Testing of U Type sheet piles for interlock slippage.
This was a continuation of the previous Hat Type Sheet pile driving test which we posted 2 weeks ago. In the previous test (See above link), we showed that Type 4 U Type JIS Piles was driven with the same amount of speed as NSP 25H Hat Type JIS piles.
In this test, we continue to examine further the various characteristics of the U Type and Hat Type piles. One of the major advantage of Hat Type piles is its interlocks are away from the neutral axis. This negates the Oblique Bending Moment phenomenon or interlock slippage when the piles are undergo loading.
NSP 25H Specification
Width: 900mm Height: 300mm Thickness: 13.2mm Weight: 113 kg/m Section Modulus: 1610 cm3/m Moment of Inertia: 24400 cm4/m
Type 4 Sheet Pile Specification
Width: 400mm Height 170mm Thickness: 15.5mm Weight: 76.1 kg/m Section Modulus: 2270 cm3/m Moment of Inertia: 38600 cm4/m
By default comparison, many would simply justify that the Type 4 Sheet Pile will perform better than the Hat Type due to its superior Moment of Inertia and Section Modulus. Deflection would naturally be lower on the Type 4 as well. However, when a test rig was set up in Singapore, with similar soil conditions, the Hat Type performed similarly well as the Type 4 sheet pile with much higher sectional properties. The assumption is that the soil conditions on both sides of the wall is similar and the excavation depth is also likewise similar. The jacking speed is also the same on both sides of the wall.
Given the similar results in both sheet piles, we can conclude that the Type 4 has interlock slippage issues inherent in all U – Type piles and there is a necessity to reduce the section properties of the piles. As per Eurocode 3 Part 5, there are reduction factors which must be included in the design.
Design Moment Resistance will require reduction factor βB.
Design Flexural Stiffness will require reduction factor βD.
Hence when you take into account the reduction factors, NSP 25H can effectively replace a Type 4 Sheet Pile, NSP 10H can replace a Type 3 Sheet pile. This saves the overall weight of the system and ultimately cost to the project.