Impedance functions of an interlocking Steel Pipe Sheet Pile (SPSP) caisson foundation under nonlinear soil-structure interaction (SSI)

2017 Fiscal Year Research-status Report

• Project/Area Number: 17K14713
• Research Institution: Saitama University
• Principal Investigator: Goit Chandra 埼玉大学, 理工学研究科, 助教 (10782732)
• Project Period (FY): 2017-04-01 – 2019-03-31
• Keywords: SPSP foundation / SSI / Impedance functions / Nonlinear behavior

Outline of Annual Research Achievements

In recent years, construction of arterial highway systems, coastal highways, and access roads has increased all over the world. In these systems, large-scale bridge structures frequently span over river or a bay where the surface ground is very soft or the water is very deep. These bridges demand large scale foundation systems with strong rigidity and large vertical bearing capacity. To meet these needs, in recent years, Steel-Pipe-Sheet-Pipe (SPSP) foundations are often adopted in various parts of the world (e.g. Tokyo Gate Bridge in Japan, Nhat tan Bridge in Vietnam, and 2nd Meghna Bridge in Bangladesh, etc.). Behavior of such SPSP foundation considering the effects of soil-structure interaction (SSI) under dynamic loads, however, is still largely unknown.

To this end, experimental model testing of SPSP foundation embedded in soil is considered under a range of static and dynamic loads (simulating linear-to-nonlinear responses) in obtaining the lateral head impedance functions (IFs). Such IFs are an essential component in evaluating the response of a foundation-structure system considering the effects of SSI. A 1/16.5 model SPSP foundation (based on the law of similitude relating to prototype) comprising of 20 pile pipes arranged in a circular fashion is considered to represent the actual SPSP foundation. As for the soil material, dry Gifu sand is selected. Considering that joint behavior of a SPSP foundation dictates the response of overall foundation-structure system, study on the joint behavior is focused prior to actual testing of the SPSP foundation embedded in soil.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

As a first step of the experimental framework, the shear performance of SPSP joint is investigated. For such, elemental tests comprising of only three piles (and joints between them) each with length 990 mm, diameter 30 mm, and thickness 2 mm were considered. Testing was carried out using a vertical unidirectional actuator on a shaking table unit. Static load (both push and pull directions) was applied at the head of the center pile (with a free-tip condition) while the other two piles were fixed at the head and the tip. The length of adjacent joint parts on both sides of the center pipe is 950 mm, while the length of adjacent joint parts of other two pipes is 990 mm; difference is provided so that a constant length of 950 mm be maintained when displacement of ±15 mm is given to the center pipe. This shear resistance offered by the joint interfaces against the displacement is recorded. The model target shear resistance of the SPSP joint is approximately 490 kN/m.

Results obtained from carried out tests, however, indicate lesser value of joint’s shear resistance than anticipated. A range of further tests are being carried out as of current for attaining the target value of shear resistance. For instance, lateral confining pressure be provided to the elemental model to simulate the same confinement condition when the SPSP foundation is embedded in soil.

Design of model SPSP foundation to simulate the actual foundation is already complete and is ready to be constructed once the elemental model testing is finalized.

Strategy for Future Research Activity

After achieving the target value of shear resistance for pile joints, circular SPSP foundation system comprising of 20 pipe piles and joints will be tested. The foundation model will be embedded in dry cohesionless Gifu sand inside a laminar box of dimension 1200 mm x 800 mm x 1000 mm. The dynamic properties of dry Gifu sand is well known (Ishida et al., 1981). Moreover, strain depended behavior of Gifu sand employing dynamic shaking table test (under 1g) is also available (Goit et al., 2013). The tip of the SPSP foundation model embedded in Gifu sand will be fixed at the bottom of the laminar box. The head of the integrated foundation will be excited by a unidirectional actuator for a range of low-to-high harmonic loadings. Results will be obtained in the form of head impedance functions.

Causes of Carryover

Continuation of current experiments and purchase of relevant items. International conference and publications.