANICS

0.01

0.1

1

10

100

0 2 4 6 8 10 12 14 16 18 20

W ater Content, w (%)

Yo

un

g's

Mo

du

lus

, E

p (

GP

a)

400 qu

2 00 qu

100 qu

50 qu

Figure 7: Comparison on initial and unload-reload moduli from pressuremeter tests Figure 8: Correlation between Young’s modulus from pile load tests and saturated water content

Summary

Test data from a number of types of laboratory and field tests show reasonable scatter which is probably indicative of joint characteristics and the scale of the tests undertaken. However, there is clear indication that a reasonable lower bound modulus for all but highly fractured/faulted Melbourne Mudstone is 100 qu with most data lying in the range of 100 qu to 400 qu. A similar range of modular ratios is obtained using GSI. A value of approximately 50 qu would appear reasonable for highly fractured and faulted Melbourne Mudstone.

References

BAMFORD, W.E. (1969). Melbourne Underground Railway investigations, in Engineering Geology Extension Course Notes, Melbourne Group, Australian National Society of Soil Mechanics and Foundation Engineering, Inst. Engrs, Aust., pp. 7-7 to 7-11. BARTON N., LEIN R. and LUNDE J. (1974). Engineering Classification of Rock Masses for the design of tunnel support, Rock Mech. 6 (4) pp 189-239 BAXTER D.A. and BENNET A.G. (1981). Aspects of Design and Insitu Testing for the MURL Rock Tunnels. 4th Australian Tunnelling Conference, AusIMM pp 41-60

BIENIAWSKI Z.T. (1978). Determining Rock Mass Deformability – experience from histories, Int. J. Rock Mech. Min. Sci. Vol. 15, pp 237-247 CHIU H.K. (1981). Geotechnical Properties and Numerical analyses for socketed pile design in weak rock. PhD Thesis, Department of Civil Engineering, Monash University, Australia, 1981 DEERE D.U. (1968). Geological Considerations. Rock Mechanics in engineering practice. Edds Staff K.G. and Ziekiewicz O.C., Toby Wiley & Sons, p 1-20 HABERFIELD, C.M. (1987). The performance of the pressuremeter and socketed piles in weak rock. PhD Dissertation, Dept. Civil Eng., Monash University, Melbourne. HOBBS N.B. (1974). Settlement of foundations on rock. General Report. Proc. British Geotech Society Conf on Settlement of Structures, Cambridge, pp 98-529 HOEK, E., KAISER P.K. and BAWDEN W.F. (1995). Support of Underground Excavations in Hard Rock, Balkema HOEK E. and BROWN E.T. (1997). Practical Estimates of Rock Mass Strength, Int. J. Rock Mech. Min Sci 34, pp 12165-1186 JOHNSTON I.W. (1992). Silurian and lower Devonian engineering properties. Engineering Geology of Melbourne, Ed. Peck, W.A., Neilson, J.L., Olds, R.J. and Seddon, K.D., Balkema, A.A., pp. 95-108. MARINOS P. and HOEK E. (2000). GSI: A geologically friendly tool for rock mass strength estimation. GeoEng2000, Melbourne, Australia MCKENZIE, I.L. (1977). Plate loading test results. Australian Geomechanics Society, Victoria Group, Workshop on the engineering properties of Melbourne mudstone, 20th April, 8pp. MOORE, P.J. (1977). Engineering properties of Melbourne mudstone – Introduction. Australian Geomechanics Society, Victoria Group, Workshop on the engineering properties of Melbourne mudstone, 20th April, 6 pp. NEILSON, J.L. (1970). Notes on weathering of the Silurian rocks of the Melbourne district. J. Inst. Engrs, Aust. Pp. 9-12. PALMSTROM A. (2000). Web sit www.rockmass.net SERAFIM J.L. and PEREIRA J.P. (1983). Considerations on the Geomechanical Classification of Bieniawski. Int. Symp. Engineering Geology and Underground Construction. Lisbon. Theme II. Vol. 1, pp 11.33 – 11.42 SONMEZ H. and ULUSAY R. (1999). Modifications to the Geological Strength Index (GSI) and their Applicability to Stability of Slopes, Int. J. Rock Mechanic. Min Sci 3, pp 743-760 WILLIAMS A.F. (1980). The design and performance of piles socketed into weak rock. PhD Dissertation, Dept. Civil Eng, Monash University, Melbourne. WILLIAMS A.F. and ERVIN M.C. (1980). The design and performance of cast in-situ piles in extensively jointed Silurian modstone. Proc. 3rd Australia-New Zealand Conference on Geomechanics, Wellington: Vol. 1, pp. 115-121.

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.00 2.00 4.00 6.00 8.00 10.00 12.00

Initial Youngs Modulus, Ei (GPa)

Rel

oad

You

ng's

Mo

dulu

s, E

r (G

Pa)