Since the reform and opening up, the construction industry has developed rapidly and the quality of construction projects has improved year by year. However, engineering quality accidents still occur from time to time, causing many undue losses. This is because the engineering geological conditions in different areas are different, and the soil layer distribution and physical and mechanical properties are different. Even in the same construction site, the soil quality is often uneven. In the process of engineering investigation, engineering design and construction, if the foundation conditions are not fully grasped and handled improperly, accidents may occur, causing undue losses to the country and people. Therefore, accurate analysis of accidents and reasonable handling of accidents are major problems to be solved urgently in the current construction industry. At present, there are many ways to strengthen existing buildings, but the following two are commonly used:
(1) Grouting reinforcement: drill a hole in the foundation to the soil layer to be reinforced, and then inject the prepared cement chemical slurry into the stratum with high-pressure grouting equipment, so that the soil layer reacts with the slurry physically and chemically, and then bond through fracturing and extrusion, so as to improve the structure and performance of the soil and improve the strength of the soil. (2) Static pressure pile reinforcement: using the gravity of the bearing column of the building as the reaction force, a set of hydraulic (oil) pressure equipment is used to press the precast pile into the soil, and the upper and lower piles are welded with embedded angle iron. Pile pressing is controlled by hydraulic pressure. When the pressure reaches the design load and basically meets the requirements of planned pile length, the piling shall be terminated. The bearing capacity of single pile at the end of pile can be directly reflected from the instruments of pile pressing equipment. After the pile is finally completed, the pile head reinforcement pressed into the pile is welded with the original foundation reinforcement, and the concrete bearing platform is connected with the foundation as a whole, so that the load of the superstructure is directly transferred to the hard soil layer through the pile.
(2) Whether grouting reinforcement or static pile reinforcement is adopted, the uneven settlement of existing buildings will cause certain losses to the project, which is beyond doubt. What should be considered at this time is how to minimize the loss.
1 project example 1. 1 project overview A factory building in Heshan is a four-story building with frame structure, with a length of 55.0m and a width of 15.0m, covering an area of 825m2. The original design used manual digging pile foundation. Due to the complex engineering geological conditions of the site, the middle section of the workshop is 3 ~ 6m and the east section is 7 ~ 6m. The scheme of digging piles in some sections of the building area could not be implemented, so the scheme of sand cushion under strip foundation was adopted (the thickness of cushion was 1.00m), while the silt layer in the west section was thinner (the thickness was 1 ~ 2 m), but 16 piles were poured, so two completely different foundation types were adopted in the stress and deformation of the same building. The workshop was completed in June, 65438 +09921October +65438, and uneven settlement was found during the use, which led to serious tensile cracking and shear failure of the workshop structure, endangering the safe use. 1.2 engineering geological survey of the site According to the engineering geological survey of the workshop site, its stratum structure is in the exposed depth.
2 from top to bottom: (1) Fill: light yellow, soft soil, with a layer thickness of 0.79 ~ 2.00m. (2) Silty clay: gray ~ grayish yellow, with 0.40 ~ 0.60 m cultivated soil on the upper part, soft plastic, N=2 blows, FK = 80 ~ 90 kPa. (3) Mud (peat soil): dark brown, partially mixed with mud, flowing plastic, n = 1 ~ 2 blows, FK = 40 ~ 60 kPa, layer thickness 0.00 ~ 7.87 m, layer depth 1.00 ~ 3.75 m..(4) Granite. The soil is sandy cohesive soil, gray and light yellow, containing medium-coarse gravel, plastic, n = 5 ~ 13 blow, FK = 100 ~ 230 kPa, layer thickness 1.70 ~ 7.30 m, and layer depth 2./kloc-0. (5) Weathered residual soil of granite; The soil is sandy cohesive soil, light yellow brown, containing medium-coarse sand, plastic ~ hard plastic, n = 13 ~ 24 blows, FK = 230 ~ 320 kPa, with a layer thickness of 2.90 ~ 9.00 m and a layer depth of 4.00 ~12.26m. (6). ..
Cause analysis of 1.3 settlement
(1) Geological factors: According to the drilling, the workshop was built at the foot of gully on hillside, with abundant groundwater, the buried depth of water level is about 0.30m, the distribution of soft soil such as silt (including deadwood) is extremely uneven, and the layer thickness increases from west to east, which is an objective factor causing uneven settlement of the workshop.
(2) Structural factors: two different foundation types are used in the same factory building. The western section with good geological conditions adopts manual digging pile foundation, and the pile tip is supported in hard residual soil layer. The eastern section with poor geological conditions adopts sand cushion under strip foundation, and the thickness of cushion is only 1.0m, which has not been compacted. According to the calculation, the final settlement of the pile foundation is only 1.4mm (the pile length is 10m, which enters the hard soil layer), but the foundation treated with sand cushion under the strip foundation still has a settlement of 200mm, and the stiffness of the strip foundation cannot adjust the deformation (settlement), which leads to uneven settlement of the factory building and affects the safe use of the factory building, so it must be treated as soon as possible.
In order to ensure the safe use of the factory building, the reinforcement of the superstructure and the treatment of the foundation should be combined in the treatment of the building. First, treat the foundation to control the continuous settlement of the foundation, and then strengthen the superstructure. According to the data of drilling and deformation measurement, pressure grouting and static pressure pile can be used to strengthen the foundation of the factory building. However, according to the requirements of the owner, the pressure grouting method is adopted on the principle of not affecting the production and being affected by the engineering geological conditions of the site.
