Large Pile Vessel Centrifuge Platform Finite Element Mesh for Platform Analysis. Comparison between experimental calibration and finite element platform analysis. a) Load along center axis for maximum deflection.
Introduction
What is needed
The insertion of the pile provokes a disturbance in the surrounding soil, which translates into a change in soil stress. In the analysis of the direction of the piles, much attention has been paid to the dynamic response of the pile.
Pile driving experiments
As mentioned above, the improvement of current pile-soil models relies on the study of pile response to hammer impact. This gives, therefore, the complete actual representation of the response of the pile-soil system from the point of view of the pile and the soil.
Thesis outline .............................................................. G
- Caltech centrifuge
- Test containers
- The pile driving mechanism
- The pile static loading mechanism
The pile is placed under the wagon and is forced into the ground by the action of the model pile. 3rd sketch: top position of the piston; the pile is still at the same level in the ground.
Centrifuge Instrumentation
- Accelerometers ............................ , .......................... lG
- Displacement transducers
- Load cell
- Pressure transducers
- Strain-gaged pile
We want to know the position of the post in the ground at all times during the drive. During an axial load test, the load is applied to the top of the pile by the carriage's beam.
Data Acquisition System
- What is needed
- General purpose digital acquisition systems
- High speed digital acquisition systems
Therefore, a very high speed data acquisition system is required to capture transient dynamic signals. Because of the location of the centrifuge, connecting to a mainframe computer for high-speed data acquisition presented some difficulties.
Signal conditioning
- Amplification and offset adjustment
- Filtering
Appendix C.4 shows the complete circuit diagram of the signal conditioner designed and built by John Lee. Offers AC or DC coupling options, offset input, signal amplification and filtering.
RPM counter
Some of the transducer's signals, such as the strain gauge bridge output from the instrument stacks, are high frequency signals and also require high gain. This generated TTL signal is in turn the input to the digital counter circuits located on the S100 plug-in board of the Z120 computer, see Appendix C.6.
Test procedure
- Description of soil
- Preparation of soil model
- Equipment and instrument setup
- Signal processing implementation
- Progress of an experiment
- Calibration of transducers
The movement of the frame due to the pull down by the hydraulic piston is transmitted through the slide to the pile. Because of the slab on the ground, the pile cannot move down and therefore undergoes compression.
Selection and reduction of data
- Pile dynamics
- Soil stress field during driving
- Soil dynamics
- Pile axial loading
- Data reduction
- Overview
- Pile dynamics
The pairs of gauges allow for the measurement of the axial strain (bending cancellation occurs in the Wheatstone completion bridge), from which we get the force in the stack at different locations. Instead, the transducer is fixed in the top of the stack in the stack cap. Likewise, accelerations at the tip portion of the stack are measured by a transducer fixed in a plug inside the stack bottom.
This implies that when measurements of the point acceleration are made, the pole is necessarily a closed pole. The output of the transducers is recorded and stored at the end of each stroke. The pressure transducers have a frequency response that is high enough to be able to record transient loading of the ground, i.e. the waves propagating in the ground.
Therefore in the integration (Simpson's or trapezoidal rule) of the signal to obtain speed, ramps due to bit jumps are often encountered. High frequency noise filtering (25 KHz and above) is done in signal conditioning.
Measurement of soil stresses using pressure transducers
Static Calibration
The purpose of the static calibration of the pressure cells in the centrifuge is to obtain the necessary data to represent their behavior by a model for the specific. First, the linearity of the membrane-type cells is checked under hydrostatic pressure in the centrifuge environment. With this understanding of the mechanical system of the pressure cell under hydrostatic pressure, we proceed to calibrate the transducers embedded in the ground.
The gauges were embedded in dry, fine Nevada sand of medium density, which corresponds to the soil conditions of the piling experiments. In particular, we know that the overburden stress at a point in the bottom will follow the acceleration variation of the centrifuge, since this stress is linearly proportional to the g level. Now consider the base layer whose thickness is equal to the thickness of the pressure cell.
As mentioned in the introductory section, this relative increase in lateral tension will cause over-registration of the cell. The aim is to model the different aspects of the non-linear behavior of the cell embedded in the soil, in terms of soil tension, with as few parameters as possible.
Dynamic Calibration
Now each meter type (refer to section 2.2.5) will have a separate set of parameters to describe its behavior. Figures 5.5 to 5.8 and Figure 5.12 correspond to a type # 4 gauge showing a larger hysteresis (making it easier to follow the load path in the plots shown) than the type # 5 gauge, the behavior of which is shown in Figure 5.9. We have no equipment in our laboratory that would allow us to do a quantitative calibration of transducers under dynamic loading, nor is it easy to see how this could be achieved.
