Gravel cushion is the secondary bearing layer of pavement structure, which mainly plays the role of water permeability and frost resistance, and its thickness is generally not suitable for 15cm. If the old road is in good shape, there may be no progressive layer. The traffic volume of rural roads is small and the technical level is low. The cushion can be made of natural gravel, stone slag, slag and other permeable materials. Gravel cushion is mainly suitable for rural roads in mountainous areas, which not only makes full use of the rich local gravel resources, but also reduces the project cost. Its construction is relatively simple, and farmers along the line can be organized to carry out construction under the guidance of professional and technical personnel.

A, construction machinery

Commonly used construction machinery for gravel cushion are: trucks, bulldozers, graders, road rollers, etc.

Second, the raw material requirements

Gravel can be collected from the floodplain near the railway line. Natural gravel should be clean, uniform in particle size, basically reasonable in gradation, low in silt content, with crushing value generally not greater than 35% and maximum material diameter generally not greater than 6.5cm. If the local gravel is generally not good, the maximum particle size can be appropriately relaxed according to the actual situation.

Three. Construction technology of gravel cushion

The construction technology of gravel cushion includes: construction lofting, material preparation, gravel paving, watering, shaping and rolling.

1, survey lofting

During construction lofting, the center line should be restored, and a stake should be set every 15 to 20 meters for straight section and every 10 to 15 meters for curved section, and the design elevation of cushion should be marked with obvious signs such as red paint. Generally, the width of cushion should be wider than that of base 15cm, and the sideline can be controlled by lime spraying line.

2. Soil lifting tank

Conditional areas should hang lines on adjacent side piles on both sides of the cushion in turn to cultivate soil grooves, which are about 30 cm higher than the cushion. Temporary drainage ditches shall be excavated alternately every 5m to10m to avoid water accumulation in rainy days.

Step 3 prepare materials

Before preparing materials, the required stone quality should be calculated according to the length, width, thickness and stone density of the construction section, and the pile spacing of stones should be calculated according to the water content of the sand grams in the yard and the tonnage of the truck.

Total mass of gravel for cushion = length × width × gravel density or total mass of cushion = tonnage of transport vehicles × number of vehicles.

4. Make the schematic diagram of buffer structure (schematic diagram of material delivery vehicle)

The loading quantity of each truck should be basically the same, the unloading should be from far to near, and the stacking distance should be basically equal to avoid too much or not enough gravel after leveling; The transportation of gravel can be completed a few days before the formal construction, and the stacking time should not be too long.

5, paving test section

Before the formal construction, the test section of 200 ~ 300m should be paved, and the construction parameters such as loose paving thickness, optimum water content, rolling times, combination of construction personnel and mechanical equipment should be summarized to guide the formal construction.

6. Paving, watering and shaping

Cushion construction should be carried out in sections, and the length of each operation section is about 500 meters. Roughly level the gravel pile with bulldozer, and then sprinkle enough water evenly with sprinkler. Then use the grader to level the gravel repeatedly until the gravel is evenly spread on the subgrade to form the specified road arch. At the same time, manual cooperation is needed to prevent local material shortage or rough material concentration.

Step 7 rotate

After the gravel shaping is completed, it will be rolled by a roller. The straight line segment without superelevation and the flat curve segment are rolled from both sides to the center of the road, and the superelevation segment of the flat curve is rolled from low to high. When rolling, the wheel width should overlap 1/2. First, slow static pressure 1 ~ 2 times, then vibrate and roll for 4 ~ 6 times until the specified compaction degree is reached.

Fourth, the construction quality inspection

After the crushed stone cushion is formed by rolling, the compactness, flatness, longitudinal height, width, thickness, cross slope and strength index shall be detected in time according to the relevant requirements of construction drawing design and specifications, and the grass-roots construction can only be carried out after all are qualified.