Digital Proceeding of ICOCEE – CAPPADOCIA2017 S. Sahinkaya and E. Kalıpcı (Editors)
Nevsehir, TURKEY, May 8-10, 2017
Effectiveness of the Geogrid Wrapping on the Bearing Capacity of the Stone Columns Talha SARICI1, Bahadır OK2 and Ahmet DEMİR*3 İnönü University, Civil Engineering, TURKEY. (E-mail:
[email protected]) 1
2
Adana Science and Technology University, Civil Engineering, TURKEY. (E-mail:
[email protected]) *3
Osmaniye Korkut Ata University, Civil Engineering, TURKEY. (Corresponding author, E-mail:
[email protected]) ABSTRACT
The world population is increasing at an extreme rate. For this reason, the obligation of structuring has occurred even in the weak soils. When shallow foundations built on the weak soils, it is faced with several problems. It is necessary to improve the weak soils by using a soil stabilization method. Stone column method is one of the soil stabilization methods and its use is growing in all over the world. Soil stabilization by using the stone column method provides to increase bearing capacity of the soil and decrease amount of settlement. Some methods have been developed to improve the performance of stone columns. One of these methods is wrapping the stone column with a geogrid material. Using this method, the bearing capacity of stone columns is developed, amount of the settlement and the lateral bulging is reduced. In this paper, bearing capacity and settlement behavior of 5 cm diameter (df) model shallow foundation located on the soft clay soil in a steel tank which has a diameter (D) of 30 cm, 25 cm, 20 cm and 15 cm was investigated experimentally. Axial stress and settlement behavior of the shallow foundation resting on the unreinforced soft clay soil, stone column reinforced soft clay soil and geogrid wrapped stone column reinforced soft clay soil were researched, respectively. The effect of tank diameter was also investigated in tests. As a result of this paper, stone columns increase the bearing capacity and reduce the settlement. Performance of stone column is improved by wrapping with the geogrid. In addition, the bearing capacity and settlement behavior of stone column and geogrid wrapped stone column are influenced by tank diameter. As the tank diameter increases up to D/df=4, the bearing capacity decreases significantly. After this value, the bearing capacity decreases slightly. Keywords: Bearing Capacity, Experimental Study, Geogrid, Soil Improvement, Stone Columns
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1. INTRODUCTION
Soft soils have a great problems for geotechnical engineers due to their low bearing capacity and high compressibility. Stone columns which are the soil stabilization method provide a satisfactory method of support in soft soils, therefore there have been widely applied in practice [1, 2]. Stone columns are more popular because of their ease of installation and cost effectiveness compared to other methods [3]. Stone column can be used to enhance the strength and stiffness properties of ground intended to support large loaded areas (e.g., floor slabs, embankments) or small loaded areas (e.g., footings, strips) [4]. Also, it has been used to prevent the earthquake liquefaction of liquefiable soil, reduce the compression deformation of soil under the load [5] and create a drainage path in order to consolidation of soft clay can be achieved in a short period of time [6]. Many researchers showed that stone column method is suitable in many respects [7, 8, 9]. But, the load carrying capacity of the stone columns in very soft soils (cu < 15 kPa) is insufficient [3, 10]. Because, columns mobilize their strength from the lateral confinement provided by the surrounding soil [11]. This limitation was eliminated by wrapping the stone column with geosynthetics to provide the required lateral support to columns installed in extremely soft soils [12]. In addition, geosynthetic wrapping acts as a filter between soft soil around stone column and granular material used in stone column construction. In this way, the effective drainage and avoiding contamination of the granular material are provided. The idea of wrapping the stone columns was thought for the first time by Van Impe in 1985 [10]. In recent years, geosynthetic products have been used to improve performance of stone columns [10, 12, 13]. Hasan and