Pile foundation selection in karst area?

Karst areas in China are widely distributed, with an area of 3.65× 106 km2. In recent years, with the acceleration of urbanization, the building structure in China is becoming more and more complex. High-rise buildings in karst areas need relevant survey, design and construction personnel to strengthen the analysis of geological conditions in karst areas and select appropriate foundation types in combination with the actual engineering situation.

2 Analysis of geological conditions in karst areas

Due to the unfavorable geological conditions such as karst caves, earth caves, cracks and underground rivers in karst areas, accidents such as water inrush, ground deformation and foundation collapse are prone to occur under the action of the self-weight of rock and soil bodies or buildings and groundwater, so it is necessary for relevant survey, design and construction personnel to do a good job in analyzing the geological conditions and characteristics of karst areas.

2. 1 soft soil

In karst area, a layer of soft soil with different thickness is often distributed near the upper part of soluble rock. Under the action of groundwater, this layer of soil has a large water content and is in a soft-plastic state, and its bearing capacity is extremely low.

2.2 Groundwater

Generally speaking, karst area is rich in groundwater, which can be divided into pore water and karst fissure water according to the different occurrence media. Among them, pore water mostly occurs in alluvial-diluvial or residual soil layer with strong permeability, while karst fissure water mostly occurs in karst caves and karst fissures with a large amount of water. And the connectivity is very good.

2.3 caves

There are many caves, gullies or gullies of different sizes in karst areas. According to the different distribution depths of caves, they can be divided into shallow caves and deep caves, among which shallow caves are well developed and sometimes even distributed in beads. Shallow caves are mostly filled with flowing plastic-soft plastic, full-filled or semi-filled. Deep points are mostly ancient points, semi-filled or not. When deep holes are connected, an underground river will be formed.

3 Application Status of Pile Foundation in Karst Area

According to the geological characteristics of karst areas, at present, manual digging pile, bored pile, prestressed pipe pile or precast pile, rammed expanded pile, CFG pile and so on are mostly used for pile foundation types.

3. 1 Manual Digging Pile

Manual digging pile is convenient to control the bearing layer of foundation and sediment at the bottom of hole. But also has short construction period and low cost. At present, it is widely used in karst areas with shallow bedrock and less groundwater.

3.2 Bored (punched) cast-in-place pile

Although drilling (punching) bored piles often appear such phenomena as sticking, pile dropping, pile burying, slurry leakage, hole collapse and hole deviation. And the sediment at the bottom of the hole is also difficult to control. However, its construction speed is fast, the construction process is not affected by groundwater, and the construction is safe. At present, it is the most widely used pile foundation type in karst areas in China.

3.3 Prestressed pipe piles or precast piles

Slipping and broken piles often occur in the construction of prestressed pipe piles or precast piles. Therefore, it is rarely used in karst areas at present, only in karst areas where karst is underdeveloped and there are silt, earth holes, quicksand and underground rivers.

3.4 Rammed expanded pile and CFG pile

When the proposed building requires low foundation bearing capacity, rammed expanded piles, CFG piles and composite foundations can also be applied to karst areas, among which rammed expanded piles are mostly used in multi-storey buildings and CFG piles are mostly used in high-rise buildings.

4 Problems and suggestions of pile foundation selection in karst area

The first phase project of Wengjiang New Town in wengyuan county, Guangdong Province and Gedi Jiayuan Project in Changyuan, Guangzhou are typical karst areas. The author synthesizes the exploration data of these two projects. The selection of pile foundation in karst area is simply analyzed, and some suggestions are put forward.

4. 1 wengyuan county Wengjiang New Town Phase I Project.

(1) Project Overview

The first phase detailed survey project of the proposed Wengjiang New Town is located in wengyuan county, Guangdong Province, and the proposed 17-23-storey high-rise residential building is a shear wall structure. The basement of 1 floor is 4m deep, and it is planned to adopt frame structure and shear wall structure system, and the engineering importance level is Grade II.

