Experimental Study On The Mechanical Properties Of Hole Bottom Softening Of Nanning Argillaceous Rocks

With the vigorous development of urban infrastructure in Nanning, continuous extending of project scale, foundation works problems involved are increasingly complex, and the increasing high requirements of bearing stratum, the Paleogene mudstone layer had replaced the previous Quaternary layers, and became the first choice for the pile foundation bearing layer of many (?)arge construction projects. However, mudstone immersion softening will reduce the bearing capacity of pile foundation bearing stratum, and lead to serious loss of engineering and safety accident. Previous studies is usually fixed dimensions first, then study on water-rock interaction results,ignore the impact of size effect. Pile diameter should be considered as an important factor in the study area while studying the problem of pile foundation bearing stratum. In view of this, in this paper, the mechanical properties of hole bottom softening of argillaceous rocks are comprehensively researched and analyzed based on the size effect:First, introduced the basic characteristics of Paleogene mudstone of Nanning Basin, classified and analysed the influencing factors of the strength. Then, study on the geotechnical springback deformation characteristics caused by the construction process of the pile foundation through indoor tests;study on the changes of mudstone foundation total stress in the process of stress relief and the immersion softening mudstone foundation bearing capacity changing characteristics, compared and analyzed with field tests through model tests. Last, contact engineering examples and test results, through the analysis of failure process of manual hole digging pile, expounds the damage caused by the hole bottom mudstone immersion, and details how to control during the construction process of bored pile, so as to avoid this problem from happening.Research results show that:(1)The unloading rebound deformation characteristics and softening disintegration mechanics properties of mudstone affected by size effect. (2)On the phase of preloading consolidation of unloading rebound, load-settlement curve go through two stages as the load increases: steep descending section and slow deformation section; In the rebound phase,the curve also go through two stages with the load unload, order is slow deformation section to steep descending section. Because of the influence of the size effect, mudstone one-side immersion disintegration residual volume and time relation curve also go through two stages:slow disintegration and rapid disintegration. Compared with the small size specimen,compressed after immersion, the settlement of the large size specimen increased obviously. (3)The increase of the negative total stress of the test point in the stress relieving process affected by two factors:one is associated with the area ratio of different pile diameter; the second one is related to the relative depth of test stress points. According to the negative total stress ratio of the test point, can be divided into two areas:one is stress ratio constant zone; the other one is stress ratio attenuation zone. (4)For the unloading stress release and springback deformation, can be divided into three stages according to the relative size of change:micro velocity change stage, uniform change stage, rapid change stage. (5)Affected by the size effect, the increasing contact area of the penetration makes the ultimate bearing capacity of the mudstone decreased amplitude increases. The depth of immersion soften disintegration of the mudstone in free boundary is much greater than in load boundary, and are also more likely to result in destruction of the foundation. Before loading of the static load test, change the immersion state of mudstone, there is no effect on the change rule of P～S curve; During the test-loading process,change the immersion state of mudstone, will affect the change rule of P～S curve. (6)Through the construction process control, can effectively avoid the occurrence of serious problems that the bearing capacity of pile foundation bearing stratum will reduce as mudstone immersion softening.