The formation principle of limestone strength is to mix an appropriate amount of lime with a certain fineness in the crushed soil, and after compaction under the optimal water content. A series of physical and physicochemical reactions occur to form the strength of the limestone. Ash and soil interact in series to form a plate body that improves strength and stability. However, due to the violation of construction operating procedures, there are many common problems mentioned below.
(1) Uneven stirring
1． Phenomenon: Lime and soil mixed after the number of stirring times is not enough, the color is white. Some are locally ashless, and some are locally lime clumped. What's more, without stirring, a layer of ash and a layer of soil are formed into a filling "steamed cake".
2． Cause analysis:
(1) The original Beijing Municipal Bureau regulations stipulate that limestone is artificially mixed 7 to 8 times, and the 1976 revision of the regulations stipulates that the mixing is not less than 3 times, even this number cannot be done, in short, the number of mixing times is not enough.
(2) No forced mixing equipment, relying on labor, time-consuming and laborious. Coupled with the lack of strict management, it disregards the quality, shoddy manufacturing, mixing effort, and is unwilling to mix more.
3． Hazards: The principle of hardening of limestone is to exchange the ions in the soil through the activity of lime (CaO and MgO contained in lime), which changes the properties of the soil (dispersion, wet collapse, adhesion, expansion), reduces the combined water film of the soil, and improves the water stability of the soil. Lime (Ca(OH)2) absorbs carbonic acid gas in the air, forms calcium carbonate, colloidal in lime gradually crystallizes, lime and the active silicon oxide (SiO2) and alumina (A12O3) chemical reaction in the soil, the formation of calcium silicate and calcium aluminate, so that the mixture of lime and soil gradually harden and other physical and chemical effects, all need lime particles and soil particles evenly mixed together to complete. If the mixture is uneven, the ash is ash, the soil is soil, the interaction between soil and ash will be incomplete, and the strength of limestone soil will not reach the design strength.
4． Governance method: According to the provisions of the construction technical regulations:
Manual stirring:
(1) The prepared soil and lime are stacked on the mixing site in layers according to the calculated proportions;
(2) Stir the shovel three times. It is required to mix evenly, the color is consistent, and there is no graying phenomenon. When the soil is dry, mix with the splash. Add how much water. Controlled with optimal moisture content.
Mechanical mixing: there are many methods, using a grader to stir, special ash mixing machine to mix. Agricultural plough rake stirred. No matter what method is used to stir in place, it should be strictly operated according to the procedures. Guarantee uniformity, structural thickness, optimum moisture content. The best way to do this is to implement factory forced mixing.
(2) The thickness of limestone soil is not enough
1． Phenomenon: The limestone soil cannot reach the design thickness, especially the performance of the limestone grass layer of the sidewalk is particularly prominent, resulting in the sinking deformation of the small square brick trail.
2． Cause analysis:
(1) The road bed process is omitted, and the compactness, longitudinal and horizontal elevation, flatness and width indicators of the dirt road bed are not controlled.
(2) Do not do dirt road beds. Stir in place, when the soil is soft, the depth of mixing is deep, the layer of ash soil is thick, when the soil is hard, the depth of mixing is shallow, and the layer of gray soil is thin.
3． Hazards: the thickness of the limestone base is uneven, the bearing capacity is different, the weak parts are easily damaged, especially the sidewalk limestone base, the conventional design thickness of Beijing is 15cm, but it is often found that there are 3cm, 5cm thick, so once put into use, immediately subsidence deformation, this situation often occurs.
4． Treatment method: According to the process of the dirt road bed specified in the quality inspection and evaluation standards, the vertical and horizontal section elevation, flatness, width and compactness of the dirt road bed should be controlled. On this basis, according to (1) the common disease treatment method of "uneven mixing", the limestone soil is stirred and paved, and the thickness of the ash soil layer can ensure uniformity.
(3) Ash doping is not measured or the measurement is inaccurate
1． Phenomenon: In the process of limestone mixing, the ash addition is more arbitrary, and the loose dry bulk weight of soil and ash is not seriously tested and calculated. Or although there is metering only a rough volume ratio.
