Chapter 10
Improvement of Compressible Soil

10.1 PRELOADING WITH A VERTICAL DRAIN
10.2 VACUUM PRELOADING
10.3 PRELOADING BY LOWERING THE GROUNDWATER TABLE
10.4 GROUND IMPROVEMENT BY THE ELECTRO-OSMOSIS METHOD
10.5 VIBROCORE REPLACEMENT (STONE COLUMNS)
10.6 CEMENT AND LIME COLUMNS

When reclamation is carried out on soft clay, a long-term problem is encountered. This long-term problem is called primary consolidation. Primary consolidation settlement usually takes as long as a few decades to a century when the permeability of soft clay is low and the drainage path is long. This consolidation process will lead to the reclaimed land being at an unsuitable level after settlement. If the underlying soil is not uniform or the future load is not evenly spread differential settlement will occur. This differential settlement will damage the structure that is built on the newly reclaimed land. Therefore, this underlying soil has to be modified.

In olden days, the problem of consolidation settlement was usually overcome by preloading the ground. Preloading the ground beyond the future load will enable the underlying soil to increase its effective stress. Preloading can solve the problem of a shallow foundation as the effective stress gain is greater than the future load. This is explained in Figure 10.1.

However, for reclaimed land with a large fill area, the stress can reach to the bottom of the compressible layer. In this case, the final load can still be much greater than the effective stress gain from preloading. Therefore, preloading alone may not solve the problem for reclamation on soft clay. It will still experience future settlement. This is explained by Mitchell (1981) in Figure 10.2.

Therefore, for reclamation projects, soil improvement is required to increase the effective stress throughout the profile of the compressible layer. As such, dissipation of pore water pressure or draining out of pore water is required from the compressible layer.

Therefore, the only way to improve the soil is to provide proper drainage throughout the entire thickness of the compressible layer. The most popular method of improving drainage is to install vertical drainage columns at certain intervals in the compressible soil mass. In the 1920s, vertical drainage columns were filled with highly permeable sand to improve

the drainage (Figure 10.3). Moram (1925) utilized vertical sand drains to stabilize the mud foundation beneath the easterly approach to the San Francisco–Oakland Bay Bridge.

Since the efficiency of vertical sand drain installations is not high, other drain material and installation methods have been developed and tested. Kjellman used tubes made of wood and cardboard material (Kjellman 1948). After developing a machine that could install the cardboard drains into the ground, these cardboard drains began to be used for soil stabilization. Though sand drains and cardboard drains were both introduced in the 1930s, sand drains were more widely used until new improved prefabricated drains were introduced in the 1970s. Sand wick drains (also known as pack drains) were also used in the past. They consisted of fabric stockings filled with sand and were subsequently placed in predrilled holes in the ground

(Dastidar et al. 1969). They were usually smaller in diameter than the typical sand drains. The advantages of sand wicks over conventional sand drains included their relatively low cost, ease of installation, and their flexibility which ensures the reliability of the drains during installation.

Nowadays, most of the vertical drains are prefabricated plastic drains. The reason for the popularity of the prefabricated vertical drain (PVD) is its low material cost and efficient installation methods. At present, there are nearly 100 types of prefabricated vertical drains. The types of prefabricated vertical drains available in the market are shown in Table 10.1.

With a vertical drainage system, the consolidation process can be completed much faster because of the shorter drainage path. The basic principle of eliminating future settlement within a short period is shown in Figure 10.4. Huge amounts of PVD were used in the Changi East Reclamation project, and details on the implementation of soil improvement works can be found in Bo et al. (2000b).

10.1 PRELOADING WITH A VERTICAL DRAIN

The installation of a vertical drain will only improve the drainage system. However, for the reclamation project, land will sink with settlement and eventually the land may be found to be below an acceptable level. As such, topping up sand fill during the period of consolidation is necessary in order to maintain the land level required. However, this method may make analysis and assessment difficult. Because several stages of loading are involved, several segments of hyperbolic curves will be obtained. The point of pore pressure measurement will also experience a rise and fall of excess pore pressure. Therefore, an alternative way is preloading a certain thickness of surcharge equivalent to the expected magnitude of settlement. An example is shown in Figure 10.5. In that case, assessment based on settlement or pore pressure becomes much easier.

