Application of Combined Vacuum Preloading Method in Soft Ground Reinforcement

Application of vacuum combined with preloading method in soft ground reinforcement Pan Changsheng! , WU Yuedong 2 (1. Water Resources Bureau, Linhai City, Zhejiang Province, Linhai 317000, Zhejiang, China; 2. Hehai University, Nanjing, Jiangsu 210000, China) Taking the reinforcement of subgrade as an example, the practicality of using vacuum combined with preloading to reinforce soft ground is introduced. . Practice has proved that the use of a vacuum combined with preloading method to strengthen the soft base has the advantages of shortening the construction period, solving the stability problem during the filling process, and reducing post-construction settlement, especially for the reinforced area and bearing capacity. ), Male, Linhai, Zhejiang Province, engineer, graduated from college, mainly engaged in water conservancy engineering design.

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Vacuum one-load combined preloading; soft ground reinforcement 1 vacuum one-stage loading combined preloading reinforcement principle Vacuum preloading method and heap preloading method both belong to drainage consolidation method, first set up plastic drainage board in soft foundation Or bagged sand well as a vertical drainage body, and a sand cushion is laid on the surface of the soft ground as a horizontal drainage body, and the pore water pressure in the soil is generated by vacuum pressure (negative pressure) or surcharge (positive pressure) Under unbalanced water pressure, the pore water is gradually discharged through the vertical drainage body under the effect of this unbalanced force, thereby causing the soil to undergo consolidation and deformation.

In order to recognize and distinguish the reinforcement mechanism of vacuum preloading method and surcharge preloading method, according to the effective stress principle of Taisaji: only the effective stress (!) is increased, the strength of the soft foundation is further improved, and the soft foundation reinforcement is played. the goal of. During 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 turns into effective stress. The soil strength can be improved, while the vacuum preloading method is the same as the total stress (!), and the pore water pressure (") is reduced by the evacuation to reduce the pore water pressure, and the pore pressure dissipates into the effective stress. Soil strength further improved. Therefore, "(potential) imbalance is a necessary condition for consolidation, and soil water is reduced and "reduced!" Increases are the determining conditions for consolidation. The vacuum one-load combined preloading method is a soft foundation reinforcement method developed on the basis of vacuum preloading method and surcharge preloading method. The method has the dual effects of vacuum preloading and surcharge preloading; Under pressure, because the drainage distance of the foundation soil is shortened, the pressure difference of the pore water pressure (") increases, the drainage rate is accelerated, the effective stress is further increased, and the strong B degree can be greatly improved; at the same time, due to the negative pressure generated by the vacuum, The inward contraction and deformation of the soil body can counteract the outward extrusion deformation caused by the pile load, and the foundation will not cause stability problems due to the high filling rate.Therefore, the vacuum one-load combined preloading method is better than the surcharge preloading. The law is safe and reliable, with better results.

2 Engineering Geological Conditions According to the drilling data and indoor test results in the Engineering Geological Survey Report, the section of the strata from top to bottom are artificial fill, silt layer, sandy clay layer and sandy sub-clay layer. Their characteristics are simple. As follows: Artificial fill layer. Most of them are filled with plain soil and their components are sandy and sub-clay. They are naturally stacked and have a layer thickness of 0.86.8m which is soft and uneven.

silt. Gray black, soft plastic flow, is a super soft soil layer thickness of 0.9 9.6m, the average water content of 82.46, porosity ratio of 2.21. Sandy clay. Alluvial origin, yellow to brownish, plastic, sandy.

Sandy sub-clay layer. The cause of weathering residue is plastic to hard plastic, and the quality of the underlying layer is better.

From the perspective of the physical and mechanical properties of the soil layer in the reinforced zone, the soft soil layer has high water content (highest value = 94.36), large liquid plasticity index (maximum value /' = 2.59, / (= 26.46), high compressibility> 1.6 MPa-1), Poor permeability, low strength, poor quality properties of the mud. Due to the large viscosity of the soil, low shear resistance, and poor stability, it takes a long time to complete the consolidation. Soft soil shear strength is low, cohesion! Less than 10.0kPa, friction angle! Also lower, it is a marine formation with high organic content and therefore has medium and high sensitivity.

