1 Characteristics and causes of common quality defects in subgrade engineering
1. 1 feature
The whole or local uneven settlement of subgrade; Longitudinal and transverse cracking of subgrade; Subgrade sliding or slope collapse.
1.2 reason
The engineering geological conditions are poor and the original foundation is weak (such as swamp area, etc.). ). If the soil is not changed or the soft foundation is not treated before filling, it is easy to form compression settlement or extrusion position;
The engineering terrain conditions are complex. When the embankment passes through the valley, the fill in the center of the valley is the largest, and gradually decreases at both ends, resulting in uneven settlement due to different fill heights.
Hydrological and climatic factors, such as excessive rainfall, flood, freezing, snow or excessive temperature difference, may cause uneven settlement of high embankment; Embankment filler, if the filler is mixed with inferior soil such as planting soil, humus soil or swamp, or the soil contains unbroken large pieces of soil or frozen soil, the stone-filled road stones have different specifications and uneven properties, and the rubble gap is large, which may cause obvious local subsidence in a certain period (such as rainy season); In terms of design, such as unreasonable section size, inappropriate slope value, improper drainage, protection and reinforcement, no checking calculation of the stability of high fill embankment, and no special requirements for construction technology and filler;
In terms of construction, the filling sequence is improper, the filling is not layered in the whole width range, the filling thickness does not meet the requirements, the filling quality does not meet the requirements, and the water stability is poor. The original roadside slope was not cleared of vegetation and roots, and no step treatment was carried out; Due to the difference of compressibility and water resistance of different soils, the mixed filling of different fillers leads to uneven settlement, the water content of subgrade fillers is not strictly controlled, and there is no large-scale leveling and rolling equipment, which makes the degree of compaction fail to meet the requirements. No attention was paid to drainage during construction. In rainy days, there is serious water accumulation in subgrade, so it is impossible to drain water by itself. Part of the accumulated water is immersed in the subgrade to form a water bag. In sunny days, it is not excluded to control the water content of accumulated water to continue the construction, resulting in hidden dangers. The construction unit has a weak sense of responsibility and the self-inspection control is not in place.
2 Preventive and therapeutic measures
2. 1 design
2. 1. 1 Do a good job in geological exploration and investigation.
Investigate the topography, geomorphology and hydrogeological conditions of the route in detail, especially provide detailed design data for special subgrade sections. For sections with poor surface, measures such as changing soil or mixing lime, cement and laying geotextile can be considered in the design.
2. 1.2 Ensure the minimum filling height of subgrade.
It is necessary to ensure that the minimum filling height of subgrade will not reduce its stability due to the influence of surface water, groundwater, capillary water and frost heaving. According to the requirements of subgrade design code, the minimum filling height of subgrade should be determined according to the dry and wet types of soil foundation and the height of capillary water level. When the height of subgrade filling is limited and can't meet the specification requirements, corresponding treatment measures should be taken, such as replacing permeable materials such as gravel and stone slag, setting isolation layer or building underground seepage ditch to prevent surface water and groundwater from infiltrating into subgrade and affecting subgrade operation area.
Earth excavation subgrade should be filled with gravel not less than 60cm, stone excavation subgrade should be provided with gravel cushion of 30cm, and blind ditches should be added to the sections with poor lateral drainage.
2. 1.3 define the quality standard requirements of subgrade filler.
In the construction drawing design of railway engineering at all levels, it is necessary to clarify the CBR value (minimum strength) and maximum particle size requirements of subgrade fillers with different filling heights. It is strictly forbidden to directly fill subgrade with inferior soil such as planting soil, humus soil, silt frozen soil and strong expansive soil. Gravel (breccia) soil should be preferred as subgrade filler, and fine-grained soil with poor soil quality can be filled at the bottom of embankment.
2. 1.4 Improve the comprehensive drainage design of subgrade
In the design of railway engineering at all levels, we must follow the principle of adjusting measures to local conditions and making overall consideration, and carry out longitudinal and transverse drainage design of subgrade to avoid long-term flooding of subgrade on both sides of subgrade and settlement deformation of subgrade bearing capacity decline surface. Drainage ditch must be set in the village section, and longitudinal slope must be set in the side ditch in the flat slope section to ensure smooth drainage.
Centralized drainage measures are taken for high fill sections, and warning piles and collision walls are considered as a whole. It is required to set simple or permanent drainage troughs at intervals of 20-40m and major slope change points. In the excavation section, the intercepting ditch is designed according to the uphill catchment, and considering the slope soil and slope, a retaining wall is set to prevent landslide. The lower section of subgrade can be paved with gravel layer and seepage blind ditch, and drainage measures such as enlarging and deepening side ditch can be taken.
2. 1.5 to ensure the stability of subgrade slope.
The slope of high-filled deep excavation subgrade should be determined according to the specifications of filler type, slope height and engineering geological conditions, and the stability of subgrade must be checked for high-filled embankment. When the fill slope is too high, it can be considered to add a slope platform in the middle of the slope.
