Comparison of soil pressure measurements around square box culverts using different techniques

H. El Naggar

In the proceedings of: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: FUNDAMENTALS - VIII Physical & Numerical Modelling

ABSTRACT: Tactile pressure sensors are measurement devices that have the ability to monitor the normal stresses on structures andboundaries embedded in soil bodies. Centrifuge tests were performed at the RPI geotechnical centrifuge facility toinvestigate the effect of soil arching and soil culvert interaction. Two different relative densities of Nevada sand wereinvestigated. To examine the validity of using tactile pressure sensors for providing acceptable estimates of soil pressures,the results were compared tosoil pressures obtained from strain gauges and numerical modeling using the FLAC software.This paper presents a description of the sensors used and their calibration. Examples of the vertical and horizontal soilpressures measured on the top slab and sidewall of the box culvert is discussed.R•SUM•Capteurs de pressiontactilessont des appareilsde mesurequi ont la capacit†de surveillerles tensions normalessur lesstructureset les limitesincorpor†esdans les organesdu sol.Essais encentrifugeuseont †t† effectu†es‡ l'installation decentrifugeuseg†otechniqueRPIpour †tudier l'effetdevo…tedu solet de l'interactiondu ponceaudu sol.Deuxdensit†srelatives diff†rentesde sabledu Nevadaont †t† †tudi†s.Pourexaminer la validit†de l'utilisation decapteurs de pressiontactilespour fournirdes estimations acceptables depressionsdu sol,lesr†sultats ont †t† compar†s‡ des pressionsdusolobtenus ‡ partir dejauges de contrainte etmod†lisation num†riqueen utilisant lelogicielFLAC.Cet article pr†senteunedescription descapteurs utilis†setleur calibrage.Des exemples despressionsverticales et horizontalessol mesur†essur la dalleet le c—t†paroi sup†rieure dudalotest discut†e.1INTRODUCTIONThe study of soil-culvert interaction problems is consideredto be one of the more complex challenges in the field ofsoil-structure interaction. There are several factorsaffecting the response of box culverts to the surroundingsoils. Soil arching,which can lead to an increase ordecrease in the soil pressures around the box culverts dueto the stress redistribution, is one of the major factors thatcontrol the response of box culverts(Terzaghi, 1943,Bjerrum et al., 1972, Evans 1984,Bulson, 1985).The effectof relative stiffness between the culvert and thesurrounding soil is also critical in considering complex soil-culvert interaction (SCI).Stone et al. (1991) performed centrifuge tests on arectangular box culvert to replicate a prototype dimensionof 3 m wide by 2 m high. Three different thicknesses (0.5,1.0, and 1.62 mm) were used to investigate the effect ofrelative stiffness between the culvert and the surroundingsoil on the loads attracted to the culvert. The culvert wasinstrumentedby forty miniature strain gauges on both facesto enablethecalculation ofthe bending moment and axialforce. To calibrate the strain gauges, the culverts endswere capped and vacuum applied to the inside of theculvert. Well-graded quartz sand wasusedfor the studythat was conducted from75g and 150g. It was noted thatwhile the response of the top slabswas similar for allofthemodels, the response of the side walls and base of flexibleculvertswas different to that observed for stiff culverts. Theauthorsencountered problems withfitting the bendingmoment dataandsuggested using cubic equations for theedge side andquadratic equations for the center of theslabs.Double differentiations of these functions providedshearforce and pressure distributions. The top slabpressure distributions were found to be generally parabolicin form. Confirmation of this type of results has rarely beenconducted.Several centrifuge tests were performed at theRensselaer Polytechnic Institute (RPI) geotechnicalcentrifuge facility in the United States. These tests wereconducted to investigate the response of square sectionbox culverts buried in dry Nevada sand to static andseismic loads. Aluminum hollow square section with twodifferent thicknesses was used to model the box culvert.To obtain data,twodifferent instrumentforms were used,i.e.strain gauges and tactile pressure sensorsto comparepressure distributions estimated with each method.The standard method of calibrating the tactile pressuresensors using the manufacturer machine calibration wheresensors are placed between two steel plates and subjectedto different pressures using air pressure could not be useddue to the variations instress distribution typically found insoil models. The calibration procedure for tactile pressuresensors was adopted byEl Ganainy et al. (2014) was usedfor this study.In this paper, a brief description of the centrifuge testsand numerical models areintroduced. The method ofcalibrating the strain gauges and tactile pressure sensorsare presented. Results of the soil pressures directlymeasured using the tactile pressure sensors, indirectlymeasured using strain gauge data and soil pressuresobtainedfrom the numerical model using finite differencesoftware (FLAC 2D) are all discussed.

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H. El Naggar (2016) Comparison of soil pressure measurements around square box culverts using different techniques in GEO2016. Ottawa, Ontario: Canadian Geotechnical Society.

@article{3411_0713113807,author = H. El Naggar,title = Comparison of soil pressure measurements around square box culverts using different techniques,year = 2016}