THE PERFORMANCE OF STRENGTHENING SLOPE USING SHOTCRETE AND ANCHOR BY FINITE ELEMENT METHOD (FEM)

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1 THE PERFORMANCE OF STRENGTHENING SLOPE USING SHOTCRETE AND ANCHOR BY FINITE ELEMENT METHOD (FEM) Tri Harianto 1*, Lawalenna Samang 2, Takenori Hino 3, Fakhriyah Usman 4 and Akbar Walenna 5 1 Associate Professor, Hasanuddin University/Institute of Lowland and Marine Research,Saga University; Tel: / ; Fax: ; tri@ilt.saga-u.ac.jp 2 Professor, Hasanuddin University; Tel: ; Fax: ; samang_l@yahoo.com 3 Associate Professor, Institute of Lowland and Marine Research, Saga University; Tel: ; Fax: ; hino@ilt.saga-u.ac.jp 4 Student, Institute of Lowland and Marine Research, Saga University; Tel: ; Fax: ; fakhriyah@ilt.saga-u.ac.jp 5 Student, Hasanuddin University; Tel: ; Fax: ; akbar@yahoo.com ABSTRACT The slope movement of the reservoir silo located near top of the slope was observed from field data and temporary fixed by installing bamboo piles in order to improve the stability of the slope. This technique is not sufficient to provide the slope stability in long time period. Fundamental problems that need to get slope strengthening solution are lack of drainage facilities to maintain the value of slope stability s safety factor (SF) within the acceptable value. Therefore, this study investigated the performance of the slope strengthening system with the installation of shotcrete and anchor. This study was carried out using Plaxis software. The results indicate that the stability of the slope strengthening by shotcrete and anchor increase the stability of the slope and also reduce the lateral movement significantly. Keywords: slope stability, strengthening, shotcrete, anchor, numerical modeling INTRODUCTION Slope on the location of reservoir silo is a 27 o slope which the upper side is burdened by 2500 tons of CPO reservoir silo. Layer of the upper slopes are dominated by silt and clay, whereas the bottom layer are sand and silty soil. The movement of soil on the slopes from the visual data appeared to have occurred and temporary fixed by installing bamboo piles. The main objective of this paper is to analyze the slope strengthening system with the installation of shotcrete and anchor. Fundamental problems that need to get slope strengthening solution are: a. Lack of drainage facilities to maintain the value of slope stability s safety factor (SF) within the acceptable value. b. Silo distributed load un the upper side slope of 2500 tons which is only 8 m from the edge of slope will greatly affect the decrease of slope stability from the original without any silo. INTERPRETATION OF SOIL PROFILE CROSS SECTION Fig. 1 Temporary reinforcement with bamboo piles From the results of soil investigation, cross section of the soil profile can be interpreted based on the data: Trend curve qc (kg/cm 2 ) and f r (%) sondir 2,5 ton for S2, S5 and S6. N-SPT value of boring log for DB2-DB3 for elevation ± 0.00 m and DB4 for elevation < ± 0.00 m. Value of soil parameters soil mechanics laboratory test results for undisturbed samples in DB3 and DB4. 285

2 Fig. 2 Predicted cross-section interpretation of the soil profile, the limit of standard fixities (boundary ) is hard soil NSPT > 30. Fig. 3 Ground deformations occurred for existing The soil parameter data used as an input for slope stability s calculation are shown in Tables 1 and 2. Table 1 The soil parameter from laboratory test results Soil γ wet γ dry Cu Layer v j Type kn/m 3 kn/m 3 kn/m 2 1 Silt - Clay Silt - Clay Silt - Clay Sand - Silt Sand - Silt (a) Table 2 The soil parameter from N-SPT correlation results Layer N-SPT E kn/m N-SPT correlation with E value (young s modulus): For Sand: E s 0,5 (N+15) [MPa] (1) For Clay: E s 0,6 (N+5) [MPa] (2) Fig. 4 (b) (a) Direction of soil movement and (b) the magnitude of soil movement for existing Safety factor (SF) approaches with landslide area classified up to 15 m depth (deep seated failure) from the top of slope. (Fig. 5) EXISTING CONDITION OF SLOPE STABILITY Analysis of slope stability was calculated by finite element method (FEM) use Plaxis 8.2 (2 D) software. The analysis results can provide an illustration of current s as follows: Deformation of soil at the lower slope is relatively larger than the upper slope areas. The amount of movement was up to 21 cm (Figs. 3 and 4). (a) 286

