A STDY ON QUABTATIVE THEORETICAL ZONING OF WEATHERING ZONES IN ROCK MASS

A STUDY ON QUANTATIVE THEORETICAL ZONING OF WEATHERING ZONES IN ROCK MASS*

Dexin Nie1 and Long Chen1

ABSTRACT: Weathering agency can not only result in the variation of physical components and chemical components of rock, joints and integrality of rock mass change along with weathering process. Therefore better corresponding relation exists between amounts of joints, block size in rock mass and degree of weathering. In this paper, zones of weathering are characterized by joint spacing, integrality index of rock mass and rock quality designation, integrated closely with rock mass structure and their engineering properties. New research is carried out in quantitative classification of weathering zones and good results are also achieved in the paper.

RÉSUMÉ: Les mass de roche qui ont subi une érosion doivent avoir no seulement un changement en composition chimique, mais aussi en intégralité. Il existe donc une bonne relation entre la quantité de joints, la grandeur de bloc et le degré d'érosion.. Dans cet article nous avons étudié la zonation d'érosion de mass de roche sur la base de la distance des joints, de 'index de l'intégralité de mass de roche et du degré d'érosion. Cette étude a établi une relation entre la zonation, la structure et la propriété de génie civil des mass de roche. Nous avons proposé enfin une classification quantitative sur la zonation d'érosion des mass de roche.

INTRODUCTION

The research of rock weathering is an old theme. The term weathering is popular in the geology domain and other subjects for a long time. Whereas many artificial factors and uncertainty exist, the method of naked–eye observation and the blow sound of geological hammer is still used so far. This is one convenient, economical and fast means. To recognize the partition of weathering zone by historical and developmental viewpoint not only follows the action of the previous partition in evaluation of engineering geology but also objectively points out its question. In practical use, adding some new quantificational indexes which denote weathering degree, on the one hand can foretype the variation degree, on the other hand can also reflect influencing degree of weathering to rock mass structure and engineering characteristic. The partition of weathering is not only reflect weathering and integrality and structure of rock mass, but also indicate integrated partition of rock mass engineering characteristic. It is a direction that is paid attention to in geological study.

According to above-mentioned, at present research of rock weathering, technologists must attach importance both to the change of color, structure of rock and the alteration degree of mineral, the change of knocking-sound, the developmental degree of fissure and integrality of rock mass as well as structure change. In addition, the correlation between weathering zones and mechanical character of rock mass must be paid more attention to. Some new contents should be endowed to the weathering zoning by summarizing the change of physical-chemistry and structure and engineering character of rock mass synthetically. This paper deals with some new advance in this research.

* Supported by National Nature Science Fund of China (40072090)

1.

Institute of Engineering Geology, Chengdu University of Technology, Chengdu, P.R.China

Engineering Geology for Developing Countries - Proceedings of 9th Congress of the International Association for Engineering Geology and the Environment. Durban, South Africa, 16 - 20 September 2002 - J. L. van Rooy and C. A. Jermy, editors

Géologie de l'Ingénieur dans les Pays en voie de Développement - Comptes-rendus du 9ème Congrès de L'Association Internationale de Géologie de l'Ingénieur et de l'Environnement. Durban, Afrique du Sud, 16 - 20 septembre 2002 - J. L. van Rooy and C. A. Jermy, éditeurs

ISBN No. 0-620-28559-1 1258

RESEARCH SITUATION OF ZONING OF WEATHERING

In the past, people chiefly paid attention to the qualitative description of rock weathering, and use mostly the qualitative indexes in judging weathering degree; Table 1 is the scheme of hydroelectricity department in 60 to 80’s. Table 2 is the National Standard that was issued in 80’s. According to these two tables, there are few quantitative indexes, but mostly are qualitative descriptions. Consequently it is very difficult to apply them in rock engineering because of subjectivity and uncertainty.

Table 1. Classification scheme of weathering degree in rock mass

Leading features

Classification

Colour and luster

Mineral

characteristics of

characteristic

joints destroyed except quartz Structure character have been of virgin rock on weathered appearance

Mechanical property immersed in water. Not undisturbed sample collected

blow sound

Digging method

Excessively

Entirely changed

Silent

Shovel Pickaxe

Colour of Lots of Weathering Most minerals most joints. Rock mass except quartz is

Mechanic Hand-

changed, like rubble. weathered,

property picker

Intensely only the Weathering intensely fresh primary Silent

fall notably, Rock-

middle along joint planes, minerals only in

inhomogeneous auger

part were especially in the the middle of fresh intersection of joints rock block Colour of most

Saturation

surface Most primary

Undamaged usually compressive

and joint minerals are

Weathering joints strength fall

Moderately plane weathered Metallic Blasting

Weathering intensely notably, are 2/3

changed, along joint

along joint planes to 1/3 of fresh

fracture planes

rock

part were fresh Color of

Structure Minerals re

joint Mechanic

unchanged ,

unchanged

Slightly plane property Metallic Blasting

weathering joints Argillaceous

changed decrease

tiny film or crust

slightly

Table 2. Classification of weathering degree in rock mass (After National Criterion GBJ 7-89)

Weathering degree Characteristics

2. Rock mass is blocky, cut by joints, block size is 20 to 50 cm. Joints filled with little

Moderately

weathered material. Hammer blow sound is metallic. Rock mass be not easy to smash. 3. It’s difficult to excavate by pickaxe ,but can be dig by core drill

2. Rock mass is macadam, which cut by joints, and the block size is 2 to 20 cm and can be

Intensely

break by hand.

