Pile Head Cyclic Lateral Loading of Single Pile

This paper presents an elastic continuum model using an extended nonlinear Davies and Budhu equations, which enables the nonlinear behavior of the soil around the long elastic pile to be modeled using a simple expression of pile-head stiffness method. The calculated results were validated with the measured full-scale dynamic field tests data conducted in Auckland residual clay. An idealized soil profile and soil stiffness under small strain (i.e. shear modulus, Gs and shear wave velocity, Vs of the soil) determined from in situ testing was used to model the single pile tests results. The predictions of these extended equations are also confirmed by using the three-dimensional finite-element OpenSeesPL (Lu et al. in OpenSeesPL 3D lateral pile-ground interaction: user manual, University of California, San Diego, 2010). A soil stiffness reduction factor, Gs/Gs,max of 0.36 was introduced to the proposed method and model. It was found to give a reasonable prediction for a single pile subjected to dynamic lateral loading. The reduction in soil stiffness found from the experiment arises from the cumulative effects of pile–soil separation as well as a change in the soil properties subjected to cyclic load. In summary, if the proposed method and model are accurately verified and properly used, then they are capable of producing realistic predictions. Both models provide good modelling tools to replicate the fullscale dynamic test results

Dynamic Field Tests of Single Piles

This paper presents the results of a full-scale field study of single free-head piles embedded in Auckland residual clay. Four hollow steel pipe piles, each with an outside diameter of 273 mm and wall thickness of 9.3 mm were installed at a site in Pinehill, Auckland. A series of dynamic tests ranging from low excitation (using an instrumented impact hammer and a low-mass loading of an eccentric mass shaker) to high dynamically-induced force from the eccentric mass shaker was performed during the spring and early summer after the winter wet weather, so that the soil can be assumed to be saturated to the ground surface. Results from low amplitude dynamic tests indicated a reduction in the natural frequency of the system from 9.6 Hz to 8.2 Hz after experiencing a higher level of forcing amplitude. This reduction in natural frequency demonstrated the non-linear response of the pile-soil system that was caused by the strain softening of the soil and the formation of a gap between the pile shaft and the surrounding soil

Analysis of liquefaction characteristics at Christchurch strong motion stations

The city of Christchurch and its surrounds experienced widespread damage due to soil liquefaction induced by seismic shaking during the Canterbury earthquake sequence that began in September 2010 with the Mw7.1 Darfield earthquake. Prior to the start of this sequence, the city had a large network of strong motion stations (SMSs) installed, which were able to record a vast database of strong ground motions. This paper uses this database of strong ground motion recordings, observations of liquefaction manifestation at the ground surface, and data from a recently completed extensive geotechnical site investigation program at each SMS to assess a range of liquefaction evaluation procedures at the four SMSs in the Christchurch Central Business District (CBD). In general, the characteristics of the accelerograms recorded at each SMS correlated well with the liquefaction evaluation procedures, with low liquefaction factors of safety predicted at sites with clear liquefaction identifiers in the ground motions. However, at sites that likely liquefied at depth (as indicated by evaluation procedures and/or inferred from the characteristics of the recorded surface accelerograms), the presence of a non-liquefiable crust layer at many of the SMS locations prevented the manifestation of any surface effects. Because of this, there was not a good correlation between surface manifestation and two surface manifestation indices, the Liquefaction Potential Index (LPI) and the Liquefaction Severity Number (LSN)

Soil profile characterization of Christchurch strong motion stations

This paper presents an overview of the soil profile characteristics at a number of strong motion station (SMS) sites in Christchurch and its surrounds. An extensive database of ground motion records has been captured by the SMS network in the Canterbury region. However in order to comprehensively understand the ground motions recorded at these sites and to be able to relate these motions to other locations, a detailed understanding of the geotechnical profile at each SMS is required. The original NZS1170.5 (SNZ 2004) site subsoil classifications for each SMS site based on regional geological information and well logs located at varying distances from the site. Given the variability of Christchurch soils, more detailed investigations are required in close vicinity to each SMS. In this regard, CPT, SPT and borehole data, and shear wave velocity (Vs) profiles in close vicinity to the SMS are currently being used to develop representative soil profiles at each site. Site subsoil classifications based on Vs measurements performed by the authors do not always agree with the original classifications, often indicating that a softer site class is appropriate. However, SPT N values often indicate a stiffer site class than the Vs data, in some cases also disagreeing with prior assumed classifications. Hence, the recent site investigation data presented herein highlights the importance of having detailed site-specific information at SMS locations in order to properly classify them. Furthermore, additional studies are required to harmonize site classification based on SPT N and Vs-

Snap-Back Testing and Estimation of Parameters for Nonlinear Response of Shallow and Pile Foundations at Cohesive Soil Sites

