1 Construction characteristics of subgrade excavation
Subgrade excavation, also known as cutting, is a key point in subgrade engineering construction. Especially in mountainous and hilly areas, subgrade excavation is often the key to control the progress of the project. After the highway is completed and opened to traffic, the excavation section is the maintenance focus of the highway maintenance department. Because both excavation and cutting are composed of natural strata, natural strata generally have complex geological structure in the long process of generation and evolution. The excavation cutting slope on the surface of the earth's crust is influenced by natural and human factors, including hydrology, hydrogeology, surface water, climate, topography, design and construction scheme, etc. It is more prone to deformation and failure than embankment slope.
Subgrade diseases mostly occur in cutting excavation areas, such as landslides, collapses, falling rocks, subgrade frost boiling, etc. The excavation of large-section subgrade destroyed the original mountain balance and the balance between underground and surface water. If the construction scheme is unreasonable, the slope is too steep, the waste is too close, the grass planting slope, pavement and retaining wall are not constructed in time, and the drainage is not smooth, it will cause the cutting slope to be unstable and landslide, and even affect the whole project progress in serious cases, which is a common fault in the excavation subgrade construction. Construction personnel should check design review, construction scheme selection and on-site geological and hydrological investigation, and do a good job in excavation subgrade construction.
2 Preparation before subgrade excavation construction
2. 1 survey lofting. Construction restoration alignment survey and construction lofting are the main technical work in the construction preparation stage. The contractor shall carry out retest according to the design drawings, coordinates of traverse points and elevation of leveling points provided by the supervision engineer in writing, and submit the retest data to the supervision engineer for review after closing. The contractor shall pay off the construction according to the pay-off data approved by the supervision engineer. According to the specification, before subgrade construction, the center line piles, boundary piles for subgrade land, cutting slopes, side ditches, borrow pits, berms, spoil piles, etc. Should be in accordance with the design drawings, construction technology and the relevant provisions of the recovery. The straight-line part of the road centerline pile is set every 20m, and the permanent fixed pile is set every 100m. The starting point, ending point, circular slow point and circular slow point of the curve are set with fixed piles. After measuring the centerline pile, measure the cross section, and then set out the side pile according to the subgrade cross section and the measured elevation. At the top of the slope in the excavation section, the side piles in the excavation section are generally set at a certain distance from the side piles to prevent the side piles from being lost and recovered in time.
2.2 Review and test before construction. Before subgrade construction, the constructors should make a detailed investigation on the geological and hydrological conditions of the subgrade engineering scope, determine its nature and scope through sampling tests, and understand the treatment methods of special soil by existing buildings nearby. We should master the degree of weathering and cracking, bedding, joints and schistosity of rock strata in rock areas, and pay special attention to faults and geological changes in areas prone to collapse.
Geotechnical test sampling should generally be based on the data provided by design documents, and each soil type should be sampled in not less than three groups; There are also geotechnical engineering routines or tests based on pile number sampling. According to Technical Specification for Highway Subgrade Construction (JTJ033-95), the test method of soil used as filler in excavation and borrow yard should be handled according to Standard for Highway Geotechnical Test (JTJ05 1-93).
2.3 Drainage facilities for cutting ahead of excavation. Because water is the main cause of cutting diseases, no matter what kind of excavation method is adopted, effective drainage should be ensured during excavation and after completion.
(1) Do a good job of intercepting ditch before cutting excavation, and build temporary drainage ditch when excavating in layers.
(2) Temporary drainage facilities should be combined with permanent drainage facilities, and running water should not be discharged into farmland and cultivated land, polluting natural water sources or causing siltation and erosion.
(3) During cutting construction, attention should be paid to maintaining drainage channels frequently to ensure smooth running water. Drainage ditches in permeable soil or rapids should be reinforced to prevent leakage and erosion. In areas with poor hydrogeological conditions, drainage at the top of cutting must be strictly done.
(4) Take away all the surface water and groundwater that may affect the slope stability, and keep a certain longitudinal slope (one-way or two-way) along the cutting line to facilitate drainage.
3 Cutting excavation
The elevation of subgrade in earthwork section shall be excavated, and the settlement caused by compaction shall be considered. The settlement value should be determined by the laboratory. The compaction degree of 80cm below the top surface of subgrade should reach 95%, and it should be inspected in strict accordance with the heavy compaction method in JTJ05 1-93 Code for Highway Geotechnical Test. Do not meet the requirements, take compaction or other measures to deal with, and report to the supervision engineer for approval.
3. 1 excavation principle.
(1) excavate by layers according to the designed slope ratio, and the excavation depth of each layer is determined according to the convenience of mechanical trimming of the slope.
(2) In the excavation of natural soft soil layer, the problem of transporting waste soil abroad should be considered, so as to ensure the smooth access road at the excavation site, reasonably organize on-site traffic, and consider the tonnage of unit transportation equipment.
(3) Rock blasting is mainly small blasting and loose blasting, and excessive blasting is strictly prohibited. Smooth blasting and presplitting blasting techniques are adopted within 2m of the slope.
(4) Cutting excavation shall be carried out in an orderly manner by means of "horizontal layering, longitudinal segmentation, synchronous excavation at both ends and step by step".