A more acceptable way of preloading is to place a thickness equivalent to 1.1 times of settlement to compensate for the shortfall resulting from the 10% settlement. This is because the degree of consolidation is usually aimed at 90%. It is rarely aimed at 100% because this requires a longer time.

It is known that improvement with PVD can prevent the primary consolidation settlement. However, the secondary compression problem will remain but it has been found that the coefficient of secondary

compression reduces with the stress level (Figure 10.6). As such, secondary compression will be minimized if soil is preloaded beyond its expected future stress.

To stress the soil beyond the required effective stress level, preloading

is usually implemented with an overheight surcharge. This overheight surcharge needs to be removed after the specified degree of consolidation has been completed. The removal of this surcharge will lead to a rebound of the land level because of the unloading. This rebound will also partially offset the secondary compression (Figure 10.7).

Although PVD helps to accelerate the consolidation process it has its limitations. Installing closely spaced PVD would lead to the creation of many smear columns. This will cause a reduction of the average permeability of the soil mass. If PVDs are installed too closely the whole soil mass will be completely covered with smear columns. In addition, PVD installation will take a longer time because more PVD points need to be installed. Sometimes it may not be feasible to reduce the drain spacing to accelerate the consolidation time. So far the minimum acceptable drain spacing that will not affect the consolidation process is reported to be between 1 and 1.1 meters.

If drain spacing of 1 to 1.1 meters is not sufficient to cut down the consolidation time, an alternative way is to increase the preloading magnitude to 20 – 30 % higher than the fill and future load and aim for a

lower degree of consolidation. This is explained in Figure 10.8. However, this method is only feasible with PVD since this makes the pore pressure dissipate more or less uniformly along the whole column of the soil mass. In addition, it is preferable to aim for a higher degree of consolidation since the distribution of pore pressure at a lower degree of consolidation is still very varied. Details of the design aspects of PVD is described in Chapter 12, and quality control of PVD will be explained in Chapter 14. The usage of PVD in land reclamation projects is widely discussed in Bo (2001) and Bo et al. (2003a).

10.2 VACUUM PRELOADING

To drain out the water from the soil requires a hydraulic gradient. A hydraulic gradient can be created either by increasing pore water pressure in the soil over the water pressure in the possible drainage channel or alternatively by reducing the water pressure in the drainage channel compared with that in the soil. Both ways can lead to the process of consolidation and gain effective stress.

The first way of increasing pore pressure is the preloading method and the second way is to lower the water pressure in the surrounding area by vacuum preloading. Both ways are explained mathematically as follows:

Initial condition, δ' = δ - u

where δ' is initial effective stress, δis initial total stress and u is initial pore pressure.

After preloading, when additional preload Δδ' is added, pore pressure increases to u + Δu.

where Δδ' is the additional load and Δu is the initial excess pore pressure which is equivalent to the additional load.

When pore pressure dissipates effective stress increases:

After vacuum loading, when pore water pressure is dropped by -Δu, the effective stress increases to δ' + Δδ', where Δδ' is equal to - Δu.

The principle of vacuum preloading and its set-up is shown in Figure 10.9. A case study on vacuum preloading is explained by Choa (1990) and Bo et al. (2003a). Figure 10.10 shows the settlement measurement at one of the vacuum preloading projects in China, and Figure 10.11 shows pore pressure measurement from the same site.

