Keywords: building foundation; treatment technology

 

 

1 Project Overview

 

 

 

1.1 A dormitory is located near the ring road of the Xinzhong Road overpass in a certain city. The total designed building area is 4600 m2, with a 7-story frame structure, an elevated floor below with a story height of 2.1 m; the upper floors all have a story height of 3.1 m. It was completed and built in earlier years. The total project investment was more than 6 million yuan. During construction, the schedule was tight, the climate was cold, and the foundation was poor. Most of the soil was silt, so the foundation was to be treated before construction.

 

1.2 Geological conditions: From top to bottom, the buildings on the site are ① plain fill 0.90~3.00 m thick, ② silt 5~8 m thick, ③ silty clay 4.8~9 m thick, ④ muddy medium-coarse sand 0.7~4 m thick, ⑤ sandy clay 0.5~3 m thick.

 

 

 

2 Proposal for Foundation Treatment

 

 

 

The foundation is the connecting body between the building and the ground, transferring the loads from the building's vertical system to the ground. If the bearing capacity of the ground is sufficient, the foundation distribution can be the same as that of the vertical structure; however, due to soil or load reasons, a full-slab raft foundation may be required. A raft foundation has the advantage of a large contact area with the ground, but compared with isolated foundations, its cost is higher. The concept of a foundation is to distribute concentrated loads to the ground so that the loads do not exceed the long-term bearing capacity of the ground. If the building has several stories and the foundation soil is good clay or silty clay with a low groundwater level, manually excavated cast-in-place piles can be used for support and sliding resistance. If the ground is very soft and the building is very tall, a raft foundation is required. The vertical structural walls and columns of most buildings can be supported separately on the soil foundation by their respective foundations.

 

If the bearing capacity of the ground is insufficient and it is soft soil, measures must be taken to treat the soft foundation. Soft foundation refers to foundations composed of muddy soil, hydraulic fill, miscellaneous fill, or other materials. During investigation, the uniformity, composition, distribution range, and soil properties of the soft soil layer should be identified, and corresponding parameters should be provided for the selected foundation treatment scheme. In preliminary calculations, it is best to calculate the approximate weight of the building structure, assume it is uniformly distributed over the entire area, and thus obtain the average load, which can be compared with the bearing capacity of the ground itself. If the allowable bearing capacity of the ground is greater than 4 times the average load, isolated foundations may be more economical than raft foundations. If the allowable bearing capacity of the ground is less than 2 times the average load, then it is more economical than building a raft foundation covering the entire area. If it is between the two, pile foundations or caisson foundations should be used.

 

 

 

3 Foundation Treatment Scheme

 

 

 

When the foundation soil is silt and the upper soil layer is thin, measures should be taken to avoid disturbance of the silt and muddy soil during construction. For hydraulic fill and construction waste, when uniformity and density are good, they can be used as a bearing layer. For miscellaneous fill such as domestic waste with high organic content and industrial waste that is corrosive to the foundation, it cannot be used as a bearing layer without treatment. When selecting a foundation treatment method, factors such as engineering geological and hydrogeological conditions, building requirements for the foundation, building structure type and foundation type, surrounding environmental conditions, material supply, and construction conditions should be comprehensively considered, and the best option should be selected after comparative analysis of technical and economic indicators.

 

During foundation treatment, effective measures must be taken to strengthen the stiffness and strength of the superstructure to increase the building's adaptability to uneven foundation deformation. For the selected foundation treatment method, necessary testing should be carried out, and relevant basis for construction quality should be provided. After foundation treatment, the foundation deformation of the building should meet the requirements of current relevant codes, and settlement observation should be conducted during construction; if the foundation contains under-consolidated soil, expansive soil, or collapsible loess, appropriate reinforcement bodies and construction techniques should be selected.

 

Common foundation treatment methods include: replacement and filling method, dynamic compaction, sand-gravel pile method, vibroflotation, cement-soil mixing method, high-pressure jet grouting method, preloading method, compacted cement-soil pile method, cement-fly ash gravel pile method, lime pile method, lime-soil compaction pile method, and soil compaction pile method, etc.