3. It can be excavated by pickaxe, digging by bit brace is difficult

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Since 70’s, many test indexes are introduced to the weathering evaluation. For instance, Chengdu Hydroelectricity Investigation and Design Institution of Station Power Corporation puts forward to the diversity of compression strength and water absorption between weathered rock and fresh rock.

［］Mugongyibang, Kehe Wei1use point load strength index of rock to describe weathering degree quantitatively.

But that index of rock can not completely reflect the character of rock mass, so it is difficult to operate in the evaluation of engineering rock mass. To solve these problems, many scholars in different departments

［］［］［］［］［］［］investigate weathering zoning making use of some characteristic indexes of rock mass234567. For

example, Patton, Deere (1976) used RQD and rock penetrability in weathering zoning in the magmatic rock and metamorphic rock (Table 3); W.R.Dearman (1978) used RQD and seismic wave velocity to research weathering classification of granite (Table 4); The Chinese GB50287-99 standard uses wave velocity ratio to

［］research rock mass weathering17. All these quantitative indexes, such as RQD, longitudinal wave velocity

of rock mass and wave velocity ration etc, further the development of weathering classification research. In a way, RQD reflects the integrity of rock mass; longitudinal wave velocity of rock mass reflects density of medium in rock mass and compactibility of rock mass.Physical and chemical component change due to weathering, so does integrity of rock mass. From intensely weathered zone to moderately weathered zone to slightly weathered zone to fresh rock, the quantity of fissure gradually reducesand finally stabilizes at a certain value. This displays the character of crack propagation and increase and new fissure formation under the action of weathering agency, at the same time it embodies the variety of integrity. In engineering geology domain, quantitative research of weathering zoning must attach importance to the change of physical and chemical component as well as the change of integrity and structure of rock mass. Structure of rock mass controls

［］engineering geology character of rock mass and the stability of rock mass17, therefore those indexes which

have been applied in the research of rock mass structure extensively can be introduced into the classification of weathering zones. Undoubtedly, this will bring new insight to the research of weathering.

Table 3. Characteristics of magmatic rock and metamorphic rock in different weathering zones (After Patton,

Deere, 1976)

Core Infiltration Name Zone Characteristic RQD% Strength recovery -rate ratio

Soil on top, root, eluvial low~ 0 0 Moderate~low organism, much void in ⅠA—A moderate leached layer

Structure without residual Ⅰ Residual Low，high deposit, ferreous and 0 0 Low when ⅠB—B clay aluminiferous and cementation siliconous, cementation

Like silty sand to sand low~ 0 0~10 Moderate ⅠC—C Structure of virgin rock moderate

low~ like rock and soil, ⅡA 0~50 10~90 High moderate Transition spheroidal weathering, rock Ⅱ crushed Weathered

ⅡB zone Joint planes rusted, feldspar Moderate~

50~75 >0 Moderate~high Weathered and mica unchanged high rock

Feldspar and mica were Ⅲlow~ high~ unchanged, joint planes >75 100 Unweathered moderate very high were not rusted rock

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Table 4. Engineering property of granite in different weathering zones

(After W.R.Dearman, 1978)

Fresh rockAbsolutelyResidual clay Engineering SlightlyⅡ ModeratelyⅢ IntenselyⅣ characteristic Ⅰ Ⅴ Ⅵ

Fit to build Fit to build Fit to build Suitability for comcrete Fit to build not fit to build comcrete dam comcrete dam small earth engineering dam and earth dam dam and earth dam and earth dam dam earth dam

Lie on the Digging Demolishing Demolishing effect and burst effect Excavating Excavating method digging digging amount of joints Rock drill Burst Rock drill

1:1

1:0.25 1:0.25~1:1 Surface 1:1~1:1.5 Steepness of Notice Notice application Surface 1:1.5~1:2 1:1.5~1:2 slope obsequent obsequent Notice application structure structure obsequent

structure

Heavy steel Support except Steel Heavy steel, Support and lining Tunnel fissures zone Light steel structure, lining. except Dangerous support and obsequent structure part Dangerous to fissures zone to wet structure reinforce wet excavation excavation

Rock quality 0 or 0 or designation 90% 75~90% 50~75% 0~50% inapplicable inapplicable RQD