The soft soil in the reinforced area belongs to the super-soft foundation with high water content, high compressibility, low strength, poor permeability, and medium-high sensitivity. Due to the high requirements of post-construction settlement in municipal engineering, effective soft-base reinforcement must be carried out on such a road.

3 Construction observations 3.1 Construction conditions The reinforcement project is 250m long. Vacuum preloading and reinforcement of the soft foundation construction began in early November 2000. The installation of the drainage filter pipe, the sealing film and the vacuum system were installed until January 12, 2001. 1 On the 13th of January, vacuum was applied. On January 15th, the degree of vacuum under the membrane reached 65kPa. On January 19, the vacuum reached about 80kPa, and it remained at about 80kPa afterwards. From the vacuum point of view, the sealing film, the membrane surface and the membrane periphery are well sealed, the vacuum degree under the membrane reaches the design value, the holding load is stable, and the vacuum preloading construction is successful.

3.2 On-site observations Observations. The on-site observations in this paragraph include: vacuum degree (mains, sand cushion), surface settlement, pore water pressure, groundwater level, etc.

The observation instrument is buried. There are 6 pieces of surface sedimentation plates, 4 pore water pressure gauges, 14 vacuum gauges under the membrane, 4 vacuum gauges, and 1 groundwater level hole.

Surface settlement observations. In the settlement block of the whole process block, the evacuation lasted for one month, the maximum settlement has reached 584mm, plus the settlement after the construction of the drainage plate, the total settlement has reached 800mm, according to the requirements of the design, consolidation The degree has reached 702, indicating that the reinforcement effect is very significant. The maximum sedimentation rate is 35.0mm/d, the minimum sedimentation rate is 7.7mm/d, and the average maximum sedimentation rate is 20.1mm/d. It can be seen from the time-dependent surface settlement curve that the sedimentation rate at the initial stage of evacuation is relatively large, over time. In the extension, the sedimentation rate gradually slows down, indicating that the soil primary consolidation change rate is also a gradual convergence process, which is basically the same as the surcharge preload, but the settlement rate is larger than the preload, and the convergence rate is slower, indicating that the vacuum preload The pressure is better than the surcharge preloading.

It can reflect the changing characteristics of soil in the process of reinforcement.

Pore ​​water pressure. In the vacuum preloading stage, the four points at different depths of pore pressure decrease with time, especially at the 2.5 m point, the pore pressure has dropped to a negative value as the vacuum level increases, indicating that the vacuum pressure is The vertical drainage body (plastic drainage plate) transmits well to the depth of the foundation and the reinforcement effect is significant; the measurement point at 10m is slower due to the slower vacuum transfer and the slower rate of hole pressure decline, while the length of the plastic drainage plate is about 10m, indicating that the underlying layer of soil The effect of body reinforcement is also very obvious. The pore pressure observations show that the depth of the soft foundation reinforced by the vacuum preloading method is larger, which can eliminate the partial secondary consolidation of the underlying soft soil layer, thereby reducing post-construction settlement.

Vacuum degree observation. Vacuum was started on January 13, and the vacuum level continued to rise. 19 turns reached 80 kPa. From the vacuum change curve, it can be seen that the vacuum on February 13 lasted for one month, and the vacuum under the film was basically maintained at 80 kPa. Around, meet the design requirements.

Groundwater level observation. The vacuum preloading is to generate negative pressure in the reinforcement zone by vacuuming. The negative pressure is used to discharge part of the water in the pores in the soil to achieve the purpose of reinforcing the soil. Groundwater levels slowly decline over time.

