International Journal of Engineering and Innovative Technology (IJEIT) Volume 2, Issue 6, December 2012

Transcription

1 International Journal of Engineering and Innovative Technology (IJEIT) Volume 2, Issue 6, December 2012 Study Of Instrumented Segmental / Meter Panels For Basements: Comparison of Theoretical and Actual Behavior D.K. Kanhere Joint Executive Director, STUP Consultants Private Limited, Navi Mumbai, India Dr R A Hegde, Professor, Sardar Patel college of Engineering, Andheri-Mumbai, Abstract: Growing need of basements for vehicles storage and services in city of Mumbai is problematic. Proximity buildings, services, narrow lanes, traffic and crowds preclude open excavations or diaphragm walls. Building subways under crowded roads with utilities and closure is not possible, led to using segmental / meter panel construction. These are panels of small width designed as cantilever sheet piles adequately anchored in sub-stratum to develop necessary passive resistance and balancing couple. When constructed in succession, full length of the wall can be constructed. This is a fast, flexible and cheap engineering solution to the problems encountered in crowded localities. A number of such structures have been constructed. Some panels were instrumented for verification of in-situ behavior. Points of interest concerning stress patterns and distribution were monitored. A comparative study of the observed readings of the instrumented meter panels and the results of theoretical analysis and design principles are presented. I. METHODS OF ANALYSIS OF RETAINING STRUCTURE/SYSTEMS A. Analytical Methods Usually, following methods for analysis of cantilever pile walls, all of which involve simplifications of net pressure distributions to allow calculations of critical retaining height are in use: a. Factored Moment Method (FMM) b. King s Method (1995) c. Day s Method (1999) d. Water Lum s Rules of Thumb (2000) B. Empirical Design Methods a. Strut Load: An important aspect in excavation support design is to estimate the strut load. Most of the methods are empirical and based on limit equilibrium of wedge (Terzaghi, 1941) and the apparent earth pressure diagrams (Terzaghi. 1943; Peck. 1969) b. Base Stability: Another important aspect is the effect of base heave. (Bjerrum and Eide 1956) proposed the use of stability numbers against base heave. C. Field Measurements: Ground movement is a major concern in the design of excavation support systems. In built-up areas where the ground contains a soft layer, thorough investigation is required. The following observations are made for analysis: Dr. V.T. Ganpule V. T. Ganpule & Associates, Mumbai, India a. Lateral wall deflections b. Ground surface settlements c. Stress state of soil near excavation D. Finite Element Analysis It is generally accepted that the Finite Element Analysis (FEM) is the major technique used in the numerical analysis of geo-technical problems. FEM in excavation related problems was first carried out between 1969 and Some fundamental techniques in excavation process simulation were developed in E. Design Considerations For general design purpose it is recommended to use the trial and error method for analyzing the system and its stability. Meter panels like circular touching piles or diaphragm walls are structural elements, which are constructed underground. These are basically used as retention systems and permanent foundation walls. These can be installed in close proximity to existing structures, with minimal loss of support to existing foundation. In addition construction dewatering is not required. Structural meter panel can be employed for forming underground retaining or load bearing walls for practically any purpose. Panels of small width are designed as cantilever sheet piles adequately anchored in sub-stratum to develop necessary passive resistance and balancing couple. When constructed in succession, full length of the wall can be constructed. The stability of excavation/meter panel, control of wall deflections, ground movements/ ground water problems have been analyzed with instrumentation as well as with FEM models as a case study. The most important consideration in proper design and installation is that the retained material is attempting to move forward and down-slope due to gravity. This creates soil pressure behind the wall. The same is to be balanced by passive earth pressure on other side. Further couple formed by active and passive earth pressure has to be balanced by trial & error method. The section of meter panel happens to be depth of cantilever and is function of excavation depth and soil properties of retained earth. The enabling structure derived its support from the passive reaction offered by 258

