Specifications, strength and bearing capacity of jointed-timber piles in soft ground

Single-timber piles were used as the pile foundation of small-scale buildings in the soft ground of the Saga lowland, Japan, where differential settlement occurred. The tip of the timber pile was required to reach the supporting stratum at a greater depth to overcome the problems. To reach the stratum, these timber piles are connected with several joints. However, there is no detailed study describing the strength of these jointed-timber piles (JTPs) and their material specifications. This study describes the specifications of the materials used and then explains the laboratory tests that evaluate the strength of the JTPs. Further, it discusses the method for determining the bearing capacity of the JTPs in terms of bearing capacity coefficients α and β. Later, the back-calculated values of α and β from the relationship of axial load with depth obtained from the strain gauge field measurements verified the bearing capacity evaluation method. The test results from the laboratory demonstrated that the concrete–timber stake joints reduced the compressive strength by around 2%, and the upper timber stake–lower timber stake joints reduced the strength by 11%. However, reduction factors of 10% for the concrete–timber joints and 15% for the timber–timber joints were adopted for safe construction.