ELSEVIER GEO-ENGINEERING BOOK SERIES VOLUME 5 Part 8 doc

390 Tunnelling in weak rocks

poor management condition affected tunnelling rate more adversely than poor rock mass

condition.

The third factor pertains to the breakdowns or hold ups during various operations in

tunnelling cycle. These hold ups cause delays which are random in nature. Based on the

data collected from many projects, Chauhan (1982) proposed a classification for realistic

assessment of rate of tunnelling presented in the following sections.

27.2 CLASSIFICATION OF GROUND/JOB CONDITIONS FOR RATE OF

TUNNELLING

The rate of tunnelling is seriously affected by the ground conditions. The factors, under

the ground condition, affecting the rate of tunnelling are (Terzaghi, 1946; Bieniawski,

1973, 1974; Barton et al., 1974).

(i) Geology, such as, type of rock, RQD, joint system, dip and strike of strata,

presence of major fault or thrust zones and their frequencies and type and rock

mass properties,

(ii) Method of excavation including blast pattern and drilling arrangement,

(iii) Type of support system and its capacity,

(iv) Inflow of water,

(v) Presence of inflammable gases,

(vi) Size and shape of tunnel,

(vii) Construction adits whether horizontal or inclined, their grade size and length and

(viii) High temperature in very deep tunnels (H > 1000 m).

On the basis of the above factors affecting the rate of tunnelling, the ground conditions

are classified into three categories – good, fair and poor (Table 27.1). It means that for

the good ground conditions the rate of tunnelling will be higher and for the poor ground

conditions the rate of tunnelling will be lower. The job/ground conditions in Table 27.1

are presented in order of their weightage to the rate of tunnelling.

27.3 CLASSIFICATION OF MANAGEMENT CONDITIONS FOR RATE OF

TUNNELLING

The rate of tunnelling may vary in the same ground condition depending upon management

quality. The factors affecting management conditions are:

(i) Overall job planning, including selection of equipment and decision-making

process,

(ii) Training of personnel,

(iii) Equipment availability including parts and preventive maintenance,

Rate of tunnelling 391

Table 27.1 Classification of ground/job condition (Chauhan, 1982).

Job conditions

S. No. Parameter Good Fair Poor

1. Geologic structure Hard, intact,

massive stratified

or schistose,

moderately

jointed, blocky

and seamy

Very blocky and

seamy squeezing

at moderate

depth

Completely crushed,

swelling and

squeezing at great

depth

2.(a) Point load strength

index

>2 MPa 1–2 MPa Index cannot be

determined but is

usually less than

1 MPa

(b) Uniaxial

compressive

strength

>44 MPa 22–44 MPa <22 MPa

3. Contact zones Fair to good or poor

to good rocks

Good to fair or poor

to fair rocks

Good to poor or fair to

poor rocks

4. Rock quality

designation

(RQD)

60–100 % 25–60 % <25%

5.(a) Joint formation Moderately jointed

to massive

Closely jointed Very closely jointed

(b) Joint spacing >0.2 m 0.05–0.2 m <0.05 m

6.(a) Joint orientation Very favorable,

favorable and

fair

Unfavorable Very unfavorable

(b) Strike of tunnel

axis and dip with

respect to tunnel

driving

(i) Perpendicular

20 to 90◦

along

dip, 45 to 90◦

against dip

(i) Perpendicular

20 to 45◦

against dip

(i) Parallel 45 to 90◦

(ii) Parallel 20

to 45◦

(ii) Irrespective of

strike 0 to 20◦

–

7. Inflammable gases Not present Not present May be present

8. Water inflow None to slight Moderate Heavy

9. Normal drilling

depth/round

>2.5 m 1.2 m–2.5 m <1.2 m

10. Bridge action

period

>36 h 8–36 h <8 h

Note: The geologist’s predictions based on investigation data and laboratory and site tests include information

on parameters at S. Nos. 1 to 6. This information is considered adequate for classifying the job conditions

approximately.

392 Tunnelling in weak rocks

(iv) Operating supervision,

(v) Incentives to workmen,

(vi) Co-ordination,

(vii) Punctuality of staff,

(viii) Environmental conditions and

(ix) Rapport and communication at all levels.

