The following is the relevant content about highway subgrade construction technology brought to you by Zhongda Consulting for your reference.
1 Subgrade filling and compaction
The strength and stability of the highway subgrade largely depend on the nature of the subgrade fill and the degree of compaction. Starting from the existing conditions, improving the filling requirements and compaction conditions is the most effective and economical way to ensure the quality of the roadbed.
1.1 Roadbed filler. The specification stipulates the conditions for the selection of roadbed fillers. Quantitative standards are given for the minimum strength and maximum particle size of subgrade fillers, the CBR value is used to characterize the strength of subgrade soil, and the concept of road bed is introduced. Restrictive conditions are put forward for the filling of the upper road bed. The CBR value of the road bed filling 0~30cm below the bottom of the highway and first-class highway pavement should be greater than 8. The lower road bed and the filling soil below are also given corresponding specified value.
1.2 Roadbed compaction. In current roadbed construction, large-tonnage road rollers are generally used, and the rolling effect has been significantly improved. It plays a very good role in improving the compaction of subgrade soil. The specification stipulates that the compaction degree of the upper embankment 80~150cm below the bottom of the road surface of expressways and first-class highways must be ≥95%. When paving high-grade pavements for other grades of highways, the compaction degree should also be the same as that of expressways and first-class highways. standards adopted.
1.3 Compaction of subgrade soil in special humid areas. In particularly humid areas, compaction on the roadbed is quite difficult, and the specification has made several adjustments: First, the compaction standard can be determined based on test data or reduced by 2 to 3 percentage points from the listed value; second, for natural When clay soil with a consistency less than 1.1, a liquid limit greater than 40, and a plasticity index greater than 18 is used as filler for the lower road bed and the embankment below it, the prescribed light compaction standards can be used; the third is to improve the properties of the filler and use it in the soil. The desired effect can usually be achieved by adding quicklime, and new water-absorbent materials can also be used for reinforcement.
1.4 Loess roadbed filling and compaction. (1) During the construction of loess embankment, the interface between fill and excavation should be combined (vertically), weeds on the slope should be removed, and inward-sloping steps should be dug. If the joint surface is steep and cannot be dug into steps, geotechnical nails can be used to strengthen the joint. If the foundation soil layer has strong collapsibility or high compressibility, and the allowable bearing capacity is lower than the self-weight pressure of the embankment, the use of heavy hammer tamping and lime pile compaction reinforcement may be considered. (2) If the moisture content of the loess is too small, water should be added evenly before rolling; if the moisture content is too high, it can be loosened and dried until the moisture content is required before rolling, or an appropriate amount of lime can be added to reduce the moisture content. After mixing with ash, the soil and ash should be mixed evenly, and the maximum dry density should be determined through a compaction test. (3) Old loess has poor water permeability, is difficult to adjust dry and wet, and large pieces of soil are difficult to crush. Measures should be determined through testing before use. Old loess shall not be used as road bed filler. New loess is a good filler and can be used to fill road beds. Loess embankments should be filled in layers and compacted in layers. Blocks larger than 10cm must be broken up and compacted close to the optimal moisture content for compaction. (4) According to the design, build the water retaining structure, intercepting ditch structure and rapid chute on the outer edge in time to lead the water away from the foot of the slope. For embankments with a height greater than 20m, the design should allow for the damage caused by the compaction and consolidation of the embankment after completion. The amount of compression sinkage. (5) Special attention should be paid to roadbed drainage in loess areas. The principles of interception, dispersion, erosion prevention, anti-seepage, and remote transmission of surface water should be adopted. Comprehensive drainage facilities should be built in a timely manner according to the design to quickly divert water away from the roadbed. When the water from the side ditch is drawn out at the junction of fill and excavation, the water outlet should be reinforced.
2 Subgrade pavement drainage
Water is another important factor affecting the strength and stability of the subgrade. Many subgrade diseases are caused by water erosion. In addition, from the perspective of protecting the environment and not Considering the damage to local farmland water conservancy facilities, roadbed drainage must also be done to form a drainage system and coordinated with regional drainage planning. During roadbed construction, attention should be paid to construction drainage to prevent flooding caused by various reasons and causing unnecessary losses to the roadbed and pavement construction.
2.1 Ground drainage. The most commonly used surface drainage facilities are side ditches, interception ditches, falls, rapids and surface drainage pipes. For drainage ditches on highways and first-class roads, paving protection is generally required. Mortar masonry is commonly used for reinforcement, and cement concrete prefabricated panels have also begun to be widely used. For highways and first-class highways that pass through water network sections, the past practice of setting up culverts at every ditch has been improved in some places. The irrigation ditches on both sides of the route have been systematically rearranged, eliminating the need for drainage and irrigation culverts across the route, thereby improving the efficiency of the roadbed. Engineering quality of roadbed.
2.2 Pavement drainage. The task of pavement drainage is to quickly remove precipitation within the pavement range and reduce water seepage from the pavement so that it does not wash away the roadbed slope. The cross slope of the road crown should be ≥2%.
There are two ways to drain rainwater from the road surface. The first is centralized drainage. A water block of cement concrete prefabricated blocks or cast-in-place asphalt concrete is set outside the hard shoulder to form a triangular water collection tank with the hard shoulder pavement. A drain outlet is set up every 20 to 50 meters apart from the hard shoulder. The rapids on the embankment slope are connected to drain the rainwater into the drainage ditch at the foot of the slope.
