Safety protection of adjacent buildings in the rec

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Safety protection of adjacent buildings in the reconstruction of the old urban area of the city center

in the reconstruction of the old urban area of the city center, due to various reasons, there have been some "half cut" projects. Some "half cut" projects have been shut down and idle after the excavation of the foundation pit. There are many problems in this kind of idle project of foundation pit excavation, especially the problems encountered in the process of suspension and continuation of construction are relatively complex. a. There are folds around the cup that affect the accurate measurement of ear making rate. This article introduces the safety problems and treatment measures encountered with adjacent buildings during the reconstruction of Longfeng building project in Daoli District, Harbin in 2002, 7 years after the excavation of the foundation pit

I. project overview

1. surrounding environment

Longfeng building project is located at the intersection of Xinyang Road and qule street, Daoli District, Harbin city. Two 7-storey residential buildings are located 16m away from the foundation pit in the south, and a 6-storey new hotel is located in the West; The East and north sides are close to the main traffic arteries of the city. (as shown in Figure 1)

Figure 1 location plan of Longfeng building

2. engineering conditions

Longfeng Daxia project covers a total area of 4300m2, with a total construction area of 74000m2, 2 floors underground and 27 floors above the ground. The foundation is CFG pile foundation and the structure is frame shear wall structure. The average design depth of foundation pit is 11m

the foundation pit of Longfeng building was excavated in 1994. At that time, the average depth of foundation pit excavation was 8m. Due to various reasons, the work was stopped immediately after the foundation pit excavation until 2001, a total of 7 years. The work was resumed in March 2002

to the west of Longfeng building is a newly built dynasty hotel with a distance of only 10cm. The foundation of the building is a bored grouting pile. The buried depth of the project pile is 12m and the pile diameter is 600mm. It has 1 floor underground, 4 floors above the ground and 6 floors of local frame structure. As the foundation beam of the building is 1.2m into the foundation of the dragon and Phoenix building, the elevation of the pile foundation top of the hotel is -6.5m on the natural ground, while the elevation of the pile top of the new dragon and Phoenix building project is -11m on the natural ground. Therefore, during the construction, the foundation pile of the hotel is exposed for 4.5m long (that is, the friction force outside the friction pile is weakened) (as shown in Figure 2)

Figure 2 Schematic diagram of foundations of two adjacent buildings

the north side is only 6m away from Harbin landscape Avenue - Xinyang Road. Curtain piles with pile diameter of 600mm, pile spacing of 900mm and burial depth of 14m are set on the foundation pit wall. The drilling grouting pile process is adopted. The original curtain pile is buried 3M below the bottom elevation of the foundation pit, and some of the original concrete is weathered. At the same time, the secondary grouting amount of the grouting pile is insufficient, and some of the main reinforcement of the pile is exposed. In addition, the local pit wall is only 4m away from Xinyang Road (as shown in Figure 3)

schematic diagram of foundation on the north side of the figure

the foundation pit on the south side is 16m away from the two residential buildings. The residential building was completed in 1995. It has one floor underground and seven floors above the ground. It is a pressed pile foundation. The structural form is a brick concrete structure. The side retaining wall is a steel sheet pile with some concrete piles and two anchor bolts. Due to a long time, the groundwater rose twice, some anchor bolts failed, the anchor head fell off, and 11 concrete piles have been dumped (as shown in Figure 4)

Figure 4 Schematic diagram of foundation on the south side

adjacent class II Street on the east side - Gongle street. Curtain pile is concrete pile, and the pile type and diameter are the same as above. Two anchor bolts are relatively stable. However, due to long-term shutdown and two times of groundwater scouring, most of the soil between piles has been washed away, resulting in the separation of pile body and retaining soil, forming a cavity

630kn is arranged in the East and south for construction M、1200kN. M two tower cranes (as shown in Figure 5)

Figure 5 Schematic diagram of the foundation on the east side

II. Geological conditions

Longfeng building is located in Hasi Daoli District. The terrain is low, only 2km away from Songhua River. The groundwater level is high, mainly composed of sand layers. It is a typical flood plain geology. The perennial static water level is -3.6m-7.2m away from the natural ground. The geological profile is shown in Figure 6

