Analysis of Strengthening Effect of Vacuum Preloading on High Filled Road Sections

Land subsidence plate (D deep subsidence mapping inclined pipe pore water pressure gauge Figure i Observation equipment Plane layout Loading process Vacuum was started on April 10, 2001. It reached 80 kPa on the second day and reached 90 kPa on April 14th. It has been stable above 90kPa. On May 17th, 2001, embankment construction began. Each layer was filled with a gap for a period of time. Due to the high filling height of the road in this area, in order to meet the construction progress requirements, the filling time of each layer is relatively low. The thickness of each layer after compaction is approximately equal to approximately 70cm except August 16th, 2001 to August 20th, 2001. The thickness of the other layers is relatively uniform, approximately 30cm. As of November 2001, it was filled to the top (excluding the pavement structure layer) on about 1 day. It took 168 days for filling, and it stopped vacuuming on December 10, 2001. The loading duration curve is as shown.

April 10, May 3G, July 19, September 7th, 1G, 27th, December 16th, February 4th, March 4th, March 26th, date loaded, duration curve, 5 Measured settlement, external load, 21 buried settlements Board, observed from April 8, 2001. According to the settlement observation on April 15, 2002, the average settlement was 218cm. At the beginning of vacuuming, the sedimentation rate was larger and later it became flat. Due to the small soil consolidation coefficient, the pore water could not be discharged quickly. The settlement curve does not reflect the loading process well during the process. There is no obvious difference between the sedimentation rate during the loading period and the intermittent period. At the end of loading, the settlement curve is similar to a straight line and can be visually displayed.

6 Loading schedule and load control Vacuum combined with preloading of embankment can accelerate the speed of embankment earthworks and improve the stability of roadbed. However, at this stage, there is no appropriate load control index to guide the construction of vacuum combined surcharge preloading. To adapt well to the requirements of construction, it is necessary to quantify the load control indicators, mainly from the three aspects of settlement rate, horizontal displacement and pore water pressure.

The existing load control indicators prescribed by various ground treatment specifications mainly include: vertical deformation no more than 1~15mm per day, horizontal displacement of side pile no more than 4~5 per day, and the ratio of pore water pressure increase value to load growth value is not large. The control index is for conventional surcharge preloading, and vacuum combined with surcharge preloading causes the soil mass to contract inwards due to the vacuum pressure, allowing the sedimentation rate to exceed the limits of the above specifications, horizontal displacement and pore water pressure growth values ​​and load growth values. Than the control index unchanged. The control loading speed is mainly based on the horizontal displacement. This is because the instability of the soil is mainly caused by the outward displacement, which is not directly related to the settlement rate. The horizontal displacement control is within the safety range and the safety of the roadbed can be basically guaranteed.

It can be seen that the maximum sedimentation rate occurs in the initial stage of vacuum preloading, and there will be no destabilization at this time. The average settlement rate in the reinforced area after the embankment is applied is 9.5 mm/d, and the maximum single settlement rate is 21 mm/d. Multiple occurrences above 15 mm/d all indicate that the settling rate exceeds the specification of 10 mm/d.

A horizontal displacement observer was embedded on the edge of the reinforcement zone and measurements began on April 3, 2001. When the initial vacuum was applied, the embankment converged to the inside by 15mm due to the action of vacuum. After May 8, 2001, due to the increase of embankment filling load, the embankment began to move to the outside and was measured until February 6, 2002. The measured displacement was actually measured. The maximum value is 62mm. According to the measured data, select the representative data, the displacement time curve is as shown.

Displacement (mm) As can be seen from the figure, in the initial stage of vacuum evacuation, due to the atmospheric pressure difference in the reinforcement zone and the consolidation and deformation in the reinforcement zone, the soil outside the reinforcement zone is deformed and contracted into the zone. The amount of soil shrinkage is different, and the size is related to the soil properties of the soil. The maximum value of shrinkage deformation in each zone varies from 20 to 70. The maximum value of shrinkage deformation occurs within a depth of 5m and the inward contraction of the ground surface is the largest. As the embankment fill load increases, the soil body gradually displaces outward, and it deviates from the maximum in the reinforcement zone. Deformation occurred at a depth of 6~9m, and the deformation was 1060mm. Horizontal displacements affected the depth. The horizontal displacement of the soil occurred within the setting depth of the drainage plate. The maximum depth of influence was about 20m, and obvious lateral displacement mainly occurred. Within a depth of 10m, there is basically no lateral displacement of the soil below the depth of the drainage plate. The depth of influence is a gradual process. With the increase of vacuum time, the vacuum degree gradually spreads downward, and the depth of influence of horizontal displacement gradually deepens.

