There are three main types of drain used for accelerating consolidation: The sand drain, the sandwick drain and the band drain. The band drain has become the most popular and it is also the most economical due to its fast rate of installation. The soil profile suitable for band drain installation, however, needs to be fairly soft or loose as the mandrel installing the drain is driven into the ground. Drilling methods are mainly used for sand and sandwick drain installation and thus they can be installed in stiffer and denser soil profiles.
The Sand Drain
This was one of the first systems used for consolidation drains and is simply a column of highly permeable sand formed in the ground. A hole is initially formed by either drilling or driving and this is filled with sand of a suitable grading. If the drilling technique is used then
Various problems have been experienced with this form of drain which have seriously reduced the efficiency of the drains. In some cases disturbance of the soil during the installation procedure has reduced the permeability of the soil immediately around the drain thus reducing the effectiveness of the system. In other cases the column of sand has become necked due to incorrect installation procedure or due to high lateral soil pressures and this has also reduced the effectiveness of the drains. These negative case histories have tended to reduce the use of sand drains to situations where there is confidence that these problems will not occur.
The Sandwick Drain
The sandwick drain is very similar to the sand drain with the exception that the sand is contained within a sock made out of a geofabric. The sand filled sock is referred to as the sandwick. The hole is formed in the ground in a similar manner to that used for the sand drain and then the sandwick is lowered into the hole. The diameter of the wick is between 50 and 75 mm so the installation can be readily achieved by conventional drilling techniques. The presence of the sock assists the contractor with the installation of the drain and reduces the risk of discontinuities and necking. Problems can still be experienced with the installation procedure affecting the permeability of the soil immediately around the drain.
The Band Drain
The band drain has taken over from sand type drains to a large degree. The main reasons for this are the fact that band drains are very economical, they are strong and able to resist necking, squeezing and buckling and the small mandrel used to install them causes a minimum of disturbance to the surrounding soil. In ideal conditions the speed of installation is very fast with it taking only two to three minutes to install each drain. The soil profile does, however, need to be soft or loose as the mandrel used to install the drain is driven into the ground by means of a vibrator.
The band drain itself is about l00 mm wide and between 2 and 7 mm thick. It consists of a strip of flexible cardboard or plastic which has longitudinal drainage channels formed in it. In some cases this strip is fitted with a surrounding filter sleeve. The band drain is supplied in large rolls.
The equipment for installing band drains consists of a crane with a leader, a vibrator and a hollow mandrel. The mandrel is rectangular in shape with a length exceeding the depth to which the drains need to be installed. Connected to it at the head and at various intervals over its length are guides located in the leader. The vibrator is clamped to the head of the mandrel. The band drain is fed in at the top of the mandrel and emerges at the bottom, allowing a plastic anchoring shoe to be attached to it. The band drain is supplied in a coil which is mounted on a spool on the leader.
The vibrator drives the mandrel with the anchor shoe and band drain into the ground. The coil of band drain unwinds as the mandrel penetrates. When the desired depth has been reached the mandrel is extracted leaving the band drain behind in the ground. Once the mandrel is clear of the ground a cut is made through the band drain just above ground level. Another anchor shoe is attached to the piece protruding from the tip of the mandrel and the crane moves to the next position.
The installation of the mandrel does cause some disturbance to the surrounding soil but experience has shown that this is limited to 2d where d is the diameter with the same circumference as that of the band drain.
Other forms of Drains
Whilst the three types of drain mention above are the most common used there are other forms which can be considered. Stone columns have a low permeability and can function as drains as well as act as structural columns (See SECTION 13.6). Lime columns can also act in a similar manner (See SECTION 13.9).
Drainage Blanket
The water which flows up through the drains under a preloaded area has to be led off to the sides of such an area. This is achieved by placing a layer of sand with a high permeability over the tops of the drains prior to placing the remainder of the fill or surcharge material. This layer, which is normally about 500 mm thick, is referred to as a drainage blanket. In certain instances there may be a natural drainage layer at the surface in which case there is no need for any additional measures to be taken.
Design
13.8 JET GROUTING
Jet grouting involves the mixing and partial replacement of the in-situ soil with cement slurry as opposed to the conventional grouting which involves the injection of cement slurry into the voids in the soil. In its simplest form the process involves the ejection from a rotating grout tube of cement slurry under very high pressure. The jet cuts a path outwards from the grout tube in a radial manner, the cement mixing with the coarse particles in the soil while replacing the fines. The combination of rotation and gradual withdrawal enables a large diameter grout column to be formed in the ground.
Positive features
• A wide range of soil types can be treated. • It is a vibrationless system.
• Noise levels are low and limited to engine noise only.
• It has features which provide unique solutions to difficult geotechnical problems. Negative features
• It is a relatively expensive technique.
SUITABLE SOIL PROFILES
The ideal soil type for jet grouting is a clean loose medium to coarse sand. The sand particles are readily eroded away by the grout jet and thus the jet is able to penetrate up to half a metre with a single jet and up to a metre with air and water assisted grouting. Gravels are also amenable to treatment using jet grouting especially the finer gravels. Larger particles such as cobbles will tend to shield the jet and limit the size of the grouted column and make the cross-section very irregular.
Cohesion in the soil tends to reduce the ease with which the particles are eroded. The diameter of the grout column will thus reduce as the silt and/or clay content increases. In silty sands the reduction in diameter can be of the order of 15 to 30 percent. With purely cohesive soils such as silts and clays the diameter is even further reduced to roughly half that in clean sand. The stiffness of the cohesive soil is also important and only soils with a very soft and soft consistency (SPT value of up to 6 ) should be regarded as being suitable.
Jet grouting is unaffected by the presence of a water table.
INSTALLATION TECHNIQUE
The equipment consists of a crawler mounted drill rig, grout mixing plant, the high pressure grout pump and the grouting tube fitted with high pressure jets. The grout tube which is usually 50 to 75 mm in diameter is either drilled in by the machine itself or can placed in a
The jetting operation involves the pumping of a cement slurry under high pressure so that it emerges from the jet at the base of the grout tube at a high velocity. If a cylindrical column is required the jet tube is rotated and gradually raised at a constant rate. Other shapes, such as flat panels, can be formed by not rotating the grout tube.
The simplest form of jet grouting involves a single jet of cement slurry which is pumped in at pressures of up to 600 bar. This has been developed further with the introduction of a jet of air which acts as a shroud around the cement slurry jet and is referred to as the double jet system. An air pressure of between 2 and 15 bar is used.
This system has been further improved with the addition of a high pressure water jet for eroding away the soil, known as the triple jet system, which can achieve twice the radial penetration of the single jet system. The water pressure is high ( up to 500 bar) but the grout pressure is reduced to between 5 and 30 bar.
Any excess suspension flows up the annular gap between the grout tube and the soil to the surface from where it is channeled into settling ponds for subsequent removal from site. The addition of an air jet assists this excess material to rise to the surface and keep the annulus clear. Blockage of the annulus can cause a build up of pressure which can result in soil fracturing and resultant ground heave and should be avoided.
The diameter of the grout column is a function of the speed of withdrawal, the soil type, the system of jets and the pressures used. Different pressures and/or rates of withdrawal have to be used in the different soil strata to produce a grout column of reasonably constant diameter. Field tests are necessary to determine what these parameters should be for the various soil strata.