Mechanisum of Creep Movement

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Engineering Geology 53 (1999) 311325

Mechanism of creep movement caused by landslide activity and

underground erosion in crystalline schist, Shikoku Island,

southwestern Japan

G. Furuyaa,

*, K. Sassaa, H. Hiura

b, H. Fukuoka

a

a Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto 611-0011, Japan

b Faculty of Agriculture, Kochi University, Nankoku, Kochi 783-0093, Japan

Received 3 April 1998; accepted 30 November 1998

Abstract

The mechanism of creep movement of the Zentoku landslide in crystalline schist has not been studied in detail

because of the steepness of the slope, very slow movement, low population density and complex topographic and

geologic characteristics. Sassa et al. (1980: Proc. INTERPRAEVENT 1, 85106) and Sassa (1984: Proc. 4th

International Symp. on Landslides, Toronto, vol. 2, pp. 179184; 1985. Geotechnical classification of landslides, Proc.

4th International Conference and Field Workshop on Landslides, Tokyo, pp. 3140; 1989: Landslide News, Japan

Landslide Society, No. 3, pp. 2124) monitored landslide movement and groundwater level at the Zentoku landslide

on Shikoku Island, southwestern Japan, and suggested that the mechanism may be caused by underground erosion.

To study the influence of underground erosion at this site, continual monitoring of suspended sediment and water

discharge from a groundwater outlet (i.e. a spring) was implemented. The locations of groundwater flow paths were

determined, as were the amounts of discharged sediment. Slope deformation was monitored by means of a borehole

inclinometer. The conclusions were as follows: (1) the flow paths were found to be on or above the shear zones in

which underground erosion has occurred; (2) in addition to being a result of precipitation and groundwater discharge,

sediment discharge is affected by landslide activity; and (3) the mechanism of creep movement is an interrelated chain

process that combines underground erosion caused by landslide activity with landslide activity caused by underground

erosion. Thus, landslide activity increases erosion susceptibility and transportation of soils within the mass, and

underground erosion causes instability of the landslide mass, in turn.

This mechanism can explain the observed phenomenon that the Zentoku landslide not only moves actively duringheavy rain, but also continues to creep throughout the year. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Crystalline schist; Erosion; Landslide activity; Monitoring; Sediment discharge

* Corresponding author. Fax: +81-774-384300.

E-mail address: [email protected] (G. Furuya)

0013-7952/99/$ see front matter 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 0 1 3 - 7 9 5 2 ( 9 8 ) 0 0 0 8 4 - 2


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312 G. Furuya et al. /Engineering Geology 53 (1999) 311325

Bonnard, 1983; Kronfellner-Kraus, 1980; Sassa

et al., 1980; Sassa, 1984, 1985, 1989). However,1. Introduction

the mechanism of movement, especially in the caseof creep movement of landslides, is not yet wellMany landslides occur in areas of crystalline

schists distributed along orogenic zones in Circum- understood. This is partly because landslides in

crystalline schists have more complex topographicPacific and European countries (e.g., Baumer,

1988; Riemer et al., 1988; Oyagi, 1989; Gillon and and geologic features than those in sedimentary

clays and shales. Many landslides in crystallineHancox, 1992; Noverraz, 1996). Most studies in

these areas have been carried out as geologic schists have occurred on Shikoku Island, south-

western Japan (Ando and Ohkubo, 1970; Katoinvestigations (e.g., Zischinsky, 1966; Mahr and

Nemcok, 1977; Chigira, 1984, 1985), laboratory and Hada, 1980; Fujita et al., 1973). The Zentoku

landslide (Fig. 1), which has a large number oftests (Yagi et al., 1989), and monitoring (e.g.

Fig. 1. Location map and plan of the Zentoku landslide.


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313G. Furuya et al. /Engineering Geology 53 (1999) 311325

homes on it, and is one of the largest crystalline (1 ) temporary discharge caused by heavy rain; and

(2) continuous discharge throughout the year.schist landslides in Japan, was selected for this

study. Their studies were concerned only with the rela-tionship between landslide activity andSince 1972, movement of the Zentoku landslide

has been monitored by extensometers, and water underground erosion. Thus, they did not ade-

quately deal with quantitative considerations oflevels have been monitored by borehole water

gauges (Sassa et al., 1980; Sassa, 1984, 1985, 1989). the mechanism of creep movement.

