50th INDIAN GEOTECHNICAL CONFERENCE th Cigs/ldh/files/igc 2015 pune...50th INDIAN GEOTECHNICAL CONFERENCE 17th – 19th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering

50th INDIAN GEOTECHNICAL CONFERENCE 17th – 19th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India

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DENSIFICATION OF SAND USING BURNT BRICK COLUMN

P. Gouri1, M. Siyoos2, A. S. Johnson3

ABSTRACT

India has large coastlines exceeding 6000 km and in view of the increasing development in these coastal areas, a number of construction activities are undertaken here. In addition, the availability of land for the development of commercial, housing, industrial and transportation, infrastructure etc, are scarce particularly in urban areas. This necessitates the use of land, which has weak strata, wherein the geotechnical engineers are challenged by presence of different problematic soils with varying engineering characteristics. Most of the coastal areas are covered with loose sand of uniform gradation, with very low strength and high compressibility. Replacing this stratum of loose sand deposits during the construction of structures may not be economical. Although deep foundations can meet all the design requirements, negative drag force and length required often results in prohibitive costs. So we need to consider the densification of the top loose sand stratum that could be used as the load bearing stratum.

The improvement methods using compaction sand piles available these days need machinery and are expensive. In addition, the difficulties in compaction sand pile installation, which is mainly due to the filling of sand in non-uniform manner leads to the density of filled sand less than the surrounding soil which results in differential settlement. In order to overcome these defects, a new method is proposed in this paper to densify the soil by inserting burnt brick columns into the loose stratum by displacement method (adopted in compaction sand piles). Bricks have been found to have density almost equal to that of sandy soil. With the introduction of these columns we could actually distribute the incoming load equally to bricks and the surrounding soil. The primary function of brick column is to transfer the load to both brick column and the densified soil.

This research focuses on the densification of loose sand by the introduction of brick columns of different dimensions. Laboratory model studies in the form of mini plate load tests were conducted on loose sand, sand densified using single brick column and group of brick columns. In the present study, relative density of the test bed was kept around 50 % of the soil sample. Columns of different sizes 35 x 25 mm,

1 Gouri P, PG Student, College of Engineering Trivandrum, India, [email protected] 2Siyoos M, PG Student, College of Engineering Trivandrum, India, [email protected] 3Arvee Sujil Johnson, Associate Professor, College of Engineering Trivandrum, [email protected]


45 x 30 mm and 55 x 35 mm with varying length of 150 mm, 200 mm and 300 mm were considered for the tests. For single brick column, the test was conducted in a cylindrical tank of 400 mm height and diameter of 400 mm and for group columns the tank was of 600 mm diameter and 400 mm height. Effects of various parameters, such as length and size of columns are studied in these tests. The test results were compared with three-dimensional, finite element analyses of the model foundation.

The paper describes details of experimental works carried out, numerical analysis using MIDAS GTS NX software and comparison of both experimental and numerical results. The tests results showed improvement in load carrying capacity of loose sand with the introduction of brick columns. The results indicated that brick columns with higher sizes have a beneficial effect on densifying the loose soil.

Keywords: Brick column; Sand; Densification; Bearing capacity; Settlement; Model tests; Finite Element

Analysis


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DENSIFICATION OF SAND USING BURNT BRICK COLUMN

P. Gouri, PG student, College of Engineering Trivandrum, [email protected] M. Siyoos, PG student, College of Engineering Trivandrum, [email protected] A. S. Johnson, Associate Professor, College of Engineering Trivandrum, [email protected]

ABSTRACT: In view of the increasing development in coastal areas, a number of construction activities are undertaken in these areas. Most of this area contains loose sand of uniform gradation as the top layer. A new method is proposed in the present work to densify the soil by inserting burnt brick columns into the loose stratum. Laboratory model studies in the form of mini plate load tests were conducted on loose sand, sand densified using single and group brick columns. Effects of various parameters such as length and size of columns are studied in these tests. The results obtained are analysed using finite element package MIDAS GTS NX. The paper describes the details of experimental work carried out, numerical analyses using finite element package, comparison of results and discussions.

INTRODUCTION India has large coastline exceeding 6000 km. In view of the developments on coastal areas in the recent past, large numbers of construction activities are undertaken in these areas. In addition, the availability of land for the development of commercial, housing, industrial and transportation, infrastructure etc. are scarce particularly in urban areas. This necessitates the use of land, which has weak strata, wherein the geotechnical engineers are challenged by presence of different problematic soils with varied engineering characteristics. Many of these areas are covered with loose sand of uniform gradation having very low shear strength and high compressibility as the top layer. Replacing this stratum of loose sand deposits may not be economical. So we need to consider the improvement of the top loose sand stratum that could be used as the load bearing stratum. A proper ground improvement method should be adopted for the same.

