Keywords: civil engineering; foundation construction technology; technical points; innovation

 

Introduction:

    Generally, in civil engineering construction, common foundation construction content includes site leveling, excavation of foundation pits, roadbeds and pipe trenches, excavation for civil air defense projects, filling and placing of floors and roadbeds, and backfilling of foundation pits, as well as auxiliary works such as dewatering, drainage, and earth wall support. Civil engineering construction is usually known for its large engineering volume, relatively heavy labor, and complex conditions. Moreover, during construction, it is seriously affected by factors such as climate, hydrological conditions, geology, underground obstacles, and site restrictions, which in turn increase construction difficulty. Therefore, before civil engineering construction, it is necessary to analyze in detail various technical materials (such as topographic maps, engineering geological and hydrogeological survey data, conditions of underground pipelines, cables, and underground and aboveground structures, and earthwork construction drawings), repeatedly verify data, and conduct on-site investigations. During the construction stage, it is also necessary to formulate scientifically and technically sound, economical, and reasonable construction plans according to actual construction conditions, so as to ensure project quality and improve efficiency. In continuous civil engineering construction practice, people have also discovered problems with traditional foundation construction technology, and therefore in recent years many new technologies have emerged in civil engineering construction that are worth studying and applying.

I. Requirements of Civil Engineering Foundation Construction Technology

The foundation construction process in civil engineering mainly includes: excavation, support, soil wall stability, drainage, filling and compaction, and foundation concrete pouring.

Earthwork excavation of foundation pits should follow the principle of “layered excavation, support first and then excavate.” First, the excavation sequence and layered thickness should be reasonably determined to prevent disturbance to the foundation soil, and only then can continuous construction proceed. During excavation, inspection and control work is essential. When excavating soil below the groundwater level, scientific dewatering measures should be taken. In the process of filling and compaction, when selecting soil materials and stones, attention should be paid to the fact that under the same conditions of crushed stone or sandy soil and moisture content, building coatings can be selected as cohesive soil with the same compaction requirements, and attention must be paid to ensuring compaction. Filling should proceed from high to low, and the entire width should be compacted layer by layer from top to bottom. Construction should be compacted on the same day, and it must meet the specified compaction requirements.

II. Key Technical Points of Civil Engineering Foundation Construction

1. Excavation technology for soil above foundation pits and trenches

The soil quality of the construction site, building excavation depth, dewatering and drainage conditions, and even climatic conditions all affect earth slopes. Under normal drawing conditions, and when the excavation depth of shallow foundation pits does not exceed the specified limit, slopes may be omitted or support may be added. However, when excavating deep foundation pits, temporary retaining is usually used to ensure stability, or precast panels, double-row cast-in-place piles, etc. are used to retain soil, so as to ensure the stability of permeable retaining support, and cantilever or anchored retaining structures are used to support water-stop retaining structures. Excavation should proceed from top to bottom, and it is strictly prohibited to cut off the toe of the slope first or to dump soil on the landslide body.

There are also certain requirements and techniques for using machinery during excavation. Bulldozers are suitable for site clearing and leveling, excavating foundation pits with a depth within 1.5 m, filling ditches and pits, and cooperating with scrapers and excavators. Commonly used methods include downhill bulldozing, side-by-side bulldozing, trench-shaped bulldozing, multi-blade gathering and transport, and bulldozing with side plates attached to the blade. Scrapers are suitable for large-area site leveling and excavating large foundation pits, filling embankments, and roadbeds. They are most suitable for excavating loose soil and ordinary soil with a moisture content not exceeding 27%. Circular routes and “8”-shaped routes are used. Common construction methods include downhill scraping, cross-scraping, and assisted scraping.

2. Key technical points for soil wall stability

    After excavation, the stability of the foundation pit soil wall is very important for subsequent construction. The key point of foundation pit soil wall stability lies in balancing the cohesive force and friction within the soil mass. Once the balance is seriously lost, it will lead to foundation pit collapse and cause serious accidents. Therefore, stabilizing the soil wall requires ensuring sufficient slope release and setting slopes as required. Soil wall support methods include: cross bracing, sheet piles, cast-in-place piles, deep mixing piles, underground diaphragm walls, etc. The slope gradient design should also be based on the actual conditions of the construction site and determined by referring to multiple factors such as local soil quality and construction conditions. For example, slopes of cohesive soil should be steeper; when using open ditch drainage, manual excavation, or mechanical excavation above the pit, the slope should be gentler; when there are major buildings near the foundation pit, the slope should be h1.0-h1.5; when the construction period is short and there is no groundwater, if a vertical trench is left without sloping, the excavation depth must not exceed the following values: 1 m for dense and medium-dense sand and gravel soil (with sand as fill); 1.25 m for hard-plastic and plastic light loam and loam; 1.5 m for hard cohesive and plastic clay and gravel soil (with cohesive soil as fill); 2 m for hard clay. To reduce earthwork volume and reduce the construction surface, support should be installed at the foundation pit. In bad weather, such as during the rainy season, excavation should not be carried out in landslide areas. Once danger occurs, excavation must be stopped immediately and countermeasures should be taken to handle it properly.