10.3 PRELOADING BY LOWERING THE GROUNDWATER TABLE

It is basic physics that the density of material is reduced when it is submerged in water because of its buoyancy weight. Therefore, the density of material below water is generally 10kN/m³ lower than that above water. For a reclamation project carried out at a foreshore area the profile of fill should be below the normal groundwater level. If the groundwater level can be

lowered by one meter, it would be equivalent to the loading of 10 kN/m². Therefore, if preloading is only required for 20 – 30 kN/m², this could be achieved by lowering the groundwater level by 2 – 3 meters. Loading can be removed by allowing the recovery of groundwater. It is diagrammatically explained in Figure 10.12.

10.4 GROUND IMPROVEMENT BY THE ELECTRO-OSMOSIS METHOD

Hydraulic modification is one of the most popular methods among practicing engineers. However, hydraulic modification requires introducing a hydraulic gradient either in the form of increased pore pressure in the soil by placing additional loads or by reducing pore pressure in the soil by means of vacuum pressure.

Both methods require additional materials such as fill material or sealing geo-membrane. Another way of consolidating soil to improve the drainage is by electro-osmosis consolidation, which was first initiated by Casagrande in (1937), followed by Bjerrum et al. (1967), Fetzer (1967), and Wade (1976). Electro-osmosis properties of other clays, such as Ontario clay, Gloucestor clay, and Wallacebury clay (Shang and Ho 1998), Leda clay (Lo et al. 1991), Bangkok clay (Nayar 1997 and Dinoy 1999), Singapore marine clay (Bo et al. 2000e and 2001b) are described elsewhere.

10.4.1 Principle of electro-osmosis

Electro-osmosis is the process in which positively charged hydrogen ion in the form of water moves from the anode to the cathode upon application of direct current. In a compressible saturated clayey soil with two phases—a solid phase and a liquid phase—the liquid phase is formed with two layers of water hall. One is a fixed part and the other is a diffused part. When direct current is applied to the soil-water system, cations in the diffuse layer move toward the cathode. This cation carries the water flow toward the cathode. Due to this process consolidation occurs. This process is graphically explained in Figure 10.13.

Consolidation caused by electro-osmosis was proposed by Schaad and Haefeli (1947) by combining hydraulic and electrical potential gradient.

where u = flow velocity, m/s

K_h = coefficient of hydraulic permeability, m/s

g_w= unit weight of water, kN/m³

E = electrical potential, V

K_e = coefficient of electro-osmosis permeability, (m/s)/v/m)=m²/(s.v)

Therefore, the coefficient of electro-osmosis permeability (K_e) plays a major role. Mitchell (1976) and others reported that K_e is relatively constant regardless of soil types. The values are found to range between 10^-4 and 10^-3 mm²/(s-v).

Laboratory measurements of the electro-osmosis process have been interpreted by Esrig (1968), Johnston and Butterfield (1977), and Wan and Mitchell (1976). Time factor curves to predict the degree of consolidation have been proposed by Esrig (1968) and Johnston and Butterfield (1977), as shown in Figures 10.14a and 10.14b. Shang and Ho (1998), Bergado et al.

(1999), and Bo et al. (2000e and 2001b) have carried out laboratory tests to determine the electro-osmosis properties of soil. Figure 10.15 shows some results from electro-osmosis tests carried out on Singapore marine clay at Changi (Bo et al. 2001b). It was found that void ratio change due to the electro-osmosis process was significant. The higher the voltage the lower is the final void ratio. Preconsolidation pressures of soil were increased and compressibility was reduced. In the electro-osmosis consolidation process, not only vertical strain but also lateral strain will be experienced. Volumetric strain contributed from lateral deformation is even greater than that caused by vertical deformation.

10.4.2 Application of electro-osmosis in the field

Improvement of soil such as silt, silty clay, and soft sensitive clay by electro-osmosis has been successfully applied in the field (Casagrande 1948, Soderman and Milligan 1961, Bjerrum et al. 1967, Fetzer 1967, Wade 1976). In most cases, electro-osmosis was only used for temporary stabilization, such as dewatering and excavation. Bergado et al. (1999) have tested the performance of electro-osmosis on vertical drain in the laboratory. Improvement of dam foundations has also been carried out with the electro-osmosis method (Fetzer 1967). Electro-osmosis has also been applied in friction pile stabilization. The penetration resistance is increased when the pile acts as an anode, and is decreased when the pile acts as a cathode (Husman 1990). Soderman and Milligan (1961) have reported that the bearing capacity of steel pile can be doubled when treated by electro-osmosis.

