Analysis of Effect of Soft Soil Subgrade Reinforced by Vacuum Preloading

Detection analysis.

The embankment of 981.11~DK49+044.40 is a soft soil roadbed, which is located on the coastal alluvial plain and has a flat topography. The surface is a silt layer at the bottom of the pit, and the following is a 10~18m thick silty silty clay, silt and silt clay. Soft soil layer. Due to the short formation time of soft soil and poor engineering mechanical properties, soft soils are characterized by low shear strength and high compressibility. The design requires vacuum preloading for reinforcement.

1 Technology Principle Vacuum preloading method is a reinforcement method that eliminates harmful deformation of soft soil subgrade and improves the performance of soft soil subgrade. It forms a sealed drainage system by inserting a plastic drainage plate in a soft ground as a drainage channel and a pre-paved sand cushion. The atmospheric pressure difference formed by the evacuation of the reinforced soft soil subgrade is used as a preload, and is transmitted to the design depth through the plastic drainage plate, so that the vacuum pressure is distributed along the depth of a substantially rectangular shape, and no physical loads need to be applied. Under the action of vacuum suction and extrusion, the soft ground discharges fine water and bubbles in the pores of soft soil so as to accelerate the consolidation settlement of foundation soil.

2 The main construction process First, clean up the debris on the surface of the clean foundation, fill in the plain soil cushion and sand cushion, and roll compaction and compaction, then insert the plastic drainage board, after the plastic drainage board is all set up, open the sealed trench A vacuum filter tube is arranged in the sand cushion layer, and the filter tube is connected to the vacuum pump through the vacuum tube out of the tube film ejector and the sealing film. After the completion of the pipe laying, cover the polyethylene gas-tight film. The edge of the film is buried in the sealing groove. The groove is backfilled with clay to form a water-proof cofferdam above the film, and then the film is covered with water. The overlying water on the membrane should be performed when there is no problem with the sealing system.

3 The instrument was buried to better guide the construction organization, and the reinforcement effect of the soft foundation was verified. Before the construction, two sections were selected for observation, various physical and mechanical indexes of the undisturbed soil and vacuum tests were performed, and a cross-board shear strength test was performed. After the vacuum preloading reinforcement is completed, the same test is performed at similar positions, and the physical and mechanical indexes before and after reinforcement are compared and analyzed.

4 Reinforcement effect analysis This section of soft soil road is based on the setting of plastic drainage board on July 16th. Vacuum is started on August 20th. The vacuum pressure under the membrane has been stable above 90KPa. The pre-pressure settlement is stable and regular and long. The average sedimentation rate was less than 1.5mm/d on November 20th. The average settlement of the soft ground surface was 766mm and the minimum was 696mm. The actual surface average settlement was 724mm. 4.1 Physical and mechanical index changes before and after consolidation are shown in Table 1. Reinforcement The water content, void ratio, and compressibility of the post-foundation soil are all reduced much more than before reinforcement, and the bulk density and cohesion of the soil are greatly improved.

Vacuum pre-compression construction diagram Table 1 Changes of mechanical properties before and after reinforcement of soil layers Soil name Soil thickness Water content w/(%) Pore ratio e Cohesion force C/kPa Compression modulus Es/MPa Reinforcement after reinforcement Reinforcement before reinforcement Reinforcement before reinforcement Reinforcement before reinforcement Reinforcement after reinforcement Reinforcement before reinforcement After reinforcement before reinforcement Reinforcement Mud clay Clay sludge Silt clay Mud clay Clay silty clay Soil description: A negative number in the table indicates that the reinforcement is larger than that before the reinforcement, and a positive number indicates that the reinforcement is smaller than that before the reinforcement.

4. Strength variation of the cross plate The strength test results of the cross-slabs of the foundation soil before and after reinforcement show that the strength of the soil at different depths after vacuum preloading is greatly improved. The detailed data are as shown.

Cross-slab strength curve before and after reinforcement of foundation soil 4. Surface settlement analysis Above, it has been maintained above 80kPa, and the maximum reached 90kPa plus membrane surface water, the entire equivalent reinforcement load exceeds 90kPa. It can also be seen from the edge of the reinforcement zone The settlement is relatively small, and the total settlement in the center of the reinforced area is relatively large. With the long time of vacuum preloading, the amount of surface settlement gradually increases. In the stage of rising vacuum, the sedimentation rate changes greatly, and then the sedimentation rate gradually becomes slower, indicating that the consolidation rate of the soil is a rate in the vacuum preloading stage. Gradient convergence process.

4. Stratified sedimentation analysis The sedimentation rate of the compression curve of each soil layer changes rapidly during the vacuuming period, which shows that the vacuum preloading effect is obvious, and as the time goes by, the inflection point of the curve is continuously moving down, indicating that the soil reinforcement The depth is continuously extended, and the depth of influence can reach about the bottom of the plastic drainage plate.

4. Analysis of Pore Water Pressure The pore water pressure dissipated at each measuring point showed a rising trend. Open experiments showed that after a few days of vacuuming, the vacuum level rose to 80 kPa. The clay loess was uniform in soil quality, with large pores, being non-weight. Collapsible loess, semi-hard hard plastic.

Dark red and blue-gray, distributed under the loess, thickness greater than 20m, the composition of sandstone, granite, limestone-based. Particle size composition:: 20~ filled with sand, partially with slight cementation, wet, dense or dense.

Since this section of the line is located in the 8 degree seismic zone, the design of the road side slope should take into account the stability, durability of the cut slope, and the economy of the cut section, taking into account the convenience of construction and maintenance. On the one hand, due to the low shear strength of loess and minimal tensile strength, the soil has a certain degree of structural integrity. When the slope is excavated, the slope can be maintained upright before a certain critical height. When this height is exceeded, or when there is a large overload on the ground and environmental factors change, soil instability will occur, resulting in overall destruction of the soil, which is called landslide in the project; moreover, the loess has special vertical joints and inclined joints. Landslides are sudden and difficult to predict. On the other hand, the stability of gravel soil cutting slopes is closely related to the cause, structure and density of gravel soils. The upper part of the stratum is collapsible loess, and the lower part is pebble soil. The total height of the slope is 30m. According to the design specifications, the slope slope ratio is 1:1.25 to 1:1.5, and the pebble soil and the loess boundary and slope height. When it is more than 10m, the platform shall be grading. According to this principle, the construction is difficult, the earth and rock works are large in quantity, occupy a large area, have a large amount of demolition, a long construction period, and require slope protection.

In light of the above-mentioned engineering geological conditions and practical difficulties, after careful analysis and research, the self-drilling anchor beam grid beam high retaining wall is actively promoted in the engineering design of this section. The design of high retaining wall is divided into 3 levels, each height is 6m, and the total height of retaining wall is 18m. The total length of self-drilling bolt 969 holes is used in order to verify the performance and design rationality of self-drilling bolts. In this section, a typical stratum was selected for the pull-out test. The anchor rods were respectively placed in the clay loess and cobble soil. The bond strength between the anchors and the surrounding rock was: the viscous loess layer was 0.08 MPa, the pebble land layer was 0.25 MPa, and the hole diameter after anchoring was 150. Design requirements have achieved the intended purpose.

5 Conclusion Based on the survey, research and analysis of the geological hazards of the loess cutting slopes of the existing railways and related buildings, Bao Lan fully recognized the importance of the yellow earth disasters and adopted reasonable engineering measures in light of the specific conditions of the project. After the test and operation of the rainy season, the technology is feasible, the measures are reasonable, and the project is reliable. It has promotion and reference value.

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Tolerance of the cutting length ±2mm.
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Un-powered.
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