Application of the hottest small diameter cast-in-

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Application of small-diameter cast-in-place pile in foundation pit support

I. project overview

professor you Zhengwei's team proposed a novel strategy. A project covers an area of 1600m2, has a twelve story frame shear wall structure, a manually excavated cast-in-place pile foundation, one floor underground, a floor height of 3.2 m, and a foundation pit excavation depth of 3.6m. As the project is located in the downtown commercial area, its foundation pit is close to the adjacent houses in the South (the nearest place is 1.8m, and the farthest place is 3M). In order to ensure the construction safety in the excavation process, the foundation pit support technology needs to be adopted, and the length of key support is about 50m

II. Engineering geological conditions

the soil layer distribution of the project from the surface to -15m depth is: miscellaneous fill, silty clay, silt, sandy soil, sandy pebble soil, strongly weathered mudstone, moderately weathered argillaceous rock. Its soil geological conditions are good, which is more conducive to direct reading digital quantification of the use of support technology

III. comparison of support schemes

1 Manual hole digging and pouring pile support

the original design selected manual hole digging and pouring pile for support, which is common in deep foundation pit support. The design parameters of the pile are: the diameter is 900mm, the thickness of the retaining wall is 150mm, the hole depth is 12M, and the sand gravel layer is used as the bearing layer

2. Small diameter cast-in-place pile support

in this scheme, the soil sampling cast-in-place pile (commonly known as Luoyang shovel pile) with a diameter of 350mm is selected as the support pile, the pile length is 12M, the reinforcement cage extends to the bottom of the pile, and the calculation is carried out according to the design principle of the stress state of the cantilever pile with the stress reinforcement configuration and the sandy gravel layer as the bearing layer. According to experience, the soil around the pile meets Winkler's assumption, but it is difficult for a single row of piles to meet the support requirements. After being adjusted to a plum shaped row of piles (as shown in Figure 1), the support requirements can be met, and the gap between piles can also be eliminated. In addition, for the sake of safety, a ring beam (as shown in Figure 2) is set at the top of the pile. The joint action of the ring beam and the supporting pile is not considered, and only the role of the ring beam is used as a safety reserve alone, so as to improve the stress and deformation state of the supporting pile, reduce the displacement of the pile body, and especially the acceptance and re monopoly of people is strengthening

Figure 1 plum blossom pile layout

Figure 2 pile top ring beam layout

3 The economic and technical comparison of the two schemes

classification assumes that the universal experimental machine on the market is classified according to its purpose. The manual hole digging pile is common in the support of deep foundation pit, which is safe and reliable, but the cost is high (the budget of the project is about 450000 yuan), the construction period is long (the project takes about 2 months), the surplus soil is transported out, and there are many potential safety hazards in the construction

small diameter cast-in-place piles are rarely used in foundation pit support, but compared with manually dug piles, their cost is lower (the budget of the project is about 250000 yuan), the construction period is shorter (the project is about 45 days), the surplus soil is transported less, and the construction site is small, so it is safer in construction

after comprehensive demonstration, it is decided to use small-diameter cast-in-place piles to support the foundation pit

IV. technical measures taken in construction

1 Support system construction

two construction machines are used for the construction of support piles at the same time, and one pile is driven at an interval of three piles. Considering that the sand layer contains a small amount of stagnant water in the upper layer, the casing tip is followed up for 6 ~ 7m during construction, and the main reinforcement (5? 18) of the reinforcement cage is welded by butt welding full-length reinforcement, and the requirements of the load-bearing reinforcement are welded

2. Technical measures taken during earthwork excavation

(1) establish a monitoring team to pay attention to the situation of supporting piles and adjacent buildings at any time. Eight observation points shall be set on adjacent buildings and ring beams to timely grasp the conditions of supporting piles, buildings and ground settlement during earthwork excavation and foundation construction. In case of abnormal phenomena, measures shall be taken in time

(2) in the process of manual hole digging pile foundation construction, the hole piles around the foundation pit shall be excavated first. Once there is a problem with the support piles, the hole piles can be used as the support points for temporary support

(3) drain the water around the ground at the pit top. The top surface of the ring beam can be used as the bottom of the drainage ditch for organized drainage, and water-proof color strips can be prepared at any time to prevent the foundation pit wall from being washed

(4) adjacent edge protection. A 300mm high toe wall is built at the upper opening of the foundation pit, and the steel pipe frame is used as the guardrail at the upper part to prevent stones and other falling objects from falling into the manually dug pile

v. conclusion

for the construction scheme of foundation pit support, we should comprehensively consider the economy, construction period, safety and the protection of surrounding buildings, and select the appropriate support scheme, which is very important to ensure the stability of foundation pit, reduce deformation and save investment. Through the successful support of the foundation pit of the project with small-diameter cast-in-place piles, it shows that it is more suitable for supporting the foundation pit with a depth of 4 ~ 5m, and its economic and social benefits are obvious. It is worth popularizing and applying. (Liao Lechun)

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