Studies on the Behaviour of Flyash Aggregate as Column Material in Soft Clay
STUDIES ON THE BEHAVIOUR OF FLYASH AGGREGATE AS
COLUMN MATERIAL IN SOFT CLAY
S. Vidhyalakshmi
P.G. Student, Division of Soil Mechanics and Foundation Engineering, Anna University Chennai, Chennai–600025, India.
E-mail:
V.K. Stalin
Astt. Professor, Division of Soil Mechanics and Foundation Engineering, Anna University Chennai, Chennai–600025, India. E-mail:
KE.A. Palaniappan
Professor & Head, Department of Civil Engineering, Panimalar Engineering College, Chennai, India.
E-mail:
ABSTRACT: In this investigation an attempt is made to use fly ash aggregate waste as column material in the place of conventional stone aggregate to improve the soft clay deposits. The consolidation and load tests were conducted on soft clay with one, two and three columns of stone aggregate and fly ash aggregate. It is found that t90, (Cvr/Cv) ratio and Cc of clay with fly ash aggregate column are comparable with stone column irrespective of number of columns. It is observed that increase of load carrying capacity of clay + three stone column is 695% and that of clay + three fly ash aggregate column is 565%, compared to clay without column. Hence it is concluded that the fly ash aggregate may be used as a column material in the place of conventional stone aggregate.
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Studies on the Behaviour of Flyash Aggregate as Column Material in Soft Clay
1. INTRODUCTION
Fly ash is an industrial byproduct from thermal power plants with current generation of approximately more than 108 million tonnes and its proven suitability for variety of applications as cement, concrete, mortar and lime pozzolana mix etc. Major areas of Fly ash utilization are building materials as bricks, blocks and tiles etc. Manufacturing of light weight aggregates, controlled low strength material, ferro cement, grouting, engineered fills for low lying land spaces, use in road construction and construction of ash dykes and embankments. Stalin et al. (2004) used concrete waste for the improvement of soft clay. The study leads to the conclusion that the concrete waste can be a column material in the place of stone aggregate for the improvement of soft clay deposits. Bijen (1986) concluded that there are many processes to manufacture artificial aggregates from fly ash. Verma et al. (1998) reported that the sintered fly ash light weight aggregates can be used as replacement of conventional aggregates in concrete for making some building components such as hollow and solid blocks. Ramamurthy Hari krishnan (2006) reported that the property of aggregates depends on the type of binder and its dosage. It is observed that when bentonite is added with fly ash that there is significant improvement in 10% fines values and reduction in water absorption of sintered fly ash aggregate. Kayali (2008) concluded that the concrete made with fly ash aggregate were 25% stronger than that made using palletized fly ash based light weight aggregates. Stone aggregate has its high demand in various civil engineering construction activities such as buildings, retaining wall, bridges etc. In this paper, attempt is made to study the performance of fly ash in aggregate form as column material in the place of conventional stone aggregate for the improvement of soft clay.
2. MATERIALS
The natural soil sample was collected from Vellachery, Chennai. It is high compressible clay having liquid limit of 76%, used for consolidation and load test. The details of physical properties of natural soil are summarized in Table 1. From the fly ash brick manufacturing unit located in Tiruvallur, Chennai, fly ash bricks of size (230 × 110 × 70) mm with compressive strength of 100 kg/cm2 were used for preparing fly ash aggregate. The composition of bricks includes fly ash powder, cement, lime, gypsum, sand, dust, and chips. Fly ash Bricks were crushed and sieved to get sizes ranging from 5 mm to 10 mm. This Factory made fly ash bricks were taken into consideration as column material in this present study. The Fly Ash Aggregate (FAA) has crushing strength of 43% and Conventional Coarse Aggregate (CCA) has 1.8%.
