ORIGINAL PAPER

Deep-Seated Slope Failures Induced by InappropriateCutting in China

Hong-Yue Sun • Yu Zhao • Yue-Quan Shang •

Yang Yu • Quan-Li Zhao

Received: 5 December 2011 / Accepted: 14 July 2012 / Published online: 25 August 2012

� Springer-Verlag 2012

Abstract Given insufficient geological investigation and

inadequate interpretation of geological settings, remedial

works for localized cut slope collapses may induce large-

scale failures and cause remarkable damage, as well as

economical loss. A number of recent reports have addres-

sed individual large-scale failures due to inappropriate

cutting, but the systematic classification of failure patterns

has received less attention. In this study, a re-profiling

triggered landslide is described in detail. The deep slip

surface is located by field measurements; then, the stability

of the slope before and after cutting is assessed with the

limit equilibrium method. Three types of slopes prone to

deep-seated failures are introduced: the loose deposits type,

the ancient landslide type, and the deep adverse disconti-

nuities type. The mechanism of each failure pattern is

illustrated with a case study. The stability analyses indicate

that inappropriate slope profiling may greatly reduce the

factor of safety (FS) of a slope. Recommendations are

given for mitigating the deep-seated landslide induced by

inappropriate cutting, and a case history of successful

measures is presented.

Keywords Cut slope � Deep-seated landslides �Geological investigations � Profiling �Limit equilibrium method

1 Introduction

During expressway construction in mountainous areas,

slope cuts are frequently designed to run along the toe of a

natural slope, as shown in Fig. 1. Landslides may occur

within slope cuts (Voight 1979) due to inappropriately

designed profiles without due geological investigations

(Fuchsberger 2008; Lee and Hencher 2009) or rainfall-

induced pore water pressure (Okagbue and Ifedigbo 1995;

Sugiyama et al. 1995; Raj 1998; Yoo and Jung 2006;

Abderahman 2007; Lee and Hencher 2009). Moreover,

human activities and erosion/slaking encourage the

undercutting or degradation of slope cuts, which also affect

slope stability (Odemerho 1986; Malkawi and Taqieddin

1996).

To mitigate cut slope failures, re-profiling is one of the

most commonly used corrective measures. The use of

re-profiling may increase the factor of safety (FS) of the cut

slopes while reducing the FS of the natural slopes, since the

location of the neutral points is always dependent on that of

the slip surfaces (Hutchinson 1977). Generally, little

money is spent on ground investigation for cut slopes in

China, and the target area of investigations and stability

analysis is always limited to the zone of cut slopes. Thus,

the potential for large-scale landslides along deep slip

surfaces is frequently ignored. Although individual deep-

seated landslides induced by inappropriate cutting were

reported in China (Deng et al. 2009) and in Korea (Lee and

Hencher 2009), less attention has been paid to the geo-

logical conditions that are prone to such failures.

In this study, a re-profiling triggered landslide is first

described in detail. The deep slip surface is located by field

measurements, and then the stability of the slope before

and after cutting is assessed with the limit equilibrium

method. Three types of slopes prone to deep-seated failures

H.-Y. Sun

Department of Ocean Science and Engineering,

Zhejiang University, Yuhangtang Road,

Hangzhou 310058, China

Y. Zhao (&) � Y.-Q. Shang � Y. Yu � Q.-L. Zhao

Department of Civil Engineering, Zhejiang University,

Yuhangtang Road, Hangzhou 310058, China

e-mail: zhao_yu@zju.edu.cn

123

Rock Mech Rock Eng (2012) 45:1103–1111

DOI 10.1007/s00603-012-0292-4

are introduced, and the mechanism of each failure pattern

is illustrated with a case study. The description of rocks

and soils in this paper mainly follows the terminology used

by Geotechnical Engineering Office (GEO 1988).

2 A Typical Large-Scale Landside Induced

by Toe Disturbance

Although the proportion of more intensely weathered rock

is generally closer to the ground surface, the existence of

deeply embedded weak planes should not be neglected.

