Chapter 5
Implementation of Prefabricated Vertical Drain Project

5.1 SELECTION OF VERTICAL DRAIN
5.2 PLANNING AND SELECTION OF VERTICAL DRAIN RIGS
5.3 DIFFICULTIES IN INSTALLATION
5.4 TYPES OF MANDREL
5.5 TYPES OF ANCHOR
5.6 INSTALLATION
5.7 QUANTITY CONTROL OF VERTICAL DRAINS

Implementing a prefabricated vertical drain project consists of several processes. Firstly, it is important to select the type of vertical drain that reflects the criteria assumed in the design phase. Secondly, the type of vertical drain rig must be selected taking into account the nature of the underlying soil, the depth of installation and the difficulties expected in installation. The types of mandrel and anchor also need to be selected so as to be able to install the PVD with minimum disturbance. During the implementation process, quality control and measurement are also important factors. This section describes the implementation process in depth.

5.1 SELECTION OF VERTICAL DRAIN

There are several types of vertical drains. However, most vertical drains are basically of the same dimension: 100 mm in width and 4–6 mm in thickness. Vertical drains are fabricated with drainage cores and filter jackets. The brands of vertical drains available in the market are given in Table 5.1. Most drains provide sufficient discharge capacity to drain out the pore water from compressible soil and efficient filters that can retain the surrounding fine grained soil. The cross-section and shapes of various types of drains and cores are shown in Figs. 5.1 and 5.2, respectively. The details of

drain parameters are discussed in an earlier section. The parameters of various drains are shown in Table 5.2. However, different clients asked for different specifications. Most of the specifications were just a duplication of those commonly used in the market, and only a few were based on the design requirement. Table 5.3 shows the various specifications called for by various clients. Although some specifications were called to suit the actual site condition, environment, geotechnical parameters, and depth of installation, it had never been possible to obtain a particular type of vertical drain that fulfilled the client's or the designer's particular requirements. Eventually, selection had to be made from among the vertical drains which were available in the market and the specifications of which were close to requirements.

5.2 PLANNING AND SELECTION OF VERTICAL DRAIN RIGS

Selecting a suitable type of installation rig is essential in implementing the vertical drain project. The selection can be based on the

following factors:

1. Bearing capacity of platform
2. Depth of installation
3. Type of soil
4. Production capacity of rig

The size and weight of the rig should be suitable for the prepared platform with a certain bearing capacity. Mobilization of an over-weight rig will lead to instability of the equipment. On the other hand, a low capacity rig with light weight may not provide sufficient installation power and reaction against penetration resistance. Therefore, it may not have the desired penetrability.

The other factors are depth of installation and the type of soil. Rigs with suitable height need to be mobilized depending on the depth of installation. Drain installation rigs of height 20–54 m can be found in the market. The capacity and the type of rig also need to be selected on the basis of the type of soil expected to be encountered. Another important consideration is the productivity of the rig. If a project requires the installation of a large quantity of vertical drains, installation rigs with a minimum capacity of 8000 m/day are required in order to be able to cope with the project schedule. The maximum production rate so far recorded in the Changi East reclamation project for 14 h working days is 30,000 linear meters.

The types of rigs normally used in PVD projects are:

The types of rig used in the Changi East reclamation projects are shown in Table 5.4. As can be seen from the table, there are various types of PVD rigs driven by different mechanisms. Some are driven by chain and others by the pulley and roller system. Some rigs have additional hydraulic cylinders to penetrate hard and difficult ground and others rigs have additional clamps to push down the mandrel. These various types of rigs with different push-in mechanisms are

shown in Figs. 5.3–5.7. In addition, some special installation equipment should also be made available in case there are difficulties in installation. Table 5.5 shows a specialized rig used for troubleshooting.

The special rigs are useful in troubleshooting for difficult penetrating ground. The details of troubleshooting using these rigs are explained in detail in the following section.