We can only perform a qualitative calibration to assess the rules that govern the behavior of the soil cell system under dynamic loading. A check of the transducer responses was performed by changing the length of the affected rod. The cell at the bar-bottom interface sees the stresses generated by the wave bouncing back and forth in the bar.
The frequency of the rod stress pulse is inversely proportional to the length of the rod.
Numerical Modeling of pressure transducer response in sand ............... 7.5
- Static loading
- Dynamic loading
- Obtaining the soil-cell model parameters for a pile driving experirncut79
Another assumption is to consider the behavior of the soil cell system as the same (non-linear) under static or dynamic loading. This means, in the case of piling, that we need the dynamic load history of the transducers at each impact. With the many calibration tests carried out, it has been observed that the stiffness of the cells and the soil are of the same order of magnitude.
Therefore, before starting an experiment, we have a record of the charging history of the cells. But we must keep in mind that there are some limitations in using the model. The effect of the stress field on the properties and characteristics of the foundation cell system is neglected.
During unloading, the cell cannot return to its original configuration due to the inelastic deformation created in the soil. Thus, the hypothesis about the definition and application of the finger cell model (Eqn.
Experimental results
Pile dynamics
The model pile driver directly impacts the pile cap, in which the top accelerometer is attached, and another accelerometer is located at the bottom of the pile in the plug. The observed delays in the signals correspond to the propagation time of the wave traveling in the stack. At the pile tip, therefore, a free end boundary condition will dominate, and the pressure wave will be reflected into a tension wave.
The impact of the hammer on the pile head does not give rise to a sharp pulse, but rather a long pulse due to prolonged contact between the pile and the driving mechanism. Upon reaching the top of the pile, the tensile wave will reflect as a compression, which propagates along the pile. Due to the soil resistance at the pile tip and along the pile, energy is dissipated in the ground and the wave propagating in the pile is damped.
The acceleration is measured in the pile cap and the force is measured directly on the pile, through the strain gauges at 2.67 pile radii from the top. The dynamic response of the pile to the hammer impact is well observed and the results show that a simulation of pile driving is obtained in these centrifuge experiments.
Soil stress field
- Measured static stress field: linear assumption
- Radial distribution of the radial measured stress
- Vertical distribution of the radial measured stress
- Comparison of radial and vertical measured stresses
- Measured soil stress contours
- Peak measured radial stress variation ........................ l 05
- Dynamic measured stresses, wave speed and stress decay
- Radial distribution of dynamic radial measured stress
- Vertical distribution of dynamic radial measured stress
- Conclusion
- Calculated static stress field: soil-cell model assumption
- Implications of the SCM assumption
- Calculated stress distribution
- Discussion and Conclusion
Shear and pressure waves are propagated in the ground by the movement of the side and tip of the pile. It should be noted that the entire dynamic response of the pile and the ground is over before the next impact on the pile occurs. We present the total static stress history, at the points of interest in the ground surrounding the pile, as a function of the depth of the pile tip.
The disturbance resulting from the insertion of the post is felt as a substantially uniaxial compression. In the area where the transducer is located, the ground is subjected to large deformations and significant compactions to allow the penetration of the pile. The difference explains the compaction and reshaping of the soil caused by the insertion of the pile.
The duration of the compression-relaxation cycle is also shorter as the transducer gets closer to the pile axis. This coincides with the difference in the form of the static stress field described in section 6.3.1.3. The incremental state of stress in the soil after each blow of the hammer on the pile is the result of the transient stress history.
In Figure 6.3.3.3.b, the normalization of the static radial stress by the overload stress is no longer meaningful.
Conclusions and recommendations
Proceedings of the International Seminar on the Application of Stress-Wave Theory on Piles, Stockholm, junij str. Proceedings of the Third International Conference on the Application of Stress-Wave Theory to Piles, Ottawa, maj str. Skov, "Investigation of the Stress" -Wave Method by Instrumented Piles," Proceedings of the Third International Conference on the Application of Stress-.
Zbornik druge mednarodne konference o uporabi teorije napetostnih valov na pilotih, Stockholm, maj str. Choice of Methodology," Proceedings of the Third International Conference on the Application of Stress-Wave Theory to Piles, Ottawa, maj str. Sakai , T., "The Wave Equation for the Pile Driving Analysis," Proceedings of the Third International Conference on the Application of Stress-Wave Theory to Piles, Ottawa, maj pp.
Proceedings of the Second International Conference on Numerical Methods in Offshore Piling, Austin, Texas, april pp. Warrington, D.C., ‘A New Type of Wave Equation Analysis Program’, Proceedings of the Third International Conference on the Application of Stress- Wave Theory to Stapels, Ottawa, mei pp.
