Lime soil is one of the most widely used materials in municipal road engineering. The application of its initial stability and its strength, hardening and water stability with age in road engineering has obvious advantages and important technical and economic significance. This paper discusses the construction technology of lime-soil base in municipal road engineering.
First, the construction preparation
(1) technical preparation
(1) raw material testing. Before the construction of lime-soil base, the following tests should be carried out on the representative soil samples in the designated yard: particle analysis, liquid limit and plasticity index, compaction test, crushing value of crushed stone or gravel, organic matter content (if necessary) and phosphate content (if necessary). In addition, the effective calcium oxide and magnesium oxide contents in lime also need to be tested. For example, materials with poor gradation, such as experimental macadam, macadam soil and gravel soil, should be improved first.
(2) According to the soil type and lime quality, determine the mixture ratio, optimum water content and maximum dry bulk density of lime.
(3) Before construction, carry out the construction of100m ~ 200m test section to determine the mechanical combination effect, compaction virtual paving coefficient and construction method.
(2) Material requirements
(1) soil: cohesive soil with plasticity index of 10 ~ 20 is applicable; When lime is used to stabilize graded macadam, graded macadam and unscreened macadam without plasticity index, about 15% cohesive soil should be added; When testing cohesive soil with large plasticity index, it should be crushed. The maximum particle size of crushed soil should not be greater than 15 mm, the soil organic matter content is less than 10%, and it is not suitable to stabilize it with lime when the sulfate content exceeds 0.8%. Use special types of soil, such as graded gravel, sandstone, miscellaneous fill, etc. It should be decided by experiment. The crushing value of crushed stone or gravel shall meet the following requirements: the base of urban expressway and main road shall not be greater than 30%; The base used for secondary roads should not exceed 35%.
(2) Lime: The lime should be 1 ~ 3 grade fresh lime. Hydrated lime that has been stored for a long time or has passed the rain period should be inspected and the use method should be decided according to the content of active oxides. Considering the specific situation, it is suggested to use bagged slaked lime and ground quicklime, and it is not suitable to dissolve the lump ash on site. If necessary, hydrated lime should be sieved (10mm square hole).
(3) Water: All drinking water (including livestock drinking water) can be used for lime-soil construction.
(4) Admixtures: When graded macadam, sandstone and other materials are used, the maximum particle size should not exceed 0.6 times of the layered thickness and should not be greater than 100mm, and the dosage should be determined according to the test.
(3) Machine preparation
(1) Main lime-soil machinery: bulldozer, grader, vibratory roller, tire roller, loader and water wheel. Forced mixers, road mixers, disc harrows, plows, etc. are selected for factory mixing. Selected for road mixing.
(2) Small machines and testing equipment: frog rammer or impact rammer, four-toothed rake, two-wheeled vehicle and shovel; Level meter, total station, 3m ruler, flatness meter, sand filling pipe, etc.
(4) Operating conditions
(1) The lower bearing layer has passed the acceptance of various indicators, and the surface is smooth and solid. The indicators such as compactness, flatness, longitudinal elevation, centerline deviation, width, cross slope and gradient must comply with relevant regulations.
(2) Before construction, the lower bearing layer should be cleaned and properly watered.
(3) Restore the center line of the construction section, and set a middle pile every 20m for straight lines and every 10m for horizontal curves.
(4) The embedment and backfilling of relevant underground pipelines have been completed and accepted.
Second, the construction process
Lime-soil base construction is divided into factory mixing method and road mixing method. The construction techniques of the two construction methods are different and each has its own characteristics.
(A) the factory mixing method
The construction technology of factory mixing method is lime-soil mixing, lime-soil transportation, lime-soil paving, rough leveling, voltage stabilization, finishing leveling, rolling survival and maintenance.
(1) Lime-soil mixing: After the raw materials are qualified, lime-soil will be produced according to the production mix ratio. When raw materials change, the lime-soil ratio must be readjusted. The water content of calcareous soil should be comprehensively considered according to the weather conditions at that time, and it is generally 1% ~ 2% in sunny and windy weather. The water content and ash dosage of calcareous soil should be monitored in time, and they can enter the site only after passing the inspection.
