| Desiccation cracking and exfoliation are the major deterioration problems of the surface of indoor earthen sites in wet environment,causing damage irreversible to earthen sites and being the key point and difficult point in research.But,at present,it is uncertain about the cracking and extension property of desiccation crack of earthen sites in wet environment,it is not accurate about crack spacing calculation and crack pattern prediction,and especially it is not adequate to the study on exfoliation evolution feature.Taking tawny tamping layer of soil city wall section in Moon Bay,the ancient sites of Sanxingdui as study object,this thesis has studied the deterioration mechanism of tawny tamping layer using on-site environment monitoring,disease investigation and indoor experiment and combining theoretical analysis;it has revealed the cracking and extension property of desiccation crack in tawny tamping layer and proposed theoretical calculation formula of critical length of unstable crack extension;it has obtained the calculation method of crack spacing changing range,given critical length ratio of crack pattern feature change and put forward the deterioration function of tawny tamping layer deterioration process.The study results provide control suggestion for the protection of indoor earthen sites in wet environment.The main content and conclusion of this thesis are as follows:1.It found the correlation between rate of water content in tawny tamping layer and relative humidity of environment,described the distribution law of pattern feature and deterioration rate of tamping layer cracking and obtained the primary factor of deterioration in tamping layer.(1)The changing range of rate of water content in tawny tamping layer of city wall section in Moon Bay was 12.8～21.8%,decreasing with the increase of the height.Fluctuation frequency of rate of water content in tamping layer was consistent with relative humidity of environment but there was 12-hour difference in phase between them;change of relative humidity of environment was the primary cause that rate of water content in tawny tamping layer fluctuated.(2)Crack network in tawny tamping layer transited from parallel crack to polygon crack with the increase of tamping layer thickness.The bigger the tamping layer thickness was,the more complicated the crack network was.The deterioration rate of tamping layer firstly increased and then decreased with the increase of city wall height and the damage was the most serious in the place of 1.98 m with deterioration rate being as high as 0.51.(3)The primary cause of the deterioration in tawny tamping layer was water loss and dryness,and the dry-wet cycle of tamping layer and primary factor was boundary condition of tamping layer and size effect.2.It found the cracking,extension and development law of tawny tamped earth sample in Sanxingdui city wall under dry condition and proposed theoretical calculation formula of critical length of unstable crack extension(seen in the published thesis [1] and [2]).(1)The cracking process of sample of earthen sites was between liquid limit and plastic limit.There were two cracking modes for crack: single successive cracking and multiple simultaneous cracking.There were two cracking methods for crack: direct cracking from the boundary and gradual cracking from the defect.The above 90% crack cracked from the defect near the boundary.The defect led to the concentration of stress in its pointed end and lowered the needed stress of cracking.The smaller the distance between defect and boundary was,the smaller the critical size triggering cracking was.(2)The requirement that unstable crack extension was: with the increase of rate of water content,stress intensity factor was bigger than fraction toughness.Hence it proposed calculation method of critical size of unstable extension.With the decrease of rate of water content from 45.6% to 29.1%,the ratio of critical length of unstable extension and sample width decreased from 0.109 to 0.024.The lower the rate of water content was,the easier the crack was to be in the unstable extension process;a subtle defect in cracking part might trigger the fast extension of crack.3.During the drying process,there were two free boundaries,two slip boundaries and one fixed boundary in the bottom of earthen sites sample.With the desiccation in the bottom of earthen sites sample,the length of free boundary increased,the length of fixed boundary decreased,and the length of slip boundary basically kept no change.Through the theoretical analysis,it inferred the calculation formula of tensile stress under the conditions of different boundaries and proposed the calculation method of changing range of crack spacing under the conditions of three boundaries(seen in the published thesis [3]).4.With the increase of crack width,and the decrease of ratio of length and width,crack number and fractal dimension in unit area gradually increased,and the largest stable block area and the largest frequency block area firstly increased and then decreased.When sample width was smaller than critical value(the sum of free boundary length and slip boundary length),and the ratio of length and width was bigger than 1,crack was parallel crack network.When sample length and width were bigger than critical value,crack pattern was affected by crack cracking mode.If crack cracked simultaneously,then the crack pattern was regular hexagon.If crack cracked successively,when the ratio of length and width in each block was bigger than 1,crack was parallel crack network;when ratios of length and width in blocks had two different situations,namely bigger than 1 and smaller than 1,crack patter was echelon form.5.It found earthen sites in Sanxingdui the development process and distribution feature of deterioration rate under the influence of dry-wet cycle;it proposed deterioration function of deterioration process in tamping layer and divided deterioration process into development period,incubation period,fast deterioration period and residual period of crack to provide control suggestion for the protection of indoor earthen sites in wet environment. |
PDF Full Text Request