5.3 DIFFICULTIES IN INSTALLATION

While installing vertical drains, several difficulties may be encountered. The difficulties may be due to hard ground condition or very soft ground condition. Each type of difficulty and the suitable solutions are explained here.

5.3.1 Top Hard Crust at Seabed

Sometimes recent alluvium deposit such as sand can be found overlying a soft compressible deposit. The density of such a deposit varies

and depends on the depositional environment at the river mouth. In such cases, penetrating the dense sand deposit at shallow depth can be difficult. This can be overcome by loosening the dense granular material with a penetration rig that has a high capacity water jetting system. Also, very stiff to hard clay may be encountered at shallow depth. This is caused by desiccation of the top part of the marine deposit during the fluctuation of the sea level. This

problem can be overcome using prepunching equipment or auguring equipment.

5.3.2 Intermediate Hard Crust at Intermediate Depth

During the geological history of deposition, some nonconformity or change in type of deposition could have occurred, especially by a change in the environment. An example is the desiccation of a young deposit due to the recession of the sea level before the deposition of the next younger marine deposit. Another example is the deposition of an alluvial deposit during a break in marine deposition. In such cases, an intermediate hard layer of clay or dense layer of sand can be encountered. The degree of hardness of the desiccated clay and the extent of desiccation depends on the duration and thickness of the cohesive deposit exposed to the atmosphere. The problem in

penetration of an intermediate hard layer can be solved by using the same type of equipment described in the previous paragraph or, alternatively, a vertical drain rig with a vibratory system.

5.3.3 Hard or Dense Layer at Deep-seated Formation

On rare occasions, a thin layer of hard or dense sand can be encountered at a deep-seated location. If a significant thickness of soft clay

underlies such a hard layer, it cannot be left untreated. In such a situation, the use of the equipment described above is impossible and will consume a lot of time and money. Therefore, in the Changi East reclamation project high-powered, low speed vertical drain installation rigs fabricated by Cofra b. v. were used for punching through the deep-seated hard layer. The specification of this special equipment is shown in Table 5.6. This equipment is able to penetrate a hard layer having SPT blow counts of 30at a depth of 30–40 m from the installation platform level. During the geological history of deposition, some nonconformity or change in type of deposition could have occurred, especially by a change in the environment. An example is the desiccation of a young deposit due to the recession of the sea level before the deposition of the next younger marine deposit.

5.3.4 Installation Through Ultrasoft Clay

Another major problem in using vertical drains is the installation through ultrasoft clay. Extrusion of mud along the annulus of penetration hole can contaminate the drainage layer. Intrusion of material into the mandrel can lead to unsuccessful anchoring of the vertical drain. Both problems can be minimized by introducing a water balancing system to counterbalance the water pressure encountered in the formation. A smaller dimension mandrel with a smaller anchor is also suitable for such a situation.

5.4 TYPES OF MANDREL

Basically there are four types of mandrel:

1. Rhombic mandrel
2. Rectangular mandrel
3. Square mandrel
4. Circular mandrel

The first two types are the most commonly used mandrels, and the last one is least commonly used. The shapes of the mandrels are shown in Fig. 5.9. On the one hand, the mandrel must be strong enough to be able to penetrate the formation vertically. On the other hand, the mandrel must not be so big as to disturb the soil to a great extent. Normally, smaller dimension rhombic mandrels

are preferable, because soil disturbance and smear effect due to such mandrels are minimum.

However, to penetrate the firm-to-stiff ground, the rhombic mandrel may not have enough stiffness, and the rectangular mandrel made with stronger thick steel is more suitable.

5.5 TYPES OF ANCHOR

The anchor serves two purposes. It must be strong enough to anchor the vertical drain on firm ground. Also, it must be capable of preventing soil from ingressing into the mandrel. The types of

anchors available are:

1. Steel bar
2. Flexible metal plate
3. Vertical drain material

Normally, small steel bars are preferred because disturbance due to its penetration to the soil is less. To use a steel bar, the mandrel has to be equipped with a mandrel shoe, the opening of which is reduced to fit the steel bar dimension.