The complex discontinuities of a slope require detailed

geological investigation before cutting a slope. Further-

more, due attention should be paid to the possibility of

further damage in a wider range once a localized failure

occurs. Despite the lack of sufficient geological investi-

gations and stability assessment, a slope was re-profiled

after a small-scale failure along the S26 expressway in

Zhejiang Province, China, and, consequently, deformed in

a wider range.

2.1 General Description of Hongyan Landslide

In 2005, it was decided to form a cut slope in a natural

hillside near Hongyan (longitude 120�300E, latitude

28�520N) of Xianjun County in China, to allow the con-

struction of the Zhuyong (S26) expressway. The two-lane

carriageway was designed to pass the toe of a natural slope

with an original slope angle ranging from 25� to 40�. The

right lane (close to the river) was on the bridge, while the

left lane (close to the mountain) was beside the cut slope.

The original design of the cut was less than 10 m, and no

large-scale excavation was involved. In August 2005, a

temporary cut of 6 m was formed at the toe to construct the

bridge, as shown in Fig. 2. During a 593-mm rain event

from 9 November to 14 November [China Meteorological

Administration (CMA), China Meteorological Data Sharing

Service System; http://cdc.cma.gov.cn/], the groundwater

table was uplifted, and water started to seep out from the

cut face. The groundwater level indicated by waters at the

toe and boreholes is shown in Fig. 3. At about 2 pm on 16

November 2005, a landslide mass (HP1) slid out after

continuous rain and turned over the bridge peers, as shown

in Fig. 2. The landslide was approximately 75 m long and

80 m wide, with a total volume of about 45,600 m3. The

scarp is located at the elevation of 430 m, approximately

40 m above the toe (Fig. 3).

2.2 Geological Setting

The study area is located within very thick (5–15 m)

quaternary deposits (el-dlQ) that are mainly composed of

yellowish-gray clay with gravel contents. The gravel level

of the quaternary clay-gravel is seen just above the older

Fig. 2 Photograph of the Hongyan landslide

Fig. 1 A natural high slope above the cut slope

0

Loose deposits

410

430

390

20 40 60 80 100 120

Landslide HP1

Original terrain line

Hubakeng RiverSlip surface

Tuff rock

Elevation(m)

Highly weathered

Moderately weathered

Slightly weathered

Groundwater table

Fig. 3 Cross-section of the Hongyan landslide (HP1)

1104 H.-Y. Sun et al.

123

rocks (Upper Jurassic J3x, gray-purple crystalline tuff

rocks). Three major joint sets trending 330�/76�, 118�/73�,

and 25�/8� (direction/dip) fragment the rock mass into

pieces of about 0.2–0.5 m in length. The joints are mostly

steeper than the slope angle, which generally do not form

direct sliding planes.

The soil of the quaternary formation is highly porous

and permeable, and contacts with the underlying weathered

and jointed rocks. Rainwater generally seeps quickly

downward through the quaternary deposits into bedrock

fissures and stops by the intact rock. The groundwater

outflow at the outcrops of the toe indicates that the

downslope flows are mainly through the stratum of highly

weathered bedrock.

2.3 Remedial Measures

At the beginning of 2006, another geological investigation

was conducted. In accord with the engineering conditions,

several reinforcement methods were installed to improve

the stability of the slope, as shown in Fig. 4. First, stabi-

lizing piles were designed to penetrate the loose deposits

and highly weathered rock to reach stable rock. From

August 2006 to December 2006, two rows of stabilizing

piles (rows A and B) were constructed in the upper portion

of the slope. Between July 2007 and August 2007, the

slope was excavated to seven levels to reduce the weight of

the slide body and to allow the construction of bridge

peers. However, the piles were not long enough to stabilize

the slope due to poor interpretation of the borehole data.

During the excavation works, two large tension cracks

(L1 and L2) appeared above the highest cutting face, and

both cracks were continuously deformed. Construction was

forced to stop. In 2007–2008, the owner chose another

corporation to take over the project due to the poor per-

formance of the previous company. The new company

carried out another investigation and proposed two more

rows of stabilizing piles (rows C and D) to be installed, as

shown in Fig. 4. Field measurement of the slope started at

the beginning of 2009.

