A Fundamental Approach for an Investigation of Behavior Characteristics of the Vegetation Structures Using Seeded Sandbags

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1 A Fundamental Approach for an Investigation of Behavior Characteristics of the Vegetation Structures Using Seeded Sandbags H. T. Kim & S. D. Yoo Dept. of Civil Engineering, Hongik University, Seoul, Korea. S. S. Park Technical Division, LG Engineering & Construction Corp., Seoul, Korea. J. H. Lee DUSON Landscape Architecture Development Co,. Ltd, Dajeon, Korea. C. J. Lee Construction Division, Samsung Heavy Industries Co., Ltd, Seoul, Korea. ABSTRACT : In this study, a novel green retaining wall technology, named as GRW (Green Retaining Wall), was developed to enhance constructability, cost-effective and geoenvironment friendly system. Up to now, the GRW system has been investigated mainly focusing on an establishment of the design procedure. The analytical procedure for sand bag design has been proposed to evaluate frictional characteristics and stability of the GRW system. In addition, large-scale direct shear tests were performed to establish the frictional characteristics between the seeded sandbag and the connection unit of the GRW system. A stability analysis formula was proposed for the GRW system that can be applied to riverside or shoreline. Hydraulic experiments were performed to quantify the loss rate of sand from the sand bags. By conducting the safety analysis for the flow pressure proposed in this study, the maximum allowable velocity and the minimum required weight were calculated and compared with those of the conventional design cases. The insights into the behavior of the GRW system observed in this study will be reported and discussed. 1 INTRODUCTION The reinforced earth retaining wall system is defined as the construction method to secure stability and exclude or minimize the earth pressure by restraining the horizontal displacement using reinforced material containing high frictional forces with soil to improve the safety of embankments. Likewise the reinforced earth retaining wall system, the fabricated retaining wall system is the reinforced earth method to pile up concrete panels or blocks. The fabricated retaining wall system is known to have disadvantages such as the damaged scenic view, difficult delivery or constructability, and environmental contamination due to the usage of concrete panels and blocks. Recently, the green retaining wall system using seeded sandbags has received much attention because of its enhanced constructability, cost-effectiveness and environment-related issues. However, the green retaining wall system provided no design criteria for the frictional forces between sand bags and connection unit. The design criteria have not covered for the loss rate of sand in the sand bag and maximum allowable velocity in the green retaining wall system. Hence a new design procedure of GWR system is to be proposed when the GRW system is applied to riverside or shoreline. 225

2 GRW SYSTEM WHICH APPLIED SEEDED SANDBAGS 2.1 GRW System The GRW system studied in the current research is consists of sandbag, connection unit and geogrid. The connection unit of 0.286 m 0.16 m0.

2 and its properties are summarized in table 1. Fig. 1. Connection unit Fig. 2. Sandbag Table 1.

2 2 GRW SYSTEM WHICH APPLIED SEEDED SANDBAGS 2.1 GRW System The GRW system studied in the current research is consists of sandbag, connection unit and geogrid. The connection unit of m 0.16 m0.042 m has a spike and a hook to increase adhesion and disperse stress when the grid is pulled out as shown in Fig. 1. In addition, the sandbag used at the front wall is shown in Fig. 2 and its properties are summarized in table 1. Fig. 1. Connection unit Fig. 2. Sandbag Table 1. Properties of Sandbag Type of sandbag Type 1 (non-woven geotextile) Type 2 (non-woven geotextile) Type 3 (heavier non-woven geotextile) Size (m m) Facing area () Tensile strength (kn) Tearing strength (kn/) Permittivity (L/sec/) GRW system has advantage over the conventional methods in terms of constructability, cost and environment-related issues because it is built of sandbags and connection unit. Fig. 3 shows the plan and crosssectional view of the GRW system. (a) Cross-sectional view of GRW system (b) Plan view of GRW system Fig. 3 Plan and cross-sectional view of GRW system The sandbag which is used in the GRW system is lighter than concrete panel used conventionally and it has advantage of convey, store, and treatment. In addition, constructability is improved since there is no requirement of installing and leveling pad. Finally, there is not much possibility of development of crack because front facing is flexible, and hence it is expected that the sandbag can be possibly filled with other materials rather than sand. 226

