Latest Ground Improvement Techniques and Vacuum Consolidation

Latest Ground Improvement Techniques & Vacuum Consolidation of Soft Soil:

1. Vibro Compaction;
2. Vacuum Consolidation;
3. Preloading of soil;
4. Mechanically stabilized earth structures;
5. Soil nailing;
6. Micro-piles;
7. Grouting;
8. Soil stabilization by heating or vitrification;
9. Ground freezing;
10. Vibro-replacement stone columns.

Following are the recent methods of ground improvement Techniques used for stabilization of the soft soil improvement:

Applications of Vacuum Consolidation of Soft Soil:

• Replace standard preloading techniques eliminating the risk of failure.

• Combine with a water preloading in scare fill area. The method is used to build large developments on thick compressible soil.

• Combine with embankment pre-load using the increased stability

Preloading or Pre-Compression of Soil for Ground Improvement:
Applications of Preloading of Soil

• Reduce post-construction

• Settlement

• Reduce secondary compression.

• Densification

• Improve bearing capacity

Vaccum reloading has been used for many years without change in the method or application to improve soft soil properties. Preloading or pre-compression is the process of placing additional vertical stress on a compressible soil to remove pore water over time. 

The pore water dissipation reduces the total volume causing settlement. Surcharging preloading is an economical method for ground improvement. However, the consolidation of the soils is time dependent, delaying construction projects making it a non-feasible alternative.

The soft soils treated are Organic silt, Varved silts and clays, soft clay, Dredged material The design considerations which should be made are bearing capacity, Slope stability, Degree of consolidation.

Soft Base Processing Specification

Soft Base Processing Specification

Layout the base:

1 Try to use the way of cross-laying, so that the width of each compartment corresponds to the width of the embankment as much as possible, because the two-way strength of the compartment is consistent of the soft base improvement;
The 2 cells are pulled vertically in the direction of the bank axis, and no slack is allowed. It should be manually tightened. The geocell can be sewn as needed;
3 by weighting (referring to the filling in the grid of the marginal compartment) or inserting the nail to position the compartment on processing of the soft ground, without moving due to filling;
4 Check the cell for damage before paving the soft soil. If found, it should be disposed of immediately;
5 After paving the high-strength geocell, it should be filled on the soft base within 48 hours. 

Filling the soil:

A. There should be no slack in the laying of high-strength geotechnical cells, and try to tighten them manually before filling.
B. Fill the soil symmetrically from the center to the sides, and it is inverted U-shaped. The filling is used to keep the high-strength geocells under tension. C. The height of the earth unloading shall not exceed 1m to prevent local subsidence.
D. Pay attention to the size and weight of the PVD construction machinery to avoid the occurrence of ruts larger than 7~8cm. 
E. The filled soil is 50cm after being crushed.

More tips or information about vacuum preloading soft soil improvement, please contact our sales engineer or visit our official webiste at http://www.sunzo99.com.

Foundation Treatment Method Selection Principle

Foundation Treatment Method Selection Principle

 The soft foundation treatment project must ensure the quality of the project, economic rationality and advanced technology. China has a vast territory, and the engineering geological conditions are ever-changing. The construction machinery conditions, technical level, experience accumulation, and the variety and price of construction materials vary greatly. When selecting the ground treatment method, it must be adapted to local conditions. Specific project analysis should give full play to local advantages. Use local resources. There are many ground treatment methods, each of which has a certain scope of application, limitations, advantages and disadvantages. No ground treatment method is omnipotent. According to the specific project conditions, the appropriate ground treatment method should be determined according to local conditions. Blindness should be overcome when referring to a method of a foreign or external unit, paying attention to regional characteristics. Adapting to local conditions is an important selection principle for the selection of ground treatment methods such as pvd wick drains.

 First, it is determined whether the soft foundation improvement needs to be treated according to the various requirements of the foundation (structure) and the natural foundation conditions. If the natural foundation can meet the requirements of the foundation of the building (structure), natural foundation should be used as much as possible. If the natural foundation does not meet the requirements of the building (structure) to the soft clay foundation, then the scope of the natural formation to be treated and the requirements for foundation treatment need to be determined by vacuum preloading.

       When the natural foundation cannot meet the requirements of the foundation of the building (structure), the superstructure, soft foundation should be considered together. When considering a ground treatment plan, attention should be paid to the interaction of the superstructure, foundation and foundation. It is not only possible to consider the reinforcement of the soft foundation, but also consider whether the upper structure is reasonable and whether the overall rigidity is sufficient. When determining the ground treatment plan, the scheme of treating only the foundation should be considered at the same time, or the combination of strengthening the rigidity of the superstructure and the treatment of the foundation should be selected, otherwise it will not only cause unnecessary waste but may bring about adverse consequences.