We are working on the development of methods for analysing the earthquake response of foundations that make use of Soil-Foundation-Structure-Interaction (SFSI) as a means of incorporating nonlinear soil deformation effects and nonlinear geometrical effects into the earthquake resistant design of foundations. There are three challenges in this work. First, to incorporate adequately the nonlinear response of the soil during the earthquake. Second, to account for geometrical nonlinearity during the earthquake - that is loss of contact between various parts of the foundation and the underlying and/or adjacent soil. Examples of this are the gapping that develops between a pile shaft and the surrounding soil during cyclic lateral loading and the uplift beneath parts of a shallow foundation subject to rocking. Third, to obtain appropriate values for the soil parameters which describe the nonlinear response of the foundations. The main thrust of this paper is to show how snap-back testing is a most effective means of evaluating nonlinear soil behaviour. It will be demonstrated that snap-back testing is more convenient than using a shaking machine which applies sinusoidal excitation. The results will show how for the rocking of a shallow foundation and the cyclic lateral loading of a single pile, the damping and the stiffness can be estimated at increasing levels of lateral loading

Macro element for pile-head cyclic lateral loading, Special Topics in Earthquake Geotechnical Engineering, Geotechnical

Interaction between laterally loaded piles and the surrounding soil is a complex phenomenon, particularly when nonlinear soil behaviour is involved; so complex that usually design calculations rely on computer software based on discrete spring formulations using empirically derived nonlinear p-y relationships. This chapter explores a macro element, Davies and Budhu (1986), as an alternative which uses relatively simple formulae that are available for evaluating the lateral stiffness of long elastic piles embedded in elastic soil and an extension to handle nonlinear soil-pile interaction. The predictions of these equations are confirmed using the three dimensional finite element software OpenSeesPL, Lu et al. (2010), as well as data from field lateral load testing on driven piles in a stiff residual soil at a North Auckland site. Furthermore, in this chapter an extension of the macro element to cyclic loading is presented and this is shown to model well the field data and also the predictions of OpenSeesPL. The pile head macro element method is not completely general as it applies only to a homogeneous soil profile, but, since we deal with long piles, the soil homogeneity needs to extend only over the pile shaft active length. Measured lateral load response of the piles at the Auckland site indicates that it is necessary to distinguish the “operational” modulus of the soil from the small strain modulus; the field data indicates a value of about one third to one quarter of the small strain value

Initial Shear Modulus of Auckland Residual Soil From Field and Laboratory Tests

This paper describes the experiments performed to investigate the initial shear modulus of Auckland residual soil. Firstly, WAK (wave-activated stiffness) tests and spectral analysis of surface waves (SASW) tests were conducted at a residual soil site by applying impact and harmonic loads on a circular steel plate in vertical direction. A sledgehammer equipped with a dynamic force transducer was used to produce the impact load while harmonic loading was applied using an eccentric mass shaker to generate steady-state excitations. The initial shear modulus of soil was obtained by considering the soil to be vibrating as a single degree of freedom (SDOF) system. Next, undisturbed soil samples from the site were subjected to consolidated undrained tests with three submersible miniature linear variable differential transducers (LVDTs) mounted on the sides of the specimen to measure the small strain stiffness. The LVDTs were capable of resolving displacements of less than 1 μm and measuring axial strains ranging from less then 0.001% to 2.5%. The small strain stiffness obtained from laboratory tests compared very well with those determined from geophysical tests

Snap-Back Testing for Estimation of Nonlinear Behaviour of Shallow and Pile Foundations

We are working on the development of methods for analysing the earthquake response of foundations that make use of Soil-Foundation-Structure- Interaction (SFSI) as a means of incorporating nonlinear soil deformation effects and nonlinear geometrical effects into the earthquake resistant design of foundations. There are three challenges in this work. First, to incorporate adequately the nonlinear response of the soil during the earthquake. Second, to account for geometrical nonlinearity during the earthquake - that is loss of contact between various parts of the foundation and the underlying and/or adjacent soil. Third, to obtain appropriate values for the soil parameters which describe the nonlinear response of the foundations. The main thrust of this paper is to show how snap-back testing is a most effective means of evaluating nonlinear soil behaviour. We consider that snap-back testing is more convenient than using a shaking machine which applies sinusoidal excitation. The results from rocking of a shallow foundation and cyclic lateral loading of a single pile enable damping and stiffness to be estimated at increasing levels of lateral loading

Lateral Dynamic Response of a Single Pile Model

The dynamic response of a single pile with an attached mass giving a single degree of freedom (SDOF) system subjected to lateral pile-head loading is described. Two types of dynamic tests were performed, primarily free-vibration and dynamic forced-vibration tests. The main aims of this paper were to provide a basic understanding of the single pile head action and to evaluate the applicability of the elastic continuum method (ECM) in estimating the dynamic parameters of the pile-soil system. The single pile model was constructed using an instrumented steel pipe with an outside diameter of 50.65 mm and a wall thickness of 1.05 mm. The measured data from the frequency domain analysis were analysed to determine the natural frequencies fn, damping ratios ζ, and lateral pile-mass connection displacement uLat of the pile-soil system. The experimental results obtained were then compared with those calculated from the analytical calculation based on the ECM. The analytical prediction was based on a perfect pile-soil bonding with the pile embedded in a homogeneous soil layer with a constant Young’s modulus with depth. The natural frequency obtained from the free-vibration test was greater than that from forced-vibration test. This was found to be in general agreement with other related published works. The experimental and analytical results were shown to be reasonably matched, thus, the ECM demonstrates a good potential application in estimating the dynamic parameters and the lateral displacement