In this study, the authors have quantitativelyFrom these monitorings, it was pointed out that

when the peak borehole water level reached a correlated landslide displacement, precipitation,

sediment and discharge at a groundwater outletcertain critical level, large movement occurred,

while when the peak borehole water level did not (i.e. a spring). The existence of eroded soils result-

ing from factors other than precipitation (ground-reach a certain critical level, small movement

occurred (Fig. 2). Sassa (1985, 1989) pointed out water discharge) also has been examined. On the

basis of these analyses, a mechanism for creepthat the large movement, residual-state sliding,

occurred when the stress on the slip surface reached movement of landslides in areas of crystallineschists has been proposed.the residual-state failure envelope, while small

movement, creep, occurred when the stress on the

slip surface did not reach the failure envelope.

Sassa (1985, 1989 ) explained that creep was caused 2. Brief view of monitoring site

by underground erosion and transportation of the

soils in the surrounding groundwater flow paths. The Zentoku landslide is located in a crystalline

schist area south of the Median Tectonic Line onHiura et al. (1991) and Matsunaga et al. (1993)

suggested that two types of sediment discharge Shikoku Island (Fig. 1). On the middle and lower

parts of slope, the landslide occurs mainly in peliticcause underground erosion:

and partially green schist; on the upper slope, the

slide may be in psammitic schist (Fig. 3). Deepershear zones (slip surfaces) have formed at depths

of 3060 m and shallower ones at 1520 m. The

shear zones are clayey, but include gravels and

sands. In these zones, rocks have been pro-

gressively crushed and oxidized (they are almost

brown in color). The dip of the bedding is nearly

parallel to the slope; the crushed zones and the

concave shape of the bedrock have been detected

by seismic exploration. The mean slope is ca 28

the length of the landslide is approximately

1300 m; and the maximum landslide width is

approximately 500 m. Landslides in crystalline

schists on Shikoku Island are known to have a

high frequency of occurrence where the schist is

pelitic, the dip of bedding is nearly parallel to the

slope, and the mean slope is from 25 to 30 (Fujita

et al., 1973, 1976). The Zentoku landslide is one

such as this.

Sliding blocks ( Block 1, 2, 3-1, 3-2, 3-3) are

shown in Figs. 1 and 3. The dimensions of these

blocks have been approximated by the followingFig. 2. Relationship between the peak borehole water level andlandslide movement at the Zentoku landslide (Sassa, 1984). means of monitoring and investigation:


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Fig. 3. Geologic cross-section through the Zentoku landslide along the line of extensometers.

(1 ) Installation of 29 long-span extensometers in tored at Springs 1 to 4 (Fig. 1). In this paper, the

uninterrupted sequence. results of measurements at only Spring 1 are(2 ) Installation of 100 three-dimensional shear presented for the following reasons:

displacement meters transversely across the (1) because Spring 1 is located at the toe of BlockZentoku landslide to measure three-dimen- 1, it seems that sediment can be transportedsional slope movement (Hiura et al., 1992). along the slip surface (shear zone) of Block 1,

(3 ) Drilling of 11 boreholes for geologic investiga- which must be related to the landslide activ-tion and six boreholes for installation of ity; andinclinometers. (2) because Spring 1 is located near the monitor-

Blocks 1 and 2 have deeper shear zones, while ing points of B2 (extensometer S14) and 4-25Blocks 3-1, 3-2, 3-3 have shallower shear zones (point of borehole inclinometer measure-(Furuya et al., 1997) . Sometimes these blocks have ment), it is possible to compare landslidebecome independently active, sometimes displacement and sediment discharge at

dependently. Spring 1.Fig. 4 shows the apparatus for monitoring sedi-

ment discharge at Spring 1. The initial type appara-

tus used from April 1984 to June 1995 is shown3. Monitoring of underground erosion and landslidein Fig. 4(a). In using this apparatus, the sedimentmovementtransported by groundwater flow through the land-

slide is collected by a stainless steel pipe and3.1. Monitoring method for groundwater andaccumulates in the bucket. The accumulated sedi-sediment discharge through groundwater outletsment is removed approximately monthly (except

during the winter), and is weighed after beingIn the Zentoku landslide, sediment discharge

caused by underground erosion has been moni- dried. Fig. 4( b) shows the improved apparatus,


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gauge located near the line of three-dimensional

shear displacement meters shown by 6 in Fig. 1.