A flexible metal plate can also be used with a small size mandrel. Upon penetration, the flexible metal will be folded to the size of the mandrel tip and also serves as a close valve for the mandrel. However, steel bars are not suitable for rectangular mandrels without shoes. In some projects, the vertical drain itself is used as an anchor.

5.6 INSTALLATION

The vertical drain should be installed on the land platform that is graded to be a flat plane. Only with the verticality of the rig can the verticality of the drain be achieved. The verticality of the rig can be checked with a spirit level. The vertical drain should normally

be installed at the general platform level, that is one meter above the groundwater table. The groundwater table used to be at about high tide level after reclamation. However, there are some projects that install vertical drains in offshore conditions (Fig. 5.10). In addition to the verticality of the rig, it is important to maintain the mandrel in a good and straight condition. Any bending of the mandrel at the tip will make the vertical drain deviate from the vertical. Operators should also be careful in installating vertical drains that penetrate through different layers that have different densities. Density changes at the boundary of two layers can make the mandrel slide along the boundary and thus make the drain deviate from the vertical. Normally, the vertical drain installation points are set out by using anchor materials. The anchor can be attached with the help of a high capacity stapler or a portable sewing machine. Splicing should be made with a minimum of 300 mm overlapping, and either a stapler or a sewing machine can again be used for splicing (Fig. 5.11). The end portion should be sealed with tape after splicing in order to prevent the soil from ingressing through the gap. The cutoff level is normally 100–300 mm above the ground level.

5.7 QUANTITY CONTROL OF VERTICAL DRAINS

Normally, it would require a few million meter lengths of vertical drain to be installed to improve about 100 hectares of land. As such, some form of prediction of vertical drain length (penetration length) and quantity control of vertical drains is required. In the

preliminary investigating stage, the penetration length can be predicted from seismic reflection surveys. A seismic reflection survey carried out with correct geophone spacing, together with systematic interpretation, will provide a good indication of the bottom of the unconsolidated soft material. An example of isoline of thickness of compressible layer interpreted from a seismic reflection survey is shown in Fig. 5.12. The results of the seismic survey can be verified by supplementary boreholes. In addition, an accurate penetration depth of vertical drain can be measured during the implementation

stage with the help of cone penetration tests. The isoline of depth to the bottom of compressible layer estimated by CPT penetration tests is shown in Fig. 5.13. Another simple way of determining the penetration depth is by measuring the penetration resistance during installation or just by determining the depth to the refusal with a high capacity rig.

However, physical supervision at site is still important to record the penetration length of the vertical drains. There are various methods and mechanisms to record the penetration length; the three basic types of measurement are:

1. Scale on the mast,
2. Dial gauge and
3. Automatic digital counter.

The first one is the simplest. Here the penetration of the mandrel top can be physically measured with the help of the scale on the mast as shown in Fig. 5.14a. The second one is the dial gauge method where the scale is calibrated from the number of rotation of driving chain sprocket to get a penetration depth (Fig. 5.14b).

The last one is similar to the dial gauge but it can count the number of reference points passing through the recorder; from the

count numbers the penetration length is calculated. The recorder has a photo cell connected to the automatic data recorder. This digital counter is required to calibrate with actual measurement during penetration. Various types of digital counters are shown in Fig. 5.15.

The penetration length of each and every point can be recorded on the computer, and as-built drawings of penetration depth can be produced by using simple software. During project implementation,

the predicted length against the actual penetration length can be checked. An as-built record of PVD penetration length produced from a digital counter is shown in Fig. 5.16. The use of PVD in land reclamation projects and implementation of soil improvement works are also discussed in Bo (2001), Bo et al. (2000), and Choa et al. (2001).