The purpose of pressure grouting reinforcement is to improve the bearing capacity and compression (deformation) modulus of soft soil foundation through the seepage pressure of slurry, and gradually reduce the settlement of foundation to meet the use requirements of factory buildings.
2. 1.65438+ on the other hand, it causes compression, dehydration and consolidation of soft soil, thus achieving the purpose of reinforcement.
2. 1.2 process flow pulping → pore-forming and pipe laying → hydraulic grouting → splitting → extrusion filling → consolidation.
2. 1.3 technical measures Grouting hole arrangement: four holes are arranged around each column position, with the hole spacing of 1.5 ~ 1.9m and the reinforcement depth of 4.0 ~10.5m. The bottom of the deep hole enters the residual soil layer. Grouting adopts low pressure, slow irrigation and multi-volume technology to inject more slurry. Repeated grouting: In order to enhance the grouting effect of thick holes in soft soil, an irregular curtain is often formed in a certain range of slurry pressure during the first grouting, and then the grouting holes are flushed for secondary grouting, so that the slurry can be fully and effectively filled. First down and then up, or first up and then down, layered grouting: first down and then up means that the slurry pipe is lowered to the bottom of the hole after the hole is formed, grouting is started from the bottom of the hole, and then the grouting pipe is pulled up while grouting, so that the slurry spreads from the bottom of the hole, which is beneficial to strengthening soft soil; Grouting starts from the depth of the foundation slab and then gradually extends to the bottom of the hole, so that the sand cushion can be effectively strengthened first.
2. 1.4 settlement analysis of pre-grouting construction effect: It is understood that the workshop was completed in June of 1992 10. During the construction period, the maximum self-inspection settlement of the construction unit was 80.00mm, and only intermittent observation was made after delivery: June1February 2, 9931June 65438. It is also observed that the distance between the top (ceiling) of expansion joints has been obviously widened, and cracks have appeared in the beams, plates and walls of some columns. Settlement during grouting construction: According to observation (* * * 44d), the settlement of No.24 column foundation is 40.08mm, No.25 column foundation is 34.77mm, and No.26 and No.26 column foundations are 30.53mm During this period, due to grouting construction, the foundation stress is released and the settlement is increased. Settlement at the later stage of grouting: According to observation (lasting 14 months), the settlement of the foundation of No.24 column is 28.93mm (2.07 mm per day), that of No.25 column is 39.22mm (2.80 mm per month) and that of No.26 column is 45.64mm (3.26mm per month). The above data show that after chemical grouting treatment, the column foundation
2.2 Static pressure pile reinforcement method Based on the analysis of the above reasons, the reinforcement scheme adopted this time must be effective and in one step, and rework is not allowed. Finally, "static pile underpinning" is selected for reinforcement.
2.2. 1 mechanism Through a set of hydraulic equipment, the precast pile is pressed into the soil in sections (2.0~3.0m for each section) and the embedded angle iron is welded in sections. The precast pile is a 250mm×250mm square pile, and the estimated pile length is 15m. The piling is controlled by hydraulic pressure, and the piling load is controlled between 45 ~ 45~55MPa. The settlement control standard is to keep the pile pressing load at 50MPa, impact once every 5min, * * * impact three times, the settlement is ≤3mm, and it is stable for 65438+0h, and the final allowable settlement is ≤ 65438+0mm. After pile pressing, the pile is connected with the original foundation cap, and the load of the superstructure is transferred to the hard residual soil through the pile foundation.
2.2.2 design technical parameters: maximum pressure 1000kN, single pile bearing capacity of 550～900 kn, precast square pile size of 250mm×250mm, concrete strength C30, pile pressing speed of 2cm/s, allowable deviation of pile pressing position and stress axis of 100mm, and inclination of pile axis at pile connection.
2.2.3 During the construction, the pile foundation pit is excavated at the designed pile position, and the reaction force is provided by the reaction steel clip installed on the column. After the static pressure pile of each pile cap is pressed, the precast pile is connected with the original pile cap, and the structural load is borne by the precast pile. During the construction, the level was used to track the construction observation, and the lifting amount of the control column was 65438 0 mm. During the construction, it was found that the surface water was rich, and it was necessary to pump water with a pump before underground operation. However, in the process of pumping, it was found that the settlement speed of the column was accelerated (0.50mm/h) through liquid level monitoring, and pumping was stopped immediately. After research, it is required to unload the column before pumping water to avoid additional settlement of the pile foundation and ensure the safety of the factory building. That is, the upper part of the column is unloaded and transferred to the soil around the column (with a radius of about 3.0m) by the unloading device, and the upper load is shared by * * * *. After the column was treated for 3 days, that is, after the concrete was basically solidified, the unloading device was removed, and no obvious settlement change was found in the unloaded column within 6 ~ 8 days, indicating that the unloading treatment effect was remarkable.
2.2.4 The monitoring of the construction effect and the reinforcement effect are directly related to the safe operation of the factory building. In order to test the reinforcement effect, the main measure is settlement observation, and the result is also the main technical basis for the completion acceptance of this project. The quality index of the settlement value of this reinforcement treatment is: under normal circumstances, the cumulative settlement value of the reinforcement column during construction is ≤ 10mm (referring to the start of construction to the end of construction), and the settlement after completion should be ≤2mm/ month. Anyone who meets this quality index can be judged to have achieved the strengthening effect. According to the observation data after construction, the first settlement observation (33d) after completion, the maximum settlement is-1.7mm, which meets the quality index that the settlement value after reinforcement is less than 2mm/ month, and achieves the expected effect of reinforcement with good quality.
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