Brief Discussion on the Control of Post-construction Settlement after Vacuum-Surface Preloading Consolidation of Soft Soil Subgrade

Incorporating the combined preloading and reinforcement of Pang Ligui (the first engineering limited liability company of China Railway Siban Group, Hefei 230000, Anhui, China) until the new balance ().

The consolidation results from the imbalance of the pore water pressure*, which is caused by the increase in effective stress and consolidation in the mutual transformation. The vacuum-surcharge combined preloading method has the dual effects of vacuum preloading and surcharge preloading. Through the vacuum load and heap load (embankment), the pore water pressure in the soil is greater than the pore water pressure in the vertical drainage body, so that the vertical Drainage forms a hydraulic gradient inside and outside the body, which promotes the drainage of pore water and achieves the purpose of strengthening the roadbed and eliminating post-construction settlement.

The second test section (K0+276% 10+834.45) of the soft soil foundation road and bridge design parameter test project is based on the Quaternary Neolithic Chonghu Lake. The surface layer is (1) clay, grayish yellow, soft plastic + hard plastic, layer thickness 0.76m~3.60m; below it is (2) silty silty clay, dark gray, flow plastic, containing a small amount of rotted plants, local With a thin layer of silt, with high compressibility, low strength, high thixotropic characteristics, thickness 3.2m+16.5m; the underlying layer is (3) layers of clay, silt and silty clay, soft plastic% hard plastic . The physical project unit of each soil layer Natural water content of the soil Severe natural void ratio Unconfined compressive strength Compression coefficient Coffee compress modulus Compression index/Preconsolidation pressure Rebound index Test section K0+276.51-K0+456 Section uses vacuum - Surcharge preloading combined with soft ground treatment. The vacuum was started on April 23, 2003. After 4 days, the vacuum level rose to 80kP:, the upper sand cushion was filled on April 28, and the upper roadbed was filled on June 13th. It was filled out on September 15. Earth height 5.2m. road. Bridge and construction In order to fully understand the deformation of soil, the change of vacuum under the membrane, and the transmission of negative pressure, the soil in the reinforcement zone was subjected to surface subsidence, stratified subsidence, earth pressure, pore pressure, and membrane Vacuum monitoring and other projects.

Analysis of the influence of construction process on post-construction settlement The use of vacuum-surcharge preloading to reinforce soft soil roadbed, if reasonable construction techniques are selected and the various processes are reasonably controlled, the subgrade settlement can be completed in advance. According to the vacuum-surcharge combined precompression reinforcement mechanism and the measured data of the reinforced area, we can see that the impact of the construction process on the subgrade settlement includes the following aspects.

4.1 Influence of the stability of vacuum on the subsidence of subgrade In the process of reinforcement, the vacuum fluctuation will affect the consolidation rate of the soil, reduce the amount of pre-settlement of the soil, extend the pre-pressure period, and increase the settlement after work. In this project, from April 29 to May 7, the vacuum under the membrane was reduced from 85kP+ to about 30kP+, and then returned to about 80kPa, for example.

The settling rate and the vacuum duration curve are similar in the trend of the two section sedimentation rates. As the vacuum level changes, but there is a certain lag time, the relationship curve between the sedimentation rate and the change in vacuum level reflects that: after the vacuum level rises to 80kP+, the sedimentation rate rises to about 33mrn/d, and when the vacuum level decreases, The rate decreases, but when the vacuum rises again to 80kP+, its settling rate does not recover, which reduces the amount of settlement in the early stage.

4.2 Influence of fill rate on subgrade settlement In soft soil subgrade treated with vacuum surcharge and preloading, the soil undergoes lateral contraction and deformation due to the vacuum load, and the subgrade soil undergoes lateral extrusion deformation due to the earth load, and the two can interact with each other. Offset partial deformation% Under the vacuum load, the pore pressure dissipates quickly, the strength increases quickly, and the bearing capacity is improved, which can make the filling rate large and the subgrade will not destabilize and damage or cause additional settlement. You can increase the filling rate, shorten the pre-pressure period, reduce the amount of post-construction settlement.

The time ((1) K0+342-1 cross-sectional loading and settlement rate curve of the cross-sectional time curve from the K0+342 cross-section load and settlement rate can be seen that the rate increases with each layer of soil. , and then gradually converged, in August 23 soil compression curve vacuum - surcharge preload consolidation after soft soil subgrade settlement control is a complex system engineering, the need for comprehensive control in the survey, design, construction, currently The design calculation method relies excessively on the physical and mechanical parameters of the soil provided by the experiment. From the experimental methods obtained from various physical force parameters, it is known that their accuracy is not very high, and that the vacuum surcharged combined precompression reinforces the soft soil. The experience of subgrade settlement calculation parameters is also not very rich, and often results in a certain difference between the calculated value and the measured value, thus affecting the determination of the pre-pressure period, resulting in the construction schedule, the total duration, etc., being affected and causing losses to the country. Based on the consolidation theory of Etherecap and Barron, using the indoor test parameters as control indexes, the average consolidation and compression parameters of the soil are inverted from the measured data. The reliability is measured by the correlation between the predicted curve and the measured curve (if the measured data is accurate, the correlation coefficient is generally above 0.98). According to the characteristics of the subsidence of deep soft foundation engineering and a large number of examples, the inversion prediction curve adopts an exponential curve. The total settlement of the roadbed is relatively accurate, and maintaining the stability of the vacuum, increasing the filling rate, and repeatedly loading and unloading the unloading method during the construction can accelerate the soil consolidation, shorten the pre-pressure period, and reduce The role of settlement after work.

From 2004-1 to September 16th in Anhui construction, as the filling rate increased, the sedimentation rate reached a peak of 2.6cm/d, and it occurred in the vacuum load to maintain 80kP+ unchanged, through the previous joint preloading On the basis of, that is to increase the rate of filling, can settle the amount of soft foundation in advance.

The effect of unloading on subgrade settlement is the advantage of unloading the vacuum load, vacuum loading, combined preloading with overload or equal load, etc.) Preloading is incomparable, which is that loading and unloading are easy and can be extremely short. The vacuum degree is raised to above 80kP+ in the time, and the vacuum degree can also be reduced to 0 in a very short time. The effect of the vacuum load on the settlement of the roadbed mainly comes from the isotropic rebound. After unloading, the vacuum load causes The negative pressure seepage field will disappear, the ball stress in the seepage field will disappear, and the soil body will produce an isotropic springback. Due to the action of the earth load, the soil's vertical rebound is very small. , mainly horizontal lateral rebound, which will cause the generation of vertical additional settlement. According to the compression curve of the soil, after the final load is added, the settlement curve starts to converge, and after approaching the unloading standard, it can pass Repeated loading - unloading to further eliminate post-construction settlement.

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