BEHAVIOUR OF SHALLOW FOUNDATIONS OVER SOFT CLAY DAMPERS

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1 IGC 2009, Guntur, INDIA BEHAVIOUR OF SHALLOW FOUNDATIONS OVER SOFT CLAY DAMPERS Sanjib Singha Research Scholar, Department of Civil Engineering, National Institute of Technology, Silchar , India, Ashim Kanti Dey Professor, Department of Civil Engineering, National Institute of Technology, Silchar , India, ABSTRACT: Indian subcontinent witnessed a number of severe earthquakes during the last two decades. Each earthquake left behind a huge loss of precious life and property. To avoid the loss of life and property, extensive research work is being carried out on ductile detailing, seismic dampers, base isolators, retrofitting etc. The base isolators mainly consist of rubber pads, laminated rubber bearings, rollers, etc. The isolators are costly and almost unaffordable by ordinary people. On the other hand, clay is naturally available material and is present in most of the parts of the country. The damping ratio of the clay increases with increase in water content. The main difficulty with clay is its behaviour of high compressibility. However, if soft clay is enclosed in a high strength geomembrane in the form of geocells, its compressibility can be arrested. Since geomembranes are moisture barriers, hence the moisture content can also be retained in case the ground water remains high throughout the year, as in the case of southern plain lands of Assam. In the present study soft clay enclosed in geomembranes were used as dampers. Ground vibration was created by dropping a heavy weight from a reasonable height. Vibration of footing without dampers and footing with clay dampers were measured. It was observed that almost 50% of peak amplitude was arrested by the clay dampers. Some important conclusions were drawn from the study. 1. INTRODUCTION Earthquakes are one of the most destructive natural hazards. Each earthquake leaves behind a number of shortcomings which are being rectified subsequently, but still today people are helpless to combat with large size earthquakes. Since long people are trying to bypass the ground shaking by putting base isolators but a large scale application in Indian context is not seen. One of the reasons may be the high cost of the present isolators and second may be the unwillingness of people to accept any new technique. Maintenance of the isolators in good condition is also expensive. Thus, there is a need for advancing the seismic isolation concept to develop simple and inexpensive systems that can offer the advantages of isolation to a much wider application worldwide. The base isolation may be provided by using natural rubber bearings, high damping rubber bearings, lead rubber bearings, resilient friction bases, friction-pendulum system, flexible first storey system, sliding joint, laminated neoprine bridge bearing, neoprine pads without slip plates, etc. But little effort have been carried out to introduce soft clay as a damper. In the present study, very soft clay has been used to absorb the vibration and isolate the structure. The clay is enclosed in a geomembrane and is placed in layers below the footings. Vibration was imparted to the soil and the response of the structure was measured. Analysis and discussions of the test results are presented demonstrating the feasibility of soft clay as dampers to reduce response of structures to ground motion. 2. ORIGIN OF THE PRESENT WORK A laboratory test was conducted with a model footing resting on clay kept in a steel tank. An impact load was applied to the tank and the responses of tank and the footing were measured. It was observed that the peak acceleration experienced by the footing was quite less than the acceleration of the tank. More tests were carried out with soft clay wrapped with plastic sheets and put in layers. Water content of the soft clay was varied. It was concluded that two layers soft clay dampers with water content higher than liquid limit shows the best result of reduction of stress/peak amplitude (Singha, S. & Dey, A.K., 2005, 2007). Present work is an extension of the laboratory work whereby the footings were tested in the field. 3. TYPES OF BASE ISOLATION There are two basic types of isolation systems. The most widely adopted system in recent years is typified by the use 691

