Research On Damage Mechanism Under Earthquake And Seismic Technique Of Subgrade Engineering

Subgrade engineering was largely damaged during Wenchuan earthquake. It takes a toll on the road and railway engineering to be constructed. Restore measures of road engineering after earthquake, damage effect of soil mass under earthquake and seismic design method of subgrade engineering are the key scientific issues calling for immediate solution at present.Based on field investigation, large-scale shaking table model tests and numerical calculation, damage effect of soil mass, deformation effect and seismic design of subgrade engineering under earthquake and earthquake sequences were studied. The main work and conclusion are as follow:A field investigation on slopes reinforced by shotcrete-bolt system and road embankments along Dujiangyan to Yingxiu highway was conducted. The investigation shows that the seismic deformation and failure modes of soil mass lateral unrestraint as slopes and road embankments were characterized by subsidence and lateral landslides, while soil mass lateral restraint by retaining wall was characterized by subsidence and concavo-convex deformation, i.e., alternative settling and bulging deformation. Seismic subsidence rate and concavo-convex amplitude rise with the increase of seismic intensity, and obey a normal distribution. Seismic damage level of the upper slope is affected by plane alignment of highway. The slope's damage of convex-curve section is more serious than concave-curve section, and the damage of linear section is most light. Two shaking table model tests of soil mass model lateral unrestraint and soil mass lateral bound were conducted, and the results were consistent with field investigation, and it was found that a threshold of peak ground acceleration which converted from subsidence to concavo-convex deformation was about 0.6g.Action process of seismic sequences of medium to large earthquake, i.e. foreshock to mainshock type and large to medium earthquake sequences, i.e. mainshock to foreshock type on road embankment and the damage development were reproduced by shaking table model tests. It is shown by tests phenomena and damage identification that damage degree of model under medium to large earthquake sequences is much larger than that model under large to medium earthquake sequences. As to the seismic wave with a PGA of 0.616g, the decrement of natural frequency of the model of large to medium earthquake sequences is 16.02% larger than that of medium to large earthquake sequences, and the increment of damping ration of the model of large to medium earthquake sequences is 38.66% larger than that of medium to large earthquake sequences. Firstly, soil mass was damaged by large earthquake, then damage cumulated under medium earthquake, so soil mass was cracked under large to medium earthquake sequences. While to the action of medium to large earthquake sequences, soil mass was compacted and consolidated under medium earthquake, so it decreased the sensitivity of soil mass under larger earthquake, and made the change of natural frequency and damage of soil mass smoothly.Failure modes, mechanism and sliding surface of earthquake-induced landslide were studied by means of field investigation, shaking table model tests and dynamic-strength-reduction method. The earthquake-induced failure surfaces commonly consist of tension cracks and shear zones, which is different from gravity-induced failure surface made by shear zone only.Depth of dynamic sliding surface will be deeper with the increase of PGA. Within 5 m from the top of the slope, the dynamic sliding surface will be about 1 m shallower than the pseudo-static sliding surface in a horizontal direction when the peak ground acceleration (PGA) is 1 m/s2; the dynamic sliding surface will be about 2m deeper than the pseudo-static sliding surface in a horizontal direction when the PGA is 10 m/s2, and the depths of the dynamic sliding surface and the pseudo-static sliding surface will be almost the same when the PGA is 2 m/s2. Based on these findings, it is suggested that the key point of anti-seismic design, as well as for mitigation of post-earthquake, secondary mountain hazards, is to prevent tension cracks from forming in the upper part of the slope. Therefore, the depth of tension cracks in slope surfaces is the key to reinforcement of slopes.Road embankment seismic damage of Wenchuan Earthquake was investigated, which found that the main seismic damage type of road embankment were rip of upside and bulge of downside of the slope. By means of shaking table model test and numerical simulation, it is found out that the magnitude and the increment of acceleration and dynamic shear stress are the largest in the upside of the road embankment, which is in the same position of the rip. Also, dynamic earth pressure and displacement are the largest above the berm, which is in the same position of bulge. All of rip and bulge are in the depth of 3 m, which show that the road embankment damage is a superficial zone damage. By dynamic numerical analysis finding out that magnification factor of PGA shows three types:monotone increasing as the height increases (h≤10m), increasing to decreasing and later increasing again as the height increases (h>20 m), and the form lies between the formers (10m<h≤20m). Lastly, the mechanism of acceleration response of high road embankment by use of propagation theory of seismic waves is explored.To study the seismic design of subgrade engineering, subgrade seismic damage of Wenchuan 8.0 Earthquake was investigated. Two damage types of un-reinforced road embankment under earthquake were found, including slope collapsing and pavement cracking, while there was nearly no damage to road embankment whose upper part was reinforced by geogrid. It was shown by shaking table model tests that body sliding was restrained by reinforcement on the top of the embankment with long geogrid, which prevented the fissures from developing downward, and slope collapsing was precluded by reinforcement on the upper part of the embankment with short geogrid, which restrained the lateral deformation of soil mass. According to conventional design of geogrid-reinforced road embankment, optimized seismic design was proposed! Dynamic-strength-reduction method was employed to compare the seismic performance of conventional design with that of optimized seismic design of geogrid-reinforced road embankment. The result showed that geogrid used would decreased by 23.1%, maximal seismic displacement of embankment would decreased by 19.0%, and maximal stress of geogrid would decreased by 14.1% after optimized design. Optimized design not only decreased consumption of geogrid, but also restrained the lateral deformation of soil mass, thus the seismic behaviour of road embankment improved.To research the anti-seismic behavior of rock slope along cutting of highway, on the basis of field investigation on damages of rock slopes in Wenchuan seismic area, some typical slope along Sichuan-Tibet highway was taken for example to explore the seismic response behavior and anti-seismic effects of anchored rock mass, combing block theory analysis with dynamic numerical calculation. It shows that:Joints may cause significant amplification of seismic motion, with amplification factor of unanchored wedged block mass 3 times larger than that of wholesome rock mass, and amplification factor of anchored wedged block mass will decrease by 30% to that of unanchored wedged block mass. According to spectrum analysis results, the frequent range of seismic motion amplification in jointed rock mass is larger than that in the integrity of rock mass. Anchor reinforcement will make the natural frequency of jointed rock mass go away from the domain frequency of seismic waves, decrease the amplitude of seismic response, therefore improve the seismic capacity.