Process overview

Sand and gravel cushion is a layer of sand and gravel laid at the bottom of embankment. As the bearing layer of foundation, it is compacted in layers to improve the foundation strength at the lower part of foundation, and reduce the compressive stress and deformation of foundation through the pressure diffusion of cushion. At the same time, the cushion can play a drainage role, and the pore water in the foundation soil can be quickly discharged through the cushion, which accelerates the settlement and consolidation of the lower soil.

This technology is suitable for the surface construction of soft soil subgrade which is rich in local sand and gravel resources and treated by drainage consolidation method (such as bagged sand drain, plastic drainage board, sand drain, etc.). ), pile handling and replacement.

6.2, technical requirements

6.2. 1, construction preparation

1. construction site

(1) Clean up the site, and dig out all the silt, roots, turf and humus on the original ground within the subgrade. The original ground base is dense and flat, and the potholes should be filled up to form an outward drainage slope of about 2%. Construction of temporary drainage ditch will drain the accumulated water in the site, so that the surrounding water will no longer enter the site, and rainwater and seepage will be discharged at any time. At the same time, do a good job of temporary storage of sand and gravel cushion materials.

(2) Build or expand existing roads, and prepare roads for the entry of bedding materials and machinery.

(3) Power-on preparation: connect to the local power grid or prepare a generator to meet the construction power demand.

(4) Water supply preparation: drilling wells or pulling water. When the cushion is medium-coarse sand, the irrigation settlement method should be adopted.

2. Material preparation

Choose sand and gravel cushion filler.

3. Mechanical equipment preparation

Entry, overhaul, maintenance and trial operation of mechanical equipment, and purchase of spare parts and consumables.

4. Technical preparation

(1) compaction process test, the length of the test section shall not be less than 100m in principle, so as to determine the mechanical configuration, loose laying thickness, compaction times, roller speed and other parameters, and report to the supervisor for approval. Prepare cushion construction scheme, and make technical disclosure to operators after approval.

(2) Collect site engineering geological data and hydrogeological data.

(3) Check the design geology, and use test equipment to check if necessary to determine the scope and thickness of cushion construction.

6.2.2, survey lofting

1. According to the control points provided by the design, use the total station to accurately set out the plane scope of cushion construction, and use the level to accurately measure the thickness to be replaced.

2. After acceptance, stake the original ground to control the elevation, so as to control the thickness of each layer of cushion filler.

6.2.3. Clean and level the original ground. Manually clean and level the ground with bulldozers and dump trucks, and roll the base according to the design.

6.2.4. Using grader to make 2% ~ 4% lateral dirt road arch is beneficial to drainage.

6.2.5, layered filling and rolling:

1. The sand and gravel cushion should be filled and rolled in layers, and the thickness of each layer is 20 ~ 30cm. Construction process of each floor: sanding ~ sprinkling ~ compaction ~ testing, and rolling by vibratory roller. The optimum water content of gravel cushion is 8% ~ 12%.

2. When used as humiliating cushion of composite foundation, it shall not be vibrated and rolled to ensure the strength of foundation pile is not damaged.

3. During the longitudinal sectional construction, the joints shall be made into slopes or stepped lap joints, with each layer of sand and gravel packing staggered by 2.0m, and the joint sections shall be fully compacted.

4. When the cushion is laid, it is forbidden to disturb the weak layer at the bottom of the cushion to prevent it from being trampled, frozen or soaked and reduce its strength. If there is a muddy soil layer with small thickness under the cushion, when riprap can squeeze into the bottom of this layer under the rolling load, silt squeezing treatment can be adopted.

Compaction test

According to the test method specified in Code for Geotechnical Test of Railway Engineering (TB 10 102). The relative density of sand cushion should be detected by ring cutter method, and the foundation coefficient of gravel cushion should be detected by K30 instrument.

6.2.7 After the cushion is laid, the next working procedure shall be carried out. Cars and people are forbidden to walk on the sand. They should walk on the mat if necessary. In order to prevent the loss of sand and gravel cushion, the slope toe on both sides of the cushion should be dry-laid in time according to the design, and the filter layer should be made at the same time.

8, safety and environmental protection considerations

8. 1 construction safety

1. Construction machinery shall operate in parallel, strengthen site safety management and prevent mechanical injury accidents.

2. Train the mechanical operators and safety inspectors, and hold relevant certificates after passing the examination.

3. Safety inspectors should implement the "three inspections" system, that is, inspection before work, supervision during work and review after work, so as to eliminate hidden dangers of accidents.

8.2, environmental protection

1. The site working face shall be provided with drainage facilities, which shall be combined with permanent drainage. The discharged rainwater shall not flow into farmland and cultivated land; It will not cause ditch siltation and subgrade erosion. Paddy fields and weitang areas should be drained and dredged or dried and compacted according to their specific conditions.

2. The spoil is piled up in the designated spoil ground and protected. It is strictly forbidden to put it in a heap.

9, quality records

(1) Make test and measurement data in time;

(2) If the quantity of works needs to be changed, the information signed by the construction, supervision, design and owner shall be prepared in time;

(3) Complete the inspection lot and submit the materials;

(4) Construction image data.