- 48 ­

LIST OF FIGURES

Figure Page No. No.

Development of a Possible Debris Dam located below Landslide B

at Landslide B

1 Location Plan 50

2 Site Location Plan for Landslides B and C 51

3 Regional Geology 52

4 Past Landslides 53

5 Plan of the Source Area of Landslides B and B1 54

6 Section A-A through Landslide B 55

7 Section B-B through Landslide B 56

8 Section C-C through Landslide B1 57

9 Plan of Upper Section of Debris Trail for Landslide B 58

10 Plan of Lower Section of Debris Trail for Landslide B 59

11 Section D-D through Landslide B 60

12 Plan of the Source Area of Landslide C 61

13 Section E-E through Landslide C 62

14 Plan of the Debris Trail for Landslide C 63

15 Section F-F through Landslide C 64

16 Schematic Representation Showing the Inferred 65

17 Location Plan of Ground Investigation Works 66

18 Sketch of the Eastern Main Scarp 67

19 Stereoplot of the Measured Orientations of Discontinuities 68

20 Sketch of West Face of Trial Trench LKTT1 69

- 49 ­

Figure Page No. No.

25 Rainfall Distribution between 07:00 hours on 13 April and 74

21 Sketch Plan of Fractures/Joints on Floor of Landslide B 70

22 Stereoplots of Main Release Surface, Landslide B 71

23 Profile of Main Release Surface, Landslide B 72

24 Rainfall Recorded at GEO Raingauge No. N07 73

07:00 hours on 14 April 2000

26 Maximum Rainfall Preceding the Landslides of 14 April 75 2000 at Raingauge No. N07

27 Location Plan of Photographs Taken 76

- 52 -

The site

N

Leung King Estate

Landslide B Landslide C

LEGEND:

SUPERFICIAL DEPOSITS (Onshore)

Alluvium - Clay/silt, sand and gravel; well sorted to semi-sorted

Debris flow deposits - Unsorted sand, gravel, cobbles and boulders; clay/silt matrix

GEOLOGICAL LINES

Geological boundary, superficial deposit

Geological boundary, solid rock

Fault

Mineral vein

Photogeological lineament

SOLID GEOLOGY

Andesite with tuff and tuffite

Fine grained granite, 0.06-2mm

Megacrystic

Mylonite

Schist

STRUCTURAL SYMBOLS

Jointing

Foliation

Intrusive contact

MINERAL SYMBOL

Mo Molybdenite

q Quartz

Note: Extract from Sheet 5 of Hong Kong Geological Survey 1:20,000 Scale Map Series HGM20 (GCO, 1988)

Figure 3 - Regional Geology

- 65 -

Ephemeral stream

(a) Pre-failure channel profile

Landslide debris - cobbles and boulders in a silty gravelly sand

(b) Failure debris coming to rest between Chainages 65 and 95 damming the natural drainage channel

Eroded overflow channel

(c) Development of an overflow channel along the western flank of the dam

Original ground profile Traces of debris on flanks of channel

Abandoned channel

Flow sorted cobbles and boulders

(d) Eastern section of dam breached forming a new drainage line

Note: Not to Scale.

Figure 16 - Schematic Representation Showing the Inferred Development of a Possible Debris Dam located below Landslide B

- 67 -

Det

ail o

f jo

inti

ng/f

ract

urin

g

F

Rel

ease

sur

face

A~8

5°/2

23°

30 m

mdi

amet

er p

ipe

0.7m

wid

e qu

artz

vei

n

Rel

ease

sur

face

B~7

0 °/2

53°

Col

luvi

um-c

obbl

es a

nd b

ould

ers

ofgr

anit

e in

a s

ilty

san

d m

atri

x

Rel

ease

sur

face

C~7

0 °/2

34°

Roc

k m

ass

is v

ery

clos

ely

join

ted.

Join

ts a

re o

pen

(up

to 1

20 m

m)

and

infi

lled

wit

h sl

ight

ly s

andy

cla

y,be

com

ing

silt

y sa

nd c

lose

to th

eor

igin

al g

roun

d su

rfac

e. J

oint

orie

ntat

ion

is g

ener

ally

par

alle

l to

the

grou

nd s

urfa

ce (

roug

hly

38°/

156°

)

Flo

or o

f la

ndsl

ide

Roc

k fr

agm

ente

din

pla

ces

Voi

d ~1

0 m

m

San

dy c

lay

infi

ll

Sli

cken

side

s

Sli

cken

side

s

Roc

k m

ass

show

s si

gns

ofdi

stur

banc

e up

slop

e of

fau

lt

App

roxi

mat

e S

cale

0 0.