2 Behaviour of Shallow Foundations over Soft Clay Dampers of elastomeric bearings, the elastomer made of either natural rubber or neoprene. In this approach, the building or structure is decoupled from the horizontal components of the earthquake ground motion by interposing a layer with low horizontal stiffness between the structure and the foundation. This layer gives the structure a fundamental frequency that is much lower than its fixed-base frequency and also much lower than the predominant frequencies of the ground motion. The first dynamic mode of the isolated structure involves deformation only in the isolation system, the structure above being to all intents and purposes rigid. The higher modes that will produce deformation in the structure are orthogonal to the first mode and consequently also to the ground motion. These higher modes do not participate in the motion, so that if there is high energy in the ground motion at these higher frequencies, this energy cannot be transmitted into the structure. The isolation system does not absorb the earthquake energy, but rather deflects it through the dynamics of the system. This type of isolation works when the system is linear and even when undamped; however, some damping is beneficial to suppress any possible resonance at the isolation frequency. The second basic type of isolation system is typified by the sliding system. This works by limiting the transfer of shear across the isolation interface. Many sliding systems have been proposed and some have been used. In China there are at least three buildings on sliding systems that use specially selected sand at the sliding interface. Another type of isolation containing a lead-bronze plate sliding on stainless steel with an elastomeric bearing has been used for a nuclear power plant in South Africa. The friction-pendulum system is a sliding system using a special interfacial material sliding on stainless steel and has been used for several projects in the United States, both new and retrofit constructions. The present study uses soft clay dampers which have low horizontal stiffness. 4. EXPERIMENTAL SETUP The experimental setup consists of four pits of size 1.2 m 1.2 m 1.5 m depth. In one of the pits marked as P1, a normal isolated footing of size 1.0 m 1.0 m was casted. Similar footings were casted over clay dampers in pits marked P2 and P3. In pit P2 the clay damper consists of two 100mm thick soft clay layers wrapped by geomembranes, This damper is designated as type1. In pit P3 the clay damper consists of a bottom layer of 100 mm thick and a top layer of three cells, each of size 1.2 m 0.4 m 0.1 m. This damper is designated as type 2. The free ends of the geomembranes were placed below the footing to avoid any leakage of soft clay under pressure. The fourth pit P4 was kept open and was used as a source of vibration by dropping a 65 kg hammer from a height of 4.88 m. The pits and dampers are shown in Figure 1. CONNECTED WITH FFT ANALYZER 1.00m Footing without clay damper m m 1.50m SECTIONAL ELEVATION P3 DROP WEIGHT 1.00m Footing with clay damper type1 P1 P4 P2 PLAN Fig. 1: Plan and Sectional Elevation Showing the Location of the Pits. A layer of sand cushion was placed over the dampers. Footing was constructed over the sand cushion. Surcharge load was also applied over the footing. Vibration of footing without dampers and footings with clay dampers were measured. 4.1 Data Acquisition Ground vibration was created by dropping freely a weight of 65 kg from a height of 4.88 m. Accelerations of footings resting on clay dampers and without clay damper were measured with the help of accelerometer, type 4507 (B&K make) with software Pulse Lab shop version The responses of vibration were recorded with the help of FFT analyzer. 5. RESULTS Figure 2 shows a typical response of model footing without clay damper under the given impact and Figure 3 is the response of the footing resting on damper type 1. It is found that the peak acceleration in footing without clay damper was m/sec 2 and that in footing with clay damper type1 was m/sec 2. It is observed that the clay damper type 1 reduces the peak acceleration. 692

3 Acceleration, Fig. 2: Plot of Variation of Horizontal Acceleration vs. Time for Footing with No Clay Damper Fig. 3: Plot of Variation of Horizontal Acceleration vs. Time for Footing with Clay Damper Type 1 Figures 4 and 5 shows the horizontal acceleration response of footing without clay damper and footing with clay damper type 2 for a given impact load. It is found that the peak acceleration in footing without clay damper was m/sec 2 and that in footing with clay damper type 2 was 5.45 m/sec 2. It is observed that the clay damper type 2 reduces the peak acceleration Fig. 4: Plot of Variation of Horizontal Acceleration vs. Time for Footing with No Clay Damper Fig. 5: Plot of Variation of Horizontal Acceleration vs. Time for Footing with Clay Damper Type 2 Figures 6 and 7 shows the horizontal accelerations in footing with clay damper type 1 and with clay damper type 2 respectively for a given impact load. It is found that the peak acceleration in footing with clay damper type1 was 8.09 m/sec 2 and that in type 2 was m/sec 2. It is observed that the clay damper type 2 is more efficient in reducing the stress wave amplitude than the damper type 1. This shows the applicability of soft clay filled geo-cells as a possible seismic isolator. Acceleration, m/sec m/sec Fig. 6: Plot of Variation of Horizontal Acceleration vs. Time for Footing with Clay Damper Type Fig. 7: Plot of Variation of Horizontal Acceleration vs. Time for Footing with Clay Damper Type 2 Figures 8 and 9 shows the vertical acceleration response of footing with clay damper type 1 and without clay damper respectively for a given impact load. It is found that the peak acceleration in footing with clay damper type 1 was 5.55 m/sec 2 and that in footing without clay damper was 8.81 m/sec 2. It is clear that the clay damper also reduces the vertical component of seismic excitation Fig. 8: Peak Vertical Acceleration Response for Footing with No Clay Damper 693

