b497bd01

8/2/2019 B497Bd01

http://slidepdf.com/reader/full/b497bd01 1/4

A STUDY OF STABILITY IN A LARGE EXCAVATED SLOPE

WITH LOCAL TORRENTIAL RAINFALLS

Byung Sik, Chun

1

, Soon Sung, Nam

2

, Hae Sig, Lim

3

, Yoo Hyeon, Yeoh

4

ABSTRACT

In this study, the stability analysis of slope failure that occasionally occurs when rain falls was carried out.

This paper focuses especially on the stability of large-scale cut slope exposed to heavy rainfalls.

The study was carried out on a cut slope which is 52 meters high and consists of soil and rock. Gradual

landsliding is in progress due to continual heavy rainfalls.

This paper describes slope stability analyses performed to assess the appropriate methods of stabilization.

For the numerical analyses, the slopes were divided into several parts by soil layers, slope height, and the

scale of the landslide.As a result, two different types of stabilization methods are recommended according to the condition of the

cut slope and the working area. For slopes with relatively low height and enough working area, it is

recommended that retaining walls are constructed at the lower end of the cut slope and soil-nailing method is

applied to flatten the slope. For the other slopes, soil-nailing method is recommended.

INTRODUCTION

At a highway construction site in the middle west part of Korea, landslides of large-scale soil and rock

slopes have occurred due to intense rainfalls. Therefore, proper stabilization methods were required to allow

construction continue. The studied slopes are largely excavated ones, which consist of soil and rock layers.

The maximum height of slope is 52m. Landslides were gradually developing, so the exact areas of landslide

and future movements were unpredictable.

Based on the information gained from geologic site exploration of the failed slopes, the slopes were

divided into several cross-sections (zones for analysis). For each cross section, slope stability analysis was

carried out and suitable stabilization methods were proposed.

SITE AND SLOPE CONDITION

According to geological investigation, from the ground surface the distribution of strata is alluvial soil

layer, weathered soil, weathered rock and soft rock. Weathered soil is in completely weathered

condition(CW) and weathered rock highly weathered(HW), soft rock slightly weathered. The entire slope

was divided in two sections of different slope height, slope failure zone, and soil strata. Representative

sections and their features are shown in Table 1. The study of geotechnical properties was carried out. Thesoils were classified as SP and SM by the Unified soil classification system.

Table 1 : Characteristics of Slope Section

Section Class Height of Slope Remark

Section 1 Left side of highway 25.0 m Tension crack occurs, shallow landslide

Section 2 Right side of highway 27.0 m Shallow landslide occurs

1Professor, Dept. of Civil Engineering, Hanyang University, Korea

2

President, Eun Jin Construction Engineering Co., Korea3Director, Housing Research Institute of Korea National Housing Corporation.

4Researcher, Construction Research Institute of Hanyang University, Korea

8/2/2019 B497Bd01


ANALYSES OF SLOPE STABILITY

SLOPE/W, a slope stability analysis computer program, was used. The data table for soil properties is

shown in Table 2. Considering the local torrential rainfalls during the rainy season, a constant Ru value of 0.3

was applied to the depth of 3m from the surface of the slopes to consider rainfall seepage in the rainy season.

Table 2 : Material Properties for Slope Analysis

Alluvial Soil(AS) Weathered Soil(WS) Weathered Rock(WR) Soft Rock(SR)

Section Cohesion

(t/m2)

Internal

Friction

Angle

(°)

Cohesion

(t/m2)

Internal

Friction

Angle

(°)

Cohesion

(t/m2)

Internal

Friction

Angle

(°)

Cohesion

(t/m2)

Internal

Friction

Angle

(°)

Section

10.0 28 0.8 32 2.0 38 7.0 43

Section

20.0 28 0.9 32 2.5 40 6.5 45

Stability analyses with SLOPE/W yielded the following results. At the higher part of slopes, surface

failure was shown in both sections as Figure 1 and Figure 2. The shape of slope failures and the failure zones

gained from the SLOPE/W analyses were similar to those of the real slopes.

5m AS

6m WS

4m WR

Water Table

17m, SR

a) for Dry Season(Fs=1.43) b) for Rainy Season(Fs=1.03)

Figure 1 : Results of SLOPE/W Analyses (Section 1)

8m AS

12m WS

7m WR

3m SR

a) for Dry Season(Fs=1.12) b) for Rainy Season(Fs=0.88)

Figure 2 : Results of SLOPE/W Analyses (Section 2)

The cut slopes have been disturbed and were in unstable condition due to the intense rainfalls. As the

failure zones expand, stabilization of the slope is necessary to avoid the progress of the landslide in the future.

8/2/2019 B497Bd01


For section 2 with relatively low slope and enough spare area, flattening of the slope is carried out and

additional stabilization method is applied afterwards. For section 1, soil nailing is applied as a stabilization

method.

To check stability of slopes, numerical analysis was carried out. NAIL11 which is developed in Korea was

used to perform slope stability analyses for section 1 reinforced by means of soil nailing. NAIL11 is a

computer program developed to analyze the stability of slopes reinforced by soil nailing. SLOPE/W was

used for section 2.

For section 1, analyses were performed with various horizontal and vertical nail intervals and nail lengths.

According to the analyses results, the nail length of 10 to 12m were installed as a 1.0m-1.6m horizontally

and 1.0m-1.2m vertically spaced grid at an angle of 15 to the horizontal plane in the excavated slopes.(Figure

3)

For section 2, the analyses with SLOPE/W were carried out changing the inclination of the slopes. The

required safety factor was 1.3. The inclinations of 1:1.2 for rock layers and 1:1.4-1:1.7 for soil layers were

achieved as a result of the analyses.(Figure 4)

Figure 3 : Pattern of Soil Nailing (Section 1) Figure 4 : Pattern of Cutting Slope (Section 2)

However, estimating the exact failure zone is difficult due to continuous landslides, so retaining walls are

constructed at the toe of the slopes and nails are installed in addition to the flattening of the slope method.

The zones between the section with flattened slope and the section with soil nailing are entirely reinforced

with soil nailing method to maintain the overall slope stability.

CONCLUSION

The results of the study on the stability of large cut slope that experienced torrential rainfall can be

summarized as follows.

(1) The studied slopes are largely excavated, and the highest slope is 52m. Due to intense rainfalls during

the rainy season, the slopes are in unstable state with developing landslide. According to geologic siteexploration, the slopes were divided into several areas based on soil layer condition, the slope height

and the landslide scale. Stability analysis was carried out for each area.

(2) As stabilization methods, for relatively low slope with enough spare area, flattening of the slope is

carried out and additional stabilization mainly soil nailing is applied afterwards. For other slopes, soil

nailing is applied as a stabilization method.

(3) The zones between the section with flattened slope and the section with soil nailing are entirely

reinforced with soil nailing to maintain the overall slope stability.

REFERENCES

Gassler,G.(1988). "Soil Nailing-Theoretical Basis and Practical Design", Proceedings of the International

Geotechnical Symposium on Theory and Practice of Earth Reinforcement, Japan

8/2/2019 B497Bd01


FHWA (1994). "Advanced Technology for Soil Slope Stability"

Bruce, D. A. & Jewell, R. A.(1986). "Soil Nailing : Application and Practice part 1", Ground

Engineering, pp.10-15

GEO-SLOPE International Ltd.(1997). "User’s Guide : SEEP/W Version 4”, Calgary, Alberta, Canada

b497bd01

Documents