(5) Pay attention to civilized construction at the excavation site to ensure orderly construction, safe production and civilized construction.
3.2 Cutting excavation method. According to the depth and longitudinal length of cutting, the excavation methods of earthwork cutting can be divided into three types: horizontal excavation, longitudinal excavation and mixed excavation.
(1) horizontal excavation method. The method of gradually excavating the width and depth of the whole section of cutting from one end or both ends is called transverse excavation method or one-layer transverse full-width excavation method, which is suitable for cutting with small excavation depth and short excavation depth. Multi-layer transverse full-width excavation method is suitable for deep cutting and short-circuit cutting excavation. When the earthwork volume is large, each floor should be pulled apart vertically to realize multi-layer and multi-directional excavation, which can arrange more labor and construction machinery and speed up the construction progress. The excavation depth of each layer depends on the convenience of work and the safety of construction. In the construction of manual horizontal excavation method, it is generally1.5 ~ 2.0m; When the mechanical horizontal excavation method is adopted, the depth of each step can be increased to 3m~4m. Horizontal excavation method is suitable for mechanized construction, and it is more advantageous to pile up soil with bulldozers and cooperate with loaders and dump trucks to transport soil. Slope trimming and drainage ditch construction are completed by manpower and grader.
(2) Longitudinal excavation method. Layered longitudinal excavation method: the operation method of excavating along the whole width of the cutting with a small depth is called layered longitudinal excavation method, which is suitable for long cutting excavation. During construction, when the cutting length is short (no more than 100 m), the excavation depth is no more than 3m, and the ground is steep, bulldozers should be used for operation. The transportation distance should be 20 ~ 70 m, and the longest distance should not be greater than100 m. When the cross slope of the ground is gentle, the surface should be shoveled horizontally and the lower soil should be pushed longitudinally. When the lateral width of the cutting is large, two or more bulldozers should be used. When the cutting front is close to the steep hillside, it is advisable to pile up soil with an oblique shovel. Tunnel longitudinal excavation method: excavate a tunnel longitudinally along the cutting, and then widen the tunnel to both sides. After the upper tunnel is widened to the cutting slope, the lower tunnel is excavated in this direction until it reaches the elevation of the top surface of the excavated subgrade. This is an effective method for rapid construction. Tunnel can be used as a road for machinery to pass through and transport earthwork vehicles, which is convenient for the flow operation of earthwork excavation and transportation. Segmented longitudinal excavation method: select one or several suitable places along the longitudinal direction of the cutting, excavate the cutting horizontally on the thinner side, divide the cutting into two or more segments according to the longitudinal pile number, and then excavate each segment longitudinally. This method is suitable for cutting excavation along mountain roads, where the cutting is too long, the transportation distance of spoil is too long, or the cutting wall on one side is not thick. At the same time, it should also meet the conditions that there is an approved spoil ground in the middle section and there are redundant excavation and spoil in the earthwork allocation plan.
(3) Mixed excavation method. In other words, the horizontal excavation method is mixed with the longitudinal excavation method, which is suitable for cutting with large longitudinal length and excavation depth. First, cut is excavated longitudinally, and then along the transverse slope, so as to increase the excavation slope. Each slope should be equipped with a mechanical construction team for operation.
3.3 Matters needing attention in the process of excavation.
(1) Do a good job of cutting the top and intercepting the drainage, and pay attention to check it at any time. Temporary drainage facilities are combined with permanent drainage facilities.
(2) During the excavation, assign special personnel to carefully investigate the stability of the excavated slope, reinforce it in time when problems are found, and do a good job in the investigation and exploration of underground equipment.
(3) Strengthen survey control, and shape with excavation to keep the slope smooth.
(4) During excavation of soil cutting in rainy season, excavation shall be carried out in layers, with the longitudinal slope of the bottom surface of each layer greater than 1%, and 30cm thick shall be reserved along the slope of the excavated slope. After the rain, it shall be renovated to the design slope line, and the cutting shall stop excavation when it is 30cm away from the top surface of the foundation, and then it shall be excavated to the design elevation after the rainy season.
(5) In winter construction, when the unfinished soil cutting and foundation pit start, the surface of the excavation surface should be turned loose for 30~40cm and raked flat as the insulation layer to prevent freezing; After the excavation, the surface layer shall be pre-covered with soil or covered with straw bags to loosen the soil, which shall be removed when the construction continues. Immediately after earthwork excavation, the superstructure shall be constructed to prevent the basement from freezing; In case of process intermittence, it shall be treated as winter protective measures.
(6) During earthwork excavation, properly protect underground pipelines, cables, cultural relics and other structures. When excavating earthwork near residential areas, take effective measures to ensure the safety of residents and construction workers, and provide effective temporary access roads or temporary bridges for the lives of nearby residents.
(7) The elevation of subgrade top surface of earthwork section shall be determined by experiments considering the settlement caused by compaction. The compaction degree of 30cm below the top surface of subgrade shall not be less than 95%.
(8) For excavation of rock blasting subgrade, a warning line should be set up with complete safety signs, and professional personnel should be set up for supervision and management to ensure the safety of production and life.
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