Samadhiya [14] carried out the laboratory model tests and numerical analyses on reinforced granular piles in very soft clay. They took into account short term loading condition. Reinforcement was performed in the form of vertical wrapping, horizontal strips and combined vertical-horizontal reinforcement. They examined the effect of reinforcement, shear strength of clay, encasement stiffness and length of granular piles. The results of their studies showed that significant improvement occurs in ultimate load intensity and stiffness of treated ground due to inclusion of geosynthetic. Ghazavi and Afshar [15] performed large scales laboratory tests on single and group stone columns with diameters of 60 mm, 80 mm, and 100 mm. Reinforcement with different lengths and reinforcing material were used in their tests and they were compared with unreinforced stone columns. Their study showed that bulging failure mode occurs at a depth of dc to 2dc from the surface (dc=stone column diameter) in single stone columns but failure mode in stone column group is a combination of bulging and lateral deformation. They found that as increasing the length and strength of reinforcing encasement, the ultimate capacity and stiffness of stone columns increase. In addition, reinforced stone column have shown that value of the load ratio with the same area replacement ratio depends on geometrical configuration of columns. Murugesan and Rajagopal [16, 17] investigated the performance of unreinforced and geosynthetic reinforced stone columns through numerical and experimental studies. They suggested that wrapping the stone columns with suitable geosynthetic is one of the ideal methods of improving the performance of stone columns and this phenomenon makes the stone columns stiffer and stronger. They also found that the load capacity of reinforced stone columns is not as sensitive to the shear strength of the surrounding soils as compared to unreinforced stone columns. Unit cell concept was used by researchers [12, 14, 18] to simplify the analysis of group of stone columns. The behaviour of group of stone columns under a uniformly loaded area was simplified as a single column constructed at the cylindrical unit of the approximate circular area representing the influence zone of a stone column. Although this concept is now well established, little research has been undertaken on the influence zone of geogrid reinforced 2
column under column area alone loaded.
Figure 1. Unit cell concept [12]
This paper investigates the results of a series of model tests that were undertaken to understand the behaviour of shallow foundation resting on the unreinforced soft clay soil, stone column reinforced soft clay soil and geogrid wrapped stone column reinforced soft clay soil. Model tests with column area alone loaded were used to find the settlement and axial stress behaviour of shallow foundation. All model tests were carried out on a 50 mm diameter shallow foundation in cylindrical tanks. A detailed experimental study on behaviour of shallow foundation is carried out by varying the diameter of the tank to find influence zone.
2. MATERIAL AND METHODS
The experimental program was carried out using the facility in the Geotechnical Laboratory of the Civil Engineering Department at the University of Osmaniye Korkut Ata. 2.1. Soil Properties Clay, crushed stone, and geogrid were used for model tests. Clay was dried and pulverized. The clay was sieved through 2.00 mm sieve to remove the coarser fraction and for easy processing and uniform water content. After conducting required conventional laboratory tests (sieve and hydrometer analysis, moisture content analysis, unit weight analysis, liquid and plastic limit analyses, unconfined compression test) the clay was prepared for model tests. The characteristics of the clay determined through an extensive testing program that consisted of a combination of laboratory and in situ tests were given in detail by Demir [19]. The properties of clay used in model tests is shown in the Table 1. The stone columns were formed from crushed stones, which was classified GP. Crushed stones (aggregates) of particle sizes between 10 and 2 mm. The particle size distribution for stone column and clay materials are shown in Figure 2. The properties of the crushed stones are given in Table 2.