2.4 Field Measurement

To observe the behaviors of the slope, both shallow and

deep deformation was monitored from January 2009.

Figure 5 plots the variation of the horizontal displacement

of four monitored points measured at the slope surface

(P04 and P08 are on the slope surface, not on top of the

piles). Over a period of 9 months, the displacement grew

linearly at a rate of 0.1–0.2 mm/day. The movement of the

monitored points seems to show a similar trend, which

indicates that the slope was moving as an entire body.

Three inclinometers were installed at cx01, cx03, and

cx06 to measure the deep displacement (Fig. 4). The dis-

placement is first plotted on the cross-section, and then the

maximum dislocated locations of the inclinometers are

connected with a smooth curve. Combined with ground

crack locations, two potential sliding surfaces are obtained

as shown in Fig. 4. It is obvious that the supporting piles

were too short to provide any resistance at any of the

potential sliding planes.

2.5 Slope Stability Analysis

The stability of the natural slope at different excavation

stages was assessed with the Morgenstern–Price method

580

100

560

540

520

500

480

460

440

420

400

380

360

120 140 160 180 200 220 240 260 280 300 320 340 360 380

P04

P08

P16

P19

cx06cx01

cx03

Stabilizing piles of row CStabilizing piles of row D

Crack L1

Crack L2

Stabilizing piles of row A

Stabilizing piles of row B

Expressway

Projectedslip surface

Initial profile

Original terrain

Modified profile

Groundwatertable

Fig. 4 Profile of the Hongyan

slope: reinforcement measures,

monitoring points, and potential

slip surfaces

Deep-Seated Slope Failures Induced by Inappropriate Cutting in China 1105

123

(Morgenstern and Price 1965), within the framework of

limit equilibrium methods. The mechanical parameters at

the slip surface are not accessible due to the complex

geological structures of the slope. Thus, the parameters C

and u were obtained by inversion analysis assuming that

the slope was in a critical state after re-profiling (the lowest

slip surface and the groundwater table in Fig. 4 are used).

Then, the FS of the slope along the same slip surface was

calculated at the original terrain and after the initial cut. As

illustrated in Table 1, the FS decreased by less than 0.03

after the temporary cut was formed, but it decreased by

0.41 after re-profiling. Thus, re-profiling was the main

factor that induced the movement of the slope along the

deep slip surface.

3 Deep-Seated Landslides Induced by Inappropriate

Slope Cutting

Generally, slope classifications based on geological

conditions are developed from the RMR classification

(Bieniawski 1989) and the Q classification (Barton et al.

1974). SMR (Romana 1991) is a well-known geological

conditions-based method that uses the summation of

weighted contributing factors. It is also used to qualita-

tively assess slope stability (Umrao et al. 2011). However,

it is not suitable to be implemented in this study for several

reasons. First, it is less effective when failures of slopes are

dominated by one single contributing factor. Second, the

study contains not only rock slopes but also slopes covered

with loose deposits. Third, a deep-seated landslide often

covers a large area where the geological conditions varied

spatially; thus, it is difficult to collect complete data or to

choose representative parameters. Therefore, the case his-

tories are practically classified using major geological

features instead of the more sophisticated SMR method.

Besides the high quasi-homogeneous type, deep-seated

landslides induced by toe disturbance usually take place in

three types of geological environments: slopes with thick

deposits, ancient landslides, and slopes where adverse

discontinuities were excavated and exposed. Landslides in

thick deposits comprise 80 % of all landslides in Zhejiang

Province, China (The Ministry of Land and Resources

2003). The deposits are usually formed from slope sedi-

ments, collapse deposits, landslides deposits, and a small

amount of fluvial sediments. Even cutting merely 3 m into

the toe may substantially increase the risk of landslide

(Zhou et al. 2006). In addition, underground water is usu-

ally active at the interface between loose deposits and

bedrock due to the difference between their permeability.

Under the effect of self-weight and the flows of ground-

water, a large amount of fine clay particles and organic

matters deposit near this interface that becomes a potential

slip surface. Cutting into the passive section of such a slope

may induce creeping deformation and may later cause non-

continuous sliding along certain secondary slip planes.