According to the drilling data, the foundation soil in the site is composed of artificial fill and cultivated soil (Qml+pd), Quaternary alluvium (Qal) and eluvial layer (Qel), and Carboniferous slightly weathered limestone (C) from top to bottom. There are local cracks in the bedrock and dissolution along the crack surface.

(2) Basic selection and suggestions

According to the drilling data and geotechnical test results, the foundation soil of the proposed site is mainly composed of Quaternary alluvium and residual silty clay, with general engineering geomechanical properties and low bearing capacity, which cannot be used as the bearing layer of the proposed building. The rock surface in the site fluctuates greatly, the geotechnical properties change greatly, and the slightly weathered limestone rock surface is inclined and has high bearing capacity, which can be used as the bearing layer at the pile end. Therefore, it is suggested to use pile foundation for the proposed building, but the rock surface of the site is inclined. Pile construction is prone to pile breakage or pile deviation, so it is suggested to use bored pile as the pile foundation type.

4.2 Guangzhou Changyuan Brothers Jiayuan Project

(1) Project Overview

The brother Jiayuan of Changyuan is located in the west of Yingbin Avenue in Huadu District. It is planned to build a 12 high-rise residential building with floors of 18-26, and it is planned to adopt a shear wall structure. The height above the ground is 6 1.20-86.00 meters, and the basement 1-2 floor, and the buried depth of the basement is 5m and 9m respectively. It is proposed to adopt frame structure and shear wall structure system, and the engineering importance level.

According to the drilling data, the foundation soil in the site is artificial fill (Qml), Quaternary alluvium (Qal), eluvial layer (Qel) and Carboniferous slightly weathered limestone (C) from top to bottom. The bedrock stratum is inclined and karst caves are developed.

(2) Basic selection and suggestions

According to the drilling data and geotechnical test results, the foundation soil of the proposed site is mainly composed of silty clay, silty fine sand and coarse sand, with general engineering geomechanical properties and low bearing capacity. There are soft soil layers such as muddy soil in some areas, which have poor engineering geomechanical properties and are easy to sink and settle, and cannot be used as the bearing layer of the proposed building. The rock surface in the site fluctuates greatly, and there are many kinds of rock and soil, and their properties change greatly. Weathered limestone rock surface is deeply buried and has high bearing capacity, which can be used as the bearing layer at the pile end. Therefore, combined with local architectural experience, it is suggested that the proposed building should adopt pile foundation instead of shallow foundation and hammering precast pile. Due to the inclination of the site rock surface, pile breakage or pile deviation is easy to occur in pipe pile construction, so it is suggested that drilling (punching) pile foundation be used as the pile foundation type.

5 conclusion

Through the above analysis, the following points should be paid attention to in the selection of pile foundation in karst area.

(1) When the overlying soil layer is thick and the side friction resistance is large, the friction pile should be given priority to effectively avoid the lower karst cave; When the overlying soil layer is thin and the side friction resistance is small, the end-bearing pile with low bearing capacity can be considered to reduce the load on the roof of the karst cave.

(2) Manual digging piles can be considered when the local groundwater is less and the bedrock is shallow, and bored piles, prestressed pipe piles and precast piles can be considered when the local groundwater content is high. However, no matter what kind of pile type is used, the development and filling of the lower karst cave should be found out to ensure the construction quality and safety of the pile foundation.

(3) The Code for Design of Building Foundation stipulates that "there should be no weak interlayer, fracture zone and karst cave in the range of 3 times the pile diameter below the pile end of rock-socketed cast-in-place pile; And there should be no free surface of rock mass within the stress diffusion range at the bottom of the pile. " The geology in karst area is complex, the height of rock surface changes greatly, and there are many cave fillings. There is no reliable geophysical exploration method to fully find out the distribution of underground karst, so as to avoid the risk of karst caves at the bottom of piles.

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