chp 8 (construction in rock)

3/20/2013

1

CHAP 8: CHAP 8: CHAP 8: CHAP 8: Civil engineering structures & construction Civil engineering structures & construction Civil engineering structures & construction Civil engineering structures & construction

activities in rock massactivities in rock massactivities in rock massactivities in rock mass: : : :

Construction problems in rock mass is shown in Fig.

8.1. It involves factors of:

Rock mass properties (intact rock + discontinuities).

In-situ & induced stresses (due to geological

conditions e.g. fold & construction activities).

Groundwater conditions.

The above factors, together with the design

procedures & construction methods must be

carefully considered so that disturbance induced

into the rock mass is as minimal as possible.

Engineering structures & construction activitiesEngineering structures & construction activitiesEngineering structures & construction activitiesEngineering structures & construction activities: : : :

Disturbance induced into the rock mass (e.g.

construction induced stresses) must be kept to a

minimal level !!

WHY??? This is to maintain the integrity& the

inherent strengthof the rock mass hence, ensuring

long-term stability for the structure. This reduces

cost on stabilisation & maintenance after

construction.

3/20/2013

2

Figure 8.1: Construction problems in rock masses Figure 8.1: Construction problems in rock masses ––

consists of intact rock & discontinuities together with consists of intact rock & discontinuities together with

the effect of stress (in situ & induced) & groundwater.the effect of stress (in situ & induced) & groundwater.

Rock material (intact) and rock mass (discontinuous)Rock material (intact) and rock mass (discontinuous)

3/20/2013

3

Engineering structures & construction activities Engineering structures & construction activities Engineering structures & construction activities Engineering structures & construction activities

Construction in rock mass – disruption/disturbance

to the rock at the vicinity of the construction area

must be minimised so any induced negative impacts

occur in a controlled manner and covers a smaller

area/volume (see the following figure).

As soon as a tunnel is excavated, surrounding rock mass will be

disturbed – formation of yield zone. Design & method of construction

must be carefully considered so that disturbance to surrounding

rock is reduced (thinner yield zone, less affected volume).

YIELD ZONE

(ZON ALAH)

CIRCULAR

TUNNEL

SURROUNDING

ROCK MASS

3/20/2013

4

As soon as a slope is excavated, surrounding rock mass will be

disturbed – formation of yield zone. Design & method of construction

must be carefully considered so that disturbance to surrounding

rock is reduced (thinner yield zone, less affected volume).

YIELD ZONE

(ZON ALAH)

CUT SLOPE

SURROUNDING

ROCK MASS

Stress distribution around circularStress distribution around circular--shaped tunnelshaped tunnel

3/20/2013

5

Stress distribution around oblongStress distribution around oblong--shaped tunnelshaped tunnel

Stress distribution around rectangularStress distribution around rectangular--shaped tunnelshaped tunnel

3/20/2013

6

Stress distribution around a cut slopeStress distribution around a cut slope

Stress distribution in a rock sample at different Stress distribution in a rock sample at different

level of stresslevel of stress

Loading

at 10 kN

Loading

at 100 kN

Loading

at 200 kN

3/20/2013

7


Category of rock engineering structure include:

Structure foundations: A rock body is used to bear

load of a structure (e.g. a building founded on

bedrock). This can be a deep-seated rock mass. Rock

is usually an excellent foundation materials, but near

surface rock can be fractured & weathered. It is

necessary to establish the competence of the rock to

bear the designed load. Important rock properties –

triaxial compressive strength; strain at failure &

modulus of elasticity.


Rock slopes: The basic modes of failure of rock (plane,

wedge, toppling, flexural etc.). Shear strength, types &

orientation of weakness planes are among the

important parameters in slope design. The potential

for failure in any of these modes can be identified

(Part 5). The need and scope for a more detailed

analysis can then be evaluated.