Numerous colluvium slopes slide along the interface

between the loose deposits and bedrock during the cleaning

process of small-scale failures.

Ancient landslides are remarkably sensitive to environ-

mental changes. The foot of an ancient landslide is often

the passive section and, thus, is the key part to sustain the

stability of the slope. Removal of the toe will probably

reactivate the ancient landslide. In the Austrian Alps, the

stability of an ancient landslide was ignored prior to

expressway construction. A minor excavation near the toe

reactivated the landslide, and the expressway was pushed

outward (Fuchsberger 2008). In the West Hubei Province

of China, digging a trench for natural gas first induced

a localized slope failure and then triggered the huge

Baiyangping landslide (Zhu 2010).

Discontinuities, such as faults, joints, schistosities, and

contact interfaces, endow slopes with a complex structure

and discontinuous mechanical properties. The physical and

mechanical properties of rock masses vary spatially. In

many situations, adverse discontinuities might not be rec-

ognized prior to the construction of a cut slope. If due

attention is not paid to the adverse discontinuities exposed

during construction, the cut slope and even the natural

0 20 40 60 80 100 120 140 160 180 200 220 240-10

0

10

20

30

40

50

60

70D

ispl

acem

ent i

n ho

rizo

ntal

dir

ectio

n /m

m

Time /d

P04 P08 P16 P19

Fig. 5 Horizontal displacement of monitoring points during the

period 10 March 2009 to 10 November 2009 (refer to Fig. 4 for point

locations)

Table 1 Parameters of the Hongyan slope obtained from the back-

ward and forward analysis

Unit

weight

(kN/m3)

Cohesion

C0 (kPa)

Friction

angle u’

(�)

FS after

re-profiling

FS before

re-profiling

FS of

original

slope

22.5 35 42 1.00 1.41 1.44

1106 H.-Y. Sun et al.

123

slope behind it may be endangered. For instance, com-

pressive discontinuities dipping downslope were exposed

during slope profiling for the Meihe Expressway KII1 in

Guangdong, China; consequently, a deep-seated landslide

was triggered (Wang 2006). In the following subsections,

one case is presented for each type of failure pattern.

3.1 Loose Colluvium Slopes

The Xiaodan landslide in Southern Zhejiang Province,

China, is a typical example of the loose deposits type. An

existing national road was to be widened for the con-

struction of a new expressway; the cut slope was designed

as 4 m maximally (Fig. 6). The natural slope is 30�–40� at

the base and 15�–25� at higher ridges. Soon after the toe

was removed, transverse cracks appeared above the cutting

surface, and then localized slope failures took place. In

addition to the low strength of clayed gravel, precipitation

in the rainy season also made the steep slope prone to

localized failures. The collapses were considered to result

from the poorly designed slope profile and the rainy season.

Thus, the collapsed soil was cleaned in the usual way

without any extra attention, causing progressive cracking at

the upper part of the slope.

A reinforced-concrete retaining wall (4 m high) was

then installed at the toe to prevent it from collapsing again.

However, the drainage system of the retaining wall was

blocked due to the poor construction conditions in the rainy

season. As a result, groundwater stopped and accumulated

behind the wall, which increased the groundwater level

near the toe. Obviously, this failed to prevent further

cracking at the slope surface behind the wall (Fig. 7). To

continue steady construction without a detailed geological

investigation, large-scale re-profiling was performed to

prevent further collapses. Then, the Xiaodan landslide

occurred along the deep slip surface, as shown in Fig. 7.

The maximum length and width of the slide mass was

approximately 140 and 135 m respectively, with a thick-

ness of 15–20 m.

Assuming that the strength and other parameters of the

deep slip surface were the same before and after profiling,

the slope stability was assessed with the methodology

discussed in Sect. 2.5. As shown in Table 2, the FS

decreased by merely 0.01 after initial profiling, but there

was a decrease of 0.13 after large-scale re-profiling. The

massive landslide was triggered by two serious mistakes in

the remedial measures. First, a timely geological investi-

gation was not conducted; thus, it was ignored that the

entire slope might slide along the deep sliding plane.