Out of the several techniques available for improving loose soils, compaction sand piles have been used to a large extend for several applications. However, installation of compaction sand piles creates a difficulty which mainly arises due to the filling of sand in a non-uniform manner. This may lead to differential settlement. In order to overcome these demerits, a new ground

improvement technique is proposed here to densify the soil through the insertion of burnt brick columns, instead of filling sand. To study the stiffness and deformation behaviour of the improved ground for various column size, length and spacing, experimental study has been carried out in a cylindrical tank representing brick columns and surrounding soil. Tests have been carried out for different dimensions of brick columns. The experimental results are compared with finite element analysis using MIDAS GTS NX and found to be comparable. EXPERIMENTAL WORK The experimental study is carried out with following objectives:

i. To study the effectiveness of new ground improvement method in sand – brick column technique

ii. To analyse the effect of various parameters such as column size, depth and spacing on the improved ground

Accordingly, tests are carried out in test tank where brick columns having different sizes of 35 x 25 mm, 45 x 30 mm and 55 x 35 mm and length of 150 mm, 200 mm and 300 mm are constructed at the centre of a cylindrical tank (one at a time), which is filled with sand at 50 % relative density


[1]. To study the load carrying capacity of the column, column area alone is loaded.

Experimental Set-up Typical test arrangement is shown in fig 1.

Properties of sand The sand used is sea sand from Veli Beach, Trivandrum. Sand obtained is sieved and modified to get required particle size for use in the model tests. Properties of sand used are:

1 Load

2

3

4

5

400 to 600mm Φ

400mm

Specific Gravity : 2.6 Max. Void ratio, emax : 0.86 Min. Void ratio, emin : 0.57 Effective size, D10 (mm) : 0.3 Angle of Internal Friction : 42.5° Max. Dry density, ϒmax (kN/m3) : 17.2 Min. Dry density, ϒmin (kN/m3) : 14.6 Medium sand (%) :77 Fine sand (%) :23 Properties of Burnt Brick First class burnt bricks used are from a construction site within College of Engineering Trivandrum campus. Typical properties of aggregates for burnt bricks for brick column are: Average Dimension (mm) : 225 x 105 x 65

1. Loading plate 4. Brick column 2. Sand 5. Concrete shoe 3. Cylindrical tank

Fig. 1. Typical test arrangement

Cylindrical tank of height 400 mm is used as model tank. Diameter of the tank is taken as 400 mm for single column testing and 600 mm for group column testing. For group column testing, triangular pattern with spacing of 2.5a is assumed [2]. In the tank sand is placed in four layers, each having 100 mm height at 50 % relative density with brick column at centre.

Experimental set-up comprises of a cylindrical tank filled with sand and with a brick column at its centre. Vertical load is applied directly over the brick column. Load was applied through a proving ring of 20 kN capacity.

Properties of Materials used Two basic materials are used for this study: Sand and Burnt bricks

Dry density (kN/m3) : 15.7 Wet density (kN/m3) : 17.2 Water absorption (%) : 11 Compressive strength (kN/m2) : 17.5 Preparation of Sand Bed Sand is filled at 50 % relative density in the tank

in layers of 100 mm each giving uniform compaction to achieve a uniform dry density of 15.4 kN/m3. Construction of Brick column The column is constructed by displacement method as shown in fig 2. Thin open ended rectangular steel pipes having 35 x 25 mm, 45 x 30 mm and 55 x 35 mm inner dimensions and lengths of 150 mm, 200 mm and 300 mm with a concrete shoe at bottom are pushed into the sand at the centre of tank. Similar sized bricks are placed one by one as per the height of column required. Size of bricks used for the test program is shown in Table 1. Casing pipe is raised in stages ensuring minimum 5 mm penetration below the bricks placed. 20 – 30 g sand is added while the pipe is raised, to fill up the


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brick joints within the column. A light compaction is also adopted to ensure proper placement of bricks. The procedure is repeated until the column is completed to the full height.

Brick column

Concrete shoe

Fig. 2. Displacement Method

Table 1: Test program

25 mm. load is applied in equal increments through a proving ring of 20 kN capacity and corresponding settlement value for each increment is noted. Vertical settlements are measured using two dial gauges (25 mm) capacity with a sensitivity of 0.01 mm. Dial gages are placed on top of the loading plate in a diametrically opposite direction. FINITE ELEMENT ANALYSIS Predictions of the load test results by a finite element analysis were compared with the model test results to evaluate the ability of the method to model the actual behaviour of brick column effectively. The finite element analysis is done using a package – MIDAS GTS NX, developed for analysing geotechnical problems. Validation of the package is done and reported below.

