All types of construction cannot do without foundation construction, and the quality of foundation engineering has a significant impact on the overall quality. During actual construction, strict control should be exercised over the construction quality of the building foundation, and attention should be paid to the handling methods for the following issues.
1、 Improper control of precipitation
During the construction process of building engineering, it is difficult to handle the dewatering work of the overall rock foundation. Due to the impermeability of rock foundation structures, water infiltration in rock foundations is distributed according to the direction of rock fissures. In the case of extremely small spacing between rock fissures in rock foundations, groundwater cannot be discharged normally. If there are too many stone joints, it is still possible to use the method of replacing and filling the entire foundation. If there are too few rock joints in the rock foundation, causing local water accumulation, using the method of replacing the entire foundation will inevitably result in high costs.
Preparation for precipitation
(1) Identify the engineering geological and hydrogeological conditions of the site.
(2) Identify the distribution of groundwater.
(3) Identify the distribution and types of various pipelines, their ability to withstand differential settlement, and determine whether reinforcement measures need to be taken in advance.
(4) Check the condition of surrounding ground and underground buildings, and determine whether the buildings need to be reinforced before precipitation.
(5) Analyze the changes in land bearing capacity after precipitation and the potential impact on foundation construction.
Necessary measures for foundation pit dewatering
Foundation pit dewatering will inevitably form a dewatering funnel, causing settlement of the surrounding ground. However, as long as the waterproof curtain does not leak and measures such as reasonable use of well points are taken, such impacts can be controlled within the acceptable range of the surrounding environment.
(1) Firstly, conduct a pumping test to predict precipitation and settlement, and control the depth of water level reduction.
(2) According to the geological survey report, arrange well points reasonably and slow down the slope of the precipitation funnel line.
(3) During excavation of foundation pits, it is necessary to avoid ground subsidence around the pit caused by sand flow at the bottom of the pit.
(4) When excavating deep foundation pits in areas with strict requirements for ground subsidence control, the method of dewatering inside the pit should be used as much as possible, and a waterproof curtain should be set up to cut off the inflow of groundwater outside the pit.
2、 Insufficient foundation testing
Insufficient foundation testing can affect the bearing capacity of the foundation, ignore quality hazards, and determine the timing of rework measures, which can have a significant impact on the foundation engineering.
Foundation testing content
(1) Investigate the conditions of foundation landslides, deformation and cracking of special soil, and determine the stability of the foundation.
(2) When testing the foundation of a house, it is necessary to use in-situ testing methods, in-situ soil indoor physical and mechanical property testing methods, or in-situ exploration methods based on the actual conditions of the house and the type of foundation soil, to conduct foundation bearing capacity testing.
(3) If the on-site conditions permit, the bearing capacity of the foundation can be determined through load testing.
(4) Check for diagonal stepped cracks, horizontal cracks, and vertical cracks at the connection between the foundation and the load-bearing brick wall of the prefabricated house; Check for horizontal cracks at the connection between the foundation and the base of the frame column.
(5) For shallow buried foundations, excavation can be used for inspection if necessary.
(6) For deep foundations (or piles), effective documents from the original design, construction, testing, and engineering acceptance can be used. If necessary, small-scale local excavation can be carried out to obtain testing data on the material properties, geometric parameters, and appearance quality of the prefabricated structure.
(7) When uneven settlement of the foundation causes excessive tilting of the building, structural cracks, deformation of doors and windows, damage to decoration and pipelines, obstacles to elevator operation, or when the foundation may continue to settle, uneven settlement monitoring of the building foundation should be carried out. The layout of uneven settlement measurement points, observation operations, and determination of whether the foundation has entered the stable stage can be carried out in accordance with the provisions of JGJ8-2016 “Code for Deformation Measurement of Buildings”.
3、 Ground collapse
1. Reasons for foundation collapse
(1) Reason for investigation: Investigation is the foundation of design. If construction is carried out without investigation, or if the investigation data is incorrect, it will cause serious settlement and damage to the house in the later stage. The above case is the best example.
(2) Design reason: Failure to accurately calculate settlement or design reasonable foundation forms and settlement joints according to the actual structure of the house during the design process can lead to settlement of the foundation.
(3) Construction reason: The foundation is a concealed project. When the construction unit does not pay enough attention to the quality of the project, cuts corners and materials during construction, and does not follow the construction drawings and specifications, it will cause the foundation to sink.
(4) Environmental impact reasons: The phenomenon of engineering construction around houses has become very common. Engineering vibration, soil squeezing pile construction, foundation pit excavation, tunnel shield tunneling, municipal pipeline excavation, and construction of new buildings in the surrounding area may all cause settlement effects on adjacent houses.
2. Handling method
(1) Testing and identification. Timely commission a housing safety appraisal agency to conduct testing and appraisal of the house, clarify the reasons for the sinking of the house and the degree of harm caused by the sinking to the house structure, and recommend corresponding reinforcement measures based on the testing results of the housing safety appraisal agency.
(2) Foundation treatment. The methods for reinforcing the foundation include: concrete pouring pile beam method, steel pipe pile beam method, triple pipe rotary spray pile directional rotary spray method, pile base frame beam support method, etc.
(3) Structural repair. After structural correction and foundation treatment, appropriate measures should be taken to promptly reinforce and repair the damaged parts of the upper structure, restore the normal function of the residence, and achieve the goal of safe bearing. The upper structure treatment generally includes crack and masonry bearing capacity treatment.
How to ensure the quality of foundation construction? Here are three practical engineering experiences worth sharing.
super high-rise building
(1) Before construction, it is necessary to make reasonable arrangements for the transportation of construction vehicles, personnel, and coordination with relevant departments according to the needs of construction technology.
