| Many countries around the world are areas with frequent earthquakes. Among those, Chile, Japan, United States and China are typical areas. There are about 5 million earthquakes happens every year around the world. Thereinto, there are about 50,000 felt earthquakes, and 20 of those result in severe catastrophe. Take China as an example, since 1900, there are more than 550,000 people die from earthquake, which is about 53% of the total number around the world. There are 22 provinces affected by earthquakes, which cover more than 300,000 square kilometers, and more than 7 million building are destroyed. In 1976, there were about 242,800 people died in the Tang Shan earthquake. In 2008, there were about 87,000 people died in the Wenchuan earthquake, and result in direct economic loss of about 845.1 billion Chinese Yuan.In order the prevent and reduce seismic catastrophe, researchers around the world conducted many studies on earthquake generating mechanism, structure earthquake resistant theory and technology for structures, secondary disaster prevention, evaluation and reconstruction after earthquake. However, most of the researches are focused on the earthquake resistance and damage evaluation of the structure in operation. There are very few studies on foundation pit seismic response and earthquake resistance performance of support structure during construction period. Along with the rapid development of city subway system, large underground supermarkets and super high-rise buildings, the number of deep foundation pit engineering grows very fast day by day. The issues of deep foundation pit construction technology, prevent and reduce disaster during construction period, temporary constructions and seismic damage evaluation for curing concrete in frequent earthquake areas will definitely become important theory and technology for the all the countries with frequent earthquakes. During foundation pit construction period, the support structures (including soil nailing wall, spray anchor, diaphragm wall, steel support, reinforced concrete support and other temporary support), construction equipment and engineering machinery have different seismic responses and earthquake resistance performances comparing with that of structures in operation. The existing research achievements con not directly apply to the foundation pit disaster prevention and structure damage evaluation during construction period.This paper look into the key technologies and theories of foundation pit construction in frequent earthquake areas. The seismic resistance stability of different deep foundation pit types under strong earthquake, seismic displacement and disaster history and distribution of foundation pit support system, strength loss and evaluation of curing concrete after strong earthquake, interior damage and evaluation of early-age concrete after strong earthquake have been studied by numerical simulation, laboratory test and theoretical analysis and some importance and useful research have been achieved as follow.1. By literature study and on site investigation of Wenchuan, the regional distribution characteristics of earthquakes around the world have been statistical analyzed. The global earthquake number accumulation along time has been studied and China is taken as example for local earthquake number distribution along time study based on statistical data. Based on the on site investigation of foundation pit and side slope destructions cases in Wenchuan, the seismic destruction model and mechanism of foundation pit under construction have been advanced and improved.2. With the data of deep foundation pit project in Chengdu area, the seismic stability of spray anchor foundation pit and steel support foundation pit with diaphragm wall has been studied by three-dimension dynamic finite element method (FEM) simulation. The foundation pit structure displacement, steel support disaster evolution and whiplash effect of the two different foundation pits under seismic load along different directions.3. The catastrophe process and failure pattern of steel support foundation pit under strong seismic effect have been studied by three-dimension dynamic finite element method (FEM) simulation of foundation pit support structure destruction. The regional distribution characteristics of foundation pit support system failure risk and support destroy phase history under strong seismic effect have been revealed, which include reciprocating vibration phase, vibration displacement rapid growth phase, structure failure phase. The conclusions have importance significance to risk prevention, disaster prevention and reduction during foundation pit construction process in the frequent earthquake area. 4. The deformation characteristics, the dilatation under compression and its dynamic mechanism of different curing period concretes during foundation pit construction have been studied by laboratory test of seismic vibration simulator. It is concluded that: 1) the volumetric strain of curing concrete after strong seismic effect possess obvious dilatation property (volume increase) and the dilatation has strong relation with concrete curing period; 2) the mechanism of concrete dilatation is mainly due to the dislocation and rotation of random distributed micro cracks generated after strong seismic effect which result in macro volume increase.5. Based on the laboratory study of after-vibration concrete deformation properties, the three stages of vibration period for crack generation and evolution in early-age concrete have been concluded: (1) vibration strengthen stage (curing age 1-10 hours, including concrete liquid status and viscous fluid status); (2) vibration damage and self-repairing stage (curing age 10-60 hours, including semi-solidification status and solidification status with increasing elastic modulus); (3) vibration damage and irreversible stage (curing age more than 60 hours, mainly final solidification status)6. The strength loss of after-vibration concrete with different curing period has been studied by laboratory seismic simulation. It is concluded that for the concrete with 3-30 vibration time, the strength loss is greatly reduced, while for that of vibration time more than 30 hours, the strength does not change much.7. The relation of concrete strength with ultrasonic wave speed has been established by laboratory tests and the scheme for on site ultrasonic wave speed inspection for concrete structure after strong seismic effect has been advanced.8. By applying three-dimension layer-analise technique to the evaluation of concrete interior damage after earthquake, the three-dimension analysis software has been developed which is based on arbitrary three-dimension orthogonal chromatography. This work provides an effective approach for visual evaluation of interior damage for concrete structure.Based on the above stated, this paper is mainly dealing with the key theories and technologies for the foundation pit construction in frequent earthquake areas. Part of the research achievements have been used for some after-earthquake reconstructions in Chengdu and Dujiangyan Dam. It provides important suggestions for risk prevention during foundation pit reconstruction after earthquake and brings useful technique and social benefits. |
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