Sequence for Building the Model

Building the models started with adopting the procedure to achieve the required relative density for each model as shown above. The models were built after placing the empty centrifuge box on the one-dimensional shaker on the centrifuge platform to avoid any disturbance in the sand model that might cause any changes in the density of the sand (Figure 3.37). Once the first five sand layers were placed, the tactile pressure sensors were attached to the box culvert model using double sided tape as shown in Figure 3.38. The culvert model was then placed and levelled in its position. A thin layer of vacuum grease was used to prevent leaking the sand between the box culvert and the Plexiglas side. After placing the sand layers on both sides of the culvert model and on top for the required height, the accelerometers were placed at their required levels inside the sand as shown in Figure 3.39; five cross bars were used to strengthen the box and to protect the Plexiglas from breaking at high g levels. Also, thin layers of blue painted sand were placed between sand layers just behind the Plexiglas to monitor any movement that might occur during testing as shown in Figure 3.40. For Case A, the three LVDTs were connected to the cross bars and placed on the sand surface, while for Cases B, C, and D, the middle LVDT was mounted on top of the foundation as shown in Figure 3.41. All sensors used in the model were checked and connected to the data acquisition system. The centrifuge was then accelerated incrementally and held at the following acceleration levels, 10g, 20g, 30g, 40g, 50g and 60g to check stability of the sensor readings. The earthquake signals were sent to the shaker at 60g. Data from all sensors were recorded continuously during the test. The entire procedure was repeated for all test cases by de-accelerating the centrifuge to 1g and stopping it to make any changes related to each case and re-running the centrifuge again.

Figure 3.37: Placing the centrifuge box on the shaker and the centrifuge platform

Figure 3.40: Photo of completed model

Case A Case B

Case C Case D

3.9 SUMMARY

A detailed description of the centrifuge modeling conducted to study the soil-culvert interaction behaviour under static and seismic loading was presented. All the materials used along with instrumentations and their calibration were explained in detail.

Specific challenges related to application of some instrumentation are highlighted along with the solutions proposed to resolve them. A newly developed procedure for installing the strain gauges inside the box culvert and the parts manufactured for this purpose were presented. Model configuration and preparation of the model container was described. The sand preparation method used to achieve the required sand density was also explained. The earthquake shakings were applied using a one dimensional box shaker, and the earthquake signals were calibrated and filtered in dummy tests, to obtain the appropriate signals that the shaker can produce.

Although there were certain challenges and difficulties that have been described in this chapter, scaled physical experimental modeling still represents an effective way to investigate soil-culvert interaction.

CHAPTER FOUR

CENTRIFUGE TEST RESULTS AND INTERPRETATIONS

4.1 INTRODUCTION

This chapter presents the results and the analysis of the data collected from a series of centrifuge tests performed as described in Chapter 3. The volume of data produced from these tests is considerable and therefore, it is not feasible to present all data obtained in this chapter. Hence selections of the data are included in this chapter and further data is summarized in Appendices. The data collected was in the form of settlement using the LVDTs, strain data using strain gauges, pressure data using tactile pressure sensors and acceleration data using accelerometers. Data processing and filtering were applied to all acceleration records to remove electronic drift from the records. Filtering is necessary to obtain the correct shape of the velocity and displacement time histories that starts and ends with zero values.

The Free Field (FF) is a well known term for the zones where the soil movement is not influenced by the presence of a structure either placed on the surface or buried inside it. The zones where there is a structure are termed as “Structural Field” (SF). To investigate the soil culvert interaction under the effects of static and seismic loading, analyses were performed to examine different aspects that involve both factors. For static loading the analyses, included the static bending moment and soil pressures. Under the effect of earthquake loading, analyses were conducted to explore the effect of the Free Field versus Structural Field responses in terms of dynamic soil properties, rocking of the box culvert and foundations, racking of the box culvert, kinematic soil culvert interaction, lateral movement of the foundations, as well as the envelope seismic bending moment.

Comparisons were also performed to show the effect of the box culvert thickness, soil density, and surface foundations. This chapter also compares the observed results from the centrifuge tests with estimates based on theoretical and/or empirical relationships. All results are presented at prototype scale unless otherwise noted.