(2) The type of mortar and brick should be reasonably selected based on design standards and engineering characteristics to ensure that the strength of the mortar meets the design requirements.
(3) During construction, it is necessary to correctly set up tie bars and strengthen settlement detection for multi story residential buildings, and unqualified construction materials should be prohibited from entering the construction site.
(4) If the foundation is a soft soil foundation, appropriate locations should be selected to set settlement joints and ensure that they have a certain width.
(5) The clear distance between the foundations of high-rise buildings should be kept at an appropriate level to avoid additional settlement caused by the superposition of base pressures.
Secondary construction project
1. Foundation leveling
Within the scope of repair, drill holes in the intervals of the settled floor or foundation and inject geological polymers through these small holes. The expanding polymer will first fill the gaps and then continue to expand, lifting the settled floor to the desired horizontal position. The entire process is monitored and tracked using precision measuring tools. The accuracy of floor lifting can reach millimeters. The injected geopolymer can cure almost immediately, and after 15 minutes of injecting geopolymer under the floor or concrete slab, the truck can be driven in the repaired area.
2. Foundation reinforcement
Geological polymer grouting technology and foundation reinforcement technology can be used, and the grouting process is monitored by radar to ensure that the soil reaches the required bearing capacity. The geopolymer injected into the soil solidifies within approximately 15 minutes, at which point the required bearing capacity of the structure can be achieved.
3. Stable foundation
The resin injection stabilization method is more efficient, cost-effective, and long-lasting compared to traditional foundation replacement. Select specific filling materials based on different ground types and foundations, and establish multiple monitoring systems for precise control to ensure that the resistance to the floor or concrete slab does not exceed 250 μ m.
4. Structural support
Geopolymer piles can be directly injected into concrete floors and building foundations. Due to the installation not requiring large pile driving equipment or load transfer structures, geopolymer piles are highly suitable for locations that are difficult to access. Suitable for treating subsidence and reinforcing very weak ground, narrow diameter piles with expandable sleeves are inserted into the ground through boreholes as small as 50mm. A highly swollen geopolymer is injected into the casing, forming a pile with a diameter of up to 400mm, which almost immediately hardens.
5. Gap filling
The process of filling voids involves the use of specific geopolymers or a combination of geopolymers and lightweight clay aggregates. Filling is achieved through the use of lightweight materials, which generally do not impose significant loads on the site.
6. Non interference swapping& Settlement treatment
Unlike traditional palletizing techniques, geopolymer methods do not rely on a large amount of hydraulic pressure, as hydraulic pressure may sometimes be slightly out of control. But rather, by lifting a large area in a completely controllable way, the possibility of damaging or breaking the floor is greatly reduced.
Rock formation architecture
1. Process flow
Positioning and laying out → excavation of foundation pit → observation and analysis of water seepage areas → drawing a plan layout of water seepage areas → planning the layout and quantity of collection pits → determining the number, spacing, width, and depth of diversion trenches → excavating collection pits and diversion trenches → backfilling collection pits and diversion trenches with 40-60mm gravel and rubble filter → making and installing water conduit sleeves → laying isolation layers → laying cushion concrete → construction of other foundation structural layers.
2. Key points of operation
(1) Determine the layout of collection pits and diversion channels based on the direction of seepage from the rock foundation.
(2) The collection pit and diversion channel should have sufficient width and depth to control precipitation below the foundation elevation by 500mm.
(3) The water conduit is made of steel pipes with a diameter of not less than 80mm and welded with waterproof steel plates.
(4) The filter pipe can be made of PVC pipe for easy drilling and permeability.
(5) The particle size of backfill stones should not be less than 40mm to prevent poor water permeability and blockage of the periphery of the filter pipe, which may result in inability to drain.
(6) An impermeable isolation layer shall be laid between the upper part of the water channel and collection pit and the cushion layer to ensure that there is no upward permeability.
(7) Cone shaped concrete should be used to fix the base around the water diversion conduit to facilitate the construction of the waterproof layer.
soft soillandLayered building
1. Foundation treatment
(1) Considering the basic conditions, soil conditions, and foundation structure: For cohesive soil, compaction method is generally used to reduce foundation interference; Generally, the method of compacting sand piles is used to improve the performance of sandy soil.
(2) Considering road grade: The road grade is relatively low, and the pavement can be laid after the foundation settles; The filling method for wide and low embankments may cause local damage, but the use of stable loading methods for high embankments is also limited; The larger the height and width of the embankment, the greater the settlement of the clay layer.
2. Leakage treatment
(1) Rebuilding clay cover: This method is suitable for single-layer foundations with good anti-seepage effect but deep impermeable layer in the dam body. The premise is that there is clay near the dam area, and the reservoir needs to be emptied or a cofferdam needs to be constructed. Otherwise, the method of throwing soil or pouring turbid water upstream can only be used, but the effect is poor.
(2) Adding clay cutoff walls or interlocking concrete well columns: This method is suitable for situations where the dam quality is good, the impermeable layer is buried shallow, and the foundation has not been excavated to the impermeable layer, provided that the reservoir needs to be emptied for construction. When the permeable layer is sand, sandy loam or other weak layers, mortar sheet piles can also be used.
(3) Building concrete impermeable walls: This method is suitable for situations where the permeable layer is deep.
(4) Grouting curtain: This method is suitable for sand and gravel or fine sand foundations, and cannot be used for emptying the reservoir during construction. According to the groutability of the basic materials, clay grouting, cement grouting, clay cement grouting, or chemical grouting can be selected separately.
(5) Construction of pressure relief wells or seepage platforms, and drainage ditches: This method is suitable for situations where there is pressurized water in the foundation, the slope gradient behind the dam exceeds the allowable value, or there is marshy phenomenon.
Article source: Architectural Technology Magazine
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