3. Key technical points for drainage

In civil engineering construction, keeping the soil mass dry is most critical. Generally, construction drainage can be divided into two types: open drainage and artificial lowering of groundwater level. Open drainage uses interception, diversion, and pumping methods for drainage. It intercepts water flow, drains accumulated water, and during foundation pit excavation, drainage ditches and sumps are set at the bottom of the pit so that water flows through the drainage ditches into the sumps and can then be pumped away. The artificial lowering of groundwater level method involves burying filter pipes around the foundation pit before excavation, pumping water from them, and always keeping the groundwater level below the bottom of the pit until foundation construction is completed. Drainage not only improves construction conditions but also keeps the foundation pit soil mass dry and prevents accidents. It should be noted that before dewatering, it is necessary to consider whether dewatering may affect the original buildings by causing additional settlement or displacement, thereby leading to cracking, tilting, or even collapse. Therefore, it is very necessary to take effective protective measures in advance.

4. Key technical points for filling and compaction

To ensure that foundation construction can meet the requirements of fill strength and water stability, a scientific and reasonable fill slope must be designed, together with the correct soil filling method. Generally, when filling, do not use soil with high organic matter content, soil with gypsum or water-soluble sulfate content greater than 2%, or peat, clay, or powdery sandy clay in a frozen or liquefied state. It is best to use the same type of soil for filling, and the fill should be spread and compacted in layers. If different soils are used for filling, soil with greater permeability should be placed below the soil layer with lower permeability. It is strictly prohibited to mix different soils unevenly, so as to avoid forming water pockets within the fill.

5. Key technical points for concrete pouring construction

    After excavation construction, foundation concrete pouring will be carried out. The pouring method mainly involves centralized mixing and transportation by concrete delivery pumps or concrete trucks. The steel bars used in processing reinforcement cages need to be tested multiple times in the laboratory in advance. After the test results are credible, appearance and quality inspection and evaluation must also be carried out on the construction site. For low-temperature welding of steel bars, operations should be carried out indoors, and the bars should be taken out only after completely cooling. After welding is completed, centralized cutting should be carried out, and overall hoisting and transportation should be done by crane. The pouring process should start within one hour after hole cleaning, using the tremie method, and the concrete should be capped and poured using the ball-shearing method.

III. Innovation in Civil Engineering Foundation Construction Technology

Traditional civil engineering foundation construction technology runs through the entire construction process, including foundation construction technology, concrete structure construction technology, and steel structure construction technology. However, in response to the higher requirements put forward by modern buildings, new construction technologies are of great significance in civil engineering foundation construction. Many new foundation construction technologies, such as new deep foundation pit retaining technologies, have gradually developed, and considerable progress has been made in the establishment of pile and pile-anchor retaining systems and the integration of retaining and load-bearing structures. For example, there is also new prestressing technology, which has formed bonded prestressing systems and unbonded prestressing systems, exerting a positive impact on project economic benefits. In the future, the economic and technical rationality of foundation construction technology will be further improved.

Conclusion:

Civil engineering projects have a large engineering volume, and foundation construction must be completed before the construction of other divisional works. Mastering the key technical points of foundation construction is crucial and is an important prerequisite for reducing project costs and ensuring project quality and schedule. Although there are still some problems in the foundation construction process, such as research theories not yet fully applicable to the needs of civil engineering practice, lack of certain building acceptance standards, and loopholes in management systems and relevant specifications, in the face of these problems, relevant technical personnel must continue to strive to develop foundation construction technology, continuously research and develop new technologies and apply them to actual civil engineering construction, extensively summarize past experience and lessons, and draw new inspiration from them for flexible application. It is believed that with the development of the engineering construction industry, foundation construction technology will also continue to improve and mature, and in the process of continuous development will bring more innovation and tremendous progress to civil engineering foundation construction technology.