3 top edge of the slope, so it needs shotcrete and anchor to support the stabilization by adding load on the bottom of the slope (Fig. 7). (b) Fig. 5 (a) Amount of safety factor and (b) the field of landslide that occurred in existing. THE PROPOSED SLOPE STRENGTHENING SYSTEM Before select the slope strengthening system, there are several things to be concerned: Silo building has shallow foundation, so it becomes distributed load above the slope (q). A reduction in safety factor of slope before and after burdened by the weight of silo (Fig. 6), it shows that the weight of silo and the distance from the top edge of slope effected the reduction of SF. Fig. 7 The alternative solution for the stability of silo foundation ANCHOR INSTALLATION METHOD To ensure that the installation of anchor is secure for the stability of silo foundation, it is necessary to explain the steps on the ground anchor installation. Skyhook anchor is designed to get tensile resistance (pull out) from the ground by utilizing frustum cone at the end of anchor (Fig. 8). SLOPE CONDITION WITHOUT SILO Fig. 6 SLOPE CONDITION WITH SILO SF = Comparison of safety factor (SF) for existing slope The principle of economical slope strengthen is by adding load on the bottom of the slope, considering the case of cut and fill on slope will be risk on the stability of silo foundation. Considering the amount of silo load reaches q=98,3 kn/m 2 with a distance of only 8 m from the Fig. 8 Frustum cone which dimension is depending on the soil shear angle, anchor dimension, overburden tension and applied load The frustum cone thickness is approximately 10 times of anchor diameter (Fig. 9). The anchor is set into the hard soil of N-SPT=20 and located in the outside of landslide slip surface. Anchor installation carried out in a practical way to locate the anchor in the desired position, and then pull back the press stick continued with load locking process to lock the tensile strength of the anchor (Fig. 10). This installation process relatively does not have risk to silo foundation stability because there is no excavation (drill) and soil counterweight at the slope. 287

4 The propose specifications of slope strengthening are: Anchor: SH40 with installed distance every 2,00 meter and 12 m depth. Soil s capacity tensile permit NSPT = 22 approaches 20 kn. Shotcrete: used concrete quality K-225 or mixture of cement: sand: split = 1:2:3 with 15 m thickness. Single reinforcement required to resist tensile, shear and momen is #. Backfill: barrow form of solid soil with wet = 18 kn/m 3. RESULTS OF SLOPE STABILITY WITH STRENGTHENING SYSTEM Fig. 9 Comparison of frustum cone dimension Analysis of slope stability is calculated with finite element method (FE) using Plaxis 7.2 software. The analysis results can provide an illustration of post-reinforcement slope as follows: Deformation of soil for the lower slope is relatively same with the upper slope. The movement approaches 6.5 cm (Figs. 11 to 13) SF approaches 1,370 with the field of landslide move until it reaches the limit of hard soil N-SPT > 30. (Figs. 14 to 15). The forces working at anchor and shotcrete is presented in Table 3 and Figs. 16 to 18. (a) Fig. 11 Direction of soil movement for strengthen (b) Fig.10 (a) Practical anchoring process and (b) field implementation by modified back-hoe with jack. Fig. 12 The magnitudes of soil movement for strengthen. 288

5 Fig. 13 Mesh displacement for strengthen Fig. 16 Moment field of shotcrete that occur in the strengthen. Fig.14 The amount of safety factor on the strengthen Fig. 17 Shear field of shotcrete occur at strengthen Fig. 15 Slip surface of landslide which is occurred in the strengthen Fig. 18 Axial field of shotcrete which occurs in strengthen. 289

6 Table 3 The forces work at anchor SH40 12 m Node-tonode Anchor F [kn/m] Fmax [kn/m] 1 9, (-) Compressive, (+) Tensile CONCLUSIONS Using shotcrete and anchor as the strengthening system of slope is recommended to support slope stabilization. The reason as follows: Deformation of soil for the lower slope is relatively same with the upper slope. Reduce the soil movement significantly from 21 cm to 6,5 cm. Increase safety factor from 1,177 to 1,370. ACKNOWLEDGEMENTS This paper is the achievement of the field research project supported by Sinar Mas Agro Resource-Sungai Buaya Mill Lampung, Indonesia. REFERENCES Abraham,L. W., Lee, T. S., Sharma, S., & Boyce, G. M Slope Stability and Stabilization Methods, 2 nd ed., Wiley, Hoboken, NJ. Bowles, J.E., 1984, Physical and Geotechnical Properties of Soils, Mc. Grave Hill, Singapore. Bromwell, E. N The Stability of Slopes, 2 nd ed., Blackie, New York. Chandrupatta, Tirupathi R. & Belegundu, A.D Introduction to Finite Elements in Engineering. Prentice-Hall, Inc. Englewood. New Jersey. Dunn, I. S., Anderson, L. R., & Kiefer, F. W Fundamentals of Geotechnical Analysis, Wiley, Hoboken, NJ. Pumnia, B.C, Soil Mechanics and Foundations, Standard Book House, Delhi Ramiah, B.K & Chikanagapa Soil Mechanics and Foundation Engineering, Oxford & IBH Publishing Co, New Delhi 290