Application and Analysis of the Sparse Pile Support for Foundations
Faculty of Engineering, China University of Geosciences, Wuhan, China
The soil arch theory has been applied to the foundation support problem. The foundation of the Huachengdadao metro station has been taken as a good example to explain the application. Based on the analysis of the geological conditions, the soil arch theory is adopted to make optimal analysis for the foundation, and the Lizheng deep foundation software is used to calculate the foundation displacement and stability, the results proved that the sparse pile support for foundation is not only stable but also economical. Therefore this support method is worth developing further.
Nowadays, assorted supports, which make full use of the soil strength, are little utilized, while single support systems are used more often. Assorted supporting structures in common use are in the following forms: compound assorted supporting structure; ladder assorted supporting structure; and arciform ranking plies or continuous wall structure. Bearing forces characteristic of the single supporting arciform defend structure is analyzed and the rules that all kinds of the factors affect functions of the arciform defend structure are discussed in References 1~3. The supporting arciform defend structure which consists of cement agitation piles is successfully utilized to maintain the safety of the foundation in Reference 4. The above-mentioned researches and applications are still limited to keep the structure of the foundation continuous in space. However, on the basis of security and economy of the foundation support, the soil arch theory of the sparse pile support, which is adopted to make the structure of the foundation discontinuous in space, is little utilized, but it is sure that the sparse pile supporting structure is safe and economical in some good geological conditions. The metro station foundation by name Huachengdadao is supported by sparse piles because of its good geological conditions. This project is taken as an example to analyze the design considerations.
ENGINEERING GEOLOGICAL CONDITION IN SITE
Engineering Geological Condition
The main fracture in site is conjectural BeiTing fracture and the main fault is TianHe syncline. The site is located in the southwest of the syncline. The trend of the fracture with a inclination of 50~700 is northeast, and The mainly fracture lies in BeiTing area, and its east part is Metamorphic rock of Cretaceous period, and its west part is Holocene epoch. In order to investigate the geological condition in site, a large number of drills are assigned. According to drilling materials, the main soil layers are Quaternary strata and weathering alluvium. Quaternary strata are silty clay, clay, silt of the alluvium and the flood deposit and alluvial silty clay from Pleistocene epoch to Holocene epoch. The upper layer with about 6.6~22.2 meter is artificial fill (plain fill). The mainly rock layers, from Huanghuagang Section Cretaceous epoch, are muddy silty sandstone and coarse (fine) sandstone, which are brownish red, muddy and calcareous bond, silty(coarse, fine) structure and middle~thick texture. The rock strata on base of drills are full weathering zone of muddy silty sandstone, intense weathering zone of muddy silty (coarse) sandstone, middle weathering zone of muddy silty (coarse) sandstone and weak weathering zone of muddy silty (coarse) sandstone. The typical geological profile can be seen in figure 2.
Hydraulic Geological Condition
The groundwater mainly is suspended water in fill layers, phreatic water in silt layers and fissure water in intense-weathering and middle- weathering bedrocks. The alimentation of groundwater mainly depends on atmospheric rainfall, and its precipitation is remarkably affected on seasons. The corrosion of groundwater in site is weak, therefore, the influence is ignored in design and construction. The seepage coefficients by well pumping tests of three drills are 0.107×10-6cm/s in ZC3-HC-15, 4.84×10-6cm/s in MZC3-HC-37 and 6.8×10-6cm/s in MZC3-HC-10. According to upper data, the other layers except for clay is weak permeable layers in the Huachengdadao metro station. It is remarkable that the groundwater is abundant in the artificial fill layer in rainy season, thus, the sealed curtain is established by agitation plies with a 5-meter length. This is no need in the other alluvium and flood layers and weak permeable layers.
Figure 1. The bolehole location and sketch map of subway station
Synthetically Geological Evaluation
The site is comparatively stable. There is no earthquake liquefaction as a result that there is no sand in site. The upper part of the strata is composed of silty clay of the alluvium, flood deposit and the eluvial layers, and the lower part of weathering zones which are thin and shallow outcrops. There is no bad muddy layer or sandy layer. The problem of anti-floating must be taken into account by reason of natural foundation. Some measures such as concrete-spouting in time must be used to prevent the softening of intense- weathering layers and eluvial layers. Combining the soil tests with the standard penetration tests, the parameters in table are adopted to calculate the displacement and stability of this foundation.
