Vacuum-Surface Compression Combined Preloading Mechanism and Its Application in Strengthening Soft Foundation of Hangzhou-Jinshan Expressway

In recent years, the construction of highways and urban roads in China has entered an era of rapid development. Most of them are built in the eastern coastal areas where economic development is fast. The soil in these areas is mostly silt, silty clay, siltous clay and silt. Mixed sand layer, belongs to saturated normal compacted soft clay. Such soils have the characteristics of high water content, large compressibility, poor permeability, high sensitivity, low strength and uneven thickness, etc. The construction of high-grade highways on such roadbed requires certain treatment measures for soft soil roadbeds. The vacuum-surcharge combined preloading method, which is one of the new technologies for the soft foundation reinforcement of economy, efficiency, and safety, has been widely used in the construction of expressways in Guangdong, Zhejiang, and Shanghai. The vacuum one-load combined preloading method is a kind of new technology for highway soft ground reinforcement developed on the basis of vacuum preloading and surcharge preloading in recent years. It has the dual reinforcement effect of vacuum preloading and surcharge preloading. . Although the lateral deformation of the foundation soil occurs during the loading process, due to the vacuum load, the lateral contraction deformation caused by the vacuum is offset by the lateral extrusion deformation caused by the surcharge; on the other hand, the vacuum load is applied. The consolidation of the underlying soil has increased and the strength has increased, which can make the loading rate very fast without destabilizing damage. For highways, the embankment itself is a very good surcharge. Because the vacuum-surcharge combined preloading is performed by evacuation and heaping of C, such as embankment filling, a pressure difference is generated in the consolidation zone, and the water in the soft soil is expelled to achieve the purpose of strengthening the soft foundation. Different from conventional preloading. In this paper, the mechanism of vacuum-surcharge combined precompression strengthening of soft foundation and its application in the reinforcement of Hangjin-Chang expressway are discussed.

Second, the vacuum-surcharge combined preload method reinforcement mechanism Vacuum-surcharge preloading method to strengthen the soft ground is based on the vacuum preloading method and surcharge preloading method developed on the basis of both, both are drainage consolidation method , The vacuum water pressure (negative pressure) and the surcharge (positive pressure) make the pore water pressure in the soil unbalanced water pressure, and the pore water is gradually discharged through the vertical drainage body under the effect of this imbalance force. Make the soil deformation and consolidation.

The reinforcement mechanism of the surcharge preloading is that when the surcharge preloading, the additional stress caused by the loading of the foundation soil begins to be borne by the pore water pressure, and the effective stress remains unchanged. As the time delays, the pore pressure gradually dissipates and Convert to effective stress as shown. It can be seen that the strength after reinforcement is changed from T. to T, and the size of the stress Moire is also changed. After unloading, the strength of the reinforced soil is retreated along the over-consolidation envelope to point B, and the strength is increased. At the same time, the reinforcement mechanism of At. vacuum preloading is to apply fluid pressure to the reinforcement zone by applying vacuum under constant external load. It acts on sand cushions and pore fluids in sand wells. In a short period of time, u in sand cushions and sand wells rapidly decreases and discharges water and gas. Since the permeability coefficients of soil and sand wells are very different, the u(potential) distribution of the soil is still maintained before the pumping starts. In this way, a difference in u (potential) is formed between the soil body and the boundary between the sand well and the sand mat. Pore ​​water under the effect of the potential difference, along with the flow of water to discharge and reduce u. The imbalance of u (potential) gradually spread from the near and far to the point farther from the boundary, forming the u (potential) difference between the far point and the near point of the sand well and the sand cushion in the soil body. Under the condition that the total stress is basically constant, the reduction value of u is the increase value, so that the soil is consolidated and compacted until the soil and the boundary reach a new balance of u (potential).

