| Strong supporting works are often required before foundation pit excavation in coastal soft soil areas.The crushed stone replacement pier formed by dynamic compaction to replace the soft soil and the soft soil work together to form a composite foundation to improve the bearing capacity of the foundation and reduce the settlement,so it can be used for the treatment of soft soil foundation.However,there are few relevant research reports on the application of the composite soil formed by dynamic ramming replacement pier and soft soil as a supporting structure in coastal soft soil foundation pit engineering.This paper studies the value of the strength parameters of the composite soil,the formation mechanism of the replacement pier,and the stability evaluation of the surrounding structure of the replacement pier.The main research work is as follows:⑴ The consolidated undrained shear test of the composite soil was carried out,and the evolution law of the shear strength of the composite soil with the replacement rate and the number of replaced piers was revealed.The results show that the effective internal friction angle of the composite soil increases with the increase of the replacement rate,and the effective cohesion decreases with the increase of the replacement rate;the shear strength increases with the increase of the replacement rate.The arrangement of replacement has more obvious enhancement effect on soft soil.The correlation between the shear strength parameters of the composite soil and the replacement rate is confirmed,and the calculation method of the shear strength parameters is proposed.⑵ Developed a model test box for simulating dynamic compaction replacement,designed an indoor model test for dynamic compaction replacement piers,and clarified the formation process and formation mechanism of dynamic compaction replacement piers.The results show that the dynamic ramming displacement pier length is positively correlated with the number of ramming strikes,and negatively correlated with the amount of filler and cone angle.The length-diameter ratio of the replacement pier and the buried depth of the maximum pier diameter are both positively correlated with the number of ramming strikes,and negatively correlated with the amount of filler and cone angle.The construction method of choosing the process parameters of flat bottom rammer and ramming pit semi-fill is put forward.⑶ The indoor model test of the dynamic compaction replacement pier enclosure structure was carried out,and the influence mechanism of the combined changes of different dynamic compaction replacement rates and slope ratios on the horizontal displacement of the foundation pit slope was revealed.The results show that when the foundation pit is excavated,the maximum horizontal displacement of the slope body does not appear at the position of the slope shoulder,but at the slope surface h/2(h is the excavation depth of the foundation pit),and the internal horizontal displacement of the slope body is given by The maximum displacement of the slope surface shows a divergent gradient decrease towards the slope body.With the same replacement rate,the larger the slope rate,the greater the horizontal displacement of the foundation pit slope and the weaker the slope stability;the same slope rate,increasing the replacement rate can reduce the horizontal displacement during the excavation of the foundation pit and enhance the slope stability..According to the three numerical models of pier-soil cost calculation,pier-soil separate calculation,and pier-soil coupling,the variation law of horizontal displacement of foundation pit excavation and the stability of slope are simulated and analyzed.It is relatively consistent,and the simulated slope stability coefficient is the largest.⑷ The change law of horizontal and vertical displacement during the excavation of the actual coastal soft soil foundation pit project is clarified,and the influence law of the replacement rate,slope rate and arrangement of replacement piers on the stability of the enclosure structure is revealed.When the slope rate is constant,properly increasing the dynamic compaction replacement rate can effectively increase the stability of the enclosure structure;when the replacement rate is constant,appropriately cutting the slope to reduce the slope rate can effectively improve the stability of the enclosure structure;The plum-shaped arrangement has little effect on the stability of the enclosure structure;the vertical excavation of steel sheet piles on the replacement pier can effectively control the deformation of the foundation pit and save the excavation space.⑸ The study establishes methods of dynamic compaction replacement pier retaining structure design and construction which develops theory of foundation pit in soft soil area and can be used for design,construction,monitoring of dynamic compaction replacement pier retaining structure in marine soft soil area. |
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