3/20/2013

8


Shafts & tunnels: Stability of these structures depends

on discontinuities & geological structures in rock, in situ stress, groundwater flow, shape of tunnel &

construction technique. Rock properties like triaxial

compressive strength, shear strength, types &

orientation of weakness planes are important design

parameters. Method of stabilisation & F.O.S depends

on the purpose of the tunnel. F.O.S. used in designing a

tunnel for civil engineering purposes (highways &

railway lines) is usually > 2.0.

Tunnel excavated in rock

Rock as foundation Slope cut in rock

3/20/2013

9

Underground cavern: Similar to tunnel but larger in

size. Methods of excavation & stabilisation are

important e.g. Power house cavern, sport complex etc.

Mining: Structure is not permanent. Objective is to

extract minerals/precious stones at maximum rate &

safety but, at minimal hazardous & pollution effect.

Thermal energy: To extract thermal energy from

earth’s crust. Deep excavation, rock permeability &

rheological characteristics are important.

Radioactive waste disposal: Excavation is deep in the

earth’s crust (safety). Rocks must be massive &

impermeble. Method of excavation is important.

Permanent structures.

Effect of size of rock mass discontinuities on size of

engineering structure:

Size of structure being constructed in rock mass is

greatly influenced by the size of structural

discontinuities (weakness planes) in the rock mass.

In Fig 8.2: smaller tunnel size means less number of

weakness planes intersecting the tunnel, i.e. more

stable. Deeper depth means less weakness planes

hence, tunnel located deep below the surface is more

stable.

3/20/2013

10

Size of rock mass discontinuities and size of

engineering structure:

Properties of rock mass (sifat massa / jasad batuan

keseluruhan) is important in designing & construction

of a structure in the field (actual conditions).

The actual rock mass properties cannot be evaluated

using small size rock samples as in the case

laboratory tests. Laboratory size samples contain only

small-scale weaknesses (e.g. lamination, micro

fractures & voids).

Fig 8.2: Effect of number of discontinuities in rock mass

on size of structure

3/20/2013

11

Fig 8.3: Effect of number of discontinuities in rock

mass on size of samples

Fig 8.4: Effect of depth of structure on conditions of rock mass; at

depth means less number of discontinuities & rock is under

confined condition (p = ρgh).

3/20/2013

12

Structural discontinuities (struktur ketakselarasan) in Structural discontinuities (struktur ketakselarasan) in Structural discontinuities (struktur ketakselarasan) in Structural discontinuities (struktur ketakselarasan) in

rock massrock massrock massrock mass::::

Based on size, structural discontinuities/weakness in

rock are categorized into 2 groups:

Large-scale discontinuities– sizes ranging between

few m and km. Affect the properties & behaviours of

rock mass. Stability of slope & tunnel is affected by

these large-scale discontinuities.

Large size samples in in situ /field tests can

accommodate the effect of these structures however,

not for small samples used in laboratory tests.

Types of large-scale discontinuities (discussed in

Chapter 5) include fault, joint & bedding plane/fold.

Inclined bedding planes (folds) in clastic

sedimentary rocks

3/20/2013

13

Joints & joint sets that present in all rock types

Inclined bedding planes in sedimentary rocks (shale, Inclined bedding planes in sedimentary rocks (shale,

Labuan)Labuan)

3/20/2013

14

Inclined bedding planes in sedimentary rocks Inclined bedding planes in sedimentary rocks

(sandstone, Labuan)(sandstone, Labuan)

Thinly bedded & folded sedimentary rocks (shale, Thinly bedded & folded sedimentary rocks (shale,

Labuan)Labuan)

3/20/2013

15

Intersecting of more than 3 joint sets is common in Intersecting of more than 3 joint sets is common in

rock mass (granite, Lahad)rock mass (granite, Lahad)

Joint can occur in various orientation in rock (granite, Joint can occur in various orientation in rock (granite,

Kuantan)Kuantan)

3/20/2013

16

Structural discontinuities in rock massStructural discontinuities in rock massStructural discontinuities in rock massStructural discontinuities in rock mass::::

Small-scale discontinuities– measuring from few mm

to cm. Direct effect on rock material properties & but

may have minimal effect on rock mass (at a larger

scale).

