Application of pile supporting structure in deep foundation pit support?

Zhuhai is located in the alternate accumulation plain of land and sea in the Pearl River Delta, and it is an important coastal open city in China. As far as the engineering geological conditions of Zhuhai city are concerned, it belongs to the alluvial plain of the Pearl River Estuary, and the upper layer is generally sedimentary soil or muddy soil, so the geology is relatively weak. In the construction of deep foundation pit, in order to ensure the safety and quality of the project, it is necessary to carry out supporting work, and the pile supporting structure is actually adopted. Project overview. The proposed project is located in the north of Renmin West Road and east of Anfu Street, Xiangzhou District, Zhuhai City, with convenient transportation. This project is a hotel project, with 14 floors, a height of 74.40m, a building area of about 5 100m2, and an indoor floor elevation equivalent to the absolute elevation of+13.00m, with a rectangular shape and a frame structure. There are 4 floors in the basement, covering an area of about 3400m2, and the safety level of the foundation pit is Grade I. The engineering importance level of this project is Grade II, the site complexity level is Grade II, the foundation complexity level is Grade II, and the geotechnical engineering investigation level is Grade B. According to the national standard "Classification Standard for Seismic Fortification of Building Engineering" (GB50223-2008), the seismic fortification category of the proposed site is Class C. ..

Geotechnical engineering conditions of site

topography

The proposed project is located in the north of Renmin West Road, Xiangzhou District, Zhuhai City and east of Anfu Street, with convenient transportation. Its original landform is alluvial-diluvial landform. During the survey, the site was flat, and the measured elevation of each borehole orifice was between12.21~13.64m. ..

Stratigraphic lithology

According to the drilling results, the buried strata in the site mainly include artificial fill layer, Quaternary alluvial and diluvial layer and Quaternary residual layer, and the underlying bedrock is Yanshanian granite. The strata developed in the site are described as follows from top to bottom.

Artificial Fill Artificial fill is brown and brown, mainly composed of cohesive soil, mixed with about 20%~25% timely particles, unevenly mixed with gravel, broken bricks, concrete blocks, etc. The particle size is generally 3~ 15cm, with occasional plant roots and domestic garbage, in which ZK4 and ZK7 contain backfill stones and concrete blocks. This layer is newly accumulated and has not been consolidated by its own weight, so the structure is wet. This layer occurs in all boreholes, with a thickness of 5.20 ~10.80m. ..

Quaternary alluvial-diluvial silty clay: brownish yellow and brownish red, mainly composed of clay particles and silt particles, containing 10% ~ 20% timely particles unevenly locally, unresponsive to shaking, slightly shiny, high dry strength and toughness, saturated and plastic. This layer is found in borehole ZK 16, ZK 18 and ZK 19, and the layer thickness is1.10 ~1.30m. ..

Quaternary eluvial gravelly clayey soil: brownish yellow and brownish red, formed by in-situ weathering of granite. The original rock structure is distinguishable, mainly composed of cohesive soil, mixed with a small amount of timely sand, unresponsive to shaking, slightly shiny, with medium dry strength, low toughness, saturated, hard plastic state and local plastic state. This layer is encountered in all boreholes, and its thickness is 3.90 ~13.80m. ..

Yanshanian granite is yellowish brown and brown, and the main minerals are syenite, feldspar and biotite, with fine-grained structure and massive structure. According to the different weathering degrees, the exposed granite in this survey can be divided into three areas: fully weathered, strongly weathered and moderately weathered.

Fully weathered granite belongs to extremely soft rock, brown and gray. Except for a small amount of weathering, most minerals are weathered soil, and the integrity of rock mass is extremely broken. The basic quality grade of rock mass is Grade V, and residual structures can be seen. The core is soil column, which is easy to soften when it meets water, and it is easy to drill with alloy drilling tools. This layer is encountered in all boreholes, and its thickness is 0.30 ~15.00m.

Strongly weathered granite belongs to extremely soft rock, brownish yellow, grayish white and fleshy red. Most minerals have obviously weathered, joints and fissures are extremely developed, and the integrity of rock mass is extremely broken. The basic quality grade of rock mass is Grade V, and the core is in the form of soil column and soil fragments, so alloy drilling tools are easy to drill. Except ZK 12, other boreholes are exposed, with the exposed thickness of 3.20~ 18.50m and the layer thickness unknown.

Moderately weathered granite is hard rock, grayish white and dark gray. The main mineral components are quartz, feldspar and biotite, with medium-fine grain structure and massive structure. Some minerals are obviously weathered, joints and cracks are slightly developed, and the integrity of rock mass is broken. The basic quality grade of rock mass is Grade IV, and the core is short column, so diamond drilling tools can be drilled. Drill holes ZK 1, ZK4, ZK6, ZK 10~ZK 12, ZK 15, ZK 17 and ZK20 to expose this layer, and expose the thickness1.20.

