Soil improvement using a combined vacuum and fill surcharge preloading method.

Chu, Jian ; Yan, Shuwang

A vacuum preloading technique and its implementation in soil improvement projects are introduced in this paper. A case study using the combined vacuum and fill surcharge preloading method is discussed to show how vacuum load can be combined with fill surcharge to consolidate soil when a surcharge higher than 80 kPa is required. Settlements and pore pressures data measured during the preloading are analyzed and used to indicate that the vacuum preloading method and the combined vacuum and fill preloading scheme adopted are effectiveness.

INTRODUCTION

One of the commonly used soil improvement methods for soft clay is vacuum preloading. This method has been successfully used in a number of countries for land reclamation and soil improvement work (Holtz 1975; Chen and Bao 1983; Bergado et al. 1998; Chu et al. 2000; Yan and Chu 2005). Sand drains and recently prefabricated vertical drains (PVDs) have often been used to distribute vacuum load and discharge pore water. A vacuum load of 80 kPa or above can be applied and maintained as long as it is required. When a higher surcharge load is required, a combined vacuum and fill surcharge can be applied. Compared with the fill surcharge method for an equivalent load, the vacuum preloading method is cheaper and faster (Chu et al. 2000). The vacuum preloading method has also been incorporated in the land reclamation process when clay slurry dredged from seabed is used as fill material for land reclamation. As the clay slurry fill is too soft for fill surcharge to be applied, the vacuum preloading method is ideally used for the consolidation of the clay slurry. Thousands of hectares of land have been reclaimed in Tianjin, China, using this method. When the reclaimed land is subsequently used for industrial or infrastructure developments, the vacuum preloading method is used again to improve the foundation soil that consists of a layer of consolidated slurry fill and the underlying seabed marine clay (Chu et al. 2000; Yan and Chu 2003).

The principles and mechanism of vacuum preloading have been well explained in the literature, e.g., Kjellman (1952), Holtz (1975) and Chu et al. (2000). For illustration, the pore water pressure and effective stress change processes in both fill and vacuum preloading cases can be explained using a spring piston analogy model shown in Fig. 1.

[FIGURE 1 OMITTED]

The consolidation process of soil under a surcharge load has been well understood and can be illustrated using the model shown in Fig. 1(a). For the convenience of explanation, the pressures in Fig. 1 are given in absolute values and [p.sub.a] is the atmospheric pressure. As shown in Fig. 1(a), the instance when a surcharge load, [DELTA]p, is applied, it is the excess pore water pressure that takes the load. Therefore, for saturated soil, the initial excess pore water pressure, [DELTA][u.sub.0], is the same as the surcharge [DELTA]p. Gradually, the excess pore water pressure dissipates and the load is transferred from water to the spring (i.e., the soil skeleton) in the model shown in Fig. 1(a). The amount of effective stress increment equals to the amount of pore water pressure dissipation, [DELTA]p--[DELTA]u (Fig. 1(a)). At the end of consolidation, [DELTA]u = 0 and the total gain in the effective stress is the same as the surcharge [DELTA]p (Fig. 1(a)). It should be noted that the above process is not affected by the atmospheric pressure, [p.sub.a].

The mechanism of vacuum preloading can also be illustrated in the same way using the model shown in Fig. 1(b). When a vacuum load is applied to the system shown in Fig. 1(b), the pore water pressure in the soil reduces. As the total stress applied does not change, the effective stress in the soil increases. The instance when the vacuum load,--[DELTA]u, is applied, the pore water pressure in the soil is still [p.sub.a]. Gradually the pore pressure is reducing and the spring starts to be compressed, that is, the soil skeleton starts to gain effective stress. The amount of the effective stress increment equals to the amount of pore water pressure reduction, [DELTA]u, which will not exceed the atmospheric pressure, [p.sub.a], which is normally 80 kPa in practice.

VACUUM PRELOADING TECHNIQUE

The schematic arrangement of the vacuum preloading method adopted is shown in Fig. 2. The construction procedure can be described as follows. 1). PVDs are installed at a closing spacing of normally 1.0 m in square grid through the entire depth of the soil that needs to be treated. If the ground is too soft, a fill of 1 to 2 m can be applied using sand or other competent fill materials to form a working platform for the PVD installation rigs. A layer of geogrid can be used in lieu of fill. Alternatively, a portable, lightweight drain installation rig may be used. 2). After the installation of PVDs, main corrugated flexible pipes of 100 mm diameter are laid horizontally to link the PVDs to the main vacuum pressure line. The pipes were perforated and wrapped with a permeable fabric textile to act as a filter layer. 3). A sand blanket of 0 .3 m on top is then placed to cover the pipes. 4). Three layers of thin PVC membrane are laid to seal each section. The membranes are anchored in a trench at the four sides and backfilled with clay to form a small dike of 0.3 to 0.5 m to contain the water that will drain out the PVDs. To cover the membrane with a layer of water can slow down the deterioration of the membrane caused by direct sun exposure. More importantly, when the membrane is immersed in water, leakage can be identified easily by the formation of air bubbles. 5) Vacuum pressure is then applied using a patented vacuum pumping system. The vacuum pressure will be applied continuously for several months until the required soil improvement objectives are achieved. A vacuum pressure of 80 kPa or above can be achieved and maintained easily using this system. The total surcharge applied will then be the vacuum pressure plus the fill or sand blanket used. When a higher surcharge is required, fill surcharge can be applied in stages after some consolidation of the soil under the vacuum load.