3.2. Monitoring methods for landslide displacement

Displacement of the landslide was monitored by

means of a bolehole inclinometer at bolehole 4-25

(70 m depth; strainer treated) and long-span exten-

someters. The extensometers had automatic, con-

tinually recording systems. However, there was no

completely stable point along the line of extensom-

eters (Fig. 1). Thus, absolute displacement value

could not be obtained. The left side of Fig. 5 shows

the geologic column and the middle graphs present

monitoring results from the borehole inclinometerin borehole 4-25. Displacement of borehole 4-25

occurred only at depths shallower than 29.5 m.

Below 29.5 m, the mass was stable. The landslide

mass included three shear zones (shear zones 1 3).

Monitoring was not continuous, occurring at

intervals of one to a few months. Therefore, contin-

ual landslide movement was calculated using a

combination of information obtained from exten-

someters and inclinometers by means of the

following procedure:

(1 ) Search for the point of minimum movementalong the line of extensometers (Fig. 1) fromFig. 4. System for catching discharged sediment.the results of cumulated records of 29 sets of

extensometers.

(2) Calculate the relative displacement at S14 nearwhich has been used since 20 June 1995. Using point B2 by using accumulated movementthis apparatus, sediment, with grain size >1 mm values between the point of minimum move-accumulates on the metal sieve and is automati- ment and point B2.cally weighed by a load cell; finer sediment (3) Calculate the composite displacement of shear(


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Fig. 5. Shear zones and groundwater flow paths in borehole 4-25.

4. Relationship between underground erosion and water level was at a depth of 63.69 m. The water

level in the borehole was monitored at a constantdisplacement of the landslide mass

depth of approximately 6364 m all year. It seems

that the borehole water level did not rise above a4.1. Estimation of groundwater flow pathsdepth of 64 m because of leaking of the water into

the bedrock fissures. The four peaks of seismicSlope failures caused by underground erosion

have been noted by Crozier (1986) and Selby output that are shown as G.W.F. 14 in the right-

hand graph of Fig. 5 occurred in borehole 4-25.(1993); however, these authors focused mainly on

erosion of the surface soil layer. In a deep shear These zones of higher seismic output undoubtedly

were due to groundwater flow paths G.W.F. 1, 3zone of a landslide mass, groundwater plays an

important role in landslide movement and and 4, which were located in or above shear zones

1, 2 and 3. G.W.F. 2 was located above the voidunderground erosion. Hence, a study was con-

ducted to detect the depth of groundwater flow (13.514.35 m). From the geological column, it

was found that the lower part of the shear zonesusing seismic detection. The method of investiga-

tion was to measure the vibration caused by agreed with the depth of highly weathered rocks

(clayey sediment is abundant); the middle or uppergroundwater flowing through the landslide mass

by inserting a seismograph into a borehole (Sassa part of shear zones agreed with the depths of

moderately weathered rocks (sands and gravels areand Sakata, 1977). The vibration was converted

into voltage by an amplifier with calibrated volt- abundant) caused by landslide movement. At

G.W.F. 1, 3 and 4, the bases of the groundwaterage. The output of the seismograph was calculated

by comparing the obtained voltage with the cal- flow paths occurred at an impermeable layer of

highly weathered rock, because the material ofibrated voltage.

The right-hand graph of Fig. 5 shows ground- moderately rock is more permeable than the mate-

rial of highly weathered rock. Thus, at the perme-water flow on 15 August 1996. The seismograph

could not be inserted deeper than 40 m because of able layer, the groundwater concentrates and flows,

and subsequently, erodes particles of rock materiala bend in the borehole. On 15 August, the borehole


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that had been crushed by the landslide activity. At tional to P on the logarithmic diagram, in spite ofsome scattering. Filled circles represents the resultsG.W.F. 2, the groundwater flow was not found to

from April to June 1995, the last part of theoccur above the voids. Upon the drilling of bore- longest inactive period between C and D. Thehole 4-25 in 1992, the groundwater path was founddistribution of the filled circles data is nearly ato at the location of the voids because they werestraight line, forming the lower boundary of allthe result of underground erosion. However, theplots. This suggests that sediment discharge in thispaths of such interconnected voids can be shiftedperiod is affected only by precipitation after a longupward. Because of the collapse of the void struc-inactive period. Hence, the sediment dischargeture, the location of the groundwater path hadassociated with precipitation can be estimated bylikely moved between the time of drilling boreholethe regression line in Eq. (2) :4-25 in 1992 and the time of measurement on 15