4 Behaviour of Shallow Foundations over Soft Clay Dampers Time Sec Fig. 9: Peak Vertical Acceleration Response for Footing with Clay Damper Type 1 Figures 10 and 11 shows the vertical acceleration response of footing with clay damper type 1 and footing with clay damper type 2 for a given impact load. It is found that the peak acceleration in footing with clay damper type 1 was 5.33 m/sec 2 and that in footing with clay damper type 2 was 2.68 m/sec 2. The reduction in peak vertical acceleration is more in clay damper type 2 than in clay damper type Fig. 10: Peak Vertical Acceleration Response for Footing with Clay Damper Type Table 1: Test Results Horizontal Acceleration (peak acceleration in m/sec 2 ) Nondamper Damper_type 1 Reduction factor Nondamper Damper_type Damper_type 1 Damper_type Vertical Acceleration (peak acceleration in m/sec 2 ) Nondamper Damper_type Damper_type 1 Damper_type Column 1 represents observation number. A cceleration, m/sec Nondamper Damper_type Observation NoNo. Fig. 12: Peak Horizontal Acceleration Response for Footing with No Clay Damper and Clay Damper Type 2 Fig. 11: Peak Vertical Acceleration Response for Footing with Clay Damper Type 2 All the tests results are tabulated in Table 1. The results obtained in the test program are presented in the form of typical bar diagram through Figure 12 for observation no 5, 6 & 7 which represents the peak horizontal acceleration response for footing with no clay damper and with clay damper type 2. A term is defined as reduction factor which is a ratio of peak acceleration in footing with clay damper to that in footing without clay damper It is observed that the clay damper type 1 reduces the peak acceleration by almost 50%. It is also observed that the clay damper type 2 reduces the peak acceleration by 60%. It is observed that the clay damper type 2 is more efficient in reducing the stress wave amplitude than the damper type CONCLUSIONS From the present study the following conclusions can be drawn Spectral acceleration of a footing decreases with application of clay dampers at the base of the footing. 694

5 Clay damper consisting of two layer of uniform thick clay damper reduces the peak acceleration by almost 50%. Clay damper consisting of one bottom layer of uniform thickness and a top layer of three cells reduces the peak acceleration by almost 60%. Soft clay filled geo cells can be used as a possible seismic isolator The reduction in the peak value of acceleration depends upon the type of clay dampers. REFERENCES Chopra, A.K. (2004). Dynamics of structure Theory and Application to Earthquake Engineering, Prentice-Hall of India Pvt Ltd. Kramer, S.L. (1996). Geotechnical Earthquake Engineering, Pearson Education. MIL-HDBK-1007/3, (1997). Department of Defence Handbook, Soil Dynamics and Special Design Aspects, Chapter 2, Murthy, C.V.R. (2004). IITK-BMPTC E/Q Tips-24. Singha, S. and Dey, A.K. (2005). Seismic Base Isolation Using Damped Soil and Frictionless Sheets An Overview, Proc. of Indian Geotechnical Conference, Ahmedabad, December, pp Singha, S. and Dey, A.K. (2007). Seismic Base Isolation Using Soft Clay Damper, Proc. of 13th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering, 10-14th December, Paper Number: / IN-24. Yegian, M.K. and Kadakal, U.K. (2004). Foundation Isolation for Seismic Protection using a Smooth Synthetic Liner, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 130, No.11., pp Yegian, M.K. and Catan, M. (2004). Soil Isolation for Seismic Protection using a Smooth Synthetic Liner, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 130, No. 11, pp