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Table 1. Properties of clay
Parameter
Value
Specific gravity (Gs)
2.6
Liquid limit (%) (LL)
55
Plastic limit (%) (PL)
22
Classification
CH
Water content (%) (ω)
40
Natural density (kN/m3) (γn)
17.4
Undrained cohesion (kPa) (cu)
4
100
Percent Passing
80 60 40 Soft Clay Crushed Stone
20 0 0.001
0.01
0.1 1 10 Particle Size (mm)
100
Figure 2. Particle size distribution for soft clay and stone column materials [20]
Table 2. Properties of crushed stones used as stone column [20]
Parameter
Value
Specific gravity (Gs)
2.85
Density (kN/m3) (γn)
16.3 3
16.9
3
Minimum dry unit weight (kN/m ) (γmin)
15.2
Internal friction angle (Degree) (φ)
440
Uniformity coefficient (Cu)
1.67
Coefficient of curvature (Cc)
1.10
Classification
GP
Maximum dry unit weight (kN/m ) (γmax)
Geogrid material was used for wrapping of the stone column for reinforcement. Geogrid used in the experimental study, is commercially available from GEOPLAS Company. The properties of geogrid taken from GEOPLAS Company are shown in the Table 3. 4
Table 3. Properties of geogrid material [20]
Parameters
Values
Type of Material
Polypropylene
Weight per Unit Area (g/m2)
200
Max. Tensile Strength, md/cmd* (kN/m)
≥30 / ≥30
Tensile Strength of 2% Elongation, md/cmd* (kN/m)
12 / 12
Tensile Strength of 5% Elongation, md/cmd* (kN/m)
24 / 24
Aperture (mm x mm)
40 × 40
*: Cross machine direction
2.2. Test setup and procedure For preparing the soft clay soil bed, a circular tanks of 30 cm, 25 cm, 20 cm and 15 cm diameter were used in the tests. Tests were conducted in a clay bed prepared at about moisture content of 40% to obtain required shear strength value. For preparation of each test bed, the soft clay soil pulverized was thoroughly mixed with required amount of water. To achieve uniform moisture distribution, the wet soil was placed in airtight plastic containers and stored for 2 to 3 days before being used in model tests. Before filling the test tank, lubricating oil was smeared along the inner surface of test tank wall to reduce friction between clay and test tank wall. The soft clay soil was placed in the test tank in layers with small quantities, which were tapped gently with a special hammer and spread uniformly. Soft soil was filled in the test tank in layers with measured quantity by weight. The surface of each layer was provided with uniform compaction with a special hammer to achieve a 5 cm height, uniform density and required shear strength as per requirement. After the test tank was filled to layer of 5 cm height, pocket penetrometer test was carried out to checked shear strength. Water content of soft clay soil was also determined at different locations. The procedure was repeated until the soft clay soil bed is completed to the full height. In all test full height of the soft clay soil is 25 cm. For stone column construction, it was decided to use the replacement method in all tests [21]. The drill rig and a thin wall tube supported and located in the two way (horizontally) controlled steel frame were used to construct the model columns. In order to minimize disturbance to the surrounding clay during the penetration of the tube, lubricating oil was smeared on the outside of the tube before the formation of every other column. A seamless tube of 5 cm outer diameter was pushed into the soft clay soil at the center of the tank up to the bottom. The soft clay soil within the tube was removed using a drill rig. Crushed stones were charged into the tube and compacted with special hammer, which is suitable for tube. This process was done in layers of 5 cm height. Density of the stone column built with crushed stone was found to be 16.30 kN/m3. Upon reaching the layer of 5 cm height, the tube is slowly withdrawn. This construction stages was repeated until the column is completed to the full height. In all test full height of the stone column is 25 cm. The case of geogrid wrapping stone column tests, wrapping was provided around the tube that was not smeared the lubricating oil. Loading tests were performed using model rigid circular 5 cm diameter of footings fabricated from mild steel with a thickness of 15mm. For all tests, loading was done only for stone column area. The model footings were loaded vertically. The loading system was displacement control and vertical displacement rate was 2.33mm/min. Load and displacement measurements were taken using a load cell and two LVTD’s. 5
A schematic diagram of the test setup is given in Figure 3. Summary of model tests is shown in the Table 4.
Figure 3. General layout of apparatus for the model test [20]
Table 4. Summary of model tests
Test Series Series I Series II
Series III
Test description Loading on unreinforced clay bed Loading on stone column reinforced clay bed Loading on geogrid wrapped stone column reinforced clay bed
Diameter of footing (df) (cm)
Diameter of column (dc) (cm)
Diameter of tank (D) (cm) 15, 20, 25 30
Total number of model tests
5
5
5
5
15, 20, 25 30
4
5
5
15, 20, 25 30
4
4
3. RESULTS AND DISCUSSION
Twelve model tests on shallow foundation resting on the unreinforced soft clay soil, stone column reinforced soft clay soil and geogrid wrapped stone column reinforced soft clay soil were conducted to find the settlement - axial stress behaviour. To present model tests results, settlement - axial stress curves are presented. In settlement - axial stress curves, vertical and horizontal axes shows the axial stress (q) and the settlement ratios (s/df), respectively. The settlement ratio (s/df) is defined as the ratio of the footing settlement (s) to the footing diameter (df), expressed as a percentage. In test Series I, model tests were conducted using shallow foundation rested on unreinforced soft clay soil deposit. Four different test tank with diameters (D) of 15, 20, 25 and 30 cm were 6
used in test Series I to understand influence zone. The settlement - axial stress curves are presented in Figure 4. It is clear from the figure that axial stress is less affected by the tank size. In conventional design, the bearing capacity of a vertically loaded shallow foundation on undrained clay is expressed as: qu=Nccu
(1)
where qu is the ultimate bearing capacity of the footing and Nc the bearing capacity factor. Based on this formula and test Series I results, it can be said that the bearing capacity is independent of the tank diameter.