August 1996. WP=100.83P1.43, (2)

in which WP

is sediment discharge estimated from

precipitation (g month1) and P is the monitored4.1.1. Ratio of monitored sediment discharge tomonthly precipitation (mm month1).that estimated from precipitation

If the sediment discharge in other periods isThe infiltration of rainfall causes a reduction ofgreater than the regression line, it is probable thatshear strength of the landslide mass by raising thethe discharge includes effects other thangroundwater level (i.e. increasing pore-water pres-precipitation.sure) and results in increased groundwater flow

In order to express effects other than precipita-and underground erosion. Thus, rainfall is an

tion, a new parameter W/WP

[in which W is theimportant effective factor in both landslide move-

monthly sediment discharge (g month1) and WPment and underground erosion. The authors exam-

is the estimated sediment discharge according toined the relationship between landslide displace-

Eq. (2 )], termed sediment discharge ratio hasment and amount of eroded sediment associated

been proposed. When W/WP=1, the sediment

with precipitation by means of monthly monitoring discharge is caused only by rainfall; whenfrom September 1993 to December 1995. Here, a

W/WP>1, the sediment discharge is also affected

parameter D/DAV

(displacement ratio, in which Dby factors other than rainfall; and when

is the monthly displacement and DAV

is the averageW/W

P1. Examining the period of high

taken as the mean amount that occurs in 30 days. values of W/WP, most of them were noted as

Fig. 6 shows the time series change of monthly appearing after the period of peak displacementdisplacement and D/D

AVfrom September 1993 ratio in Fig. 6. The authors compared the displace-

through December 1995. In this figure, DAV

isments for the active periods (AE in Fig. 6) with2.6 mm month1, and there are five peak values the values of W/W

Pfor the same periods, and for

of monthly displacement (A, B, C, D, E), which 1, 2 and 3 months afterwards. The displacementcorrespond to the peaks of D/D

AV>1. These peak in the active period, A, is the sum of 2-month

periods indicate that the landslide was active. The displacements because activity continued forother periods, when D/D

AV


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Fig. 6. Change in value of monthly displacement and D/DAV

with time ( September 1993December 1995). Capital letters AE

represent the most active periods.

landslide activity and the sediment discharge ratio after a high level of landslide activity, the sediment

discharge affected by the precipitation considerably(W/WP

) in the following four periods: open

squares, for periods of a high level of landslide decreased because the heavy rain had stopped.

However, sediment discharge caused by landslideactivity; filled circles, for a period of 1 month aftera high level of landslide activity; crosses, for activity continued at a somewhat consistent rate.

The rate of sediment discharge caused by a high2 months after a high level of landslide activity;

and filled upside-down triangles, for periods of level of landslide activity was relatively higher than

that caused by precipitation. Therefore, most3 months after great landslide activity. According

to this figure, sediment discharge can be classified values ofW/WP

in the first month after high levels

of landslide activity fell into group A. In theinto two groups:$ Group A: the sediment discharge ratio is pro- periods of 2 and 3 months after a high level of

landslide activity, the sediment discharge causedportional to landslide displacement during an

active period. by landslide activity gradually decreased; thus, the

rate of sediment discharge caused by precipitation$ Group B: the sediment discharge is almost

independent of landslide displacement. increased relatively, and many W/WP

values are

plotted in group B. The intersection point ofA high level of landslide activity is commonly

caused by heavy rain such as during a typhoon. W/WP=1 and the inclination line of group A are

shown in Fig. 9. This point is called criticalWhen heavy rain occurred on the Zentoku land-

slide, a lot of sediment discharge was monitored, displacement (Dcr

). If the displacement is larger

than Dcr

, the sediment discharge caused by land-due to the high level of landslide activity. In the

periods of this activity, the sediment discharge was slide activity can be noted.

more dependent on precipitation than on the land-

slide activity because these periods had more pre- 4.1.2. Ratio of monitored sediment discharge to

that estimated from groundwater dischargecipitation than normal periods. Hence, almost all