Table 1: Index Properties of Natural Soil
Description / ValuesSpecific Gravity / 2.70
Gravel (%) / nil
Sand (%) / 7
Silt (%) / 15
Clay (%) / 78
Liquid Limit (%) / 76
Plastic Limit (%) / 30
Shrinkage Limit (%) / 12
Free Swell Index (%) / 80
Max. dry density (kN/m3) / 14.8
Optimum moisture content (%) / 28
IS Classification / CH
3. METHODS
Consolidation tests were conducted for L/D ratio of 3.2 on clay with stone aggregate and fly ash aggregate (Fig. 1). The diameter of column is 31 mm and the length of column is 100 mm respectively. The soft clay bed was prepared in the mould by taking a known quantity of soil mixed with water content of 55% in such a way that it gives a soft consistency (consistency index of 0.45) by gentle hand-remoulding and tapped with a small wooden rod. For the formation of column with conventional stones and fly ash aggregates, the well graded crushed aggregates of size varying from 5 mm to 10 mm were used. The single column was formed by placing PVC pipe of diameter same as the diameter of column to be formed at the centre of the mould upto the bottom of clay bed. The required quantity of soil prepared as above was placed around the pipe by gentle tamping. After filling the soil, the stones of required quantity to achieve a density of 15 kN/m3 were poured into the pipe and compacted by a tamping rod. For beds treated with columns the top and bottom surfaces of the bed were made impermeable by using steel plates with suitable holes to ensure drainage only through the columns.
Fig. 1: Consolidation Test Setup for L/D Ratio of 3.2
To study the load carrying capacity of virgin clay, conventional stone-clay column and fly ash aggregate-clay column, load tests were conducted in a cylindrical tank of diameter 25 cm and height 25. The soil sample is prepared for a water content of 55% in the tank similar to the sample preparation for consolidation test. After filling the tank with clay of soft consistency, the soil sample is allowed for self-consolidation for 24 hours. For the formation of columns with conventional stones and fly ash aggregates, the same procedure is followed as in consolidation test. The load was applied on a 15 cm diameter circular bearing plate. While placing the bearing plate on the stone columns care was taken such that the centroid of the plate and centroid of stone column or group of stone columns coincide. The load was applied on to the circular plate through a proving ring by reaction method.
4. RESULTS AND DISCUSSION
4.1 The t90 Values of FAA and CCA
The Cv values of clay without column are 0.09, 0.038 and 0.030 cm2/min respectively for 20, 40 and 80 kN/m2 for a sample thickness of 10cm. From Table 2 the percentage decrease in t90 value for the pressure increment of 0-20 kN/m2 is 35% respectively for clay with single stone column and 28% for clay with single FAA column. For higher pressure increment of 20–40 kN/m2 and 40–80 kN/m2, the percentage increase of t90 is considerably decreasing as seen from Table 2. For the pressure increment of 40–80 kN/m2, the decrease in t90 is 23% for clay with two stone column and 36% for clay with two fly ash aggregate column. For the same pressure increment of 40-80 kN/m2, the decrease in t90 is 44% and 42% respectively for clay with three stone column and clay with three fly ash aggregate column.
Table 2: Comparison of Cv and Cvr Values for Soft Clay with Different Column Materials
Description / Pressure(kPa) / t90
(min) / Cvr
(cm2/min) / Cvr/Cv
ratio
Virgin Clay / 0–20 / 653 / -- / --
20–40 / 784 / -- / --
40–80 / 919 / -- / --
Clay + 1 CCA Column / 0–20 / 425 / 0.151 / 1.62
20–40 / 622 / 0.105 / 2.75
40–80 / 703 / 0.091 / 2.95
Clay + 2 CCA Column / 0–20 / 279 / 0.229 / 2.96
20–40 / 489 / 0.131 / 3.44
40–80 / 506 / 0.126 / 4.10
Clay + 3 CCA Column / 0–20 / 230 / 0.278 / 2.98
20–40 / 453 / 0.141 / 3.71
40–80 / 514 / 0.124 / 4.04
Clay + 1 FAA Column / 0–20 / 469 / 0.136 / 1.46
20–40 / 684 / 0.094 / 2.46
40–80 / 754 / 0.085 / 2.75
Clay + 2 FAA Column / 0–20 / 308 / 0.208 / 2.23
20–40 / 543 / 0.118 / 3.09
40–80 / 582 / 0.111 / 3.57
Clay + 3 FAA Column / 0–20 / 254 / 0.252 / 2.70
20–40 / 503 / 0.127 / 3.34
40–80 / 536 / 0.119 / 3.87
From Figure 2 it is observed that the t90 value decreases with increase in number of columns for single conventional coarse aggregate for a pressure increment of 0-20 kN/m2.There is a marginal decrease in t90 value for fly ash aggregate column when compared to that of stone aggregate column. Similarly for the pressure increment of 20–40 kN/m2 and 40-80 kN/m2, the t90 values of clay with fly ash aggregate is always comparable with clay with conventional stone aggregate column. The fly ash aggregate particles being very porous the dissipation of water particles are easy and hence the t90 fly ash aggregate is less. However for higher pressure increment if there is a chance for crushing of particles inside the column especially for fly ash aggregate whose crushing strength is very low (43%) and hence there is possibility for t90 value to get reduced.