Table 1. The geotechnical properties for the foundation soil
SPARSE PILE SUPPORT DESIGN
The Calculation For Sparse Pile Support
It should be pointed out that the vertical position of interior support can not be changed because of the limit of the station story while the horizontal distance can vary on the fact. In addition, the foundation can be classified into two protected levels(super level and first level) in terms of foundation depth and the relationship between foundation and pipelines around it. Super level foundation should be guaranteed that the horizontal displacement of retaining structure be less than 0.1%H (23mm); first level foundation should be guaranteed that the horizontal displacement of retaining structure be less than 30mm. 1.05 is used as important factor for the retaining structure. Additional load is 20kPa; pile diameter is 1.20m (1.5m including the guard wall). The thick pile is adopted for the 23m deep foundation; the sparse piles whose distance is 3.0 m is adopted for the 16m and 18 m deep foundations; interior support two layers (f600d14) are installed in the vertical direction, horizontal distance is 5.0m;vetica distance is 1.0m,7.0m. the middle part between neighboring piles is improved with mixing pile and sealing the underground water (suspended water of artificial fill mainly). The relationship of supporting pile, inner bracing, top beam and sealing agitation pile can be seen in the perspective of foundation support (figure 3).
Figure 2. Perspective of foundation support
The deformation can be limited in allowable scope by adjusting horizontal distance of steel supporting in some individual points with great deformation. The parameters of soil and rock mass which is used in foundation calculation can be seen in Table 1, and the results of foundation deformation in Table 2, and the results of safety factors in Table 3. The greatest deformation of this foundation (30.32m) primarily meet the canonical requirement. The safety against overturning of the foundation in Table 3 is greater than 1.248, and its global safety factor is greater than 2.786, and The safety against overturning of most drillings is greater than 2.0, and their global safety factor is greater than 4.0. It is obvious that scheme of sparse pile support has adequate safety margin.
Table 2. Calculated displacement results of foundation
Table 3. Calculated safety factors for the foundation
STABILITY ANALYSIS OF SOIL ARCH BETWEEN PILES
Based on observation of the tunnels of mine mountain and model tests of the friable medium, M•M•PuLuo,a scientist from former -USSR, brought forward to the P balance arch theory: when excavated in friable mediums, the arch separations in top of the foundation come into being by disturbing of excavation, and the soil and rock masses outside the arch separations are still in balance condition, thus, the arch separations will be slumped or in balance naturally (Figure 4). The height of soil arches of between piles and the pressures of adjoining rock can be calculated and the formula are as following:
Here, fk is competent factor or P factor, and its formula can be calculated in the following form:
Here, g is the unit weight of the soil and rock mass, and K0 is the coefficient of earth pressure at rest.
The competent factor can be set by experiences also. The competent factor of silty clay and clay can choose 1.0~1.5. According to classification in Reference ? and SPT blow count, the coefficient of earth pressure at rest choose 0.5.
The distance between sparse piles of foundation is 1.5m. Because of their small distance, the neighboring pile can form soil arch, and it delivers pressures to the supporting piles with the diameter of 1.5m (figure 5). According to P formula, the slump of soils can be calculated in different depth, and the distance of piles is 1.5m, and competent factor can be calculated by P theory and experiences. The results of the height of soil arches and pressures of adjoining rock are expressed in Table 4.
Table 4. Calculated results for the soil arch between piles
It can be seen from Table 4 that the heights of the balance arch from theory and experiments is small and the pressures of adjoining rock is not very great. Obviously, the natural balance ability of the soil arch is very strong. The soil arch, which is formed by the deep agitation piles with the length of 5m between neighboring piles and is added to artificial excavation piles auxiliarily, can not only control the juncture between agitation piles and artificial excavation piles to insure the effect of sealing underground water, but also deliver pressures to the supporting piles and increase the stability of the foundation undoubtedly. To insure the instability of soils between the soil arch and the foundation wall, the soil anchors, which are 1.0m, are set up between the piles in the process of construction of artificial excavation piles. Additionally, the soil anchors are driven into the foundation wall vertically to insure their global instability.
It may be wasteful to use conventional ‘dense’ piles to support foundations in good geological conditions. The sparse pile system is both safe and economical. An example sparse pile project of foundation has been presented. In this case the number of piles can be decreased by half, and only costs 500,000RMB. This supporting scheme of the metro station in Guang-Zhou gain the good appraisement from experts and get biding successfully because of technology advantage over the other 11 construction companies. As a result, in the similar geological conditions with this the project, the sparse-piles scheme has an advantage over others and should be considered as a viable alternative in similar projects.
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