It can be seen that the higher the degree of vacuum is, the smaller the attenuation along the depth is, and the greater the effective stress is, the better the reinforcement effect is. Since the pore pressure is the ball stress, the reduced pore pressure (increased effective stress) during vacuum preloading is equal to the equivalent, so the Moire circle size of the soil unit in the foundation does not change, but only moves to the right and reinforces it. The magnitude of post-shear stress did not change, and the strength was changed from I. After unloading, the consolidated soil was changed from normal consolidation to overconsolidation, and the strength was increased before reinforcement. At the moment, the effective stress change diagram during vacuum preloading The effective stress change diagram for vacuum-surcharge combined preloading has the dual effects of vacuum preloading and surcharge preloading, but it is not a simple superposition of both. The effective stress changes during combined preloading are shown. From the infiltration law to further analyze the reinforcement effect, according to Darcy's penetration law, the permeation speed can be directly proportional to the hydraulic slope (A/L), increase the head difference A/ and decrease the drainage distance L, both can accelerate the soil Drain consolidation. The vacuum preloading reinforcement mechanism is to reduce the pore water pressure in the soil. Even if a negative excess pore water pressure is formed in the reinforcement zone, there is a head difference between the reinforcement zone and the outside, so that a hydraulic gradient is needed for seepage; and the preloading It is due to the fact that the super-static pore water pressure is generated by the heap, and the strength is increased by the dissipation of the pore pressure. The joint action of the two increases the pressure difference between the positive and negative pore water pressures, which means that increasing the head difference causes the pore pressure to dissipate faster and the reinforcement effect is better.

III. APPLICATION OF VACUUM-SURROUNDING COMBINED PRELOADING METHOD IN SOFT GROUND STRENGTHENING OF HANGJIN-YU EXPRESSWAY 1 Project Overview The Hangjin-Chang Expressway is located in Xiaoshan City, Zhejiang Province. There are a large number of modern deposits of soft clay in this area. This soft clay layer has features such as high water content, low strength, high compressibility and poor permeability. Among them, the K5+851~K5+934 section is the head of the highway overpass, and the requirements for post-construction settlement are more stringent. After analysis of various schemes, a vacuum-surcharge combined preloading method was used to perform soft foundation treatment on the road section.

The roadbed is about 83m long and about 48m wide, and the reinforced area is about 4000m2. The first layer of geological conditions is filled with soil and is 0.30.6m thick. It is a road gravel. The second layer is silty clay, 0.41.0m thick, yellowish yellow, soft plastic - plastic, saturated, brown rust. The third layer is silt, thick 3.75.0m, gray, loose ~ slightly dense, saturated, local thin layer of clay. The fourth layer is silty clay, gray, soft plastic, saturated, with silt, see shell debris and humus.

2 During the construction process, 25m long plastic drainage boards shall be placed in the reinforced area in a triangle arrangement with a spacing of 1.6m. In the reinforced area, 40cm gravel layers and 20cm fine sand layers were laid, and gravel layers and fine sand layers were used as horizontal drainage bodies. The fine sand layers also functioned to protect the sealing membrane from being crushed by gravel. The PVC filter tube was buried in a gravel layer. The PVC tube was divided into a branch tube with a diameter of 55 mm and a tube with a diameter of 82 mm. The filter tube was punched with a filter with a diameter of 8 mm and a spacing of 4 cm, and was wrapped with a geotextile to prevent the fine sand from entering the branch filter tube during vacuuming. The vacuum pressure is provided by the vacuum jet pump connected to the main pipe. As shown.

The vacuum pressure is maintained at about 80kPa and continues to function for 4 months. During the evacuation process, the 2.8m subgrade filling was filled and played a role in the surcharge. After the subgrade is filled, vacuum is continued for a period of time, and the vacuum load and the surcharge act together. Until the consolidation of the foundation and the average sedimentation rate for 10 consecutive days meets the requirements. The loading process is as shown.

Load curve vacuum-surveillance preloading profile Schematic survey Oblique hole 彐 - sub-item A3, surface sweet sink release plate - surface sinking plate 3 reinforcement effect Observing instrument layout diagram Surface settlement surface settlement observation results are shown. It can be seen that the total settlement of the edge of the reinforced area (Al, A3, A4, A6 plates) is relatively small, which is 701mm, 828mm, 644mm, 719mm. The total settlement at the center of the reinforced area (A2, A5 plate) is relatively small. Large, respectively, 858mm, 798mm. Overall, the effect of vacuum-surcharge combined preloading method for soft soil foundation is very good, reduce post-construction settlement and construction progress than block, settlement rate can be much higher than the heap loading Pre-pressure method 10mm/d. Surface settlement with time curve of layered settlement By layered settlement observation, we can understand the layered settlement of different layers of the foundation; According to the law of layered settlement changes, further analysis of deep soil reinforcement Effects and reinforcement affect the depth. The layered sedimentation process line is shown as 0. It reflects the characteristics of sedimentation and compression at different depths of soil in different depths. The settlement decreases along the depth and shows a good regularity.