In the aspect of the horizontal displacement rate that controls the loading, there is no stability problem due to the inward contraction of the soil in the initial stage of evacuation. The rate of inward horizontal displacement is not limited, and generally there is no need to consider more, as the load of the embankment increases. The outward displacement of the soil is increasing, but it is generally no more than 5/Mountain. Only in September, the horizontal displacement rate reached 5mm/d due to the rapid overburden, which caused the suspension of the heap construction for more than a month.

The pore water pressure gauge was buried at different depths below the center line of the embankment. Measurements began on March 30, 2001 because the depth of the drainage plate was 20 m. The pore water pressure probes that exceeded this depth were mainly measured pore water below the depth of reinforcement. Dissipation. According to the measured pore pressure data, the curve of the over-static pore water pressure-load-time is drawn, and some rules can be drawn through the analysis. The pore pressure time chart is as follows: the pore pressure time curve shows a good rule of increasing excess pore water pressure, dissipating, increasing load, and stopping. In the initial stage of vacuum pumping, a negative excess pore water pressure is generated, along with the vacuum time. With the increase, the depth of influence and the negative excess pore water pressure also increase, and when the membrane is filled, the negative pore water pressure reaches a maximum, basically -40-50kPa, affecting the depth with the road filling load. With the increase, the negative excess pore water pressure gradually decreases to zero. At this time, the positive pore water pressure caused by the embankment load and the negative pore water pressure generated by the vacuum pressure are dismantled and no positive excess static pressure is caused. Pore ​​water pressure, which is very beneficial to the stability of the foundation.

When the embankment is further filled and raised, the positive pore water pressure generated by the embankment load in the foundation is greater than the negative pore water caused by the vacuum pressure, but due to the influence of the vacuum, the growth rate is slower, so that the super-static in the soil The pore water pressure becomes a positive value in the range of 1847 kPa. The average pore water pressure coefficient P in each zone during filling is less than the critical value of 0.6. Therefore, from the perspective of pore water pressure, the foundation is stable.

The hole pressure gauge with a depth of 24m is mainly used to observe the pore pressure dissipation of the soil below the depth of the installation of the drainage plate. The actual measurement results show that the pore water pressure value remains almost constant when a vacuum is applied alone, and when the heap load is applied The value of pore pressure has been rising. Even if the heap was suspended for a month in September, the pore pressure value did not decrease, indicating that the vacuum preloading has no reinforcement effect on the soil below the depth of the drainage plate.

7 The post-construction settlement analysis code specifies that the maximum length of the post-construction settlement of a highway section is 30 cm, which is calculated based on the final settlement calculated using the three-point method after the full-load settlement curve, and the measured settlement difference. As shown in the following table: Measured settlement (cm) Corresponding degree of consolidation Calculated total settlement (cm) Residual settlement (cm) In actual settlement observation, it was found that by the end of observation on April 15, 2002, the surface subsidence rate Almost zero, and the vacuum combined embankment surcharge preload is overload preload, the pavement load is much smaller than the vacuum pressure, so the settlement caused by the pavement load is very small, we can see that the residual settlement calculated in the above table is equivalent Settlement after work to meet the requirements of the specification.

8 Conclusion The preloading of vacuum combined embankment can better solve the problems of poor foundation bearing capacity and large post-construction settlement similar to the construction of expressways on the soft soil foundation in Zhejiang Province, which has a good application prospect.

In general, the use of vacuum combined embankment preloading for soft foundation treatment can fully achieve the purpose of accelerating the filling and eliminating excessive post-construction settlement. The vacuum preload of 80 kPa can be used as a super-loaded load and can be loaded at the earliest time. The preloading time is long, and the load value greatly exceeds the load of the upper pavement structure, so that the compression layer of the embankment soft soil foundation completes more than 90% consolidation settlement within a short preloading period, and the post-construction settlement of the general road section can be controlled. Within the range of 30cm specification requirements.

In highways with high-filled soft-base sections similar to those in Zhejiang Province, the method of preloading by vacuum combined embankment can greatly speed up the filling of embankments.

In the stabilization control standards for fill rate, the horizontal displacement value and the pore pressure dissipation coefficient are mainly used as control indexes, and the settlement rate can exceed the limit of 10 mm/d.

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