Data obtained from laboratory test on small size

samples can be affected by these properties.

(Bah. 3 kesan syistositi ke atas batuan jelmaan syis).

Common types - foliation, lamination, cleavage, micro-

fractures & voids.

Note: Effect of these discontinuities on rock material

properties will impose indirect impact on stability of

structures like foundation, slope & tunnel.

Effect of minerals arrangement (foliation & schistosity) on Effect of minerals arrangement (foliation & schistosity) on

strength & failure strain of small rock samplestrength & failure strain of small rock sample

3/20/2013

17

Minerals arrangements due Minerals arrangements due

to sedimentation to sedimentation

((laminationlamination) and due to ) and due to

metamorphism (metamorphism (foliationfoliation) )

are is small scale are is small scale

discontinuities rock (e.g. discontinuities rock (e.g.

shale, sandstone slate & shale, sandstone slate &

schist)schist)

Being small scale discontinuities, they occur in laboratory rock Being small scale discontinuities, they occur in laboratory rock

sample. Fracture/failure can be easily induced along the sample. Fracture/failure can be easily induced along the

lamination/foliation, but not perpendicular to it. Thus rock sample lamination/foliation, but not perpendicular to it. Thus rock sample

displaying lamination/foliation may display different strength displaying lamination/foliation may display different strength

when loaded at different directionwhen loaded at different direction

3/20/2013

18

The influence of weakness planes on strength – joint

orientation with respect to loading axis

Effect of loading orientation on UCS of sample displaying

lamination (metamorhic rock e.g. schist)

Stress versus strain

0

50

100

150

200

250

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Strain (%)

Stress (MPa)

3/20/2013

19

EXISTING FRACTURE AFECT FAILURE LOADEXISTING FRACTURE AFECT FAILURE LOAD

[a] [b] [c] [d]

Compressive Load

Existing weakness plane

Loading axis

Existing fracture plane

Effect of orientation of Effect of orientation of

existing fracture on existing fracture on

test data is more test data is more

significant for Brazilian significant for Brazilian

& Point& Point--load test load test

SLATY SLATY -- minerals rearrangement due to metamorphism; minerals rearrangement due to metamorphism;

changing of shale to slatechanging of shale to slate

3/20/2013

20

Slaty in slateSlaty in slate

SCHISTOSITY (flaky)SCHISTOSITY (flaky)-- minerals rearrangement due to minerals rearrangement due to

metamorphism; changing of shale to schistmetamorphism; changing of shale to schist

3/20/2013

21

Minerals arrangement in metamorphic rocks Minerals arrangement in metamorphic rocks –– schistosity & schistosity &

slatyslaty

Minerals arrangement in metamorphic rocks Minerals arrangement in metamorphic rocks –– schistosity & schistosity &

slatyslaty

3/20/2013

22

Failure of rock slopes in metamorphic rock slate & schistFailure of rock slopes in metamorphic rock slate & schist

Failure of rock slopes in metamorphic rock slate & schistFailure of rock slopes in metamorphic rock slate & schist

3/20/2013

23

Discontinuities in rock massDiscontinuities in rock massDiscontinuities in rock massDiscontinuities in rock mass::::

Large-scale discontinuitiesaffect the whole rock

body/rock mass i.e. rock conditions on site. Effect on

engineering structures is that these discontinuities

weaken to whole rock mass directly.

Small-scale discontinuitiesaffect rock material

propertiese.g. properties of small sample in lab test.

Strength of rock obtained from lab test is usually

higher than the actual strength of rock mass in the

field (in situ strength).

Value of FOS (e.g. > 2.0) used in design is to cater for

the weakening effects of structural discontinuities in

rock on engineering structures, as their effects are

difficult to be assessed and rated numerically.

chp 8 (construction in rock)

Documents