| Soil column method, which is P=γh, has been adopted for calculation of soil pressure ofopen cut tunnel especially the vertical earth pressure in the current design specification.However, existing researches show that this calculation method has certain limitations. Thepresent paper, mainly relying on the project of high backfilled soil of one open cut tunnel，studies on vertical loess pressure properties and the load shedding scheme of geogrid throughthree aspects of the indoor simulation test, the actual engineering test and the numericalsimulation. The current study tends to seek the best backfill scheme of the open cut tunnel onthe premise of safety and economy. The work and the results of the study are as follows:(1) According to the1:30scale model, the real backfilled open cut tunnel is simulated.Through:①Experiments without the protect arch;②Set up the protect arch during the tests，these two schemes to discuss the influence law of the earth pressure of open cut tunnel underthe different backfill soil compaction (75%,80%,85%). The results show that, with theincrease of filling height, vertical earth pressure of the tunnel rises in the form of curve andgradually levels off, when filling up to0.5m(Simulate the filling height of15m).Verticalearth pressure of the open cut tunnel decreases with the increase of the filled soil compaction.That is to say the highly filled soil compaction is conducive to the formation of "the soil archeffect".(2) Four schemes are applied to study the geogrid load reduction systematically,including:①Without setting geogrid（means without load shedding）;②Laying themultilayer geogrid without “the lightening hole”;③Setting the single-layer geogrid+“thelightening hole”;④Laying the multilayer geogrid+“the lightening hole”. In addition, thecalculation formulas of vertical earth pressure under the action of Single layer and multilayergeogrid are derived through the above schemes. The results show that lightening effect ofschemes with setting "lightening hole" is better. In the third scheme, the best effect ofsingle-layer geogrid can reach to54%. In the fourth scheme, it can reach to59%. Thetheoretical calculation formulas, fitting with the experiments very well, has certain credibility.(3) Four schemes are adopted to study the geofoam load reduction and to discuss theinfluence law of the earth pressure of open cut tunnel under the different thick of geofoam,including:①Without setting geofoam（means without load shedding）;②Setting geofoamwith the thick of5cm;③Setting geofoam with the thick of7.5cm;④Setting geofoam withthe thick of10cm. The results show that, the lightening efficiency of EPS board is between1/3and1/2. And the lightening effect of geofoam is more obvious with the increase ofthickness. (4) Two schemes are adopted to discuss the influence law of the earth pressure of opencut tunnel and the lightening efficiency of geogrids, including:①69m loess experimentsection without load shedding at the scene;②20m lightening experiment section by settinggeogrid. The results show that it has not obvious "arch effect" when filling height reaches to13m. This matches with the conclusion of indoor model test. Although the early earthpressure has a slight deviation with P=γh, with the growth of the time it increases to the γh,and "arch effect" gradually disappears. These confirm the instability of the “arch effect". Inthe second scheme, the best effect of geogrid can reach to40%. Although it is not better thanindoor model tests, it still can achieve to a satisfactory effect.(5) The plane finite element numerical calculation model is set up. Through:①Withoutload shedding and②lightening load by setting geogrid, two schemes are applied to simulatethe backfill of the real tunnel. From the aspects of numerical analysis, we contrast theconstructions reach from indoor model experiments and outdoor real constructionexperiments. The results show that, in the first scheme, when loess filling is below15m,earth pressure value of the open cut tunnel is same as the value of γh. When loess filling iaigher than15m, the vertical earth pressure of the tunnel rises in the form of curve andgradually levels off. In the second scheme, the best effect of geogrid can reach to42%,providing the effect of40%gotten from outdoor real construction experiment.(6) The constructure, the cracks, and the composite foundation bearing capacity of theopen cut tunnel are judged on the basis of the above researches. The results show that, in bothcases, safety coefficient of open cut tunnel can satisfy the requirements of the specification.When the height of backfill without load shedding up achieves to50m, the crack widthoverruns that the maximum crack width is0.21mm, which confirms the necessity oflightening. According to bearing capacity of composite foundation, it can be found that thebackfill without load shedding height achieves to13m, the maximum vertical stress offoundation has already overrun. It is better when shedding load with geogrid. However, whenthe height achieves to30m, the maximum vertical stress of foundation has already overrun aswell. So the current paper comes to the conclusion that it do not recommend to backfillcontinuously with the current height of13m and the maximum vertical stress of foundation of304kPa. When the bearing capacity of foundation is enough, it can be backfilled according tothe present paper to get an expectant effect. |
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