       Before the specific soft ground treatment plan is determined, the feasibility study of the ground treatment plan should be carried out according to the conditions of the natural formation, the principle of the foundation treatment method, the experience of the past application, the equipment and the material conditions, and various technically feasible solutions are proposed; Then, the technical, economic, and progress aspects of the proposed multiple schemes are compared and analyzed, and the environmental protection requirements are considered. One or several ground treatment methods are determined. This is also the optimization process of the ground-based treatment plan. Finally, based on the pre-determined ground treatment plan, decide whether to conduct a small field test or conduct a supplementary survey as needed. Then carry out the construction design and then carry out the foundation treatment construction. During the construction process of soft ground improvement, monitoring and testing should be carried out. If necessary, reverse analysis should be carried out. The design can be modified and supplemented according to the situation. The detailed understanding of the natural geological conditions of natural foundations in actual projects is often neglected. Many engineering accidents caused by foundation problems or soft ground treatments fail to achieve the intended purpose, often due to insufficient understanding of the engineering geological conditions. Detailed engineering geological survey is one of the important basis for judging whether the natural foundation can meet the requirements of the foundation of the building (structure). Detailed engineering geological survey data is one of the main basic data for determining a reasonable foundation treatment method if ground treatment is required.

       Through engineering geological survey, investigate the topography and geomorphology of the building site, and identify the geological conditions, including the nature of the rock and soil, the genetic type, geological generation, thickness and distribution range. It is necessary to understand whether there are alums, dark scorpions, ancient rivers, ancient wells and ancient tombs in the foundation. 

For the layer, the degree of weathering and the contact relationship of the stratum should also be ascertained, the geological structure of the natural stratum should be investigated, and the hydrological and engineering geological conditions should be ascertained to determine whether there are any unfavorable geological phenomena: such as landslides, collapses, karst, soil caves, gullies, Debris flow, shore scouring and earthquakes. The physical and mechanical properties of the foundation soil are determined, including: natural gravity, relative density, particle analysis, plasticity index, permeability coefficient, compression coefficient, compression modulus, shear strength, and the like. 

Finally, according to the requirements, the stability and suitability of the site, the uniformity of the foundation, the bearing capacity and the deformation characteristics are evaluated. In addition, it is necessary to emphasize the comparison of multiple schemes for ground treatment. For a specific ground improvement project, there are often several technically feasible ground treatment schemes. The comprehensive analysis of the technology, economy, and schedule, as well as the impact on the environment, should be carried out to optimize the ground treatment plan to obtain a better ground treatment plan.

vacuum preloading

vacuum preloading soft soil treatment work site

The Vacuum Drainage Preloading Method

The vacuum drainage preloading method is a relatively new reinforced soft soil technology. It is a kind of drainage consolidation method. It is laid by laying a horizontal drainage sand cushion and a vertical drainage body arranged in a soft foundation, and then in the sand pad. A gas-tight membrane sealing device is laid on the layer, and a vacuum is formed by means of a pipe embedded in the sand cushion layer to form a negative pressure in the soil body, thereby extracting pore water in the pores of the soil body, thereby reducing pore water pressure. Increase the effective stress, make the soil consolidation, reduce the settlement at the later stage, and improve the bearing capacity of the foundation.

Principle of vacuum preloading

When vacuuming, negative pressure is gradually formed in the surface sand cushion and the vertical drainage channel, so that a pressure difference is formed between the interior of the soil and the drainage channel and the cushion. Under this pressure difference, the pore water in the soil is continuously discharged by the drainage channel, thereby consolidating the soil. Mainly reflected in the following aspects:

1) The film is subjected to a load equal to the pressure difference between the inside and outside of the film.

2) The groundwater level is reduced and the additional stress is increased accordingly.

3) The closed bubble is discharged, and the permeability of the soil is increased.

The vacuum preloading is to draw a vacuum under the sealing film covering the ground to form a pressure difference between the inside and the outside of the film, so that the clay layer generates a consolidation pressure. That is, a method of increasing the effective stress by reducing the pore water pressure while the total stress is constant. Vacuum preloading and precipitation preloading are drainage consolidation under negative superhydrostatic pressure, known as negative pressure consolidation.