2.2 Failure Modes of GRW System The failure mode of the GRW system is similar to the that of reinforced earth retaining wall (NCMA, 1977), but it has a fundamental difference because sandbags and

In the case of external stability analysis, it is predicted that sliding, bearing, over turn, and whole failure will occur and the general stability analysis technique of reinforced earth retaining

Also, in case of internal stability analysis, pull out, tensile over stress, and internal sliding can be applied in same way as shown in Fig. 4-(b).

Failure modes of GRW system Consequently, the frictional resistance between the sandbag and connection unit, when the reinforced grids are pulled out, is the main failure mechanism and hence

3 2.2 Failure Modes of GRW System The failure mode of the GRW system is similar to the that of reinforced earth retaining wall (NCMA, 1977), but it has a fundamental difference because sandbags and connection unit is used in the GRW system instead of using concrete blocks. The failure mode of the GRW system is illustrated in Fig. 4. In the case of external stability analysis, it is predicted that sliding, bearing, over turn, and whole failure will occur and the general stability analysis technique of reinforced earth retaining wall system can be applied in same way as shown in Fig. 4-(a). Also, in case of internal stability analysis, pull out, tensile over stress, and internal sliding can be applied in same way as shown in Fig. 4-(b). However, in the case of front facing stability analysis, a distinct method is needed because sandbags and connection unit is used instead of using concrete blocks in the GRW system. Fig. 4. Failure modes of GRW system Consequently, the frictional resistance between the sandbag and connection unit, when the reinforced grids are pulled out, is the main failure mechanism and hence experimental studies are probably needed. To estimate frictional resistance more correctly, an experimental investigation may be preceded about the shear strength, adhesion strength, and bending strength of the sandbags and connection unit. Also, to estimate connection strength between the geogrid and connection unit, an experimental study about the tensile strength of Hook may be performed. Base on these results, more feasible stability analysis technique can be determined by estimating the frictional resistance of the sandbag and connection unit (Fig. 4 (c)). 3 GRW SYSMEM FOR THE RIVERSIDE OR SHORELINE 3.1 Case Study The application of the GRW system to riverside or shoreline is shown in Fig. 5, and the properties of sandbags and installation methods are summarized in Table 2. (a) Application of seashore (b) Application of river bank (c) application of channel or pond Fig. 5 Applications of GRW system to riverside or shoreline 227

Table 2. The properties and installation methods of the GRW system Size of Sandbags : 0.8 0.6 0.3m Maximum height of wave : 0.55m Minimum height of application : 1.

0m/sec Installation slope : 1 : 2.5 Size of Sandbags : 0.6 0.3 0.15m, 0.8 0.4 0.2m, 0.8 0.6 0.3m Maximum applicable velocity : 1.0m/sec Installation slope : 1:1 3.