2 formation below excavation level and the resisting counterbalancing couple developed on going further deep. The forces on enabling wall are; 1. The active earth pressure from back of wall which tries to push wall away or towards excavated side. 2. The passive pressure generated below dredge line in direction opposite active pressure which resists the movement of the wall 3. The passive pressure in the direction of active pressure which will counterbalance the moment formed by active pressure. 4. These passive forces are calculated by using equilibrium conditions namely summation of moment and summation of horizontal forces are zero at all points. 5. The passive pressure can be idealized to rectangular block in case of stiff clay or rock and as first trial considers suitable pile length below excavation level depending upon the formation 6. Using equilibrium conditions the pressure intensities are calculated and they are not equal or slightly less than permissible. The length of embedment below excavation is changed. The pressure is repeated till above condition is satisfied. 7. The maximum bending moment is calculated where shear force becomes zero. Fig 3. Step-2 Equilibrium of Horizontal Forces 0 m P3 L m Excavation level P2 P1 Fig 1. Earth Pressure Diagram (Theoretical) Fig 2. Step-1 Active Earth Pressure - Soil Fig 4. Step-3 Equilibrium of Horizontal Forces & Moments Soft Soils II. PRESENTED STUDY The past concerns about the stability of excavation have now shifted emphasis on control of wall deflections and ground movements under working conditions especially in densely built-up areas. The numerical methods with appropriate consideration of relevant factors have the potential to give relatively accurate solution for both stability and ground movements for an excavation. However, a perfect model to predict the collapse of support system and evaluation of ground movements at some distance behind the retaining wall under working conditions is still not available. III. INSTRUMENTATION SETUP In order to physically verify the developed pressure both active and reactive, worked out from theoretical consideration, the retaining elements are instrumented as described below; In arranging instrumentation of meter panel, several aspects must be considered. The use of instrumentation is complex process. Right from selection of appropriate tool for gathering the data as an even a small mistake leads to failure of investigation program In and around the city of Mumbai, a number of basements/ subways have been and are being instrumented and constructed. Normally, bending 259

3 strains in wall are measured along its depth giving bending moment response. The evaluated geo-technical properties of soil and observed bending strains along with theoretical bending moment, shear force, deflection and earth pressure are presented and comparison made between theoretical and experimental values. The measurements have been taken using electrical resistance strain gauge. These adhesive type strain gauges were mounted on the panels, had the following specifications: Type: BE 120 6AA (11) Gauge resistance: Gauge factor: % Make: Tokyo Sokki Kenkyujo Company Limited A. Strain Gauge Arrangement: Connections are made using red, black, yellow and green wires as indicated in Fig.1. B. Strain Measuring Instrument A strain measuring device in the SYSCON product range was used. It accepts input from 10 numbers of strain gauge bridges simultaneously and signals are selected one by one through an electronic channel selector to a signal conditioner which consists of a high gain, stable, low noise, low drift, instrumentation type amplifier, GF setting control and bridge configuration select facility. The conditioned outputs provide 5V DC signal for an input of 20,000 micro strains. This output is connected to a micro-controller based 41/2 digit panel meter that can display strains up to 20,000 micro strains with one micro strain resolution. This voltage output is also made available to the user on the rear panel for purpose of monitoring or recording. (Strain indicator SI 38 MS) IV. FIELD SETUP As indicated in Fig.2, the meter panel is instrumented before concreting and all lead wires are taken out at the top of the pile. V. CASE PRESENTED 1. Soil profile at Andheri site (Fig.3) 2. Observed strains at each strain gauge station i) Location of site: New Link Road, Andheri ii) Size of Meter panel: 1000mm x 600mm iii) Total depth of Meter panel: 9.00m from original ground level iv) Total depth of excavation: 6.00m v) Total number of strain gauge installed: 5 on tension side and 5 on compression side Table No 1 Observed Strains S ttension side Compression side Initial Strain Reading Final Strain reading Total train ( m/m) Initial strain reading Final strain reading Total train ( m/m) Calculated bending moment from observed strain: See table 2 Location of strain gauge from GL Area of steel (mm 2 ) Lever arm (mm) Stress in (N/mm 2 ) Bending moment in KN-m Calculation of Deflection, shear force and earth pressure: Deflection is second integral of bending moment; shear force the first derivative of bending moment and earth pressure the second derivative of bending moment. Therefore, equation of observed bending moment was worked out by using curve-fitting techniques. Origin Lab 7.0 Software was used. Lorentz curve best fitted the observed bending moment. By solving the differential equations, observed deflections, shear force and earth pressure on meter panel were calculated (See Fig.4). VI. COMPARISON OF THEORETICAL (AS MENTIONED IN 1.6 DESIGN CONSIDERATION) AND OBSERVED BEHAVIOR Tabulated results are given in Tables-3a & 3b, and for graphical representation refer Fig.11, 12, 13 & 14. Table-3a Location of strain gauge from top of the Pile in m Theoretical Earth Pressure kn/m Observed Earth Pressure kn/m Theoretical Shear forcekn Observed Shear for kn