The second type is decentralized drainage, which is mostly used in long sections with flat terrain and longitudinal slopes less than 0.3% in the northwest. In addition to hardening road shoulders and reinforcing roadbed slopes, when passing through oasis areas with high groundwater levels, it is also necessary to prevent the upper part of the slope from The upward growth of grass blocks the lateral drainage outlet and causes water accumulation on the road surface. The improvement method is to harden the road shoulders, set up shoulder drainage ditches, and increase ditch slope drainage.
2.3 Underground drainage. Underground drainage on roadbeds still mostly uses underground ditches, blind ditches, seepage ditches, seepage wells, etc., which are characterized by permeability drainage. When the water flow is large, seepage ditches with seepage pipes are often used. The traditional sand and gravel filter layer is often replaced by geotextile with filter function. The reinforced soft permeable pipe composed of steel rings, filter cloth and reinforced synthetic fibers developed in recent years has a diameter of 8~30cm, which is very suitable for underground drainage. .
3 Roadbed protection
The construction of roadbed changes the natural balance of the stratum, and the roadbed is exposed to space and is constantly eroded by various intricate natural factors. Therefore, various types of roadbed protection are required. protection.
3.1 Slope protection. The purpose of slope protection is to prevent surface water erosion, weathering and peeling of slope rock and soil, and to coordinate with the environment. In recent years, with the emphasis on environmental protection, grass protection is often used on the slopes of high-grade highways. When the slope is higher, masonry frames (square, diamond, arch, M-shaped) are used for grass protection. Due to the drought and water shortage in the west, the choice of slope grass protection type is very important. Nowadays, lawn planting belts are mostly used, that is, grass seeds, fertilizer and soil are evenly mixed and wrapped in geotechnical materials. When the grass seeds germinate, they will grow into grass and solidify. After the soil is used, the non-woven fibers will naturally decay and will not pollute the environment. The effect is very good.
3.2 Scouring protection. Direct protection is still mostly used to protect roadbed slopes along rivers from erosion. Traditional stone masonry, riprap, wire gabions, retaining walls, etc. have been improved. High-strength geogrids are used instead of wires to make gabions, and polyester or polyurethane geotextile concrete slope protection mold bags are used to protect the panels. It protects the slopes hit by water waves and is very adaptable to uneven settlement of soil.
3.3 Supporting protection. Retaining walls are still mainly used for retaining and protection. Stone gravity retaining walls are mostly used in places with rich stone materials, low wall heights and good foundations; the stress comparison of cantilever retaining walls, buttress retaining walls and slab-column retaining walls with reinforced concrete structures Reasonable and small in size, wall masonry has also been widely used in the protection of highway roadbeds. Stacked retaining walls are easy to adjust the height of the wall and are assembled using prefabricated components. They are a special type of retaining wall.
4 Soft soil foundation treatment
In recent years, with the rapid development of highways and first-class highway construction, soft soil foundations have been used to prevent embankment instability and settlement observation and control. , soft soil foundation treatment technology and other aspects have achieved remarkable results. The settlement rate of the treated soft soil foundation is used as a settlement control method for pavement paving time, which solves the key technical problem of building an advanced pavement on the soft soil foundation at one time (rather than paving a transitional pavement in the early stage).
4.1 Lime soil compaction pile. When the moisture content of the soft soil layer is too high or too low, lime soil compaction piles are used. If the moisture content is too high, you can fill the hole with dry soil powder or lime powder to absorb part of the water, or quickly form the hole for watering, or install casing while forming the hole, or add casing after forming the hole; if the moisture content is too small, you should Pre-wet the soil layer within the reinforcement range. The order of forming holes should be outer circle first, then inner circle and at intervals. The holes that have been formed should be prevented from being soaked by water and should be backfilled and compacted daily. In order to avoid necking and blockage caused by tamping, If it is cold, one hole should be drilled and the other filled; when the pile holes are dense and the soil is soft, jumping and compacting should be done at intervals.
4.2 Lightweight embankment. Filling the embankment with lightweight materials alleviates the requirements on the bearing capacity of the foundation. At present, there is successful experience in using fly ash to fill embankments in China, which can reduce the weight of embankments by about 25%. The maximum dry bulk weight measured using the heavy compaction test method is 9~12KN/m3. Silica drill type fly ash has low viscosity and no plasticity, but its liquid limit is about 64%, the optimal moisture content is 37% to 41%, and it has good compaction performance. The surface 1~2m of the fly ash embankment slope is covered with clay soil to stabilize the slope and facilitate the growth of grass. The top surface of the road bed is sealed with a coarse-grained layer 0.3~0.5m thick.
4.3 Geosynthetic material reinforcement. The thickness of the soft soil layer is 3~5m, which can be treated jointly with geotextile and sand cushion. The thickness of the drainage sand cushion can be reduced from 50cm to 30cm. There are also multiple layers of geotextiles laid between the underside of the embankment and the surface. The high tensile strength of the material is used to overcome the sliding deformation of the foundation to maintain stability. By controlling the filling rate and combining with overload pre-pressure, the foundation is quickly consolidated.
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