From Table 2 and it can be seen that the compressive capacity of the soil is mainly completed by the soil layer within a depth of 20m. In addition, it can be seen that in the early stage of vacuuming, there was already settlement at 27m below the ground. This shows that the vacuum preloading method and vacuum-surcharge combined preloading method can reach the deep soft soil layer, and its depth of action can reach 2~3m below the depth of the drainage plate. Table 2 Vacuum-pile-ear-pushing preloading Compression amount of each layer of the observed observation section of the compression layer thickness of less than 20m compressive amount of layered settlement curve with time curve soil horizontal displacement.

The lateral displacement rate of soil is one of the control indexes for judging whether the embankment foundation is stable or not. The variation law can monitor the development of lateral deformation of each layer of soil, and can calculate the settlement caused by lateral displacement. the amount. According to the measured data of lateral displacement and depth variation at different time intervals, we can understand the change characteristics of soil in different stages of soft foundation reinforcement by vacuum-surcharge combined preloading method.

From the perspective of the horizontal displacement of the 0-inclinometer tube as a function of depth, after the vacuum has been applied, each inclinometer tube has a contraction deformation toward the reinforcement area. The horizontal displacement of the K5+870 section is about 40mm for 4 days, and that of the K5+910 section is 4 days. A horizontal displacement of about 50 mm results in a visible crack of 15 m in length and 15 mm in width at 4 m outside the edge of the reinforced zone. It shows that vacuum combined with preloading can avoid shear failure of soil, which is obviously better than overload preloading reinforcement. Seen from 0, after the start of the loading process, each time the reinforcement area of ​​the loading process has outward extrusion displacement, it generally reaches the maximum value in one or two days after the loading, and then there is a faster inward contraction displacement. After four days, it stabilized. In the late stage of loading, the outward displacement of the K5+870 inclinometer is 190mm, and the maximum daily displacement is 20mm; the displacement of the K5+910 inclinometer is 220mm in two days, and the maximum displacement in the day is 25mm. The reason is considered to be twofold. (1) The vacuum preloading stage weakens the constraints around the reinforcement zone, resulting in a large outward extrusion displacement during initial loading. (2) The soil consolidation degree increases with the increase of settlement, the strength increases, and the deformation is resisted. The ability to gradually become stronger, resulting in the latter two surcharge processes outward extrusion displacement. After the completion of the loading, the overall displacement of each tube is changed inwards under the action of a vacuum load. This change tends to be stable with the extension of the preloading time. In total, the K5+870 inclinometer was displaced inward by 13mm; the K5+910 inclinometer was shifted inward by 28mm. At this stage, the maximum inward displacement of the two inclinometers was 2.5mm (starting phase); The minimum daily displacement is 0.4 mm (end phase). The displacement of the inclinometers is inwards, indicating that the vacuum load still plays a significant role in this stage, causing the soil to shrink inward.

Cross-Section 0 Lateral Displacement of Inclination Holes vs. Time Fig. 4. Conclusion The vacuum-surcharge-loaded preloading method for soft ground improvement has been gradually applied in the construction of expressways in Guangdong, Zhejiang, and Shanghai in recent years. It is to improve the stability of the roadbed in highway construction and reduce post-construction settlement. Through the above theoretical analysis and on-site observations before loading, it is believed that the vacuum-surcharge combined precompression method for strengthening soft foundations has the following characteristics: Vacuum-surcharge combined preloading In the course of strengthening soft foundations, vacuum loads make the interior of the foundation negative Pressure state; heap loading promotes the pore water pressure of the foundation to rise, both of which act together to increase the head drop of the foundation soil, thereby accelerating the drainage of the foundation soil, and the consolidation speed is faster.

The vacuum-surcharge combined preloading method can solve the stability problem during the embankment filling process in the process of strengthening the soft ground. Because the consolidation zone is always in a state of negative pressure, the soil body will shrink and deform inward, and can be partially balanced. The lateral extrusion deformation caused by the preloading of the pile is beneficial to the stability of the roadbed.

The lateral displacement and displacement rate have a significant correlation with the load size. As the load increases, the displacement and displacement rate increase, the loading stops, and the displacement and displacement rate decrease significantly. Different from the preloading, the horizontal displacement is not the outward displacement with the stoppage of the loading, because the vacuum not only consolidates the soil, but also causes the soil to shrink inwards, so it can speed up the filling rate and shorten the filling rate. Duration, bring obvious economic benefits. '

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