The basic concept
Preloading method belongs to the method of drainage consolidation treatment, which is a kind of soft foundation treatment. Before the construction of the proposed structure, in order to improve the bearing capacity and reduce the settlement amount, preloading method is carried out on the building site, so that most of the settlement of the foundation is completed in the process of preloading and the strength of foundation soil is improved.
Preloading method is suitable for soft foundation such as silty clay, silt and artificial flushing.
The preloading method includes preloading, vacuum preloading and vacuum combined preloading.
1. Heap preloading
Preloading is the preloading of heavy objects (water, soil, sand, stone, etc.) on the foundation.
According to the different preloading load is divided into overload preloading, equal load preloading and underload preloading.
Underload preloading is seldom used in highway engineering.
In order to prevent the foundation from being damaged by the pileup, graded loading is required.
2. Vacuum preloading method
The vacuum preloading method is to lay a sand cushion on the surface of the soft ground that needs to be strengthened.
Then a vertical drainage pipe was buried,
It is then sealed off from the atmosphere by an airtight film, which is buried in the earth around it.
Through the water suction pipe embedded in the sand cushion, vacuum pump is used for air extraction to form vacuum and increase the effective stress of the foundation.
When the foundation is treated by vacuum preloading, plastic drain plate or sand well must be installed to drain water vertically. A plastic drain plate or sand well can transfer the vacuum from the sand cushion to the soil and drain the water from the soil through it to the sand cushion.
Vacuum preloading method is suitable for the treatment of soft soil foundation with poor property, shortage of soil source and tight construction period.
3. Vacuum combined load preloading
When the preloading load of the designed foundation is greater than 80kPa and the vacuum preloading foundation cannot meet the design requirements, the combination of vacuum and heap preloading foundation treatment can be adopted.
Vacuum combined load preloading can be used to treat soft soil subgrade of high fill road section and bridge head road section.
The basic requirements
1. The sealing membrane should be made of impermeable material with good aging resistance, good toughness and strong puncture resistance
2. The thickness of sealing film is 0.12~0.14mm, and 2~3 layers can be laid
3. The sealing film connection should adopt the thermal bonding joint flat lap, the width should not be less than 15mm
4. The periphery of the sealing film should be buried in the sealing groove, with a width of 0.6~0.8m and a depth of 1.2~1.5m.
5. The vacuum tube routing supervisor and the filter tube shall be located in the drainage sand cushion with 0.1~0.2m thick sand covering layer.
6. The filter pipe shall not be sand permeable. Sand filling shall be used around the filter pipe
7. The preloading period and the preloading height shall be determined according to the settlement amount or consolidation degree of foundation after work
8. In the case of vacuum preloading, sand Wells or plastic drainage plates shall be set in the foundation to drain water vertically, and sand cushion and drainage pipes in the cushion shall be set.
The construction process
Fixed price and application
(I) General analysis
1. The 2018 estimate includes only vacuum preloading
(1) Take the processing area as the quota unit
(2) Take the preloading period as the standard to divide;
2. The pre-pressure quota is not included in the 2018 estimation index
"The contents not included in the special roadbed treatment shall be calculated according to the highway engineering estimated budget according to the design treatment method when necessary."
Accordingly, can apply estimate budget norm to carry on valuation.
3, 2018 quota deleted the heap load preloading quota
(1) If the design adopts heap preloading, it is suggested to apply the 2007 budget quota.
(2) 2007 fixed load preloading is divided according to preloading
(3) The 2007 quota does not include the loading of materials and the transportation of materials after unloading. If it happens, the price shall be calculated according to relevant contents
4. In the 2018 highway engineering construction bidding model, the list item is 205-1-E, including vacuum preloading and heap preloading
1) Vacuum combined heap load preloading
Press 205-1-e-1 for the vacuum preloading part
Press 205-1-E-2 for the preloading part of the heap
(2) Vacuum prepressure is measured according to the area of the subgrade covered by the sealing film along the inner edge of the sealing groove
(3) The sand cushion in vacuum preloading shall not be separately measured as the auxiliary work of vacuum preloading
(4) Preloading is measured by preloading volume
(Ⅱ) result of fixed price
1. The base price of the proposed budget is higher than the budget base price
2. The base price of vacuum preloading for three months is 62 yuan /m2
(Ⅲ) Quota description and analysis
1. The quota comprehensively considers the engineering contents and costs of filter drainpipe, cushion, film, sealing ditch and settlement observation.
2. The quota does not include the required sand cushion and vertical drainage water body
(1) When necessary, it shall be calculated separately according to the design requirements according to the relevant quota.
(2) Sand cushion refers to the sand cushion set up by sand Wells or plastic drainage boards
(3) The cushion required for vacuum preloading has been included in the quota
3. Calculation rules
According to the design needs to deal with the soft soil foundation area calculation
4. Designed for heap preloading
It is suggested to apply the 2007 budget quota, or apply the relevant quota according to the preloading
(where materials shall be available, but transportation shall be calculated)