Fig. 2: Number of Column Vs t90 Relationship for Pressure of 20–40 kN/m2
4.2 Ratio of (Cvr /Cv) of FAA on CCA
The Cv or Cvr values are the indicative of settlement rate of the given sample and is a function of permeability of the soil. Generally, increase of coarser fraction in clays increases Cv or Cvr values and vice versa. In the present case for any pressure increment Cvr values increases with number of columns. Further the ratio of Cvr/Cv is ranging from 1.6 to 3.0 for clay with single stone column, 2.5 to 4.0 for clay with two stone column and 3.0 to 4.0 for clay with three stone columns (Table 2). For clays with single, two and three columns of FAA, the ratio of Cvr/Cv are 1.5 to 3.0, 2.2 to 3.5 and 2.7 to 3.8 respectively. From Figure 3, the Cvr/Cv values increases with the number of columns for stone aggregate column and there is a marginal decrease in Cvr/Cv values for fly ash aggregate column when compared with stone aggregate column.
4.3 Variation of CC of FAA and CCA
The Cc value of clay without column is 0.1518. From Table 3, it is observed that the Cc values are decreasing with increasing number of columns, however the difference in compression index values for two column materials and number of columns is marginal. Further, Cc values for clay with single stone column is 0.142, for clay with two stone columns is 0.133 and clay with three stone columns is 0.088 which is lower than that of clay without column. For clays with single, two and three columns of flyash aggregate column, the Cc values are 0.149, 0.141 and 0.135 respectively which are lower than clay without column. The variation of Cc is in the order of clay with stone aggregate column < clay with fly ash aggregate column. It is clear that even for the wide pressure range of 0 to 80 kN/m2 the reduction in settlement at any pressure is more or less same irrespective of column materials. The variation of t90, Cvr and Cc, only suggest that FAA can be used as column material to increase the rate of settlement and also reduce total settlement in the place of CCA for the improvement of soft clay deposits.
Fig. 3: Number of Column Vs Ratio of (Cvr/Cv) Relationship for Pressure of 20–40 kN/m2
Table 3: Variation of Cc Values of Clay with and without Column
Description / Cc ValuesVirgin Clay / 0.1518
Clay + 1 SC CCA / 0.1420
Clay + 2 SC CCA / 0.1330
Clay + 3 SC CCA / 0.088
Clay + 1 SC FAA / 0.149
Clay + 2 SC FAA / 0.141
Clay + 3 SC FAA / 0.135
4.4 Load Carrying Capacity of FAA and CCA
From the Table 4 the percentage increase in load carrying capacity for 5 mm and 12 mm settlement of clay with CCA and FAA. The load of clay with two stone columns is 517.5% and clay with two fly ash aggregate column is 380%. For clay with three stone columns, the percentage increase of load is 695% and for clay + three fly ash aggregate columns the percentage increase is 565% compared to clay without column.
Table 4: Comaprison of Load Capacity for CCA and FAA
Description / Load (N) at5 mm / Load (N) at
12 mm
Virgin Clay / 39 / 40
CCA / 1 SC / 63 / 80
2 SC / 133 / 247
3 SC / 196 / 318
FAA / 1 FAAC / 39 / 41
2 FAAC / 96 / 192
3 FAAC / 139 / 266
Earlier, Shanker Shroff (1997) have shown that the load carrying capacity of stone column in triangular pattern is 374% higher than that of clay without stone column. The above variation of load carrying capacity for a given settlement only confirms that load carrying capacity of clay + FAA waste column is almost nearer to that of the capacity of clay with CCA column irrespective of number of columns.
5. CONCLUSIONS
Based on the load carrying capacity and consolidation characteristics of soft clay with CCA and FAA as column material the following general conclusions may be drawn.
1. Eventhough the crushing strength of fly ash aggregate is lower than the stone aggregate, the Cc values of one, two and three columns of FAA are comparable with CCA
2. The Load carrying capacity of clay with one, two and three columns of FAA are comparable with the capacity of soft clay with one, two and three CCA.
It is hence concluded that fly ash aggregate may be effectively utilized as column material in the place of conventional coarse aggregate in the improvement of soft clay.
REFERENCES
Bijen J.M., (1986). “Manufacturing Processes of Artificial Light Weight Aggregates from Fly ash”, The International Journal of Cement Composites and Light Weight Concrete, Vol. 8, pp. 191–199.