4 Table-3b Location of strain gauge from top of the Pile in m Theoretical bending moment kn-m Observed bending moment kn-m Theoretical Deflection mm Observed Deflection mm Tensile Stress in Steel VII. COMPARISON OF THEORETICAL (FEM MODEL) AND OBSERVED BEHAVIOR moment considered c value much less than actual. Earth pressure diagram clearly shows that the actual passive earth pressure developed is greater than the assumed theoretical passive earth pressure in rock strata. This reflects a high factor of safety taken for design of panels and an underestimated c value of rock. IX. SCOPE FOR FUTURE ENHANCEMENT There is a need to extend the work further so as to collect more information for inferring the design parameters from the field and laboratory test results of cohesion and angle of internal friction. The selection of design parameters from soil properties at present is very much empirical and based on subjective interpretation of the designer as per his experience. Some sort of relation needs to be established between the design parameters and properties of soil as revealed by investigation to serve as a guide for designers. The realistic evaluation of earth pressure, either active or passive, is possible only by collecting the behavior data of the instrumented piles / meter panels / diaphragm walls as retaining elements, particularly since the laboratory results conducted on soil samples is still a grey area. The authors are making efforts in that direction. Soil profile with properties is given below; Table-4 Depth (m.) C EP (kn/m 2 ) WP (kn/m 2 ) Total (kn/m 2 ) The water table is located at 2m below GL. Results are graphical represented in Fig.15, 16, 17 & 18. VIII. CONCLUSION On the basis of field measurements, the observed flexural behavior of cast-in-situ reinforced concrete cantilever segmental/ meter panels leads to the following conclusions: Theoretical and observed profiles of earth pressure, bending moments, shear force and deflection in the structure due to excavation are matching well. Nature of variation in theoretical and observed values of force functions along the depth of the panel wall matches well with slight differences in magnitudes. Differences in theoretical and observed values are due to variations in soil properties, inconsistent laboratory results of soil samples, changes in groundwater table, etc. Magnitudes of observed values through instrumentation are less than the theoretical values. The difference occurred because the theoretical magnitude of lateral earth pressures and consequently theoretical bending Fig 5. Strain Readout Unit Fig 6. Instrumented Plate 261

5 Fig 9. Soil Profile Fig 10. Best Fitted Curve Fig 7. Strain Gauge Arrangement Fig 8. Strain Gauge Locations Fig 11. BM Comparison 262

6 Fig 12. SF Comparison Fig 14. Observed Earth Pressure `. Fig 15. SF Comparison (FEM with Instrumentation) Fig 16. BM Comparison (FEM with Instrumentation) Fig 13. Deflection Comparison 263

7 Fig 17. Deflection Comparison (FEM with Instrumentation) Fig 18. Theoretical (FEM) BM, SF and deflection. REFERENCES [1] Charles Ng. W.W., Lings. H.L., Nash. F.T., back analyzing the bending moment in a concrete-diaphragm wall, The Structural Engineer/Volume 70/ No.23/24/8 December [2] Yoo Chungsik, Behavior of braced and anchored walls in soils overlying rock Journal of geotechnical and geoenvironmental engineering / March [3] Charles. W.W., Douglas. B.R., Sean W.L., Lei.G.H. Field studies of well instrumented barrette in Hong Kong, journal of Geotechnical Engineering and Environmental Engineering, January 2000, [4] Symons I.F. and Carder D.R. Field measurements on embedded retaining wall, geotechnique 42, No.1, [5] Kanhere D. K., Dr. Hegde R A, and Dr. Ganpule V T; Segmental / Meter Piles for Basements and Subways; Proceedings of Indian Geotechnical Conference December 13-15, 2012, Delhi. [6] Kanhere D. K., and Dr. Ganpule V T; Behavior of Segmental / Meter Panels for Basements and Subways; Published and presented in FIB International Conference, Amsterdam,