Validation of finite element package used

Finite element package (MIDAS GTS NX) is validated by analysing the load test results published based on the work done by J. T. Shahu and Y. R. Reddy (2011). The test tank is 300 mm diameter and height of clay bed is 300 mm. A group of 9 stone columns of diameter 13

Dimension of each brick

specimen (mm)

Length of brick column

(mm)

No: of brick specimen

taken

mm and height 150 mm was made in a square grid pattern and loaded with a plate of 100 mm diameter. A granular mat was placed below the footing having a thickness of 20 mm. properties

35 x 25 x 75

45 x 30 x 100

55 x 35 x 100

Test procedure

150 2 200 2.5 300 4 150 1.5 200 2 300 3 150 1.5 200 2

300 3

of clay and stones are given below Properties of clay Modulus of elasticity : 2000 kPa Poisson’s ratio : 0.33 Shear strength : 20 kPa

Properties of stones Secant modulus of elasticity : 1250 kPa Poisons ratio : 0.35 Dilation angle : 8°

After preparing the brick column, the load deformation behaviour of the column is studied by applying vertical load in a loading frame. To load the brick column alone a square loading plate of 75 mm size is placed exactly at the centre of brick column and load is applied till settlement exceeded

Angle of internal friction : 43.42°


Sett

lem

ent m

m

Fig. 3 Validation of MIDAS GTS NX

Fig. 3 compares the results obtained from model tests and finite element analysis. Load deformation behaviour from finite element analysis matches well with experimental results.

Analysis of brick column A non-linear, symmetric analysis is done. Material properties used for the analysis are given below.

Properties of sand The properties of sand used are given below. Poisson’s ratio is assumed based on Bowels (1988).

Modulus of elasticity : 2000 kPa

An equal settlement analysis is done when column alone is loaded. Failure is by axial breakage of column in model tests. No breakage of individual brick is seen as in model tests.

RESULTS AND DISCUSSIONS Effect of width of brick columns Fig. 5 shows the stress settlement curve when different columns of 35 mm, 45 mm and 55 mm width with 150 mm length are installed in sand with relative density 50%. From the figure it is clear that the bearing capacity increases with increasing column width. The experimental results also showed same trends. The curves obtained from the finite element analysis compares well with that from model tests.

Axial stresses kN/m2

0 100 200 300 400 0

4

8

12

16

Poissons ratio : 0.15 20 sand filled at min. density (exp) sand filled at min. density (FEM)

Angle of internal friction : 42.5o BC of size 3.5 cm (exp) BC of size 4.5 cm (exp) BC of size 5.5 cm (exp) BC of size 3.5 cm (FEM)

Unit weight : 15.67 kN/m3 BC of size 4.5 cm (FEM)

Properties of brick

BC of size 5.5 cm (FEM)

Modulus of elasticity : 1500 kPa Poissons ratio : 0.35 Unit weight : 17.54 kN/m3

The basic symmetric finite element mesh used to represent individual load tests are given in figure 4. Hybrid meshes are used for meshing. Along the bottom of the tank both radial deformation and settlement are restricted.

Fig. 4 Mesh discretisation

Fig. 5 Stress settlement curve for columns having 150 mm length

Effect of length of brick columns Fig. 6 shows stress settlement curve when different columns of 150, 200 and 300 mm length with width 35 mm is installed in sand with relative density 50 %. From the figure it is clear that when the length of brick column increases the bearing capacity also increases. The experimental results also showed the same trends. So the results are found to be in agreement with the experimental study conducted in the laboratory.


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Fig. 6 Stress settlement curve of column having

width 35 mm size

PARAMETRIC STUDY Effect of width of footing

A parametric study was conducted on sand densified using brick column of width 35 mm and length 150 mm by changing the width of footing. The width of footing was taken as 50 mm, 70 mm, 80 mm and 100 mm. The stress settlement graph obtained in MIDAS is shown in Fig. 7. From the figure it is clear that the bearing capacity decreases with increase in width of footing.

Fig. 7 Stress settlement curves for different width of footing

Fig.8 shows the effect of width of footing

on BCR. It is clear that BCR decreases with increasing the width of footing.

Fig. 8 Effect of width of footing on BCR

Effect of width of brick columns in group analysis A parametric study was conducted on sand reinforced with group brick column of different widths 35, 45 and 55 mm with a length of 150 mm. The stress settlement graph obtained in MIDAS is shown in Fig. 9. Load settlement curves for group testing are shown in figure. Model test results for a spacing of 2.5 times width of column is compared with the finite element analysis. For other spacing of 3 times width of column, the load settlement behaviour is predicted using finite element analysis. From the figure it is clear that the bearing capacity increases with increasing the width of columns in the group analysis also.

Fig. 9 Stress settlement curve for group column


Effect of width of brick columns on BCR in group analysis Fig. 10 shows the effect of width of poles on BCR. It is clear that BCR increases with increasing the width of group columns.

