Subgrade slope collapse is one of the most common subgrade diseases. Depending on the soil type, damage cause and scale of the slope, the main damage forms are slippage, landslide, spalling and debris collapse. . Slippage is caused by a small amount of soil moving downward along the soil slope, that is, the thin topsoil on the slope slides down. It is usually caused by precipitation, rainfall and other flowing water washing the slope or improper construction. Landslide refers to a part of the soil sliding along a certain sliding surface of the slope under the action of gravity, which is mainly caused by the insufficient stability of the soil. The main cause of landslides on embankment slopes is that the slope is too steep or the toe of the slope is hollowed out, or the filling layer arrangement is inappropriate. The main cause of landslides on cutting slopes is the difference between the height and slope of the slope and the natural rock and soil layers. The nature is not suitable. Spalling and chipping refer to the damage form of the weathered rock layer on the slope, which causes the surface rock to peel off from the slope under the influence of various external environments. Collapse usually occurs when larger rocks break away from the slope surface and roll down the slope.

1. Factors affecting slope stability and damage forms

Normally, the geological and hydrological conditions, climatic factors, geomorphological factors and weathering in the construction area have an impact on the slope. The stability will be affected to a certain extent. With the further acceleration of social development, the influence of engineering activities on slope stability has gradually increased. The forms of stability failure of roadbed slopes generally include the following forms:

1. Landslide. When factors such as rainwater erosion or vibration affect the soil or rock mass on the slope, shear failure occurs along the sliding conditions through the action of gravity, which is called a landslide.

2 shattered. Under the influence of atmospheric temperature changes, the upper layer of the cutting slope or the rock surface will expand and contract, prompting the weathering phenomenon of rocks or soil peeling off the slope surface. When old debris falls, new debris will continue to appear, which is the debris disease.

3 Slips and collapses. Slide collapse mainly refers to the overall sliding of the upper part of the roadbed slope or the rock. Compared with the chipping phenomenon, this form is more serious in scale. Collapse is when the entire rock collapses and topples under the action of gravity, and is eventually deposited at the bottom of the slope.

II. Control measures for highway subgrade slope stability

(1) Design stage

The design stage should follow the principles of “comprehensive design, local materials, and According to the basic principle of "prevention first, ensuring construction", roadbed slope protection should be set up on stable slopes. The selection of protection types should comprehensively consider factors such as engineering geology, hydrogeology, slope height, environmental conditions, construction conditions and construction period. For road sections with insufficient subgrade stability and adverse geological factors, attention should be paid to the comprehensive design of subgrade slope protection and retaining reinforcement.

The types of roadbed protection are mainly divided into two types: slope protection and slope erosion protection. At present, the common types of protection for mountain roads mainly include spray-seeded grass planting, arched frames, hanging nets and climbing vines, window protection, retaining walls and other protection types. The applicable conditions for these types of protection vary. Low-grade highways in mountainous areas (roads of Class II and below) often suffer from problems such as collapse and chipping of roadbed slopes, which are partly related to the designs of some designers. If due to construction fund factors (limited investment in mountainous roads), the entire project is underinvested, in order to reduce the project cost, the design unit cancels or reduces the number of some reasonable protection projects; the design unit does not carefully investigate the geological and hydrological conditions of the slope, and drill holes must be drilled. The road sections with clear geology have not been drilled or the number of drilled holes does not meet the requirements, the geological survey data cannot meet the basic requirements, and whether the plan has been optimized and adjusted according to the actual geological conditions during the construction stage; or the design is not comprehensive enough, and the protection design is only based on the geological conditions. , factors such as hydrology and construction conditions were not fully considered, which increased the difficulty of later construction. Attention should be paid to the importance of roadbed protection in the design stage. In-depth research should be carried out during the design stage. According to geological, hydrological conditions and local materials, local materials should be selected according to local conditions, and appropriate protection types should be selected or comprehensive measures should be taken to ensure the stability of the roadbed.

(2) Construction Stage

The construction stage is the implementation stage of the design documents, and its quality directly affects the stability and durability of the roadbed slope. At this stage, management should be strengthened and control should be strengthened from the aspects of people, materials, and processes to effectively improve the construction quality.

