ABSTRACT

Stone columns are increasingly used in offshore applications in order to improve the soil bearing capacity, reduce settlement of foundations and to mitigate the potential of liquefaction. This paper describes typical applications and state-of-the-art installation techniques. In addition, key construction aspects of offshore technology are discussed, focusing on different installation methods and technical factors to be considered when selecting the equipment.

INTRODUCTION

Stone columns are granular columns made of gravel size aggregate. The oldest method to install such columns onshore is known as Wet Top Feed. It is carried out by penetrating in to the ground a depth vibrator, or so called Vibroflot, and then feeding the gravel against the upstreaming flushing water down the hole that was previously washed out by the same Vibroflot, as shown in Figure 1. The column is built from the bottom to the top. The Vibroflot moves up/down and the number of such up/down stokes in each depth increment controls the produced diameter. In the marine environment, a 3.0 to 3.5 m thick gravel blanket is initially placed on the seabed. This blanket will feed the stone columns. The maximum stone column length that can be constructed using this method is in the order of 10 to 15 m as longer columns may be starved out of stone in the top meters of the columns.

The stability of the annular space around the Vibroflot is achieved by maintaining the water level in the hole higher that the water table in the surrounding soil. However, this is not feasible when the installation is carried out under water, as shown in Figure 2. The lack of differential water head makes this simple installation method often not suitable for offshore applications (Hamidi et al., 2013).

Therefore, the offshore installation of stone columns should use the Bottom Feed system, whereby the gravel is transported in a separate tremie pipe that is mounted alongside the Vibroflot (see Figure 3). In order to achieve a positive gravel flow (out of the tremie pipe and never in reverse direction), the tremie pipe needs to be put under an inside pressure that it at all times higher than the ambient pressure in the subsoil around the tip of the Vibroflot. Special equipment has been invented and patented (EP1367180A1) by the first author and applied on several sites to assure this controlled gravel flow at large water depths. This paper will mainly focus on the technology of this installation method.

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