10.4.3 Design and layout of electro-osmosis

In electro-osmosis, both the cathode and anode are usually in the form of a metal bar, pipe, or beam. The length of these electrodes range between 2m and 15m and the spacing between the same type of electrodes (that is, either cathode to cathode or anode to anode) can be as close as 1m. The spacing between opposite electrodes is generally between 2m and 5m (Husman 1990). Mitchell (1981) has reported that a hexagonal arrangement of anodes around a central cathode is more efficient than linear rows or square patterns. Some possible arrangements of cathodes and anodes in the electro-osmosis consolidation process is shown in Figure 10.16.

Some designs of cathodes consist of an iron pipe and an eductor pipe

installed in a predrilled hole of substantial size (about 400 mmf) and filled with clean sand. An anode is usually made of iron pipe, bar or rail (Lo et al. 1991).

10.5 VIBROCORE REPLACEMENT (STONE COLUMNS)

Soil compaction, as achieved in the vibro compaction process through the rearrangement of soil particles, is not possible for very fine-grained cohesive soils because of their inability to respond to vibration. The cohesion between the particles prevents rearrangement and compaction to occur. These particles merely slide against each other and cannot be rearranged into a denser configuration, and thus another technique is required.

The stabilization of such cohesive and very fine-grained soils is achieved by displacing the soil radially with the vibrator, refilling the created space with granular material and compacting in the same manner with the vibrator. This procedure is commonly referred to as vibro replacement (popularly known as stone columns) developed by Keller in the 1950s. In this way, a column of well-compacted coarse granular material is constructed in the soft cohesive formation which forms the load-bearing element consisting of gravel or crushed stone aggregates. By introducing these permeable columns, the consolidation process in the soft cohesive soil will be accelerated since the columns create a drainage path.

There are two methods of installation in the vibro replacement technique, namely, the wet method and the dry method. In the past several decades, both methods were used to install the stone columns successfully in numerous job sites all over the world. A schematic of a vibro replacement technique is shown in Figure 10.17.

10.5.1 Installation procedure

In general, the installation procedure consists of four stages:

1. Penetrating the vibrator to the required depth and creating a hole;
2. Filling the space created with coarse grained backfill material during the retraction of the vibrator in small steps;
3. Compacting the filled coarse grained backfill material with the assistance of horizontal vibrations; and
4. Repeating steps b and c till ground level, thereby creating well compacted, tightly interlocked stone columns

10.5.1.1— Wet method

In the wet method, a crawler crane of sufficient capacity is used to support the vibrator assembly, and penetration to the required depth is assisted by the combined action of vibrations and high pressure water jets placed at the tip of the vibrator. A schematic of the installation procedure is shown in Figure 10.18. An annular space is created between the vibrator and the hole by a flushing operation. After the vibrator reaches the required depth, stones (typically 40mm to 75mm) are fed to the vibrator point from the ground surface with the help of a loader. The aggregates sink down through the annular space created between the vibrator and a hole. This method is known as the top feeding system. The up and down motion of the vibrator is used to displace the stones laterally into the ground and at the same time compact the filled stones, thereby creating a column of well compacted and tightly interlocked stones. The vibrator is slowly withdrawn in steps of 0.7 to 1.0 m and the stone falls to the tip of the vibrator from the ground surface. The vibrator is then lowered back into the hole to 0.3 to 0.5 m depth, thereby

creating up to 0.5 m length of stone column. With stones being added as required this process is repeated up to the ground level.