1 Control of construction personnel

Subgrade slope protection is mainly operated by personnel and cannot be mechanized. Therefore, the quality of the project is directly affected by the construction personnel's own quality, construction level and quality awareness. This requires the construction unit to improve the quality assurance system, improve the management system, further improve the construction personnel's awareness of responsibility and quality, mobilize the subjective initiative of the construction personnel themselves, and effectively ensure the quality of construction. Personnel in key positions (professional quality inspectors, surveyors, material technicians, construction workers, testers, etc.) should be given on-the-job training. They can only start working after passing the assessment, and the prior control of personnel should be implemented.

During the construction process, the on-site construction management and construction status of the construction management personnel and operators should be checked at any time. If there are personnel with poor management ability and sense of responsibility at the construction site, they should be replaced in a timely manner and those with technical skills and understanding of management should be replaced. Construction personnel carry out engineering operations and continuously improve the quality and management level of construction personnel. By effectively controlling the quality of construction personnel, the work quality of construction personnel can be further improved to ensure the quality of the project.

2 Control of construction materials

As the basis of project construction quality, the quality of raw materials directly affects the quality level, external shape, structural safety and use of the entire project after completion. Function. Usually, when quality accidents are caused by the use of inferior engineering materials, it will be difficult to make up for and repair them, resulting in delays in the construction period and waste of manpower. Therefore, during the construction process, the quality of construction raw materials should be strictly controlled, and the construction materials entry inspection and approval system should be strictly implemented to effectively avoid the occurrence of such quality accidents as much as possible. During construction, we should try our best to use products from quarries with larger stone reserves and sand plants with sufficient supply and uniform quality:

2.1 Ensure the use of tested raw materials, if conditions permit. , the quality requirements can be appropriately improved to leave a margin for construction quality. According to the construction technology, site requirements, etc., select raw materials such as stones and flakes with appropriate specifications to reduce the difficulty of construction.

2.2 When cement enters the market, its grade, bagging quality, variety, packaging, and factory date batch number should be inspected on a batch basis. Check the factory inspection reports and product qualification certificates of all entering cement, use cement from the same manufacturer, variety, batch number, strength and factory date as a test to judge its strength, stability and setting time to ensure that the quality of the cement can be consistent with Comply with relevant regulatory requirements.

2.3 The sand used in masonry should be natural medium sand with strong durability and particle size less than 5mm, and avoid mud content exceeding 5%. The use of medium sand can improve the workability of the mortar and effectively ensure the cement dosage. At the construction site, the sand entering the site is randomly inspected according to the same variety, site and specifications, so that the fineness modulus and particle gradation of the machine can be mastered.

3 Control during the construction process

3.1 Before laying the foundation, the weathered and loose soil and stones on the surface of the foundation should be removed first. When the foundation pit meets the qualification requirements, it should be immediately A layer of mortar is spread and the mortar is laid. The first batch of stones selected should be relatively square, ensuring that the large side is facing down and placed stably. When laying the second batch of stones, the upper and lower staggered joints of the first batch should be connected to each other. Make the masonry walking slab smooth and tidy and close to the pit wall. After the foundation is laid, it should be backfilled in time. The backfill should be filled layer by layer and compacted layer by layer to ensure the stability of the basic structure.

3.2 It is prohibited to fill with gravel blocks and then fill them with mortar during the leveling process, or to fill only mortar and gravel blocks to avoid empty masonry and mismatched stones. The result is a sandwich wall. In severe cases, "rat holes" of varying sizes will occur, reducing the strength of the masonry. The implementation of the filling and masonry method should be to pave the mortar according to the gaps of different sizes, and use appropriate stone chips to squeeze into the gaps. For excessively large gaps, two pieces of rubble should be used to fill the gaps tightly to prevent the stones from interacting with each other. Collision, thereby effectively improving the strength of the masonry.

3.3 The selected masonry should be in a smooth and neat state, with full mortar on the surface and neat joints. During the masonry construction process, concave joints with a depth of two centimeters should be left to ensure that the width and distance between the staggered joints are consistent with the construction requirements to avoid cracks and falling off. After the masonry is completed, sprinkling and curing should be carried out in time to avoid cracking and falling off. Through timely use of covering curing and sprinkling curing, the masonry should always be in a moist state. Usually, under normal temperature conditions, the curing time should exceed 7 days. .

3. Common roadbed slope protection types

1. Grass protection.

Plant protection is to plant grass or trees on the slope to slow down the water flow on the slope, and use plant roots to consolidate the surface soil of the slope to reduce erosion, thereby protecting the slope. Plant protection can not only beautify the highway environment, adjust the humidity and temperature of the slope, and play a role in consolidating and stabilizing the slope, but it is also relatively simple and economical.

For grass planting protection, drought-tolerant grass species with developed root systems, short stems and lush leaves should be selected according to local conditions. It is strictly forbidden to use turf growing in swamps.

2. Dry-laid rubble slope protection

Dry-laid rubble is suitable for protecting slopes from atmospheric precipitation and ground runoff, as well as protecting flooded embankment slopes from water erosion. Dry rubble protection is generally not suitable for severely wet or frozen road sections and areas that have been subject to active earth pressure for a long time. It can be used for soil slope protection in lower slopes and is suitable for soil (stone) cutting edges with a slope gentler than 1:1.5. slope.

3. Mortar rubble slope protection

Mortar rubble is installed on waterlogged embankments and easily weathered rocks and soil slopes with a slope of less than 1:1 that may be washed away. When the water flow velocity is high (such as 4-5m/s), the wave action is strong, and the river may be impacted by driftwood and other impact objects, it is advisable to use mortar rubble for protection. If necessary, it can be combined with a flooded retaining wall or protective wall. Set up on the wall at the same time.

The advantage of mortar rubble slope protection is that it has good durability, is suitable for protecting the rock and soil layers in different control slopes and slopes at different positions, and the cost is moderate, so it is suitable for general slopes in upper and lower slopes.

5. Arch and grid protection.

This protection method overcomes the characteristics of the fish-scale masonry’s weak drainage and erosion resistance and large amount of pollution. It maximizes the greening of the slope and has a better appearance. It is suitable for high-fill and long straight lines. The slope protection section can achieve the dual effects of beautification and greening, and is one of the commonly used methods for highway protection in recent years. The disadvantages are that the construction is cumbersome, labor-intensive, and has strict slope requirements. The arch must be filled with soil, grass, or other engineering protection, otherwise it will be easily eroded by rain.

6. Prefabricated block paving protection. This is one of the most commonly used slope protection methods on highways. Since the prefabricated blocks have consistent specifications, are easy to construct, and have a neat appearance, they minimize the dependence of slope protection on grass planting and greening, even if the slope greening effect is not good. , it is not easy to cause serious erosion, but the disadvantage is high cost and difficult construction.

7. Shotcrete protection. The shotcrete method is divided into plain spraying method and anchor spraying method. The plain spraying method directly sprays high-grade mortar onto a roughly flat rock surface to stabilize loose particles on the slope and ensure the safety of driving and pedestrians. It is used on weathered rock surfaces where the surface layer is easily loose. The working principle of the anchor spraying method is to use anchor rods to fix the sliding body on the mountain, use the anchor rod to restrict the sliding of the mountain, and hang a steel mesh on the anchor head on the surface of the sliding body and spray concrete to form reinforced concrete on the surface of the sliding body. The plate structure can fix the loose rocks into a whole to achieve the purpose of completely eradicating landslides. The anchor spraying method is suitable for most geological structures such as rock soil and gravel soil, mountains and areas prone to landslides. It can prevent the occurrence of early landslides. This method can be used as long as the area where sliding occurs is not very large and the sliding layer is not very thick.

The quality of roadbed protection projects directly affects the quality of the entire highway project. Therefore, full attention should be paid to the design and construction of roadbed protection projects. Extensive investigation and in-depth research should be conducted during design to formulate reasonable and scientific protection plans. During construction, scientific management and meticulous construction should be carried out to improve construction quality and give full play to the comprehensive benefits of national investment. benefit.

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