Core recovery 15% 15% 90% 90% 90~75% 15~75% (N.X) (sand-like ) (sand-like) Diamond

drilling 2~4m/hr 2~4m/hr 8~10m/hr 8~10m/hr 10~13m/hr 10~13m/hr velocity(N.X)

Moderate-StronPermeability Low Low-Moderate Strong Moderate Low g

Seismic wave 3050~ 2500~ 1500~ 1000~ 500~ 500~ longitudinal 5500 4000 3000 2000 1000 1000 velocity (m/s)

Electrical

resistivity 340 240~500 180~240 180~240 180 180 (ohm)

SELECTION OF THE REPRESENTATIONAL INDEXES OF SYNTHETICAL CLASSIFICATION

OF WEATHERING ZONES IN ROCK MASS AND THEIR CORRESPONDENCE

Selection of the representational indexes of synthetical classification of weathering zones in rock mass There are many factors that can indicate the character of weathering agency, they are : ① the variation of medium in rock mass, for instance: color of rock and mineral, mineral composition, density of rock, void rate, structure of rock, compressive strength of rock and tensile strength (or point load index )etc; ② the variation structural plane(or joint): including the variation of the quantity of joint, joint sets, length, spacing, property of the structural plane and the variation of aperture and type of filling material of structural plane; ③ the variation of characteristic of the rock mass because of weathering: the variation of rock mass integrality, rock mass structure and rock mass quality etc.

The above-mentioned qualitative or quantitative factors can denote the degree of weathering on a certain extent, but it is unrealistic to apply all numerous factors to classify weathering degree in rock mass synthetically and scientifically. On the basis of the availability of engineering rock mass, emphasis should be put on rock mass structure, which is the basic condition controlling engineering geological property and

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stability of rock mass in the classification of weathering zones in rock mass. Therefore, besides of some customary qualitative terms, other indexes that can indicate rock mass structure and apt to be obtained can be chosen in weathering zoning, such as rock quality designation (RQD), spacing of structural plane, coefficient of rock-mass sound-degree (or wave velocity and wave velocity ratio of rock mass), volumetric joint count (or linear in discretional direction). Among these indexes, joint spacing, volumetric joint count and line frequency of fissures can indicate rock block size variation, and joint spacing which can be measured easily in adits and outcrop is the leading index to classify rock mass structure. But joint spacing isn’t easy to achieve in drills and RQD is easy attained. RQD is an index indicating scale variation of structural body of rock mass in fact, but it can’t indicate the size of blocks that the size is more than 10cm. Coefficient of rock mass sound-degree or wave velocity, wave velocity ratio can indicate the variation of rock density and rock mass sound-degree, aperture of rock structural and character of filling substance. All these variation actually represented concentratedly the variation of rock structure and rock quality. The authors chose joint spacing (D) and rock quality designation (RQD) and coefficient of rock mass sound-degree (Kv) and volumetric joint count (Jv) as the quantitative indexes of classification of weathering zones to study the synthetical zoning of weathering in rock mass. Because joint spacing and coefficient of rock mass sound-degree are representative indexes to quantitate the rock mass structure. Using these indexes to zone the weathering we can connect zoning of the weathering with classification of structure of rock mass.

Study on relation among the leading representative indexes

The relation between rock quality designation and coefficient of rock mass sound-degree has been studied much by precursors. Dearman pointed out the corresponding relation between different weathering zones and RQD, A.H.Merrit (1968) proposed the relation between RQD and coefficient of rock mass sound-degree (Kv). We can get the corresponding relation in Table 5 through two scholar’s results. The author attained the well-relation formula through farther study on the relation of RQD and Kv:

Kv=-0.017183+0.00861(RQD)-0.00007822(RQD)2+0.000000894(RQD)3 (1) r1=0.964, r2=0.991, r3=0.996

In the formula, Kv represents coefficient of rock mass sound-degree; RQD represents rock quality designation ,and r1 and r2 and r3—related coefficients.

Table 5. Corresponding relations between rock quality designation, integrality coefficient and degree of

weathering in rock mass

Weathering Coefficient of rock mass RQD (%) Rock quality classification sound-degree Fresh rock 90~100 0.8~1.0 Very good

Slightly 75~90 0.6~0.8 Good 25~50 0.2~0.4 Relatively bad Intensely①

0~25 0~0.2 Bad Intensely②

The corresponding relation between joint spacing (D) and coefficient of rock mass sound-degree: cutting density of joint corresponds with degree of rock integrity in existing circumstance. The degree of rock integrity decline along with the density of joint’s increases (namely joint spacing’s reduction). The density of joint cutting has two representational indexes: one is joint spacing, another is volumetric joint count. Joint spacing has been used more than volumetric joint count. In practical applications, true joint spacing of the most developed group is used by many classifications of rock mass structure and the correlative standard. The author studied further on the relation of joint spacing and coefficient of rock mass and achieved the well-relational formula:

Kv=-0.1331+0.02106D-0.0001758D2

+0.0000005331D3

r1=0.89, r2=0.995, r3=1 (2)

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