Fig. 10 Effect of width of footing on BCR (Group analysis)

Effect of tank dimension in group analysis Experiment of group analysis was done on a tank of diameter 600 mm with height of 400 mm. the analytical study was conducted on the same tank size as that of experiment and also on a tank of diameter 400 mm with 400 mm height. Fig. 11 shows stress settlement behaviour for different tank dimensions. It is clear that the bearing capacity increases with deceasing tank diameter, this may be due to more confinement effect in the case of small tanks.

Fig. 11 Effect of tank dimensions

SUMMARY AND CONCLUSIONS The present work compares the load

settlement behaviour of ground improved using brick columns from experimental work with finite element analysis. Following conclusions are drawn based on the study.

1. When column area is loaded, the failure is by axial breakage of individual bricks at the top portion.

2. The load settlement behaviour obtained from model tests compares well with that of finite element analysis.

3. Results of the model study using MIDAS GTS NX are found to be in agreement with the experimental study conducted in laboratory.

Parametric study is done by changing the width of footing in single and width of column, tank diameter in group columns. The results showed bearing capacity decrease with increase in width of footing and tank diameter. Also bearing capacity increases when the width of brick column increases. Hence, a new easy environmental friendly method is proposed to improve the field density of sandy soil and this method can be applied for the construction of low rise buildings as inferred from initial findings. REFERENCES

Shahu, J. T., and Reddy, Y. R.(2011). “Clayey soil reinforced with stone column group: model tests and analyses.” J. Geotech. Geoenviron. Eng., ASCE, 137, 1265–1274

Ambili, A. P., and Gandhi, S. R. (2007). “Behavior of Stone Columns Based on Experimental and FEM Analysis.” J. Geotech. Geoenviron. Eng., ASCE, 133, 405-415.

Ayadat, T., Hanna, A., and Etezad, M.(2007). “Failure process of stone columns in collapsible soils.” Technical Paper, IJE, 21, 135-144.

Gigi, V. V., and Johnson, A. S. (2010).” Ground improvement of loose sand using concrete poles.” 12th National conference on technological trends, Trivandrum, 106-113.

Goliat, Y. S., Sathyanarayana, V., and Raju, S. S. V. (2009). “Concept of under-reamed


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cemented stone columns for soft clay ground improvement.” IGC 2009, Guntur, INDIA.

Hamed Niroumand, Kassim, K. A., and Yah, C, S. (2011). “Soil improvement by reinforced stone columns based on experimental work.” Elec. J. Geo. Eng., 16, 1477-1499.

IS 2720 (Part 4) – 1985, Methods of Test for Soils: Grain Size analysis, Bureau of standards, New Delhi, India.

IS 2720 (Part 3/ Sec 2) – 1980, Methods of Test for Soils: Determination of Specific Gravity, Bureau of standards, New Delhi, India.

IS 2720 1888 – 1988, Methods of Load Test on Soils, Bureau of standards, New Delhi, India.

Malarvizhi, S. N, and Ilamparuthi, K. (2007). “Load versus settlement of clay bed stabilized with stone and reinforced stone columns.” Technical paper, Anna University, 322- 329.

Mani, K., and Nigee, K.(2013). “A study on ground improvement using stone column technique.” Int. J. Innovative R. Sci, 2-11, 6451-6456.

McCabe, B, A., McNeill,J. A., and Black, J. A. (2007). “Ground improvement using vibro stone column technique.” Technical Paper, Institution Of Engineers Of Ireland., 1 – 12.

Mokhtari, M.,and Kalantari, B. (2012). “Soft soil stabilization using stone columns- a review.” Elec. J. Geo. Eng., 17, 1459-1466.

Shameena, K. P., and Johnson, A. S. (2014). “Densification of loose sand using rubberized square concrete poles.” 15th National conference on technological trends, Trivandrum.,67-72.

Solymer, Z.V., Samsudin, Osellame, J., and Purnomo, B. J. (1986). “Ground Improvement by Compaction Piling.” J. Geotech. Geoenviron. Eng., ASCE, 112(12), 1069–1083.

Verma, B. P., and Char, A. N. R. (1986). “Bearing capacity tests on reinforced sand subgrades.” J. Geotech. Geoenviron. Eng., ASCE, 112(7), 701-706.

Zhmatkesh, A.,and Choobbasti, A. J. (2010). “Settlement evaluation of soft clay reinforced by stone columns considering the effect of soil compaction.” Int. J. Res.Rev. Applied Sci., 3 (2), 159 -166.

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50th INDIAN GEOTECHNICAL CONFERENCE th Cigs/ldh/files/igc 2015 pune...50th INDIAN GEOTECHNICAL CONFERENCE 17th – 19th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering