First, the common failure forms of mountain subgrade
1. Collapse
Collapse is a huge rock mass that has lost contact with the basement. Under the action of gravity, the rock suddenly collapses violently from the steep slope, and the rock mass is often smashed to pieces. Collapse belongs to slope failure, and its scale and harm degree are serious. For mountainous roads, due to the particularity of geological and geomorphological environment in mountainous areas, the cross section of subgrade is easy to form the form of embankment cutting, which increases the slope of excavated cutting slope. In addition, a large amount of rainwater flows into cutting in rainy days, which causes the subgrade sections with poor soil quality, soft lithology and severe weathering to be destroyed, which often cuts off traffic and brings inconvenience to pedestrians.
Flip milk
Boiling phenomenon mainly occurs in seasonal frozen areas in China. In the process of subgrade freezing in wet areas, the water in the soil moves upward continuously, which increases the water content in the upper part of subgrade. During the thawing period in spring, due to the excessive water content of soil foundation, the strength drops sharply. Coupled with the action of driving, there will be springs, cracks, bulges and mud sloshing on the pavement, which is called boiling. Road frost boiling in mountainous areas is mainly exposed to wet subgrade, severe freezing and lack of road construction materials. Mountain road frost boiling is also one of the main forms of subgrade damage in mountainous areas.
Step 3: water stains
For all kinds of flood disasters caused by nature, such as flash floods caused by heavy rain, roads and bridges along the river are often attacked or submerged, causing damage to varying degrees. The mountainous highway has complex terrain, steep slopes and sharp bends, narrow pavement and few protection projects. However, the mountain stream changes greatly and the flood is fierce, which often causes the roads to be flooded. At the same time, the flood contains a large number of branches and mud stones, which often block the entrance of side ditches and culverts, resulting in poor drainage, flash floods rushing straight to the road surface, forming gaps in the subgrade, and rainwater seeping down, reducing the friction between the joint surface of the overburden and the mountain rocks, and at the same time, part of the overburden is unstable, leading to the collapse of the deep excavation slope. Water damage is also one of the main forms of highway subgrade damage in mountainous areas.
4. Debris flow
Debris flow is mainly a concentrated flow caused by a large amount of precipitation in a short period of time in the waterfall rain area. However, when the rocks in the catchment area are weathered, there are a lot of loose deposits, which become the source of debris flow solids when the hillside slope is cut by concentrated catchment and gully. Debris flow is a common disease in mountain roads. When it happens, it often destroys subgrade, bridges and culverts and other buildings, cutting off traffic.
Second, the influencing factors
1. Geological and climatic conditions. The particularity of geological and geomorphological environment in mountainous areas makes the cross-section of roads diverse and requires high drainage; Hydrological and climatic factors, such as natural disasters such as rainfall, flood, freezing, snow or excessive temperature difference, will lead to some collapses, water damage and mudslides in mountainous roads.
2. Design impact. If the section size of subgrade is unreasonable, the geological, hydrological and meteorological data along the line are not fully collected before design, and comprehensive analysis and research are not carried out, it will also lead to subgrade diseases.
3. Construction impact. During the construction, the thickness and quality of the filler do not meet the requirements. Different filling materials are not divided into paving. Subgrade filling does not meet the specification requirements, resulting in uneven settlement. The quality management system of the construction unit is not perfect.
Three. preventive measure
1. Design aspect
(1) Do a good job in geological exploration, make a comprehensive and detailed investigation on the topography, landforms and hydrogeological conditions of the route, especially provide detailed design data for special subgrade sections, and determine the crossing of poor hydrogeological sections and poor surface sections. According to the standards of different grades of highway, after comparing various schemes, the route selection of highway adopts the route avoidance scheme as far as possible. When the route must pass through bad sections, measures such as changing soil or mixing lime, cement and laying geotextile can be considered in the design. In order to reduce the possibility of disaster.
(2) Correct design of subgrade cross section
Due to different filling and excavation conditions, the typical forms of subgrade cross section can be summarized into three types: embankment, cutting and combination of filling and excavation. Embankment refers to the subgrade completely filled with rocks and soil, and cutting refers to the subgrade completely excavated on the natural ground. These are the basic types of subgrade. When the cross slope of natural ground is large and the subgrade is wide, one side needs excavation and the other side needs filling, it is also called semi-filled and semi-dug subgrade. In hilly and mountainous highways, the combination of filling and excavation is the main form of subgrade cross-section.
(3) In order to ensure the minimum filling height of subgrade, it is necessary to ensure that its stability will not be reduced due to the influence of surface water, groundwater, capillary water and frost heaving. According to the requirements of subgrade design code, the dry and wet types of soil foundation and the height of capillary water level, it is necessary to ensure the minimum filling height of subgrade. When the height of subgrade filling is limited and can't meet the specification requirements, corresponding treatment measures should be taken to determine the waterproof performance of the filler. For example, replace permeable materials such as gravel and stone slag, set isolation layer or build underground seepage ditch. Prevent groundwater and groundwater from infiltrating into subgrade, which will affect the strength and stability of subgrade in subgrade operation area. Strengthen and optimize the waterproof design of roads, deepen and widen the longitudinal drainage ditch according to local conditions, appropriately deepen the gravel at the entrance of culverts, and strengthen the longitudinal and transverse drainage of roads. Waterproof protective layer is added to the pavement to prevent rainwater from invading the subgrade.
(4) Clarifying the quality standard of subgrade filler In the construction drawing design of highway engineering at all levels, the minimum strength and maximum particle size requirements of subgrade filler with different filling heights must be clarified. It is strictly forbidden to directly fill subgrade with inferior soil such as planting soil, humus soil, silt frozen soil and strong expansive soil. Gravel soil should be preferred as subgrade filler, and fine-grained soil with poor soil quality can be filled at the bottom of embankment.
(5) In order to ensure the stability of subgrade slope, the subgrade slope with high fill and deep excavation should be determined according to the specifications such as filler type, slope height and engineering geological conditions, and the subgrade stability checking calculation must be carried out for high fill embankment. When the fill slope is too high, it can be considered to set a slope platform in the middle of the slope and a broken platform at the bottom.
(6) Strengthen protection engineering design. When designing the retaining wall of subgrade slope, the topographic and geological conditions at the toe of the wall should be considered, and the stability against sliding and overturning and the stress at the base should be calculated.
2. Architectural aspects
(1) It is an important link to ensure the construction quality of high-fill subgrade to do a good job in construction organization design, reasonably arrange working procedures, clarify the connection relationship between structures and subgrade, give priority to the construction of high-fill sections, and rationally allocate personnel and equipment according to the actual situation of the construction site.
(2) Make preparations before construction, learn about the filling and excavation, geology, soil filling and excavation, and deployment of each section in detail, and focus on investigating important sections.
(3) Carefully remove the bad soil quality of the topsoil, strengthen the foundation compaction treatment, the surface vegetation, tree roots and garbage must be removed, and at the same time increase the surface compaction density, and adopt a large-tonnage vibratory roller for treatment.
(4) Water is another important factor affecting the strength and stability of subgrade, and many subgrade diseases are caused by water erosion. Before subgrade construction, drainage facilities such as intercepting ditch and drainage ditch should be well done, especially in rainy season. Subgrade underground drainage can be set with blind ditch, seepage ditch and seepage well.
(5) Before subgrade filling, soil analysis, CBR value and standard compaction test should be carried out for subgrade filler. For inferior soils such as planting soil, humus soil, silt and strong expansive soil, materials whose CBR value and maximum particle size can not meet the requirements of the specification can not be used for subgrade filling; Moreover, before subgrade filling, construction lofting should be carried out according to the design, semi-permanent temporary leveling points and coordinate points should be established and recorded. The lofting of subgrade slope toe must be accurate to ensure that the subgrade width meets the design requirements. Weeds, roots, mud, etc. The slope angle in the subgrade should be removed, and the degree of compaction should meet the specification requirements.
(6) Subgrade construction must be filled by layers and rolled by layers. The compaction thickness of each layer is l5㎝, which is ideal. It is forbidden to roll fill in the road reform project, and soils with different properties cannot be mixed. Subgrade filling must be carried out in full width, once in place, and it is forbidden to widen it. During construction, on-site inspection shall be conducted according to the compaction machinery used. After the soil in the pit is transported to the construction section for paving, the thickness of loose soil shall be measured with a ruler, and each layer of filled soil shall exceed the width of embankment to ensure the compactness of subgrade edge. Water content must be controlled before rolling. Practice has proved that the rolling effect is the best when the water content is slightly higher than the optimal water content (generally about 2% higher than the optimal water content).
Four. conclusion
With the development of society, more and more roads are built in mountainous areas, which are extremely unfavorable to road construction in terms of topography and geology, especially in mountainous areas, and the stability of subgrade can not be ignored. Based on the failure mechanism of mountain subgrade, combined with design, construction and management measures, and strengthening road maintenance, it will be very effective to prevent and control the failure of mountain highway subgrade.
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