The spherical weathered body of moderately weathered granite is brownish yellow, bluish gray and grayish white, with developed joints and fissures, hard rock, columnar shape and short core, which can be drilled with diamond drilling tools.

Quartz vein is white, milky white, granular crystal structure and massive structure. The main mineral composition is timely, and the core is mostly 3~8cm block, so it is difficult to drill with diamond drilling tools.

Foundation pit design

Analysis of foundation selection

According to the investigation results and the architectural experience in Zhuhai, the proposed building (structure) is more suitable for pile foundation, and the pile type should be prestressed concrete pipe pile or bored pile. If prestressed concrete pipe piles are used, the bearing stratum at the pile end can be completely weathered granite or its lower stratum, and the pile end should enter the bearing stratum to a certain depth. Under the condition of meeting the design pile length, the penetration or final pressure value should be taken as the standard of pile sinking control. If bored piles are used, moderately weathered granite should be used as the bearing layer at the pile end.

Excavation and support of foundation pit

Zhuhai Yatong Co., Ltd. plans to build the Lang Yue Hotel Project with four floors underground. The elevation of the basement is -2.7m, the buried depth is about 15.7m, and the safety level of the foundation pit is Grade I. The excavation stratum of the foundation pit is mainly artificial fill, silty clay and gravelly cohesive soil, among which the artificial fill is loose stratum with poor stability. Therefore, in the process of foundation pit excavation, effective support and water stop measures must be taken for the pit wall to ensure the safe and smooth excavation of foundation pit. Combined with the supporting experience of similar projects in Zhuhai, the foundation pit can be supported by pile supporting structure and water stop between piles. According to the on-site hydrogeological conditions, when the foundation pit is excavated, a collecting well and an open ditch are set in the foundation pit, the water is discharged to the outside of the foundation pit by a pump, and the foundation pit is stopped by a high-pressure jet grouting pile. It should be noted that foundation pit excavation and support is a comprehensive geotechnical engineering work, which often involves many factors such as project cost, construction difficulty, construction period, etc., and special geotechnical engineering design and construction scheme evaluation are needed to ensure the safe and smooth progress of the project.

supporting structure

The shape of the foundation pit in the supporting position is rectangular (northeast-southwest), so the supporting structure with four corners and middle support should be selected. This kind of supporting structure can prevent deep foundation pit from deformation due to weak soil, large bending moment of pile body, base fluctuation and so on. The triangular vacancy formed in the northeast can be unearthed, because there are two supports and it is in the urban area of Zhuhai, so it is more difficult. Compared with the pile-anchor supporting scheme, the pile-anchor supporting structure has high cost and long construction period, but it can better control the settlement of surrounding buildings, better safeguard the safety of surrounding buildings, roads and underground pipelines, and will not pollute the site outside the red line.

The axial force analysis of the support adopts the normative method and finite element method, and the comprehensive analysis of plane calculation and longitudinal section calculation, and obtains the best stress mode of the support. Compared with the actual detection and calculation results; The axial force of the four-corner beam is relatively small, which is mainly caused by the interaction of soil at the internal corner of the foundation pit. Even if some axial forces are large, they are caused by the irregular shape of some foundation pits. According to the comparison between the actual test results and the calculated results, the actual additional test results are close to the calculated results of the axial force of the support beam. There is a big difference between the bending moment of pile body and the actual detection, the numerical value calculated by finite element method is small, the result calculated by standard method is conservative, and the actual detection result is even smaller. This calculation result accords with the mechanical characteristics of pile-supported structure in practical application, that is, the weak support characteristics of piles, and the retaining piles have the function of transmitting earth pressure; At the same time, in the support, columns are also an important part; The column is not only affected by the weight of the supporting beam, but also by the mechanical misoperation during foundation pit excavation and the unbalanced force generated during soil excavation, so the safety factor of more than 2.0 should be adopted when redesigning the column. During construction, the calculation results of standard method should be used as much as possible to avoid potential safety hazards caused by the application of small calculation results.

Advantages of pile support Due to the continuous development of urban construction, the existing buildings are more and more dense, and the available construction space is getting smaller and smaller. The supporting structure in the cast-in-place pile is stable, occupies small space, does not exceed the red line of land use, and can better protect the buildings around the construction site. It is of great guiding significance to the process of foundation pit excavation and support in modern urban construction.

Concluding remarks

Compared with other supporting schemes, the pile-supported foundation pit support is stable and reliable. The most prominent advantage is that it does not occupy space outside the red line, and can be widely used in modern cities that pay more attention to environmental protection and reduce underground pollution.

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