Second, inappropriate slope profiling reduced the weight of

the passive section and, thus, increased the possibility of a

large-scale landslide.

3.2 Ancient Landslides Type

The failure of slope K81 along the Jinhua–Lishui–Wenzhou

expressway in Zhejiang Province, China, is a typical

example of the ancient landslide type. The expressway was

designed to bypass the toe of a natural slope whose slope

angle is 35�–42�. The slope consists primarily of volcanic

tuffs of Upper Jurassic strata, showing a fairly clear

boundary between the fractured rock and relatively intact

rock in Figs. 8 and 9. The relatively intact rock has mainly

two different sets of joints trending 320�–340�/80�–90�(direction/dip) and 215�–225�/50�–60�, respectively. Among

the complex joints in the fractured zone, the dominated set

trends 150�–160�/50�–60�, the direction of which is almost

opposite compared with that of joints in the relatively

intact rock. The contrasting joint attitudes indicate that the

rock of the fractured zone had once undergone rotation in

historical sliding.

The cut slope was initially designed to be less than 30 m

high (Fig. 10). However, a localized collapse occurred

during the excavation; the failure was attributed to the low

Highway

Horizontal Distance (m)

180170160150140130120110100908070605040302010

Bedrock

NW334

110

Elevation (m)

100

80

90

70

60

30

40

20

10

50Loose Deposits

o

OriginalRoad

160 170 180

Subgradeexcavation

Fig. 6 Localized slope failure

induced by the toe cutting for

expressway construction

Deep-Seated Slope Failures Induced by Inappropriate Cutting in China 1107

123

strength of the fractured rock. To solve the problem, the

slope was re-profiled to a 100-m cut; then, two rows of

anchors with 6-m spacing and of length 10–15 m were

installed on each stair. The spacing and length of anchors

was designed to protect each step from localized collapse.

After cutting, field engineers found that the slope

deformed in a wider range. Thus, an in-depth geological

investigation and field survey, including borehole drilling

and geological radar detection, were conducted. The

investigation permitted us to map the geological section at

A–A0 in Fig. 10. A deep slip surface due to rock friction

was evident from the exposure of the cut face and from the

video taken using a borehole camera. The slip surface has a

plane of weakness embedded that consists of discontinu-

ously distributed clay. Based on the geological conditions,

it was confirmed that the slope was an ancient landslide

type. Eventually, further anchoring into the ancient land-

slide was performed even after slight anchoring was

complete. In the adjustment, the strength parameters of the

slip surface are calculated by backward analysis assuming

the FS of the slope taking a value of 1.0 after re-profiling

(Table 3). To ensure that the FS is no less than 1.25,

another 380 anchors were installed and pre-stressed to tie

back the slope. The anchor cables had a pre-stress force of

750 kN and lengths varying from 22 to 40 m based on the

geological conditions.

The contribution of initial profiling and re-profiling was

also assessed (Table 3). The excavation of the 30-m cut

slope reduced the FS of the ancient landslide by 0.1.

However, 553 m3 of rock mass per unit width (1 m) were

excavated in the re-profiling. The slope cutting reduced the

FS by 0.11. Hence, slope re-profiling without an in-depth

geological investigation was the immediate cause that

reactivated the ancient landslide. Although the final

anchoring plan ensured the stability of the slope, it caused

enormous economic losses.

3.3 Adverse Discontinuity Type

The landslide K103 along the Hangzhou–Jinhua–Quzhou

expressway in Southeastern China was induced by the

excavation and exposure of adverse discontinuities at the

toe. The slope of the original terrain was 20�–35�, where

the massive rock at the surface is underlain by adverse

discontinuities. Initially, an 18-m cut was formed, as shown

in Fig. 11. However, during construction, tension cracks

and superficial collapses occurred several times. The cut

was then arbitrarily adjusted to a height of 45 m without an

in-depth geological investigation. A retaining wall was also

constructed to withstand the slope behind it.