Fig. 6 geological profile of Longfeng building

III. dangerous situations during construction

the project began to be fully resumed in March 2002, and the pit wall was relatively stable at the initial stage of resumption. When the foundation pit was constructed to the design elevation of -11m, the danger occurred one after another. By the beginning of June, the danger was quite serious, becoming one of the largest dangers at the construction site in Harbin

1. the dynasty hotel in the West may collapse at any time

when the earthwork of Longfeng building is excavated to the design elevation of -11m, the engineering piles of the building are exposed for 4.5m, and the sand between the piles flows, as shown in Figure 7. From May to June, there is heavy rain, the free surface of some engineering piles gradually increases, the friction force also weakens, and the foundation bearing capacity of the building drops seriously. Therefore, the foundation piles will be in danger of displacement, fracture and dumping at any time. At the same time, due to the recent opening of the hotel, the production is very prosperous. 600 people eat at the same time during the business hours every day. Once there is a problem, the consequences will be very serious. Therefore, the situation faced by the building becomes the first insurance in the construction (as shown in Figure 7)

Figure 7 Schematic diagram of West foundation

2. tower cranes on both sides are in danger of overall toppling

the foundation of 630 tower cranes on the south side is a "cross" beam, under which 5 bored grouting piles are set. Three of them have been exposed due to perennial rain erosion. The curtain pile outside the foundation has been displaced, and the tower crane is in danger of overall collapse at any time (as shown in Figure 8)

figure 8 schematic diagram of tower crane foundation

3. the third largest danger in the north and east sides

due to the perennial precipitation of the project, the foundation construction is relatively deep after reaching the design elevation (natural view -11m), the curtain pile loses a lot of soil between the piles under the scouring of rainwater, and most of the pile bodies have displacement angles. There are many collapses in the ground within 3M around the foundation pit, and the temporary facilities and mechanical equipment are in danger. Cracks have appeared in Xinyang Road in the north, and some sidewalks have collapsed. What is more serious is that the design of the original concrete curtain pile is unreasonable. The buried depth below the foundation pit bottom is only 3.0m, and the diameter of its tensile reinforcement is too small. Under the influence of earth pressure and vehicle vibration load, the curtain pile wall may collapse into the foundation pit at any time, becoming the third dangerous situation in the construction at that time (see Figure 9)

Figure 9 schematic diagram of foundation on the north and east sides

when the Longfeng building was constructed to the design elevation of the foundation, the Dynasty Hotel, tower crane, Xinyang Road and residential buildings were in danger one after another. Therefore, the first major event in the foundation construction is to protect the safety of adjacent buildings, protect the landscape Avenue from damage, and avoid major safety accidents caused by crane dumping

IV. design and construction of risk elimination scheme

1. eliminating the danger of dynasty hotel is the first problem to be solved in the foundation construction. However, all the foundation piles of the gable on the east side of the hotel have been exposed, and the main line near Xinyang Road has been disturbed by vibration load. Dynasty Hotel is in an extremely unstable state. If it is applied to other buildings, its bearing capacity may be seriously reduced, and the external force may worsen. Therefore, the determination of the risk elimination plan of dynasty hotel is greatly limited. According to the geological conditions and through the comparison of multiple schemes, the risk elimination scheme is determined as follows:

a). A 9m Deep Grouting retaining wall is set outside the foundation pile along the full length of the foundation pit. There are three purposes for setting the retaining wall: first, to stabilize the engineering pile of the dynasty hotel in an extremely weak state, to ensure that the 7.5m pile body at -11m will not have new displacement, and to strengthen the foundation to ensure the safety of the upper building. Second, after the partition wall is set, the disturbance caused by the foundation construction of the outer Longfeng building can be reduced. Thirdly, considering that the Longfeng building has 29 floors (including 2 underground floors), its settlement will be greater than that of the dynasty hotel which has been used for several years. In the process of settlement, the foundation on the east side of the dynasty hotel was disturbed due to the settlement of the foundation of the dragon and Phoenix building, resulting in uneven settlement of the dynasty hotel as a whole. After setting the retaining wall, the foundation of the two buildings with large difference in the upper load of the foundation constructed in different years was separated. It is a very necessary protective measure for the dynasty hotel with fewer floors and lower upper load that has been delivered for 6 years (see Figure 10)