4 Table 2. The properties and installation methods of the GRW system Size of Sandbags : m Maximum height of wave : 0.55m Minimum height of application : 1.1m Over the maximum wave line Installation slope : 1:1 Size of Sandbags : m, m, m Maximum applicable velocity m : 3.0m/sec m : 2.5m/sec m : 2.0m/sec Installation slope : 1 : 2.5 Size of Sandbags : m, m, m Maximum applicable velocity : 1.0m/sec Installation slope : 1:1 3.2 Stability Analysis of GRW System for the application to the Riverside or Shoreline In the case of the GRW system applied to riverside or shoreline, a stability analysis can be performed if a force due to current (F) and a current resistance force (R) are determined. The force due to current (F) can be determined from the basic equation of hydraulics (Eqn. 1). 2 v F = CD γ w ε S 2g (1) Fig. 6 Free body diagram of submerged sandbag where, C D = resistance coefficient, γ w = unit weight of water, ε = shielding coefficient, g = acceleration of gravity, F = force due to current(ton), v = current, S = projected area. Moreover, the current resistance force (R) can be determined by estimating a coefficient of friction (μ) between the sandbag and the connection unit as shown in Eqn. 2. γ w R= µ ( γc γ w) V = µ 1 W γ c where, μ = coefficient of friction, R = current resistance force, V = Volume of sandbag, γ c = unit weight of sandbag, W= weight of sandbag in air. (2) From Eqn. 1 and Eqn. 2, a safety factor of sliding (FS sliding ) can be determined by following Eqn. 3. FS sliding γ w µ 1 W R γ c = = 2 F v CD γ w ε S 2g (3) 228

In this study, shielding coefficient is assumed to be unit, because the sandbags do not resist falling. In the same way, a safety factor of overturn can be determined (Eqn. 4).

1 Hydraulic Experiment The hydraulic experiment was performed at the hydraulic laboratory in Hongik University. Fig.

(a) Front and side view of circulation channel (b) Germinated sandbags Fig.

5 It is noticeable from Eqn. 3 that the factors which impact upon the safety factor of sliding (FS sliding ) are the weight of sandbag in air, shielding coefficient, coefficient of friction, projected area, and current. In this study, shielding coefficient is assumed to be unit, because the sandbags do not resist falling. In the same way, a safety factor of overturn can be determined (Eqn. 4). FS overtern = γ w x 1 W γ c y F (4) 4 EXPEREMENTAL STUDY TO DETERMINE DESIGN FARAMETERS 4.1 Hydraulic Experiment The hydraulic experiment was performed at the hydraulic laboratory in Hongik University. Fig. 7 shows properties and dimension of circulation channel and sandbags used in the experiment. The sandbag was filled with weathered granite soil and seeds after which it was germinated for 4 weeks. (a) Front and side view of circulation channel (b) Germinated sandbags Fig. 7 Circulation channel and germinated sandbags The germinated sandbags were installed in a circulation channel and measured a percentage loss of soil in sandbags according to increment of current and elapse of time. A sample was collected at every 30 minutes for 150 minutes and collected for both courses (straight and curved) arbitrary to measure the percentage loss in same condition of current. The sandbags used in the experiment were made of certain weight of 40 kgf and measured a weight and water content of wet sandbags to estimate the percentage loss quantitatively. Moreover, the current velocity was changed from 0.2 m/sec to 1.0 m/sec at a regular interval of 0.2 m/sec to estimate the percentage loss of soil according to increment of current. This hydraulic experimental was performed in the prototype scale in order to estimate the percentage loss of soil for the accurate measurement (see Fig. 8). (a) base installation (b) side wall installation Fig. 8 Installation of germinated sandbags 229

4.2 Large scale direct shear test Large-scale direct shear tests were performed to estimate the coefficient of friction. The measured coefficient of friction is about 0.76.

1 Percentage loss according to current The percentage loss of the sandbags measured from the current experiment is shown in Fig. 10.

6 4.2 Large scale direct shear test Large-scale direct shear tests were performed to estimate the coefficient of friction. The measured coefficient of friction is about The result of large scale direct shear test is shown in Fig. 9. Fig. 9 Results of large scale direct shear test 5 ANALYSIS AND DISCUSSION 5.1 Percentage loss according to current The percentage loss of the sandbags measured from the current experiment is shown in Fig. 10. It is found from the measurement that the percentage loss has maximum value of 5.4 % after 150 minutes. It is noted that the percentage loss is almost constant independent on velocity of the current. The maximum percentage loss by current is smaller than the maximum percentage loss by opening size (11 %) which assumed that a granule smaller than opening size is perfectly loosed. It is for this reason that a decrease in contact surface with water by overlap sandbags and root effects by the increments of saturation degree and vegetation. (a) Percentage loss according to current (b) Opening size of sandbag Fig. 10 Percentage loss according to current and opening size of sandbag 5.2 Percentage loss according to locations Fig. 11 shows that the percentage loss in same current where the straight course, the inside of curved course, and outside of curved course. It was found from the results that there is little difference according to course if the current is slow, but in fast current, the maximum loss occurs at outside of curved course and inside of course, straight course in order. 230

section 3 is applied because the coefficient of friction is generally less than unit.