W/WP

values for periods of high levels of landslide The correlation shown in Fig. 9 indicates that

landslide activity has an effect on sediment dis-activity occurred in group B. In the first month


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Fig. 7. Relationship between monthly precipitation and sedi-

Fig. 9. Relationship between the displacement at high landslidement discharge for each period and assumed value of WP. P, activity and W/WP

. (A) Group affected by landslide activityMonthly precipitation (mm month1); W, monthly sedimentand precipitation; (B) group affected by precipitation; D

cr, criti-discharge ( g month1); W

P, sediment discharge estimated from

cal displacement.the above regression line (g month1).

Fig. 8. Change in value of W/WP

with time (September 1993December 1995).


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Fig. 10. Change in value of weekly displacement and D/DAV

with time ( 20 June 19953 September 1996). Capital letters AF represent

the most active periods.

charge. The monitoring of groundwater discharge DAV=0.7 mm week1 and that two periods of high

levels of landslide activity occurred:has been carried out since 20 June 1995 to deter-(1 ) from 20 June to 25 July 1995; andmine better the relationship between landslide(2 ) on 26 September 1995.activity and underground erosion. During thisA lesser amount of high landslide activity (thetime, the monitoring system of sediment dischargevalue ofD/D

AVis nearly 2) occurred on 24 Octobershown in Fig. 4(b) was improved to a semi-auto-

1995 and 16 July 1996. Slow movement took placematic procedure. Semi-automatic monitoring ofduring the other monitoring periods.groundwater discharge was carried out at Springs

Fig. 11 shows the relationship between weekly1 and 4 (Fig. 1). The monitoring data from Springsediment discharge W (g week1) ( logarithmic1 were used for analysis because Spring 1 is locatedvalue) and weekly groundwater discharge Qclose to the key borehole inclinometer 4-25 at the(l week1) ( logarithmic value). Open circles repre-center of landslide. Data from 20 June 1995 to 3sents all monitoring results from 20 June 1995 toSeptember 1996 were used. Almost the same3 September 1996 in Fig. 10. Filled circles are themethod of analysis to that described in Section 4.2monitoring results for 28 May to 9 July 1996,was employed substituting groundwater dischargewhich was the inactive period that occurred overfor precipitation. However, the time unit wasa period of ca 8 months after the high level ofchanged from months to weeks, that is, weeklylandslide activity of 24 October 1996 and beforesediment discharge W (g week1), and weeklythe high level of landslide activity of 16 July 1996groundwater discharge Q (l week1).shown in Fig. 10. The line in Fig. 11 presents the

Fig. 10 shows the time series change of landslideregression of the data shown by the filled circles.

displacement and displacement ratio, D/DAV

(D isThe sediment discharge correlates with ground-

the weekly displacement and DAV

is the weeklywater discharge as expressed by:

average displacement), from 20 June 1995 to 3

September 1996. This figure indicates that WQ=106.48Q2.00, (3)


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we could not calculate W/WQ

because the bucket,

the water tank and the V-notch weir were frozen.

To check whether or not the unknown factorwas a result of the landslide activity, a comparison

was made between the displacement during active

periods and the value ofW/WQ

. The high landslide

activity periods in Fig. 10 occurred from 20 June

to 25 July 1995 and on 26 September 1995. Activity

from 20 June to 25 July 1995 was caused by heavy

rain during the rainy season. Usually a period of

landslide activity due to heavy rainfall terminated

within 2 weeks after the rain ended. Each fortnight

for A, B, C, D and E are regarded as one group

of landslide activity. On 26 September 1995, F was

caused by the typhoon No. 24 (T9524). The arrowfrom A in Fig. 12 is an example of the comparison

between the displacement of period A (see Fig. 10)

and log(W/WQ

). The serial numbers (07) in this

figure are the number of weeks after landslide

activity A from 20 to 27 June 1995.