1 0.

2 (m)

38°/

156°

38°/

156°

34°/

176°

66°/

230°

F

Qua

rtz

vein

dis

plac

edby

abo

ut 2

5 m

m

Min

or f

ault

-wit

h 30

mm

myl

onit

e zo

ne

Roc

k m

ass

wit

h m

ediu

msp

aced

ope

n jo

ints

, wit

h 5

mm

to 2

0 m

m s

edim

ent i

nfil

l A

ppro

xim

ate

Sca

le

0 1.

5 3.

0 (m

)

46°/

230 °

79°/

265°

45°/

297°

Not

e:

Ske

tch

base

d on

Pla

tes

17 a

nd 1

8.

Fig

ure

18 -

Ske

tch

of th

e E

aste

rn M

ain

Sca

rp

(J5)

She

etin

gjo

ints

(J1

)

Eas

t rel

ease

Eas

t rel

ease

surf

ace

A (

J2)

Eas

t rel

ease

sur

face

C (

J4)

surf

ace

B (

J3)

Nor

th-s

outh

tren

ding

frac

ture

s (F

1)

Wes

t-ea

st tr

endi

ngfr

actu

res

(F2)

Fis

her

Con

cent

ratio

n%

of

tota

l per

1.0

% a

rea

0.00

- 2

.00

%2.

00 -

4.0

0 %

4.00

- 6

.00

%6.

00 -

8.0

0 %

8.00

- 1

0.00

%10

.00

-12

.00

%12

.00

-14

.00

%14

.00

-16

.00

%16

.00

-18

.00

%18

.00

-20

.00

%

Equ

al A

rea

Low

er H

emis

pher

e

76 P

oles

76 E

ntri

es

Pre

-fai

lure

slo

pe a

ngle

35°

/165

° F

rict

ion

Ang

le =

38 °

- 68 -

Pol

es

Fig

ure

19 -

Ste

reop

lot o

f th

e M

easu

red

Ori

enta

tion

s of

Dis

cont

inui

tes

at L

ands

lide

B

Roc

k on

rock

Cla

y in

fill

Roc

k on

roc

k C

lay

infi

ll

App

roxi

mat

ely

5 m

msu

b-ho

rizo

ntal

disp

lace

men

t of

shee

ting

join

t. (N

ote

trai

ling

corn

er h

as f

ract

ured

)

20 m

m th

ick

clay

infi

ll –

soft

to f

irm

ligh

t bro

wn

clay

with

occ

asio

nal s

and

grai

ns

Gen

eral

dir

ectio

nof

dow

nslo

pe

mov

emen

t

App

roxi

mat

ely

25 m

msu

b-ve

rtic

aldi

spla

cem

ent o

f jo

int

Min

or s

eepa

ge n

oted

on

shee

ting

join

t on

24.1

0.20

00,

four

day

s af

ter

a he

avy

rain

stor

m

0 0.

1 0.

2m

App

roxi

mat

e Sc

ale

- 69 ­

Figu

re 2

0 -

Sket

ch o

f W

est F

ace

of T

rial

Tre

nch

LK

TT

1

50/2

60

- 70 ­

45°/

268 ° (a

) O

rtho

gona

l Fra

ctur

es/J

oint

s

52° /

259°

Wes

t to

east

str

ikin

gfr

actu

res

gene

rally

term

inat

e at

fra

ctur

eco

ntac

t. T

he f

ract

ures

are

open

and

infi

lled

by2

mm

to 4

mm

of

clay

N

56° /

000 50

°°/26

0 °°

60° /

005°

Nor

th-s

outh

str

ikin

gfr

actu

res

/ joi

nts?

Gen

eral

ly p

ersi

sten

tov

er 2

m to

3 m

, ope

nan

d in

fille

d w

ith4

mm

to 1

0 m

m c

lay

App

roxi

mat

e Sc

ale

0 1

m0.

5

(b)

Rad

ial F

ract

ures

/Joi

nts

N

66°/

258 °

60°/

270 °

60° /

375°

Firm

, lig

ht b

row

ncl

ay in

fill

Ape

rtur

e of

ope

n jo

ints

incr

ease

s to

war

dspo

stul

ated

poi

nt c

onta

ct

App

roxi

mat

e Sc

ale

0 1

m0.