The wet method is a partial replacement process where some of the soil is partially washed out and the rest is laterally displaced and compressed. The wet method requires a continuous supply of water and the discharge of water from the hole contains soil particles which have to be removed before the water can be disposed off, which in turn necessitates open areas for settling ponds. This method has been successfully used to treat depths up to 32 meters.

10.5.1.2— Dry method

In the dry method, custom built equipment is used to support the vibrator assembly, and penetration to the required depth is assisted by the combined action of vibrations and compressed air. Normally, no water jetting is used. Stones (typically 15mm – 35mm) are fed, using a skip, to the top of the vibrator and transferred through a special stone tube attached directly to the vibrator tip—this method is known as bottom feed system. A schematic of the installation procedure is shown in Figure 10.19. With a charge of aggregates filled into the stone tube and with the help of an air compressor, the vibrator is pushed into the ground. Upon reaching the required depth, the vibrator is retracted up to 1m (depending on the surrounding soil), and the pressurized stone tube forces the aggregates to exit and fill the void created. The vibrator is then repenetrated into the infilled space, compacting and compressing the aggregates into the surrounding soil.

The building process then comprises the up and down movement of

the vibrator until the aggregates in the stone tube are exhausted, after which another charge of aggregates is loaded into the stone tube and the building process continues up to the ground surface.

The dry method is a pure displacement process where no soil is removed. Moreover, no water jetting is required which implies that water supply and disposal does not arise. It is particularly well suited for congested working areas such as inner city areas, areas adjacent to existing railways and roadways, etc. This method has been successfully used to treat depths of up to 30 meters.

10.5.1.3— Offshore method

In the offshore method, a bottom feed system is required to install the stone columns in a controlled manner starting from the sea bed level under marine conditions. A barge or pontoon serves as a working platform on which a crawler crane of sufficient capacity is mounted to support the custom-built vibro string assembly. Penetration to the required depth below seabed level is assisted by the combined action of vibrations and compressed air. The whole procedure follows the bottom feed method of installation.

A schematic diagram of a typical setup for offshore stone column installation is shown in Figure 10.20. After shifting the barge to the treatment zone, the exact positioning of the vibrator to each probe point is done by a crane using the data constantly provided by a GPS (Global Positioning System) receiver mounted at the tip of the crane boom to monitor the location of the vibrator.

10.5.2 Equipment

The equipment developed for the vibro replacement technique comprises four basic elements:

1. The vibrator, which is suspended from extension tubes, the total length of the vibrator and extension tube assembly being equal to or greater than the treatment depth. Air/water jetting systems are attached to the sides of the vibrator to assist the penetration.
2. The crane or custom-built base machine, which supports the vibrator and extension tube assembly.
3. The stone delivery tube attached to the vibrator for the bottom feed system.
4. The quality control recording unit which produces a computer record of the installation process in a continuous graphical mode, plotting depth versus time and power consumption versus time.

10.5.2.1— Vibrators

The principal piece of equipment for the vibro replacement process is the vibrator. There is a wide range of depth vibrators available in the market. Keller has developed Mono, Alpha-S and Beta vibrators for the vibro replacement process. The Alpha-S is the development of the S-vibrator and Beta is development of the Mono or L-vibrator with a specially designed stone tube and stone feeder hopper attachment. A schematic of the Mono and Beta vibrators are shown in Figure 10.21.

10.5.2.2— Custom-built equipment

Custom-built equipment is necessary for the bottom feed method of stone column installation. Keller has developed two types of custom- built equipment for the bottom feed system (dry method). The first type is the Alpha-S vibro string consisting of the vibrator, suspended from combined stone tube/extension tubes with compression chamber and a stone feeder

hopper, hanging from a high capacity crane, called a crane-hung system (Figure 10.22a). The second type is the Vibrocat, comprising a specially constructed track-mounted supporting unit, the Beta vibrator string, which incorporates a stone tube with a compression chamber and a stone feed hopper (Figure 10.22b) which facilitates a pull down thrust of up to 20 tons. Both these set-ups can be modified to suit the requirements of the wet method as well.

