Performance of Composite and Monolithic Prefabricated Vertical Drains

With the increasing use ofprefabricated vertical drains in the improvement of soft clay there are many types of these drains, both composite (with geotextile wrapping) and monolithic (without geotextile wrapping), available in Malaysia. Some engineers have the impression that all drains perform satisfactorily. However, case studies have shown that there are cases where vertical drains failed to function. Therefore, there is a need to identify the drains which are not suitable or do not perform satisfactorily. This paper describes the various tests that have been carried out on two types of drain (i.e. one composite and one monolithic) to study their performance under lateral pressure. Test results show the importance of having a filter sleeve around the drain core. They also indicate that the type of geotextile wrapping affects the performance of the drain. A relatively stiff woven geotextile seems to be the most favourable. INTRODUCTION Consolidation settlement of highly compressible cohesive soils often creates serious problems in foundation engineering. The consolidation process is governed by the rate of excess pore pressure dissipation which is dependent on the coefficient of consolidation, Cv and the thickness of the soil layer. The time required to achieve complete primary consolidation may be considerable for a thick layer of soil with a low value of Cv. To reduce the time of consolidation on such soils, preloading in combination with vertical drains has been found to be effective (Stamatopoulus and Kotzias 1985). The preload, often in the form of an embankment, causes consolidation due to excess pore pressure in the clay between the drains. The pore water can escape horizontally towards the drains and flow along the drains to a drainage blanket or a permeable layer above or below the soil layer. The rate of consolidation is increased because the shortest drainage path is reduced as vertical drains are placed at relatively close spacing (0.8 3m). Furthermore, the value of Ch in most sedimented cohesive soils is 2-5 FAISAL HJ. ALI & BUJANG B.K. HUAT times higher than Cv, especially if there are continuous sand and silt seams. The conventional type of vertical drain is the sand drain. They were first installed in the USA during the 1930s. The diameter of sand drains ranges from 18cm to 30 cm. However, there are some difficulties associated with sand drains. Firstly, the soil may be considerably disturbed during installation especially if the displacement method, where a closed-end tube is driven into the soil, is used. Necking of the drain may occur, which could reduce the effective diameter. There is also the possibility of discontinuity in the drain if excessive settlement and lateral movemen.t occur. Furthermore, suitable sand may not be easily available. The first prefabricated drain, the ~ellman Cardboard Wick, was introduced in 1937 (~ellman 1948). It was a band-shaped drain, 3.5 mm thick by 100 mm wide, made from 3 layers of cardboard; two outer layers serving as a filter and a middle layer forming 10 longitudinal flow channels. Subsequently, many types of prefabricated band-shaped drain were developed and prefabricated drains have now largely replaced sand drains. Many of the difficulties associated with sand drains were eliminated with the prefabricated drain. Prefabricated vertical drains often consist of a profiled plastic core wrapped in geotextile filter fabric (composite). There are also unwrapped vertical drains (monolithic drains). These are inserted into the ground with the help of a mandrel, sometimes to a depth of 30 40 metres. At this depth, the lateral pressure on the drain may close the drainage paths along the plastic core so much so that the flow of water is inhibited. This problem may be aggravated with time because the geotextile and the core are likely to be clogged up by the clay particles and the plastic core may deform further with time. This is because of the filter extensibility and creep of the core and filter under lateral soil stress. As noted by Holtz et al. (1987) very few long-term tests have been carried out on prefabricated drains especially when confined in soil. Scope of Investigation The design of a vertical drain system is generally based on theoretical solutions in which the drains are assumed to be functioning as an ideal well, i.e. water collected from surrounding soil is allowed to flow freely along the drain to the drainage blanket. However, such ideal conditions rarely exist in practice. Hansbo (1981) has shown that the resistance of the central core can have an important effect on the consolidation process of a vertically drained soil, particularly for greater drain lengths. Well resistance has been recognized by researchers and has been incorporated in theoretical solutions to radial drainage (Barron 1948; Yoshikuni and Nakanodo 1974; Hansbo 1981). The effect of well resistance is quantified by a parameter called discharge capacity, qw. It is defined as the rate of discharge of a drain, Q, under a unit hydraulic gradient, i.