(2) Lime-soil transportation: vehicles with covering devices are used for transportation, and the number of vehicles is reasonably allocated according to demand, transportation distance and production capacity. Transport vehicles should go in and out of the construction site according to the established route, and it is strictly forbidden to suddenly brake, make sharp turns, turn around and overspeed on the working face.
(3) Lime-soil paving: on the wet lower bearing layer, calculate the virtual paving amount of lime-soil per linear meter according to the design thickness, and assign special personnel to direct the unloading according to the fixed interval, the established vehicle type and the established vehicle number. Before paving, manually mark the elevation points with white ash according to the virtual paving thickness, and pave with bulldozers and graders, with the help of loaders when necessary.
(4) Rough leveling type: firstly, use bulldozer to rough leveling 1 ~ 2 times, then use bulldozer to release pressure 1 ~ 2 times within the whole width of subgrade to find out the potential uneven parts. For the surface with large local elevation difference (generally exceeding the design elevation of 50mm), bulldozers shall be used to level it, and graders shall be used to level it when the elevation difference is small (within 30mm).
(5) Stabilizing voltage: firstly, level the ground once with a grader, and detect its water content in time. If necessary, adjust the water content by sprinkling water and drying. After the water content is appropriate, use a tire roller to carry out full-width static pressure quickly.
(6) Fine leveling type: mark the elevation points manually, and the grader will level 1-2 times, then check the elevation, flatness and cross slope, and manually deal with the local fine aggregate concentration phenomenon. It is forbidden to directly repair the surface of lime soil with a slightly lower local elevation with a thin layer. Before repairing, loosen the soil manually or mechanically for about 100m.
(7) Rolling: Trial rolling should be carried out when the lime-soil paving length is about 50m, and rolling should be carried out when the optimum water content is-1% ~+2%. After the pressure test, the elevation shall be rechecked in time. In principle, rolling should be "slow first and then fast", "light first and then heavy" and "high first and then low". Matters needing attention are as follows: when rolling, it should overlap 200 mm ~ 300 mm, and the rear wheel must exceed two joints; The rolling speed of the roller should be 1.5 ~ 1.7 km/h for the first two times, and 2 ~ 2.5 km/h for the next two times; Roller static pressure once, then vibration compaction for 3-5 times, and then according to the compaction test results to determine the number of vibration compaction times. Finally, static pressure 1-2 times with steel roller and tire roller, and finally wheel tracks are eliminated. In culverts, bridge heads and other parts where rollers are difficult to roll, frog rammer or impact rammer shall be used for compaction.
(8) Stubble treatment: When the project is interrupted or constructed in sections, 300 mm to 500 mm of lime stubble should be reserved for non-compaction, and it should be connected with the newly paved lime soil, and it should be wetted with water when rolling; Longitudinal summer-planting joints should be avoided. When longitudinal summer-planting is needed, the summer-planting seam should be located near the road center line, and the summer-planting should be made into a trapezoid with a step width of about 500 mm
(9) Curing: Sprinkle water for curing after compaction for at least 7 days. During the maintenance period, the traffic will be closed, and the layered continuous construction will be completed within 24 hours.
(2) Road mixing method
The construction technology of road mixing method includes material preparation, mixing, paving, shaping, compaction and curing.
(1) Material preparation: the soil and lime powder are transported to the working face and distributed by grid method according to the mixture ratio.
(2) Mixing: firstly, mix the screened soil and lime 1 ~ 2 times, detect and adjust the water content, and then mix with a road mixer, generally 2 ~ 3 times.
(3) Paving, shaping and rolling construction are basically the same as the factory mixing method.
Third, the special season construction
(1) rainy season construction
(1) Construction of lime-soil structure layer in rainy season should be avoided in rainy areas.
(2) Spare lime and mounds should be piled up in large piles, the surface should be covered with plastic sheets, and drainage ditches should be dug around to prevent the aggregates transported to the pavement from getting too wet, and protective measures should be taken to prevent lime from getting wet.
(3) Shorten the paving length, and the paved lime soil will survive on the same day.
(2) Winter construction
(1) lime-soil base is not suitable for construction in winter, and the daily minimum temperature should be above 5℃ during construction.
(2) Lime soil base should be completed one month to one and a half months before the first heavy freezing (-3℃ ~-5℃).
(3) When lime soil base enters winter, antifreeze, such as 3% ~ 5% nitrate, should be added to lime soil.
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