New Data on Grouted Connections With Large Grout Dimensions

ABSTRACT The paper describes two studies into the static strength of grouted connections with low D/T ratios. These were performed as part of the development programme for the North Rankin 'A' foundation improvements. One study was on the strength of grouted pile plugs in 1.83m diameter piles and comprised tests on quarter scale and full size pile sections. The other study comprised quarter scale tests on pile/insert connections where both steel tubulars and the grout annulus had low D/T ratios; the effect of different weld bead shear connector geometries was also studied. The new data presented is complimentary to existing design guidance on the strength of grouted connections, and extends the available data into connection geometries with large grout dimensions. The results of tests on four quarter scale and two full size pile plug specimens are presented, together with the results of tests on a further ten quarter scale pile/insert specimens with different weld bead shear connector geometries, different grout types and loading arrangements. Comparison is made with the predicted strengths derived from current UK and US design guidance. The tests demonstrate that current design guidance formulae for grouted connections cannot be extrapolated to connection with tubular and grout annulus diameter to thickness ratios below the recommended lower limits of applicability if current safety margins are to be maintained. INTRODUCTION Design formulae for grouted connections are recommended by both the UK Department of Energy(1) and API(2). Both recommendations impose limits on connection geometry because of the limited extent of the test data available. Remedial measures to improve the performance of the piles foundations to the North Rankin 'A' platform used grouted connections of two different types. First, grout plugs were constructed over the bottom 15m of each of the 32 No. 1.83m diameter driven piles as a temporary measure to improve their end bearing capacity. Second, a conventional pile/sleeve type connection was used to transfer load from the driven piles to the under-reamed pile bells via an insert tubular. Few data are available on the strength of pile grout plugs, particularly so for the North Rankin 'A' plugs which were constructed of a blast furnace cement grout to reduce the risk of thermal cracking. Because of the nature of the existing piles and the proposed insert to transfer load from pile to bell, design of the pile/insert connection fell outside the design guidance provided by the UK Department of Energy. For these reasons model testing was carried out to:determine the capacity of the pile plugs, andprovide data for the design of the pile/insert connection. DESIGN OF GROUTED CONNECTIONS Pile Plugs Design can be based on a simple bond/shear approach assuming all load is transferred from pile to plug through the pile/grout interface or the weld bead hear connection.