10.5.3 Quality control

The monitoring of each stone column is performed by an automatic recording device. This instrument produces a computer record of the installation process in a continuous graphical mode, plotting depth versus time and power consumption versus time.

The information provided by the recording device also includes the stone column reference number, date and period of installation, maximum depth, and maximum power consumption. These records are the main quality control tools during the installation process. Typical records are shown in Figure 10.23 (a) and (b) for the wet and dry methods respectively.

10.5.4 Design of stone columns

The vibro replacement technique not only introduces the stone columns but also densifies the surrounding existing soil. Therefore, in estimating the total improvement, the effects of the stone columns and the densification have to be considered in terms of the equivalent composite system. This equivalent composite system exhibits the improved stiffness, shear strength, bearing capacity, and drainage characteristics. The schematically equivalent composite system of natural soil and stone columns is shown in Figure 10.24.

The design parameters of the stone column design include diameter, spacing, layout, and depth of the columns. These parameters are determined based on subsoil conditions, engineering parameters (shear strength and compressibility) of subsoil and stones, type of structure, loading conditions, and specifications regarding settlement and stability criteria. The various steps involved in the design of the stone columns are shown in Figure 10.25. Priebe (1995) has derived design charts which give an improvement factor(n) based on the friction angle of stones (Δ_c) and the ratio of the stone column area to the area being treated by the column(A_c/A), as shown in Figure 10.26, to arrive at the improved composite soil parameters.

Furthermore, he considered in his analyses the compressibility of the column, the overburden pressure, and compatibility conditions to provide a conservative and practical approach to the design of stone columns in soft weak soils. For more details, refer to H.J. Priebe (1995), “The design of vibro replacement”.

The degree of improvement is largely dependent upon the layout, diameter and spacing of the columns, and subsoil conditions. Stone columns have been proven to be an effective method for treating soft cohesive soils with undrained shear strengths of less than 10 kPa. The application of stone columns in very soft cohesive soils typically results in improved shear parameters ranging from an undrained cohesion of 5 to 20 kPa, and friction angle of 20º to 30º.

10.5.5 Performance of stone columns

To assess the performance of stone columns, load tests are carried out with designed loads on a single column or a group of four-columns, as shown in Figure 10.27. A graphical plot between the loads and settlements shows whether stone column treatment satisfies the settlement criteria or not. In addition, during the construction of the intended structure, the performance of the stone columns will be assessed with the help of instrumentation such as rod settlement gauges and inclinometers.

10.5.6 Applications of stone columns

Typical applications of stone columns include individual footings, earth embankments, highway embankments, reinforced earth walls, railway embankments, and industrial structures (storage tanks). A typical cross-section of each of these applications is schematically shown in Figure 10.28.

10.6 CEMENT AND LIME COLUMNS

Cement and lime columns are generally used to improve the foundation of shore protection slopes instead of a sandkey. By introducing cement or lime columns, the stability of the slope will increase. Settlement will be reduced if cement or lime columns are used for improvement of embankment foundations.

Soft to very soft inorganic clay or silty clay with a water content of less than 100% to 120% can usually be stabilized with lime, and a relative increase of 10 to 20 times the initial shear strength can be expected. For this case, the maximum lime content is 10 – 12 % of soil by dry weight (Broms 1999).

Lime and cement columns can be applied to the foundations of shore protections instead of using a sandkey. It will improve the stability of the shore protection structure. Lime and cement columns have also been used in retaining walls, quay walls and revetments. Details on lime and cement columns can be found in Broms (1999).

The installation of cement or lime columns can be achieved with similar equipment, explained in Section 10.5. However, the dry method together with compressed air is usually used. Dehydration of the cement or lime is achieved when water from the soil is absorbed by the cement or

lime. The treatment for cement or lime can be carried out in a similar way to stone columns. In addition, a sample of the cement or lime column created can be collected and the strength determined in the laboratory. The application of cement or lime columns is the same as for stone columns.