Fig. 13 shows the distribution of log(W/WQ

)

from 5 to 7 weeks after high landslide activity

against its displacement. In this figure, the open

circles are for 5 weeks, the filled diamonds are for

6 weeks and open upside-down triangles is forFig. 11. Relationship between weekly discharge, sediment dis-

charge, assumed values of WQ. Q, Weekly groundwater 7 weeks after high levels of landslide activity. Whendischarge (lweek1); W, weekly sediment discharge the displacement is larger than Dcr

and(g week1); W

Q, sediment discharge estimated from above

log(W/WQ

)>0, the displacements of high levels ofregression line (g week1).

landslide activity (AF) are proportional to

log(W/WQ

) from 5 to 7 weeks. Notably, the scatter

of data for 6 weeks after high levels of landslidein which WQ

is the sediment discharge estimated

from the groundwater discharge (g week1) and activity is very small. These results mean that,

when the displacement exceeded a certain criticalQ is the groundwater discharge (l week1) moni-

tored weekly. Thus, the effect of the landslide value, the sediment discharge was caused by land-

slide activity. These considerations correspond toactivity can be evaluated by W/WQ

, as well as by

W/WP

. the presumption presented in Section 4.2. Thus, it

has been proved that the eroded and transportedFig. 12 shows the time series change of W/WQ(logarithmic value). In this figure, large values of sediments are mainly the result of landslide

activity.log (W/WQ

) occur in the left side of the figure for

the first half of all monitoring periods (i.e. from

18 July to 19 December 1995). In the right half of

the figure, the values of log(W/WQ

) occuring from 5. Mechanism of landslide movement caused by

landslide activity and underground erosion13 February to 3 September 1996. It seems

that some unknown factor causes sediment dis-

charge to a greater amount than groundwater Sassa (1984, 1985, 1989) has proposed that

creep movement at the Zentoku landslide is causeddischarge does in the first half of all monitoring

periods. From 26 December 1995 to 9 January by underground erosion. It would be expected that

if underground erosion were to continue for a long1996 and from 23 January to 13 February 1996,


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Fig. 12. Change in value of log(W/WQ

) with time (20 June 19953 September 1996). W/WQ

, Sediment discharge ratio for ground-

water discharge.

period of time, the amount of erosion-prone mate-

rial being removed would decrease and the creep

movement would gradually terminate. However, it

has been shown that creep has continued at the

Zentoku landslide (Sassa, 1984; Furuya et al.,

1997). It is insufficient to explain this movement

alone as an effect of the underground erosion.Consequently, it can be hypothesized that the

movement mechanism is not the only process of

underground erosion that causes landslide activity,

but also that there is some other process.

Fig. 14 illustrates the overall mechanism of a

landslide in crystalline schist as studied at the

Zentoku landslide; this mechanism includes two

interrelated processes. On the left of this figure the

process of landslide activity caused by

underground erosion is shown, which has been

previously proposed by Sassa (1984, 1985, 1989).

The soils and other fine materials surrounding the

groundwater flow path are eroded and transported

by groundwater flows. Thus, the voids tend to

enlarge there, which makes the landslide mass

unstable. Hence, landslide activity (creep move-

ment) occurs. On the right in Fig. 14 is shown the

process of underground erosion caused by theFig. 13. Relationship between displacement during periods of landslide activity, as indicated by the results ofhigh levels of landslide activity and log(W/W

Q) for the period

this study. The fine-grained materials are producedof 57 weeks after this activity. W/W

Q, Sediment discharge ratio

by disturbance and mechanical weathering of geo-for groundwater discharge; Dcr

, critical displacement; r, correla-

tion coefficient. logic materials in the shear zone due to the land-


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the sediment affected by high levels of land-

slide activity. Especially, the sediment dis-

charge ratio in the period of 12 months (from5 to 7 weeks) after a high level of landslide

activity is proportional to the landslide dis-

placement. Greater landslide activity caused

greater sediment discharge.

Therefore, creep movement of this landslide in

crystalline schist has occurred over a long period

of time as the result of two interactive processes:

(1 ) landslide activity that produced fine-grained

materials; and

(2 ) underground erosion of these materials by

groundwater, which in the landslide mass

helped to cause the landslide activity.

6. Conclusions

The mechanism of creep movement caused by

landslide activity and underground erosion in crys-

talline schist is proposed; the Zentoku landslide,

Shikoku Island, serves as an example. This mecha-

nism is based on the results of:

Fig. 14. Chain interrelationship of processes of erosion and (1 ) measurements of sediment discharge from

landslide movement in the Zentoku landslide. groundwater outlets (springs);(2) measurement of landslide activity; and

(3) investigation of the groundwater flow (paths)slide activity. Erosion susceptibility inside the

landslide mass increases. The latter process is within the landslide.