Before the vacuum preloading construction, the construction preparation work should be fully prepared and the construction technical measures should be formulated. Check whether the sandbag, PVC film and medium coarse sand meet the technical standards and design requirements, and submit the inspection report to the supervision engineer for review and approval. After obtaining the approval of the supervision engineer, the next step of vacuum preloading is started.

Vacuum preloading construction process

Measure the discharge line → lay the main branch filter drain pipe → lay the upper sand cushion layer → sand surface leveling → lay the PVC film → construct the sealing groove → set the measurement mark → install the vacuum pump → vacuum preload the consolidated soil layer.

The vacuum drainage preloading method is mainly composed of four parts:

(1) Construction of a vertical and horizontal drainage channel, ie construction of plastic vertical drains（PVDS） and sand cushions;

(2) Construction of a sealing layer that ensures the insulation of the foundation from the atmosphere is ensured;

(3) To set up a set of high-efficiency vacuuming devices. That is, the main pipe and the filter pipe network are laid in the sand cushion layer, and a vacuum pump or the like is installed outside the sealing system;

(4) It is necessary to set up a set of inspection systems that can be constructed according to the design requirements.

The basic introduction to surcharge preloading techiniques in China

The basic introduction to surcharge preloading techniques in China

The foundation is drained and consolidated under the preloading load, the foundation is deformed, the strength of the foundation is improved, and the prefabricated load is removed, and then the building (structure) is constructed, which not only improves the bearing capacity of the foundation, but also has small settlement after construction.
China's soft soil areas are widely distributed. Its characteristics are: large water content, low strength and high compressibility, which is difficult to meet the needs of engineering construction. However, with the development of the national economy, many engineering projects have to be constructed on soft soil foundations. Therefore, it must be reinforced, otherwise it will cause large settlement or even damage.

In engineering practice, the traditional and effective soft soil foundation reinforcement method is the preloading method. However, due to the insufficient source of materials, transportation shortage, and difficulty in turnover, the construction materials have a direct impact on the construction schedule, resulting in long construction period and high project cost. To this end, the vacuum preloading method at home and abroad solves the problem of insufficient material for stacking and long construction period. However, with the improvement of reinforcement requirements, the vacuum preloading method cannot meet the requirements of engineering practice. Therefore, a new method of vacuum combined with preloading preloading soft soil foundation has been created.

The surcharge preloading method has been widely promoted and used in domestic port engineering, industrial and civil construction, airport runway construction and other foundation treatment projects. It has achieved good reinforcement effect and objective economic benefits, and accumulated a large amount of practical experience. At present, the method has matured after more than ten years of engineering application.

land reclamation and soft soil improvement

Land reclamation and soft soil improvement in China

Large-scale reclamation and land reclamation in the South China Sea, thousands of workers in thousands of cutters gathered in the South China Sea, and many islands emerged from the sea almost overnight, which really attracted the attention of the whole world. Depending on the blowing material, some islands use seabed mud and some use sand. Different fillers lead to different post-treatment methods.

The mud-filled islands, which are mainly silt and silty soils, are typical weak foundations characterized by high porosity and high compressibility. The natural consolidation speed is extremely slow, and uneven sedimentation occurs, causing cracks, tilting, and normal use in the building. Manual reinforcement measures must be taken when constructing the silt. If the piling method is adopted, the cement pile may be tilted or even broken due to the instability of the entire foundation. To deal with this soft foundation, we use the vacuum preloading method. This method is suitable for the treatment of soft soil foundation. It was invented by the Swedes in 1952. China was introduced from the 1980s, but due to the backwardness of materials at that time, the vacuum pressure could only reach 60kpa. Later, with the progress of China's domestic processing capacity and materials science, this method has been widely promoted and applied, and achieved good results. Vacuum preloading for soft soil improvement in China now has the most mature construction practice experience in the world.

In the sand-filled islands, the overall bearing capacity of the soft foundation is much higher. After the dredging, as the pore water in the sand dissipates, the bearing capacity increases rapidly, and some islands do not even need special treatment, and only need to be structurally designed according to the bearing capacity. I am now in the process of land reclamation in the Gulf of Jakarta, Indonesia. There is no problem with the island carrying capacity of sand-filled land, but there is the possibility of earthquake liquefaction. Those who have studied geology know that although sand is a good foundation material, foundation liquefaction is also very dangerous.