4 Comparison of vacuum one-load preloading method and surcharge preloading method The following features of using vacuum one-load combined preloading method are as follows: 1) Shorten the construction period. The soft soil layer is characterized by high water content, high compressibility, low shear strength, poor permeability, and high sensitivity. The construction of high-grade roads or municipal roads in such soft soils, and the speed of filling embankments are the key factors that restrict the construction period. Taking the soft base section of a highway in Guangdong Province as an example, the construction period comparison results of the vacuum one-load combined preloading method and the overload preloading method are listed in Table 1. Table 1 Comparison of the construction period Comparison of the soft foundation reinforcement method Preparation of the road embankment During the pressure period, the vacuum one-stage combined preload (vacuum is 80kPa) (including vacuum preloading 44d, overload preloading (calculated as overloaded 1.8m)) (using thin-layer wheel addition method, the vacuum preloading method is used to shorten the construction period. The cumulative settlement and settlement rate are much greater than the preload, which means that the consolidation effect of vacuum preloading method is more significant than that of the surcharge preloading method.) Solve the stability problem during the filling process. In the process of consolidating soft foundations, the combined preloading method is always in a state of negative pressure in the reinforcement zone.In the negative pressure state, the soil body undergoes inward contraction deformation, and the inward contraction deformation can counteract the outward movement caused by the filling. Extrusion deformation, which is very beneficial to the stability of the roadbed.Nanchang Road in the right before the start of vacuum preloading K0+340-K0+440 has emerged the phenomenon of outward slip.In addition, the cumulative settlement of measured data reflect the pre-reinforcement zone (vacuum pre Step The cumulative settlement of the section is large, about 600 mm can be settled in one month, and the total settlement is about 800 mm after the completion of the construction of the drainage plate. The large settlement indicates that the consolidation of the soil is fast and the foundation is fast. The intensity increases, thereby increasing the anti-destabilization ability of the foundation.

3) Reduce post-construction settlement. A large number of studies have shown that the post-construction settlement of highway soft ground mainly comes from two aspects: the secondary consolidation of soft soil layers; the consolidation and settlement of the underlying soft soil layer. The measured data of pore water pressure reflect that the use of a vacuum-loaded combined preloading reinforcement soft foundation can achieve reinforcement depths up to the length of bagged sand drains or drainage slabs, which can eliminate partial consolidation settlement of the underlying soft soil layer. In addition, when the filling reaches the design elevation, the vacuum load is equivalent to an overload of 3-4 m (vacuum of 80 kPa), which is also beneficial for eliminating post-construction settlement.

PV, due to the constant peripheral speed "1, the inlet relative velocity 1 changes and becomes V, and its direction is no longer consistent with the direction of the blade inlet bone line. Conversely, when the guide vane opening is less than the design opening, the runner inlet Absolute speed !1 becomes !r, the relative speed 1 becomes, its direction is no longer consistent with the direction of the blade inlet bone line.It can be seen that whether the guide vane opening is larger or smaller than the design opening, it will destroy the original Hit the inlet, resulting in an import impact loss.

At the exit of the runner, due to the restriction of the flow path of the blade, when the opening degree of the guide vane increases, 2 will increase to 2' because the flow increases, but the direction will not change; when the guide vane opening decreases, the flow decreases. Small, make 2 reduce to 2', the direction does not change. 2 increase, will produce a circumferential component opposite to the direction of rotation! "2; 2 reduction, will produce the same circumferential component with the direction of rotation + visible from this, when the guide vane opening deviates from the design opening degree, regardless of the opening degree is increased or decreased, the impact of the importer will have impact loss , And the rotating wheel output will produce a rotating component of 7!82, leading to the reduction of the recovery efficiency of the tail water pipe kinetic energy, which is the main reason for the efficiency of the turbine deviating from the design conditions.

4 Conclusion The non-impact import and normal export are the necessary conditions for the optimal working conditions of the turbine. Turbine design conditions generally meet these two conditions, so the operating efficiency is high under this condition.

The non-impact inlet and the normal outlet can only correspond to a certain head and flow. In the actual operation of the turbine, due to the change of the reservoir water level and the external load, the water head and flow rate often change, so the turbine often deviates from the optimal operating conditions. Operation, no-impact imports and normal exports are destroyed, resulting in import impact losses, reducing tailwater pipe kinetic energy recovery and reducing the efficiency of turbines. The farther away from the design conditions, the more the efficiency is reduced. In severe cases, the cavitation in the cavity at the upper end of the draft tube will cause vibration. ) Axial-flow paddle-type turbines are generally designed to operate at 85%-90:over-water flow rates. When the water flow rate of a turbine with a lower specific speed is about 30:1 of the design flow rate and the flow rate of the turbine with a higher specific speed is about 45:1, the efficiency is reduced to nearly zero, and severe cavitation occurs. And vibration. Therefore, the axial-flow paddle-type turbine should avoid operating under no-load and light-load conditions for a long time, and it is best to operate under a slight load change.

4) In order to reduce the adverse effects of large load changes during stand-alone operation, the power station can be equipped with multiple identical turbines to adapt to changes in load, or to install small units to open small units in low or light loads. Or when the heavy load was put into operation, the large units were put into operation.

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