2. 1.6 actively adopt the form of comprehensive protection of subgrade.
Actively promote the comprehensive protection form of combining plant protection with hard protection, and take plant protection measures such as planting grass, spreading turf and planting trees on relatively stable soil slopes. Engineering protection measures such as facing wall and masonry can be adopted for the fill slope with severe rock weathering, developed joints, soft rock, loose broken (gravel) rock soil and water erosion, and direct protection measures such as retaining wall, masonry slope protection and gabion riprap can be adopted for the subgrade along the river and the sections damaged by ice and erosion.
2.2 Construction aspect
2.2. 1 It is an important link to ensure the construction quality of high-fill subgrade to do a good job in the construction organization design, reasonably arrange the sequence of construction sections, clarify the connection relationship between structures and subgrade, give priority to the construction of high-fill sections, take the construction organization design as the leader in the construction, and rationally allocate personnel and equipment according to the actual situation of the construction site.
2.2.2 Make preparations before construction, carefully review the design documents before construction, learn more about the filling and excavation conditions, geological conditions, soil filling and excavation conditions and deployment conditions of each road section, focus on investigating important road sections, further check the design data, and report to the owner in time if any errors are found in the design documents, so as to properly handle them.
2.2.3 Carefully remove the bad soil quality of the topsoil and strengthen the foundation compaction treatment. Surface vegetation, tree roots, garbage and poor soil quality (saline soil, expansive soil, etc.). ) must be deleted. At the same time, the compaction density of the surface should be increased and treated with a large-tonnage vibratory roller.
2.2.4 Before subgrade filling, the vertical and horizontal drainage systems on both sides of subgrade must be dredged to avoid subgrade being soaked by water. In particular, the foundation soil is fine-grained soil such as loess and clay, which has strong structure and bearing capacity in dry state (optimal water content). Once soaked in water, it is easy to form frost boiling or subgrade settlement. Therefore, it is particularly important to make drainage unblocked before subgrade construction, and project supervision and construction quality self-inspection personnel should carefully supervise it; Secondly, we should strictly choose the soil for subgrade filling. Before determining subgrade filler, soil analysis, CBR value, standard compaction test and other tests are needed. For inferior soils such as planting soil, humus soil, silt and strong expansive soil, and materials whose CBR value and maximum particle size can not meet the requirements of the specification, they cannot be used for subgrade filling; Moreover, before subgrade filling, construction lofting should be carried out according to the design, semi-permanent temporary leveling points and coordinate points should be established and recorded. The lofting of subgrade slope toe must be accurate to ensure that the subgrade width meets the design requirements. Weeds, roots, mud, etc. The slope angle in the subgrade should be removed, and the degree of compaction should meet the specification requirements. In the widening, half-filling and half-digging section of the old road, the inner inclined steps with a width of not less than 6m should be made.
2.2.5 During the construction of stone-filled subgrade and chicken-claw-shaped section subgrade, heavy tamping equipment can be used for dynamic compaction, or geogrids (geotextiles) can be laid horizontally in layers in the stone-filled embankment to prevent or slow down the flow of fine materials in the filler gap.
2.2.6 Subgrade construction must be filled by layers and rolled by layers. It is forbidden to roll fill in the road reconstruction project. Generally, the compaction degree of the road section is not more than 30cm, and the loose laying thickness on both sides of the structure (bridge and culvert head treatment) is not more than 20cm. Soils with different properties cannot be mixed, and the filling thickness of the same soil should not be less than 50cm (two layers). Subgrade filling must be carried out in full width, once in place, and it is forbidden to widen it. During the rolling process, the water content should be well controlled, and the subsequent construction can only be carried out after the compaction degree meets the specification requirements. Each layer of 2000m2 (less than 2000m2) shall be tested for at least 4 points of compaction. According to different fill types and compaction thickness, the compaction equipment of gravel soil vibratory roller with double functions of rolling and vibration is selected, and the effect is good.
2.2.7 Cutting construction should ensure smooth drainage, and attention should be paid to ensuring slope stability during uphill construction to avoid under-excavation or over-excavation. Small and medium-sized guns and smooth blasting shall be used as far as possible in rock blasting to avoid large-scale loosening and slope instability caused by large-scale blasting. During mechanical excavation, the slope shall be equipped with grader or manual trimming. If there is overbreak on the top surface of the road bed, the loose blocks should be removed, backfilled with permeable materials, and the compaction degree should be controlled according to the road bed engineering standard.
2.2.8 During subgrade construction, drainage works and temporary drainage facilities near the construction site should be done according to the design requirements, so as to keep the subgrade in a dry, solid and stable state. The top surface of subgrade is made into 2%~4% cross slope to facilitate the timely discharge of surface water.
2.2.9 During or after subgrade earthwork construction, subgrade protection works shall be constructed and maintained in time. All kinds of protection and reinforcement should be carried out on a stable foundation or slope. Mortar and concrete for protection works should be mechanically mixed, which should be used as it is mixed, and attention should be paid to health preservation.
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