[FIGURE 2 OMITTED]

Fielding monitoring of vacuum pressure applied to the drain, settlements and pore water pressure changes are an essential part of a vacuum preloading project, as whether the method is effective or implemented properly or not can only be evaluated using the field monitored data. The field monitoring scheme and the methods of evaluation will be discussed using the following case study.

CASE STUDY

The described vacuum preloading method has been used in many projects in China and elsewhere (Shang et al. 1997; Chu et al. 2000; Yan and Chu 2003). The case presented by Yan and Chu (2005) is used as an example to illustrate the use of the vacuum preloading technique in soil improvement works.

A storage yard of 7433 m2 was to be constructed at the Tianjin Port, China. The storage yard was located on a 16 m thick soft clay layer. The top 3 to 4 m of the clay layer was reclaimed recently using clay slurry dredged from seabed. The rest 16 to 19 m was original seabed clay. The soil in both layers was soft and was still undergoing consolidation. This soft clay layer needed to be improved before the site could be used as a storage yard.

Preloading using fill surcharge alone was not feasible as it was difficult to place a fill embankment several meters high on soft clay. The vacuum preloading method could be used. However, the nominal vacuum load of 80 kPa was not sufficient for this project. Therefore, a combined vacuum and fill surcharge preloading method was adopted. Fill surcharge of a height ranging from 2.53 to 3.50 m was applied in addition to the vacuum load and a 0.3 m of sand blanket. The fill was applied in stages partially for stability consideration and partially due to practical constraints in transporting fill. The fill used was a silty clay with an average unit weight of 17.1 kN/[m.sup.3].

The layout of the storage yard is shown in Fig. 3. It was a L-shape with a total area of 7433 [m.sup.2]. For the convenience of construction, the site was divided into three sections, I, II and III, as shown in Fig. 3. The soil conditions in the 3 sections were very similar. The idealized soil profile together with the typical liquid limit (LL), plastic limit (PL), water content (w/c), and the field vane shear strength profiles as measured for Section II are shown in Fig. 4. The water content of the soil was higher than or as high as the liquid limit at most locations in the soft clays. The field vane shear strength of the soil was generally between 20 to 40 kPa.

[FIGURES 3-4 OMITTED]

The soil improvement work was carried out by following the method described in the preceding section. The loading sequence and the ground settlements induced by the vacuum and surcharge loads for Section II are shown in Fig. 5. The vacuum load was applied for about 6 weeks before fill surcharge loads were applied in stages. The total fill height applied was 3.5 m. The maximum surface settlement induced by the vacuum and surcharge loads was 1.614 m for Section II. As shown in Fig. 5, a vacuum pressure of 80 kPa or above was maintained for the whole duration of soil improvement.

[FIGURE 5 OMITTED]

Instruments including surface settlement plates, multi-level settlement gauges, inclinometers and standpipes were used. The settlements monitored by the settlement gauges installed at different depths during vacuum and surcharge loadings are plotted versus duration for Section II in Fig. 6. The reductions in the pore water pressures at different depths are also measured by the piezometers. Based on the pore water pressure monitoring data, the pore water pressure distributions with depth at the initial stage, 30 and 60 days and the final stage are plotted in Fig. 7 for Section II. The hydraulic pore water pressure line and the suction line are also plotted in Fig. 7. Before the application of vacuum and surcharge loads, the initial pore water pressures, [u.sub.0](z), were greater than the hydrostatic pore water pressure, indicating that the subsoil was still under consolidation. These initial excess pore water pressures were mainly the remaining pore water pressures generated during land reclamation and the pore water pressure induced by the placement of the sand blanket. The total fill surcharge was about 60 kPa for Section II. The initial pore water pressure distribution after the application of the fill surcharge is shown as [u.sub.0] (Z) [DELTA][alpha] in Fig. 7. The suction line for a suction of -80 kPa is also plotted in Fig. 7 as the line [u.sub.s]. The pore water pressure distributions at 30, 60 days and the end of preloading ([u.sub.f] (z)) are also shown in Fig. 7. These curves show the changes of the pore water pressure profiles with time. The area bound by the final pore water pressure curve, [u.sub.f] (Z), and the suction line, us, represents the remaining excess pore water pressures that have not dissipated.

[FIGURES 6-7 OMITTED]

CONCLUSIONS

A vacuum preloading technique and its application in soil improvement are introduced in this paper. A case study of using vacuum preloading combined with fill surcharge is presented. The study shows that the vacuum distribution system comprising PVDs at a square grid of 1.0 m together with horizontal 100 mm diameter corrugated flexible pipes was effective in distributing the vacuum pressure and collecting drained water. Vacuum load can be combined with fill surcharge to provide a combined surcharge much higher than the vacuum load alone. In the case presented, the ground settled for more than 1.6 m. The pore water pressure reduced substantially. The case study has proven that the vacuum preloading technique or a combined vacuum and fill surcharge scheme is effective for soil improvement works.

REFERENCES

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JIAN CHU

School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798

SHUWANG YAN

Geotechnical Research Institute, Tianjin University, Tianjin, 300072, China