After the expressway came into operation, the retaining

wall was found to be toppled and cracked. Numerous

cracks were evident on the middle to the upper part of the

slope. The largest crack was opened to almost 1 m wide

Loose Deposits

50

10

20

40

30

60

70

90

80

100

Elevation (m)

110

NW334

Bedrock

20 40 60 80 100 120 140 160 180

Horizontal Distance (m)

Highway

Cut Zone

10 30 50 70 90 110 130 150 170

o

Fig. 7 The Xiaodan landslide triggered by large-scale slope cutting

140400 20 60 80 100 120

Elev.221m

Elev.246m

Elev.280m

340240160 180 200 220 260 280 320300 360

water interception trench

Fig.9

fractured rock mass

relatively intact rock mass fractured rock massboundary line offractured rock mass

A

A'

Fig. 8 Rock profile of the cut

face of the K81 slope

Table 2 Parameters of the Xiaodan slope obtained from the back-

ward and forward analysis

Unit

weight

(kN/m3)

Cohesion

C (kPa)

Friction

angle u(�)

FS after

re-profiling

FS before

re-profiling

FS of

original

slope

20.3 19 20 1.00 1.13 1.14

1108 H.-Y. Sun et al.

123

and sheared 0.5–0.8 m out-of-plane. The cracked and dis-

located retaining wall, tilted drainage ditch, and cracked and

uplifted road all indicate that the entire slope was losing its

stability and threatening the safety of the expressway. As

shown in Fig. 11, the landslide is 400 m long and 140 m

high, with a thickness of 15–40 m. The total volume of the

slide mass is estimated to be 160 9 104 m3.

After the deep-seated landslide occurred, a 10�–25�dipping fault was revealed by a series of borehole drillings.

The fault zone is 1–13-m thick and mainly composed of

highly weathered tuff and tuffaceous sandstone (polyhedral

structure, taupe or maroon, some joints filled with maroon

clay). The rock masses are very weak and prone to soft-

ening in water. The rock above the fault zone is highly

weathered tuff stone and conglomerate (gray or purple,

largely disturbed), whereas the rock beneath the slip sur-

face is gray-purple conglomerate with good integrity.

The stability of slope was then assessed using the

geological cross-section in Fig. 11. Compared with the

initially designed cut slope (18 m), the re-profiling plan

(45 m) reduced the FS by 0.07 (Table 4). The exposure of

the deep adverse joins and the resistance reduction due to

the re-profiling were major factors contributing to the slope

failure.

4 Recommendations for Mitigating Slope Failures

Induced by Inappropriate Cutting

4.1 General Recommendations

Sufficient geological investigation is needed in order to

assess the stability of a slope and prevent failures. The

existence of any adverse geological features should be

determined before cut slope construction or at least after

cut slope failures. It is mandatory to describe the zonation

of rock units, detect the existence of any plane of weak-

ness, and assess all possible failure patterns.

It is important to analyze the causes of localized failures

and their effect on the stability of the natural slopes.

Table 3 Parameters of the K81 slope obtained from the backward

and forward analysis

Unit

weight

(kN/m3)

Cohesion

C (kPa)

Friction

angle u(�)

FS after

re-profiling

FS before

re-profiling

FS of

original

slope

21.8 25 34 1.00 1.11 1.21

Fig. 9 Boundary between relatively intact rock mass and fractured

rock mass

Ele

vatio

n (

m)

original slope terrain

Modified profile

Highway

anchor cable

clayey soil

failure plane

Initial designed profile

fractured rock mass

relatively intact rock mass

320

300

280

260

240

220

200

Fig. 10 The geological cross-section at K81 ? 820 (A–A0 in Fig. 8)

Mod

ified

Profil

eInitially Designed Profile

Original Terrain

200

180

160

140

120

100

80

60

40

20

80

60

40

20

Projected slip surface

Adverse discontinuities

Fig. 11 Profile of the K103

slope and the deeply hidden slip

surface

Deep-Seated Slope Failures Induced by Inappropriate Cutting in China 1109

123

Cutting the toe of a slope in a critical state likely induces

localized collapses that could be the beginning of a deep-

seated landslide. This possibility of deep-seated sliding

should be thoroughly investigated prior to further excava-

tion works, especially when the slope is a loose deposits

type, ancient landslide type, or adverse discontinuities type.