7 (a) Current : 0.2m/sec (b) Current : 1.0 m/sec Fig. 11 Percentage loss according to location 5.3 Determine the minimum weight of sandbags which applicable to riverside or shoreline It is expected that the sliding failure mechanism is dominant when the stability analysis equation suggested in section 3 is applied because the coefficient of friction is generally less than unit. Hence, the minimum weight of sandbags which is applicable to riverside or shoreline can be determined by assuming that FS sliding of 1.5, γ c of 1.6 t/m 3, γ w of 1.0 t/m 3, ε of 1.0, C D of 1.0 and g of 9.8 m/sec 2 as shown in Fig. 12 and Table 3. Fig. 12 Minimum applicable weight of sandbags Table 3. Results of comparison Case Size (m) Weight of sandbags (kg) projected area () A B C D As Fig. 12 indicates, the minimum weight of sandbags is increased according to the increment of projected area and it is in general agreement with the design criterion which is suggested in table 2 when the current is slow, but it is not in good agreement when the current is fast. Consequently, it may be needed that application the sandbags which exceed the minimum weight or use wire net to control a floating of sandbags. 231

8 6 CONCLUSION The following conclusions can be drawn based on the present study: Large scale direct shear test shows coefficient of friction of about The maximum percentage loss by current is smaller than the maximum percentage loss by opening size (11 %) which assumed that a granule smaller than opening size is perfectly loosed. It is probably due to a decrease in contact surface with water by overlap sandbags and root effects by the increments of saturation degree and vegetation. It was found from the results that there is little difference according to course if the current is slow, but in fast current, the maximum loss occur at outside of curved course and inside of curved course, straight course in order. The minimum weight of sandbags is increased according to the increment of projected area and it is in a- greement with the design criterion which was suggested in table 2 when the current is slow but it is not in agreement when the current is rapid. When the GRW system is applied to the riverside or shoreline, it is desirable that apply the sandbags which exceed the minimum applicable weight or use wire net to control a floating of sandbags. REFERENCES James G. Collin, Design Manual for Segmental Retaining Walls, 2 nd Edition, NCMA, Virginia (1977). Kim. H. T. & Kang. I. K., The stability analysis and design of reinforcing stone wall, Journal of Korean Society of Civil Engineering, Vol. 12, No. 2, pp (1992). Kim. H. T., Park. S. S., Yun. J. Y., Park. K. N., & Lee. J. H., Basic Approach of Green Retaining Wall System using the Seed Spray Sandbags, Proceedings of the Korean Geosynthetics Society Spring Conference, pp (2003). Kim. H. T., Park. S. S., Yoo. S. D., Park. K. N., & Lee. J. H., Basic approach to investigate behavior characteristics of the vegetation structures using seeded sandbags, Proceedings of the Korean Geosynthetics Society Autumn Conference, pp (2003). Lee. K. H., A Case of River or Harbor Design and Construction, DAEDO Co. LTD., pp (1977). Robert M. Koerner, Designing with Geosynthetics, Prentice Hall, pp (1994). 232

CE 4780 Hurricane Engineering II. Section on Flooding Protection: Earth Retaining Structures and Slope Stability. Table of Content

CE 4780 Hurricane Engineering II Section on Flooding Protection: Earth Retaining Structures and Slope Stability Dante Fratta Fall 00 Table of Content Introduction Shear Strength of Soils Seepage nalysis