The conclusions are as follows:supported by the following facts:

(1 ) Fukuoka (1991) carried out ring-shear tests (1 ) Groundwater flow exists in and above shear

zones in the landslide because the bases ofon samples of material from the Zentoku

landslide under a normal pressure of 294 kPa. these zones are highly weathered, clayey and

impermeable. Because of this groundwaterThese tests indicated that, as shear displace-

ment increases, the degree of grain crushing flow, underground erosion occurs in and above

the shear zones.also increases. The depth of the shear zone at

the Zentoku landslide (Block 1) is >20 m. It (2) Erosion and transportation of sediments are

caused not only by the effects of precipitationis reasonable to believe that crushing of the

rocks due to the weight of the landslide mass and resulting groundwater discharge, but also

by the effects of landslide activity. In particu-occurs in the shear zone.

(2) Seismic investigation has revealed that lar, there is a proportional relationship

between landslide displacement during highgroundwater flow paths exist in and above the

shear zones. The rocks have been crushed into levels of landslide activity and the sediment

discharge ratio (W/WP

or W/WQ

) for periodsfine-grained particles due to the landslide

activity and these particles were eroded from of 12 months (from 5 to 7 weeks) after high

levels of landslide activity. This relationshipthe mass along the groundwater paths.

(3) Sediment discharge from an outlet of the indicates that erosion susceptibility is

increased by landslide activity.groundwater path (i.e. at a spring) included


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Fujita, T., Hirano, M., Hada, S., 1976. The structural control(3 ) Active landslide movement causes disturbanceof landslides in the Kawai area, Tokushima Prefecture, Shi-and crushing of rocks into erodible and trans-koku. J. Jap. Landslide Soc. 13 (1), 2536 (in Japanese).

portable material in the shear zones. Fukuoka, H., 1991. Variation of the friction angle of granular(4 ) The mechanism of creep movement in this materials in the high-speed high-stress ring shear apparatus:landslide in crystalline schist is an interrelated influence of re-orientation, alignment and crushing of grains

during shear. Bull. Disaster Prevention Res. Inst. Kyotochain process consisting of underground ero-Univ. 41 (Part 4), 243279.sion caused by landslide activity and landslide

Furuya, G., Sassa, K., Fukuoka, H., Hiura, H., 1997. The rela-activity caused by underground erosion. The

tionship between underground erosion and landslide move-interrelationship of these two processes is the ment in a crystalline schist landslide, Zentoku, Tokushima,reason why landslides of this type have contin- Japan. J. Jap. Landslide Soc. 34 (2), 916 (in Japanese).

Gillon, M.D., Hancox, G.T., 1992. Cromwell Gorge landslides:ued to move for many years, and are nota general overview. In: Bell, D. (Ed.), Proc. 6th Int. Symp.easily stabilized.Landslides, Christchurch, vol. 2, Balkema, Rotterdam,

pp. 83102.Acknowledgments Hiura, H., Sassa, K., Fukuoka, H., 1991. On the mechanism

of a crystalline schist landslide: landslide movement and the

underground erosion. In: The Soviet-China-Japan Symp.The authors wish to thank the Yoshino Riverand Field Workshop on Natural Disasters, Shanhai, Lanch-Sabo Work Office of the Ministry of Constructionzou, Urumgi, Alma-ata, Dushanbe and Kazselezashchita,Japan for its cooperation in monitoring theU.S.S.R, pp. 2130.

Zentoku landslide. The efforts of Mr Kin-ichiroHiura, H., Sassa, K., Fukuoka, H., 1992. Monitoring system

Mukai and Mr Michifumi Mukai, who have been of a crystalline schist landslidethree dimensional displace-engaged in this monitoring and the maintenance ment meters and underground erosion. In: Bell, D. (Ed.),

Proc. 6th International. Symp. on Landslides, Christchurch,of the monitoring apparatus for several years, arevol. 2, Balkema, Rotterdam, pp. 11411146.especially appreciated.

Kato, J., Hada, S., 1980. Landslide of the Yoshino-gawa water

system and its geological aspects. Research Reports of Kochi

University, Natural Science, 28, pp. 127 140 (in Japanese).

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