The principle, ‘‘reinforcement first and cutting follows’’,

is effective to avoid deep-seated landslides. The most

effective measure is to reinforce the slope before or during

cutting in order to maintain the stability of the slope.

Re-profiling should be carried out prudently. Note that

re-profiling reduces the stability of the slope above the

excavation line.

4.2 A Case of Successful Measures

Shangsan slope failure #2 along the Shangsan expressway

occurred after the slope toe was excavated. When an 8-m

cut was formed, a series of arcuate tension cracks appeared

on the head of the cut face. Then, the part below the cracks

collapsed, and the failure propagated progressively upward,

resulting in a step-like terrain (Fig. 12). If excavation

continued, a deep-seated landslide would likely have been

triggered.

The original slope angle is 15�–25� at the base and

10�–20� at the upper portion. The construction site is

within 10–15 m of clay–gravel stratum with crystallino-

clastic tuff rock lying beneath. The range of loose deposits

and the weathering profile of rock units were investigated

and interpreted in a rational manner (Fig. 13). Field engi-

neers determined that it is a slope of the loose deposits

type. To avoid prematurely re-profiling, its stability was

assessed along with the scale of potential sliding before any

remedial plan was made. Since the contents of gravels

varied spatially, the strength parameters are difficult to

obtain by laboratory tests. During the design, the strength

parameters of the slip surface were first calculated by

backward analysis, assuming that the slope was in a critical

state after the initial cut. Then, the strength parameters

were used to search for the most adverse slip surface within

the loose deposits and to assess the stability of the slope

along the boundary between the loose deposits and the

highly weathered tuff.

According to the analysis, reinforcement is required

before further excavation to protect the cut face and the

slope from deep-seated failure along the slip surface

(Fig. 13). A row of stabilizing piles was installed at the toe,

and then the slope was re-profiled to be step-like. The

expressway was constructed smoothly, and the slope has

not experienced any failures since.

5 Conclusions

The failure of cut slopes often results from insufficient

geological investigation prior to design and the rainy sea-

son after construction. If collapsed rock/soil masses are

directly removed without a comprehensive understanding

of the geological environment, this can increase the pos-

sibility of deep-seated sliding. Once a slope is deformed in

a wide range after toe cutting, the risk of a large landslide

should be thoroughly investigated.

Re-profiling a failed cut slope may induce deep-seated

landslides for the following types of slopes: loose deposits

type, ancient landslide type, adverse discontinuities type, or

high and steep type. Once a cut slope failure occurs, the

existence of adverse geological features should be ade-

quately investigated and mapped. The type of geological

condition should be determined, and then remedial

211.52

202.72198.82

188.82

37.00

7.40

dlQ 3-4

delQ 32

dlQ 32

delQ 31

m

m

m

196.03193.43

Designed pavement level

187.83

Elevation(m)

Elevation(m)

8 m cut

Fig. 12 Cross-section of the #2 landslide along the Shangsan

expressway

1:1.5

600

778

500

250180

190

200

210

220

230

240

Expressway

Elevation (m)

201.76

209.76

Intercepting ditch

Intercepting ditch

Reinforced concrete piles180×250

Loose depositsHighly decomposed tuff

Moderately decomposed tuff

Fig. 13 Scheme of remedial measures for the #2 landslide along the

Shangsan expressway

Table 4 Parameters of the K103 slope obtained from the backward

and forward analysis

Unit

weight

(kN/m3)

Cohesion

C (kPa)

Friction

angle u(�)

FS after

re-profiling

FS before

re-profiling

FS of

original

slope

22.2 9 14 1.00 1.07 1.13

1110 H.-Y. Sun et al.

123

measures should be made accordingly. In addition, the

feasibility of re-profiling should be sufficiently discussed in

advance. If necessary, slope reinforcement should be made

before re-profiling is carried out.

Acknowledgments This research was financially supported by the

National Natural Science Foundation of China (40972187) and the

Key Innovation Team Support project of Zhejiang Province (2009

R50050).

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