b). Two plain concrete piles with a diameter of 600mm, a spacing of 600mm and a depth of 9m are set outside the foundation. C is used to obtain accurate and convincing data. FG pile plain concrete rotary spraying process. The construction process of CFG pile is as follows: drill the drill pipe to the bottom of the pile body, pump the fine aggregate concrete to the bottom of the pile tip through the drill pipe with a high-pressure pump, slowly lift the drill pipe with the concrete pumped, until the drill pipe is pulled out of the pile top elevation, and stop pumping concrete. The first is to reduce the vibration of adjacent buildings with CFG piles. Second, the pile body will not collapse when drilling. Therefore, the use of CFG Piles for reinforcement can reduce the interference and damage to the foundation of dynasty hotel. Because an underground cement slurry retaining wall has been set outside the foundation pile of dynasty hotel. In this way, two strong reinforcement and protection facilities are formed on the side of the engineering pile of the Dynasty Hotel (as shown in Figure 10)

Figure 10. Reinforcement and protection drawing of dynasty hotel project pile

c). Set bearing platform on CFG pile, and pour a shear wall on the bearing platform (this shear wall is the structural outer wall of the foundation basement of Longfeng building) (as shown in Figure 11)

d). When the foundation of the Longfeng building reaches the top of -2 floor, the gap between the shear wall and the foundation of the dynasty hotel is backfilled with medium sand layered water to restore the friction of the foundation pile of the Dynasty Hotel (as shown in Figure 11)

Figure 11 schematic diagram of the foundation of Longfeng mansion

four measures including plain mortar retaining wall, CFG pile, shear wall reinforcement and water shaking medium sand backfilling are adopted in the risk elimination scheme of dynasty hotel. The practice has proved that they have played a vital role in enhancing the stability of the hotel foundation pile and preventing the displacement, fracture and dumping of the foundation pile. However, the setting of plain slurry retaining wall and the selection of CFG pile have not caused new interference and damage to the hotel foundation pile under the condition that the bearing capacity of the hotel foundation pile is already very weak, which are two extremely prudent and strict protective measures

see table

Figure 12 tower crane plan

a). Unloading: unload the ground load within 3M around the tower foundation to reduce the pressure of the tower crane supporting pile, and dig a drainage ditch around the foundation to timely remove the surface water and ponding, so as to avoid the impact of rainwater leakage on the pile foundation and the collapse of the soil around the pile foundation

b). Reinforcement: re reinforce the original anchor bolt and re weld the anchor head. At the same time, adopt the same method as the foundation pile of dynasty hotel to reinforce the curtain pile and support pile

c). Monitoring: because the site is narrow and dangerous, the tower crane cannot be removed. Therefore, during the risk elimination process, the tower crane shall be subject to settlement and vertical observation by designated personnel. In case of any abnormality, necessary emergency measures shall be taken immediately to ensure the safety of pedestrians and adjacent buildings

d). Most of the basic functions of the controlled universal testing machine refer to the various basic functional characteristics of the instrument and equipment and their use: in a period of time after the danger is eliminated, the crane height is limited, and the hoisting weight and use time are controlled to reduce the pressure of the supporting pile. When the strength of CFG pile and shear wall reaches 100%, the full load operation can be implemented

3. scheme for protecting Xinyang Road and Gongle street

due to the poor concrete quality and insufficient burial depth of curtain piles on the north and east sides, there is a danger of collapse at any time, affecting the safety of Xinyang Road, Gongle street and underground design. Therefore, the following measures are taken:

a) gradually develop the enterprises with Yucheng industrial chain layout, remove the obstacles and temporary facilities on the side near Xinyang Road, and remove 250m2 of workers' dormitory to reduce the pressure in the sliding plane

b). When the toilet is stopped, the load will be reduced and the internal water will not seep into the pit

c). Build an earth embankment on the pit wall, 300cm high and 500cm wide, to prevent rainwater from flowing into the pit

Copyright © 2011 JIN SHI