5

Not

e:

Sket

ch b

ased

on

Plat

es 1

5 an

d 16

.

Figu

re 2

1 -

Sket

ch P

lan

of F

ract

ures

/Joi

nts

on F

loor

of

Lan

dslid

e B

- 71 ­

Figu

re 2

2 -

Ster

eopl

ots

of M

ain

Rel

ease

Sur

face

, Lan

dslid

e B

i = 1

2

(fo

r fa

iled

se

ctio

n)

i = 2

8

(fo

r u

nfa

iled

se

ctio

n)

Dir

ec

tion

of

Slid

ing

i = 1

9

(fo

r fa

iled

se

ctio

n)

i = 2

8

(fo

r u

nfa

iled

se

ctio

n)

Dir

ec

tion

of

Slid

ing

a) U

sing

80m

m P

late

b)

Usi

ng 4

20m

m P

late

No

Bia

s C

orre

ctio

ns

Max

Con

c =

38.

1703

%

Equ

al A

rea

Low

er H

emis

pher

e

104

Pole

s

104

Ent

ries

No

Bia

s C

orre

ctio

ns

Max

Con

c =

47.

1696

%

Equ

al A

rea

Low

er H

emis

pher

e

26 P

oles

26 E

ntri

es

i 1 10o (fai led)

i2 18o (unfailed )

data from North P oin t s ite (Richards & Lowland ), 1982

data from Landslide B site , Leung K ing Es ta te

1st Order As perit ies

i1

i2

i1 9o (f

i2 17 o

-0.5-0.5

-0.5-0.5

- 72 -

unfa iled fa iled u nfa iled fa iled section section section section

3.0 3.0

2.5

38o

i1

i2

2.5

Verti

cal H

eigh

t (m

)

1.0

38o

i1

i2

=

=

=

Verti

cal H

eigh

t (m

)

2.0 2.0

1.5 1.5

1.0

0.5 0.5 =

0.0 0.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Horizontal Distance (m) =i1 = 10º (failed) =i2 = 18º (unfailed)

a) using 80 mm Plate

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Horizontal Distance (m)

i1 = 9º (failed) i2 = 17º (unfailed)

b) using 420 mm Plate

data from North Point site (Richards & Cowland, 1982)

data from Landslide B site, Leung King Estate

20 18

16

14

12

10 8

6

4

2

0 0 100 200 300 400 500

Diam eter o f M eas uring Plate (mm )

Rou

ghne

ss A

ngle

, i (

degr

ee)

Figure 23 - Profile of Main Release Surface, Landslide B

- 73 -

Dai

ly R

ainf

all (

mm

)

4 00

350

300

250

200

150

100

50

0

375 m m of rain fall reco rded in t he 31 days befo re the landslides

375 m m of rain fall reco rded in t he 15 days befo re the landslides

Report ed Date o f t he Landslides (14 April 2000)

13 15 17 19 21 23 25 27 29 31 2 4 6 8 10 12 14 16

M arch 2000 Ap ril 2000

(a) Daily Rainfall Recorded between 13 March and 16 April 2000

258 m m of rain fall reco rded in t he 24 hours befo re t he landslides

234 m m of rain fall reco rded in t he 12 hours befo re t he landslides

70

Deduced t im e o f t he landslides 07 :00 hours on 14 April 2000 60

Hou

rly R

ainf

all (

mm

)

5 0

40

30

20

10

0

0 0 .167 0 .333 0 .5 0 .667 0 .833

13 A pril 2000

1 0 .167 0 .333 0 .5 0 .667 0 .833

14 A pril 2000

1

(b) Hourly Rainfa ll Recorded be tw een 00:00 hour on 13 April and 24:00 hours

on 14 April 2000

Figure 24 - Rainfall Recorded at GEO Raingauge No. N07

- 75 -

Max

imum

Rol

ling

Rai

nfal

l (m

m)

0

100

200

300

400

500

600

700

800

0.01 0.1 1 10 100

Duration (hours)

22 July 1994

5 N ovember 1993 14 A p ril 2000 (Rainfall p receding the landslides)

24 A ugust 1999

9 June 1998

1000

Note: Rainfall data shown are derived from 5-minute data at GEO raingauge No. N07.

Figure 26 - Maximum Rainfall Preceding the Landslides of 14 April 2000 at Raingauge No. N07