These factors affect the rate of tunnelling both individually and collectively. Each

factor is assigned a weighted rating (Table 27.2). The maximum rating possible in each

subgroup has also been assigned out of 100 in Table 27.2 that represents ideal conditions.

At a particular site the rating of all the factors is added to obtain a collective classification

rating for management condition. Using this rating, the management condition has been

classified into good, fair and poor as shown in Table 27.3. The proposed classification

system for management is valid for tunnels longer than 500 m, which are excavated by

conventional drilling and blasting method.

It may be noted that the rate of tunnelling can be easily improved by improving the

management condition which is manageable unlike the ground conditions which cannot be

changed. So, it is necessary to pay at least equal, if not more, attention to the management

condition than to the ground condition. Hence, there is an urgent need for management

consultancy for improving the tunnelling rate.

The key to success of tunnel engineers is evolution of a flexible method of construction

of support system. On-spot strengthening of support system is done by spraying additional

layers of shotcrete/SFRS or using long rock bolts in the unexpectedly poor geological

conditions. This is a sound strategy of management in tunnelling within the complex

geological situations. Affection is the key to success in the management. Young engineers

love challenging works. There should be no hesitation in throwing challenges to young

engineers. Otherwise these young engineers may loose interest in routine management.

27.4 COMBINED EFFECT OF GROUND AND MANAGEMENT

CONDITIONS ON RATE OF TUNNELLING

A combined classification system for ground conditions and management conditions has

been developed by Chauhan (1982). Each of the three ground conditions has been divided

into three management conditions and thus nine categories have been obtained considering

both ground and management conditions. The field data of six tunnelling projects in the

Indian Himalayas have been divided into these nine categories for studying the combined

effect. Each category has three performance parameters which are:

(i) Actual working time (AWT),

(ii) Breakdown time (BDT) and

(iii) Advance per round (APR).

Table 27.2 Ratings for management factors for long tunnels (Chauhan, 1982).

Remarks for improvement in management

S. No. Subgroup Item Maximum rating for condition

Item Subgroup

1. Overall job

planning

i) Selection of construction plant and

equipment including estimation of optimal

size and number of machines required for

achieving ideal progress.

7

ii) Adoption of correct drilling pattern and use

of proper electric delays.

6

iii) Estimation and deployment of requisite

number of workmen and supervisors for

ideal progress.

5

iv) Judicious selection of construction method,

adits, location of portals, etc.

4 Horizontal adits sloping at the rate of 7%

towards portal to be preferred to inclined

adits or vertical shafts.

v) Use of twin rail track 2

vi) Timely shifting of California switch at

the heading

2 26

2. Training of

personnel

i) Skill of drilling crew in the correct holding,

alignment and thrust application on drilling

machines

4 Proper control of drilling and blasting will

ensure high percentage of advance from the

given drilling depth and also good

fragmentation of rock which facilitates

mucking operation.

ii) Skill of muck loader operator 4

Continued

Table 27.2—Continued

Remarks for improvement in management

S. No. Subgroup Item Maximum rating for condition

Item Subgroup

iii) Skill of crew in support erection 3 A skilled crew should not take more than 1/2 h

for erection of one set of steel rib support.

iv) Skill of blastman 2

v) Skill of other crews 2 15

3. Equipment

availability and

preventive

maintenance

Time lost in tunnelling cycle due to breakdowns

of equipment including derailments, etc.

i) upto 1 h. 12–15

ii) 1–2 h. 9–11

iii) 2–3 h. 6–8

iv) >3 h. 0–5 15

4. Operation

supervision

i) Supervision of drilling and blasting

(effectiveness depends on location, depth

and inclination of drill holes, proper

tamping and use of blasting delays)

7 Improper drilling may result in producing:

i) unequal depth of holes which results in

lesser advance per meter of drilling

depth and

ii) wrong alignment of hole which may

lead to :

a) overbreak due to wrong inclination of

periphery holes and

b) secondary blasting due to wrong

inclination of other than periphery holes