Grouted Piles in Weak Carbonate Rocks

ABSTRACT This paper describes a proposed method of design for the axial capacity of grouted piles in weak carbonate rocks. The, method was developed during a detailed study of weak carbonate rocks prevalent in offshore areas of the Middle East where more than 200 offshore platforms are supported by grouted steel pipe piles. The method includes a unique relationship between the rock to grout bond strength and the unconfined compression strength of the rock, and also includes a load transfer analysis to study the brittle failure mechanisms. INTRODUCTION The axial capacity of a grouted pipe pile depends on both the ultimate steel/grout bond strength and the shaft resistance developed from the grout/rock bond. This paper only discusses the shaft resistance which for weak carbonate rocks is the factor governing axial pile capacity. It is known that the behavior of carbonate materials differs from most offshore materials, which are usually composed of other minerals. An example of this departure is the very low skin friction values widely observed from driven piles in carbonate sand and rock where conventional theory would predict higher values. No reliable theories and only a limited quantity of data are available for calcareous sand and even less for grouted piles in weak carbonate rock. During a series of offshore projects in the Middle East conducted during the period 1979 to 1983, a method specifically suited to the materials encountered in that area was developed. The materials in question are typically calcarenite and calcisiltites (carbonate sandstones and siltstones) with unconfined strengths in the range 0.5 to 5.0 Maps. Typical piles utilized are 610 to 914 mm diameter pipe piles grouted into predrilled holes usually 150mm larger in diameter than the pipes. The method is both simple and rational and is based on observations. Until a reliable theory becomes available, the method will provide a useful basis for design in weak carbonate rocks. ULTIMATE ROCK TO GROUT STRENGTH RELATIONSHIP The proposed relationship between the ultimate rock to grout bond strength and the unconfined compression strength of the rock was developed considering the following sources of information:Experience with onshore bored piles in chalk in UK.Pile load tests on rock sockets and anchors in other weak rocks (eg mudstone).Laboratory tests on carbonate rock samples.Pile load tests on grouted or bored piles in similar carbonate materials. Each of these factors are discussed in turn Below The relationship is presented in Figure 1, and has been reduced to a 3 part curve for simplicity. The cut-off value of 0.75 MPa requires to be reviewed for specific projects to take into account grout strength and steel/grout bond strengths. The relationship assumes relatively clean side walls to the borehole.