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1g Modellversuche von horizontal zyklisch belasteten Pfählen

1g Small-Scale Modelling of a Laterally Loaded Pile Under Cyclic Loading

Prof. Dr.-Ing. Martin Achmus / M.Sc. Proserpine Peralta

Fig. 1: Simplified system and principal variables for a laterally loaded pile Fig. 1: Simplified system and principal variables for a laterally loaded pile Fig. 1: Simplified system and principal variables for a laterally loaded pile
Fig. 1: Simplified system and principal variables for a laterally loaded pile

Offshore piles under wave and wind loading are subjected to high-cyclic lateral loading, inducing soil stresses and pile displacements that are difficult to quantify. In practice, p-y or subgrade reaction methods as well as finite element modelling techniques are commonly used to investigate serviceability of such piles. However, subgrade reaction methods have well-known limitations and do not explicitly take essential factors into account. Diameter effects, pile-soil stiffness, and number of cyclic loadings are obscurely and unsystematically lumped into empirical factors that, not surprisingly, greatly vary according to literature (compare subgrade reaction values from Terzaghi 1955, Sherif 1974, and API 2002). While finite element modelling can better account for the pile-soil interaction under different boundary conditions, applicable soil constitutive relations and a working model for long-term cyclic loading is still under research and remains as of yet unsuitable and costly for practical design purposes.

At the IGBE, 1g small scale model tests of laterally loaded piles in dry sand are conducted to investigate the deformation behavior of such piles under static and cyclic loading. 1g models often provide an efficient and practical means to conduct parametric studies on the physical behavior of a phenomena. Using appropriate scaling laws, the observed and/or measured changes can be extrapolated to that of a prototype. The main objectives of the model tests are:
- to investigate the pile-soil deformation behavior under constant cyclic loading amplitude, but especially,
- to investigate the influence of cyclic loading history for piles under variable loading amplitudes,
- and to gain insight on the effect of multiple load directions on the pile-soil deformation behavior.

Essential to 1g modeling in geotechnical engineering is the use of dimensional analysis. The basic principle is that any particular phenomenon can be described by a dimensionless group of the principal variables. Similarity between the model and the prototype is ensured when the dimensionless group has the same value in the model and prototype. For 1g modeling of laterally loaded piles, the assumed principal variables contributing to the deformation behavior are the applied force F, number of load cycles N, moment arm h, pile length L, diameter D, and bending stiffness EI, soil unit weight γ and porosity n, as well as pile displacement after first loading w1 and after N number of cycles wN. Secondary variables and its effects are disregarded so that pile displacement can simply be expressed as wN = f(w1,F,γ,n,L,EI,D,N).

Fig. 2: Small scale model of a laterally loaded pile Fig. 2: Small scale model of a laterally loaded pile Fig. 2: Small scale model of a laterally loaded pile
Fig. 2: Small scale model of a laterally loaded pile

In the model tests, pile length and elasticity, diameter to length ratio, soil density, and cyclic load amplitudes are varied to determine their functions and effects on the deformation behavior of a pile. Static pile load tests are first conducted to determine the ultimate lateral load for each pile-soil system. The model piles are hereafter cyclically loaded at a percentage of the ultimate lateral load for up to 5000 cycles. The cyclic amplitudes range from 10 percent to 50 percent of the static ultimate lateral load. Load sets with variable amplitudes are also applied, with each load set comprising a total of 45000 cycles. Two equivalent load sets are currently being investigated, one with initially descending load amplitudes, the other with initially ascending load amplitudes.

Publications:

[1] Achmus, M., Abdel-Rahman, K., & Peralta, P. (2005). On the Design of Monopile Foundations with respect to Static and Quasi-Static Cyclic Loading. Copenhagen Offshore Wind 2005.

[2] Achmus, M., Abdel-Rahman, K., & Peralta, P. (2005). Untersuchungen zum Tragverhalten von Monopiles für die Gründung von Offshore-Windenergieanlagen. Pfahlsymposium, Braunschweig, Heft-Nr. 80.

[3] 3. Achmus, M., Abdel-Rahman, K., Kuo, Y. & Peralta, P. (2007). Untersuchungen zum Tragverhalten von Monopilegründungen unter zyklischer Belastung. Pfahlsymposium, Braunschweig, Heft-Nr. 84.