Application of the most rigid micro pile in the co

2022-08-06
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Application of rigid micro piles in underground cavern collapse accident

the total building area of No. 41 residential building of Jinan Iron and Steel Group Corporation is 5600m2, which is divided into five units. One expansion joint is set at the and axis. The building plan is shown in Figure 1. The main body is a 6-story brick concrete structure with reinforced concrete strip foundation. The buried depth of the foundation is -2.25m. It was completed and put into use in 1996. Figure 1 building plan and location of air raid shelter in August, 1998, it was found that there were many inclined cracks under the windowsill of the outer longitudinal wall of the building, the units 1 and 2 on the east side of the building were obviously inclined to the north, the building was obviously staggered at the expansion joint, the cornice was offset by 70mm, and the heating and water pipes on the top floor were broken

1 building detection results

in order to find out the causes of building inclination, master the development speed of building settlement and inclination, and provide reliable data for building reinforcement, the following aspects of building detection have been carried out. 1.1 supplementary geological survey

in the high stress area of common components outside the building, 32 boreholes can be specially strengthened, and it is found that there is an abandoned air raid shelter under units 1 and 2 on the east side of the building (see Figure 1). The outdoor tunnel top is about 7.5m from the ground, the tunnel height is generally 1.7m, and the tunnel width is about 2.0m. As the air raid shelter has not been provided with any protective treatment, its top has collapsed, and a place on the north side outside the building has collapsed to the ground

the soil layer revealed in this survey is divided into five layers, from top to bottom: ① miscellaneous fill, grayish brown, plastic, wet, with a thickness of 1.4 ~ 1.8m; ② Loess like silty clay, yellowish brown, plastic, wet ~ saturated, layer thickness of 4.9 ~ 5.3m; ③ Silty clay, light maroon, plastic ~ hard plastic, wet, layer thickness 6.1 ~ 7.8m; ④ Gravelly soil, grayish yellow, wet, moderately dense ~ dense. 1.2 settlement observation

according to the field observation from August 12 to September 4, the maximum settlement value of units 1 and 2 on the east side of the building during this period is 28mm, the minimum is 15mm, and the maximum settlement speed of the building is 4m, so the blood transfusion tube can be made m/d. Due to the rainy season, a large amount of surface water infiltrated into the air raid shelter through cracks, resulting in the collapse of the upper soil mass of the air raid shelter, and the rapid development of building settlement and inclination. 1.3 inclination observation

according to the on-site observation on September 4, the maximum inclination value of the main structure of the building is located at the axis of the North longitudinal wall (east side of the expansion joint), with a northward inclination of 75mm, which is converted to a maximum inclination of 0.41%, which has exceeded the allowable foundation deformation value of 0.4% specified in article 5.2.4 of the code for design of building foundations (gbj7-89). However, it did not exceed the dangerous building limit of 0.7% specified in article 2.5.1.1 of the dangerous building appraisal standard (cj13-86). 1.4 building cracks

the cracks in the longitudinal wall of the building generally appear in the corner of the windowsill on the first floor, the diagonal cracks appear in the horizontal wall on the top floor, and the floor slab joints show signs of opening. As the overall rigidity of the building is good, the width of these cracks is small, generally 0.2 ~ 0.3mm, and the maximum is not more than 1mm

2 treatment scheme

2 the one with notch faces the edge of pendulum 1 design idea

as the inclination of the building is close to the limit value of dangerous buildings, and the settlement and inclination of the building are developing rapidly, it is necessary to first prevent the collapse damage of the building due to the sudden collapse of the air raid shelter, and at the same time prevent the building from becoming dangerous buildings due to further inclination. Since the current inclination of the building does not exceed the above dangerous house limit of 0.7%, and has not caused a great impact on the psychological and daily life of the residents, considering the impact of the construction on the lives of the residents of the three units in the west side, and considering all factors, it is decided not to rectify the deviation. 2.2 treatment scheme

(1) seal the air raid shelter port with fine aggregate concrete at both ends of the building. The specific method is to drill 9 holes in the air raid shelter outside the building, with a hole diameter of 250mm and a hole spacing of 750mm. The hole bottom is required to reach the undisturbed soil layer under the air raid shelter. Then, C15 fine aggregate concrete with a slump of 120mm to further improve the heat resistance of PPC is poured into the hole until it is filled, and the air raid shelter is blocked at the entrance and exit of the building

(2) rigid micro pile underpinning the collapse in the air raid shelter caused by rain, and the soil is relatively soft. In order to prevent necking or pile breaking in this part, it is decided to use rigid micro piles. The micro piles are equipped with I10, and the pile body is formed by pressure grouting cement mortar (the grouting pressure is 0.4MPa)

(3) pressure grouting due to the coarse cement mortar particles and small pressure poured into the micro pile, the air raid shelter and its upper collapsed soil mass cannot be effectively reinforced, so as to prevent this part of soil mass from generating negative friction resistance to the micro pile. Therefore, after the construction of the micro pile is completed, the air raid shelter and its upper loose soil can be reinforced by the method of pressure injection of mud. The specific method is to drill grouting holes at the proved air raid shelter with a spacing of 1.0m, and the hole depth reaches the bottom of the air raid shelter. After the hole is completed, put the grouting pipe into the hole, and inject pure water slurry under pressure. The grouting pressure is 1.5 ~ 2MPa

3 micro pile reinforcement design and construction

3.1 micro pile reinforcement design

considering that the soil above the air raid shelter has partially collapsed, the lateral friction of the soil above the air raid shelter and the bearing capacity of the soil between the piles at the air raid shelter are not considered in the micro pile design. The design pile diameter is 220mm, and the micro pile is 4.5 ~ 5m deep under the air raid shelter. In order to reduce the settlement, the pile end is required to enter the gravel soil for 0.5m

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