Preloading system construction

Preloading System Construction

The preloading load is an important part of the drainage consolidation method, which promotes the power of free water discharge between the particles. The implementation of preloading is mainly reflected in the graded loading. The stability of each stage of loading depends on the improvement of the strength of the previous stage before preloading. In order to accelerate the consolidation of the foundation, the preloading of the pile is combined with the gravel cushion and the bagged sand well to treat the substrate.

 In order to ensure the safety and progress of the construction, the observation section is set, and the settlement and displacement are observed to guide the construction. Five observation points and two level points are set for each observation surface. Before the roadbed is filled, install the subsidence plate on the cleaned surface on the road center and left and right shoulders according to the design requirements; at the same time, the displacement observation pile is buried 1 meter away from the foot of the embankment. 

During the pre-loading period, settlement observation should be carried out according to the regulations. After the embankment settlement deformation reaches the expected value of the design, the pavement can be paved, and the excess soil above the roadbed will be unloaded, and the embankment will be renovated to the elevation of the roadbed surface and meet the compaction requirements.

General construction technology and supervision points of vacuum preloading method

General construction technology and supervision points of vacuum preloading method

1). Sand cushion and bagged sand well construction

For sand cushion and bagged sand well construction, the ground can be leveled, and according to the specific conditions of the soil foundation, a certain thickness of sand cushion is laid and stabilized by machine. Position the pile tube in a vertical position, place the casing at the designed depth, place the sandbag filled with sand into the casing, and then pour a small amount of water into the casing. The most important thing in this process is to immediately fill the head of the sand well.

2). Laying vacuum filter tube

The sand bag head is buried in the sand cushion, placed flat, and then the vacuum filter tube is laid. The vacuum filter tube is taken out from the main filter tube, sealed from the membrane discharger, and finally connected to the membrane discharger and the jet vacuum pump [3].

3). Filming and lamination

A layer of non-woven fabric is applied to the sand cushion to avoid damage to the sealing film. Then, in the windless climate, a sealed polyethylene film will be set up. It should be laid layer by layer. After the first layer is checked, there is no problem. Then lay the next layer. If there is any problem during the inspection, it should be repaired in time. After the film is laid, the film setting is set, and the jet vacuum pump and the observation instrument are intermittently installed along the roadbed slope. The sealing film should be completely buried in the laminating groove, and the lamination groove should be trapezoidal and filled with clay.

4). Road filling

After the jet pump and the settlement table are placed, the readings of various side instruments can be set, and then the test pumping test can be used to check whether there are any loopholes on the membrane and take corresponding measures. After confirming the correctness, press it to the designed preloading height according to the general embankment filling construction method.

The roadbed is piled up to handle the soft soil foundation, and the original foundation will produce lateral deformation. However, due to the effect of vacuum preloading, the lateral deformation caused by the vacuum preloading and the lateral deformation caused by the stacking are mutually eliminated. Moreover, the consolidation of the foundation soil caused by the vacuum preloading makes the effect of the loading treatment better.

5). Preload control

The output power of the vacuum pump is greater than or equal to 7.5 kW, and the seal must be ensured before installing the vacuum pump. A PVC drainage filter is arranged in the middle of the gravel drainage layer, and the spacing is generally about 4 meters, and the maximum is not more than 6.5 meters. In the area where the vacuum preloading is strengthened, when the vacuum is more than 85 kPa and the vacuum is continuously pumped for 4-5 months, or the foundation consolidation degree meets or exceeds the design requirements, the unloading should be stopped and handed over to the relevant parties. acceptance.

6). Instrumentation

Before the completion of the first layer of sof soil treatment and  backfilling, the measuring point should be set up with the settlement board, the horizontal settlement tube, the earth pressure box, the single hole water meter, the inclined tube, the groundwater level well and the side pile. The pore water pressure gauge is placed in the road and shoulder position of the cross section of the subgrade, and each position is placed 48 in the depth direction. 

Twelve deep subsidence marks were placed in the road. The inclinometer is placed on the outer side of the subgrade slope and on the outer side of the side ditch. The pile is placed 80m at the foot of the slope, and one of the two observation sections is set. A settling plate is provided on the road in the cross section of the roadbed, on the left shoulder and on the right shoulder. Horizontal sections are placed in each section. A pressure box is placed at the center of the carriageway, at the center of the central divider, at the shoulders and at the foot of the slope. A single-hole water discharge machine is designed in the middle of the road and at the shoulder [4]. The groundwater level well is generally designed 2m outside the subgrade trench, and an earth pressure box is placed between each two observation sections. After all the meters are set, they should be returned to zero in time. Look at the effect every two days or so. At least 3 consecutive readings are stable.