90 75
q (kPa)
60 45
Series I (D=3df)
30
Series I (D=4df) Series I (D=5df)
15
Series I (D=6df) 0 0
10
20
30
40
50
s/df (%) Figure 4. Results of test Series I
In test Series II, model tests were conducted using shallow foundation rested on stone column reinforced soft clay soil deposit. Four different test tank with diameters (D) of 15, 20, 25 and 30 cm were used in test Series II to understand influence zone. The settlement - axial stress curves are shown in Figure 5. From this figure it is apparent that as the tank diameter increases, axial stress decreases.
450
q (kPa)
375 300 225 Series II (D=3df) Series II (D=4df) Series II (D=5df) Series II (D=6df)
150 75 0 0
10
20
30 s/df (%)
40
50
Figure 5. Results of test Series II
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In test Series III, model tests were conducted using shallow foundation rested on geogrid wrapped stone column reinforced soft clay soil deposit. Four different test tank with diameters (D) of 15, 20, 25 and 30 cm were used in test Series III to understand influence zone. Figure 6 shows the settlement - axial stress curves. As may be seen from the figure that as the tank diameter increases, axial stress decreases.
900 750 q (kPa)
600 450 Series III (D=3df) Series III (D=4df) Series III (D=5df) Series III (D=6df)
300 150 0 0
10
20
30 s/df (%)
40
50
Figure 6. Results of test Series III
By comparing all test series with each other, the effect of both the stone column and the geogrid wrapped stone column on the bearing capacity (qu) were investigated. The bearing capacity value (qu) may be defined based on the settlement such as that which causes a settlement equal to 10% of the column diameter [22]. Comparison all test series are shown in the Figure 7. It is clear from the Figure 7 that the bearing capacity carried by soft clay soil was increased by using stone column for all tank diameters. Stone columns provide to densifying the soft clay soil and reinforcing the soft clay soil creating a stiff composite soil mass. Also, the results from the Figure 7 indicated a clear improvement in the bearing capacity of the stone column due to geogrid wrapping for all tank diameters. Geogrid wrapping makes the stone columns stiffer and it gives an extra lateral confinement.
400
Series III Series II Series I
qu (kPa)
300
200
100
0 2
3
4
5
6
7
D/df Figure 7. Comparing all test series
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4. CONCLUSION
In this study, series of model tests were carried out to understand the behaviour of shallow foundation resting on the unreinforced soft clay soil, stone column reinforced soft clay soil and geogrid wrapped stone column reinforced soft clay soil. A detailed experimental study on behaviour of shallow foundation was carried out by varying the diameter of the tank to find influence zone. Based on this study the following main conclusions can be drawn. • •
•
•
Axial stress in shallow foundation resting on the unreinforced soft clay soil is less affected by the tank size. A conventional design method supports this. Axial stress in shallow foundation resting on the unreinforced soft clay soil and geogrid wrapped stone column reinforced soft clay soil are affected by the tank size. As the tank diameter increases from D=3df to D=4df, the bearing capacity decreases significantly. After this (D=4df) value when the tank diameter continues to increase, the bearing capacity decreases slightly. A significant improvement in the bearing capacity of a shallow foundation can be obtained by installing stone column in soft clay soil deposit. Stone columns provide to densifying the soft clay soil and reinforcing the soft clay soil creating a stiff composite soil mass. Bearing capacity of the stone column can be increased by all-round wrapping by geogrid. Geogrid wrapping makes the stone columns stiffer and it gives an extra lateral confinement.
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