1. Foundation treatment of a 750kV substation in Xinjiang: collapsible loess
Geological characteristics: Grade collapsible loess, maximum collapsible depth 43m, foundation bearing capacity 150kpa.
The purpose of foundation treatment: to prevent the settlement of the foundation.
Foundation treatment scheme: strong compaction.
High compaction energy level selection:
Precautions: Saturated cohesive soil is not suitable for strong compaction.
It is advisable to compaction at the optimal moisture content, and too dry or too wet will affect the effect of compaction treatment.
2. Foundation treatment of a wind power project in Ningxia Sun Mountain: silty sand
Geological characteristics: silt, fine sand, foundation bearing capacity: 120 180kpa.
The purpose of foundation treatment: to improve the bearing capacity and reduce settlement.
Foundation treatment scheme: strong compaction.
3. Foundation treatment of a 220kv substation in Shandong: large-thickness backfill
The substation site has complex topography, the terrain is high in the east and low in the west, high in the south and low in the north, and there are large mining excavation pits on the west, north and south sides of the station area, and the natural elevation of the mining pit is about 1100 meters.
The station area is low, with a natural elevation of about 1160 meters. The design elevation of the outdoor floor in the substation site is 12250m, most of the substation area needs to fill the height of about 65m~12.5m。
Due to the large filling height in the site, in order to meet the design requirements, the entire site needs to be treated with a strong compaction foundation. The construction of strong compaction should be deep first and then shallow, and the layered strong compaction.
According to the topography and design requirements of the site, the foundation was treated by the strong compaction method, and the characteristic value of the bearing capacity of the treated foundation reached 150 180kpa. The compaction range is 40m。After the foundation is completed, the foundation pit of the superstructure should be backfilled and leveled in time before construction, and the plain soil can be rammed in layers, and the compaction coefficient is not less than 094。
Note: After the completion of the project for more than a year, the large-thickness backfill area of the substation site has subsided, because the thickness distribution of the backfill area is different, and there is uneven settlement, but there is no large quality and safety hazard.
Fourth, the foundation treatment of a 220kv substation in Chicheng, Zhangbei: collapsible loess
According to the geotechnical engineering survey report of the site site, the thickness of loess soil in the site area is 20~6.Between 0m, non-self-weight collapsible site, collapsible grade.
1.High compaction method:
According to Table 6 of the Building Code for Collapsible Loess Areas1.10. The strong compaction method is suitable for collapsible loess above the groundwater level, sr (soil saturation) 60%, local or whole treatment, and the treatment depth is 3 12m.
The groundwater level in the site area of this project is deep, and the thickness of the collapsible soil layer is not more than 6m, so the strong compaction method can be used for the foundation treatment of this project.
Assessment of the impact of compaction on the surrounding environmentThe relevant scientific literature studies show that the safety distance of strong compaction to the building structure is 25 30m, and the safety distance of the impact on the human body is not less than 30mThe noise caused by strong compaction, the sound is muffled, the frequency is low, and the noise impact is not very large when it is more than 10m away from the tamping point. The substation of this project is about 700m away from the village boundary, and the adverse effects of strong compaction on the surrounding environment can be ignored.
The substation is treated with strong compaction, and the foundation treatment is completed at one time in this period. According to the principle of equilibrium of site leveling earthwork filling and excavation, the maximum processing depth of the excavation area is 60m, the maximum processing depth of the fill area is about 80m, check Table 6 of the Building Code for Collapsible Loess Areas3.6. Click on ramming to take 4000knm, ramming energy level: the main ramming takes 4000knm, full ramming 2000knm, ramming 1000knm, the main ramming twice, the low energy full ramming once (full tamping requires the hammer mark lap 1 3 hammer diameter), patting the ramming once.
2.Strong compaction + replacement method:
The cushion is suitable for treatment above the groundwater table, locally or in whole areas, with a treatment depth of 1 3m.
According to the principle of balance of site leveling earthwork filling and excavation, the maximum height difference of the excavation area is about 27m, the maximum height difference of the fill area is about 19m, so the comprehensive treatment scheme of replacement + strong compaction can be adopted according to the depth of the site collapse, that is, the depth of the collapse is less than 3 after removing the buried depth of the building (structure) foundationThe 0m area is replaced by the filling method, and the rest of the area is by strong compaction.
The replacement area is mainly located in the excavation area, and the specific method is to replace the collapsible loess (permeable materials such as sand and gravel) at the lower part of the building, and make a 3:7 ash closed layer on the surface. The treatment plan of the strong compaction area is the same as the above strong compaction plan.
The economic and technical comparison of the two different foundation treatment schemes is shown in the table below
As can be seen from the above table, the cost of the strong compaction method project is relatively low. Considering that the GIS foundation part outside the house is located in the fill area and the part is located in the excavation area, the strong compaction foundation treatment scheme of the whole station can better reduce the uneven settlement of the foundation soil, and the strong compaction method has the advantages of simple construction technology, convenient construction and short construction period.
5. Summary
The strong compaction method has a good effect in the treatment of collapsible loess and backfill sites, with excellent economy and relatively short construction period, and has good application value.
Note: The impact of noise and vibration needs to be considered when applying the high compaction method, such as the existence of nearby villages or buildings, on the one hand, it is necessary to consider the complaints and obstruction caused by construction noise, and on the other hand, the adverse effects of vibration on the surrounding houses and other facilities need to be considered when the distance is too close.