Public defence in Building Technology, M. Sc. Lehar Leetsaar

Title of the thesis: The load-bearing capacity of screw piles in silty soils based on mechanical, piezocone and seismic soundings

Doctoral student: Lehar Leetsaar
Opponent: Prof. Kenneth Gavin, TU Delft, The Netherlands
Custos: Senior Adviser Leena Korkiala-Tanttu, Aalto University School of Engineering, Department of Civil Engineering

The application of in situ testing to evaluate the load-bearing capacity of piles is a commonly adopted practice. Nevertheless, in Estonia, there exists a notable absence of correlations between the load-bearing capacity of piles and the data acquired from soundings. This issue is particularly pronounced in difficult soil conditions, such as those encountered in ancient valleys, where accurately assessing the load-bearing capacity of a pile poses a considerable challenge. 

The primary focus of this dissertation is to determine the load-bearing capacity of three types of screw piles—Fundex, DSP, and FDP—utilising direct methods such as Cone Penetration Testing (CPT) and Standard Penetration Testing (SPT). The findings will be compared with those obtained from static pile load tests conducted in silty soils situated above ancient valleys. Additionally, the research investigates the applicability of DPSH-A readings in the direct methods of SPT for assessing the load-bearing capacity of screw piles in silty soils. A secondary focus of the dissertation is to evaluate the advantages of the SCPTu probe in predicting the load-bearing capacity of screw piles, particularly in comparison to the probing techniques predominantly employed in Estonia, including DPSH-A, SDT, and CPT. The analysis is confined to an analytical elastic solution grounded in the theories proposed by Randolph and Wroth. Furthermore, the study identifies a correlation between normalised operative shear stiffness and normalised pseudo-strain through pile back-calculations. A supplementary topic involves determining the load-bearing capacity of screw piles through reliability design and the statistical analysis of sounding data, in accordance with Eurocode 7 and utilising Monte Carlo simulation. The results demonstrated a notable decrease in the variability of the computed outcomes when the cone penetration test or dynamic probing test was performed at depths exceeding several meters below the pile base. Regarding the direct methods associated with the cone penetration test, it was found that certain CPT and SPT techniques yielded satisfactory results for screw piles situated in silty soils. A thorough analysis was undertaken to explore the correlation between normalised operative shear stiffness and normalised pseudo-strain for screw piles in silty soil, resulting in a highly significant relationship. Furthermore, the load-bearing capacity of screw piles in silty soils surpasses that of other pile.

Key words: Cone penetarion test (CPT), Pile load-displacement, static-dynamic probing test (SDT), Dynamic probing super heavy (DPSH-A), Piezocone penetration test (CPTu), Seismic piezocone penetration test (SCPTu), Shear wave velocity, Bearing capacity, Pile, Static load test

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Contact information: Lehar Leetsaar, lehar.leetsaar@taltech.ee, tel: +372 53409015

Doctoral theses of the School of Engineering: https://aaltodoc.aalto.fi/handle/123456789/49