Stability And Control Technique On Subgrade Of NaQian Highway In Karst Areas

Most of the limestone area in western China is more developed karst and complex geological conditions. In the construction of expressway in this areas will encounter a series of engineering problems. The stability of karst subgrade is a important engineering problem in karst areas.Consulting the internal and overseas literature, firstly, this paper to sichuan traffic technology project "karst area of subgrade disease prevention and built bridge and tunnel relying on technology" as the basis, then use NaQian expressdway in which there are hidden karst caves under the subgrade as engineering backround, secondly, through the spot detection to understand the karst development distribution rule and the shape of the karst cave, using theoretical analysis and numerical simulation research the karst cave roof subgrade stability and the ratio of thickness to span the karst cave roof safety.This paper adopts field survey, geophysical exploration, outdoor and indoor test and theoretical analysis, numerical simulations and methods of demonstration projects, mainly on the following job:①In order to understand about the water flows and the karst situation of NaQian highway subgrade encountered, the site reconnaissance of karst areas.②Using geological radar and high density electrical detection karst cave in the site,geological radar mainly adopts 100MHz and 40MHz antenna detect,high-density electrical use Wenner and Spey electrode array method detect,after detection know that karst is more developed in K21 section and K45 section,which would be impact of NaQian expressway construction, both used in conjunction, they can more effectively detect the location of karst foundation, size and morphology.③Through the laboratory test and reference materials in the limestone experimental data of the geological exploration, by using numerical simulation software FLAC-3D establish three dimensional subgrade model, analyzing the relationship between the stability of karst subgrade and the ratio of karst cave's thickness and span.Through the research work,this paper made the following progress:①High-density electrical method and geological radar detection cave works well, but each has its advantages and disadvantages, of high-density electrical has low resolution,so it does not reflect the hierarchical filling in caves and reflect the small size of cave conditions, but its detecting depth and area widely.geological radar method has high resolution, so it can reflect the internal filling layered cave conditions and small size cave, but its effective detecting depth is only 15 meters.②In the NaQian expressway karst area, paved roads directly to the limestone roof, and meet the following conditions:1) Limestone weak weathering and fissure medium development,2) Four-lane highway,3)Cars load rating of highway-Ⅰ,4)Take the impact factor of 1.2 cars, lane reduction factor of 0.67. through the numerical simulation software FLAC-3D analysis to know:a.when subgrade karst cave's ratio of thickness and span is less than 0.6, the engineer should take some measures to avoid the collapse of the subgrade and hande the karst cave; but when the ratio is more than 0.6, we can think that the karst subgrade is stable under such karst cave. b.In the condition that the karst cave shade and limestone is the same, the greatest threat to the stability of the subgrade is when the karst cave in the middle of subgrade bottom, the secondly threat is when the karst cave at the bottom right (or left) subgrade, the minimum threat is when the karst cave at the subgrade's slope bottom.③The filler used in aggregate mixture, and considering vehicle load, through the numerical simulation software FLAC-3D analysis to know:in the process of the karst embankment filling, from 0 to 4 metres in height of the filling, as the incresement of the thickness of the subgrade's filling, the security ratio of karst cave roof's thickness and span increase too.But more than 4 meters in height, as the thickness of the filling increse continuously, the security ratio of karst cave roof's thickness and span decrease. and when the thickness of the embankment filling is larger than 17 metres, the changement of the security ratio of the karst cave roof's thickness and span is small, its value is about 0.54.Finally, applying research achievement to the engineering of K45 deep cutting section of NaQian highway and HeNi bridge change to the subgrade, and give some helpful advices to the construction of the two project.