2． Cause analysis:
(1) Managers and operators do not understand that the dose is an important factor that directly affects the strength of the clay.
(2) The management personnel have not been tested to calculate or although the test calculation is carried out, the operator is not clear.
3． Hazards: in production practice. The lime dosage should not be less than 6%. Not higher than 18%, if the measurement is not accurate, less than 6% or higher than 18% will reduce the strength of the clay.
4． Treatment method: Lime dosage of lime soil. It is calculated according to the percentage of the total dry weight of hydrated lime in the gray soil. The cost-effective dosage is 10% to 14%. The ash-containing dose of limestone structures in Beijing is usually 12%, and the lime treatment soil foundation is usually 9%. To obtain an accurate dose, it is necessary to undergo tests to obtain data such as the "Lime Volume and Mass Conversion Table" in Table 1-3-7.
If there is no test data, 12% limestone, compaction thickness 15 cm. Take artificial soiling as an example. Soil pine paving is 22~24cm, lime pine is paved 6cm; compaction thickness is 20cm, soil pine is paved 30~32cm, and lime pine is paved 8cm. According to the above proportion of soil and ash thickness. Roughly 4:1, if the lime treatment soil base is 15 cm (solid thickness), add ash 6%, then the thickness of the lime pine paving is 3 cm. If it is 9%, the thickness of the loose paving is 4.5cm.
(4) The content of lime active oxides is low
1． Phenomenon: Lime is tested for grade III ash with an active oxide content of less than 60% calcium oxide and magnesium oxide. In particular, bagged quicklime powder is widely used in current municipal works. It was found that many were below the grade III ash standard, and the ash contained a large amount of inactive quicklime flour.
2． Cause analysis:
(1) The purchase is inferior lime or inferior quicklime powder.
(2) The lime is stored for too long and fails.
3． Hazards: The influencing factors of the formation of limestone strength have two factors: internal and external. The internal factors are soil, gray, lime dose, water content and compactness. External factors include time, temperature, humidity and mechanical compaction and driving effects. The higher the grade of lime. Its calcium oxide and magnesium oxide content is also higher at the same lime dosage. The better the stabilization effect on the soil. The finer the lime, the greater the finer. The larger the ratio surface, the more fully it interacts with the soil particles under the same dose conditions, and the greater the intensity formation. When the lime grade is lower than grade III. or the lime is stored too long, the active oxide content in the lime will be greatly reduced, and the effect is similar to the role of reducing the lime dose in the limestone soil, weakening the plate body effect of the limestone. Inferior ash is often small in fineness, weakening a series of effects between lime and soil particles, so that the strength of limestone soil grows slowly. Therefore, if the active oxide content of lime is low, the limestone soil mixed with it will not meet the strength required by the prescribed ratio.
4． Governance Approach:
(1) To use lime not less than the grade III standard.
(2) The active oxide content test should be carried out on newly purchased or stored for too long lime.
(3) If the test is lower than the grade III ash standard, the lime dose can be increased according to the active oxide content.
(4) To shorten the storage time of lime as much as possible, the general production of lime is not later than 3 months into use.
(5) Dissolve the lime but do not sift
1． Phenomenon: Lime blocks and slow fossil ash blocks containing not yet completely digested are mixed directly into the soil, but sieved.
2． Cause analysis: Figure save labor, violation of operating procedures.
3． Hazards: However, after the digestion of the sieve lime is compacted in the soil, the unsolved ashes and slow fossil ash blocks that exist in them are dissolved after a certain period of time after encountering water, and the volume expands, arching the road surface and destroying the structure.
4． Governance Approach:
(1) Quicklime blocks should be powdered at least 2 to 3 days one week before the ash is used to make the ash fully digested.
(2) The method of digestion should be in accordance with the provisions of the regulations, in the place where there is tap water or pressure water head, try to use the water spray pipe, so that the water is evenly sprayed into the interior of the ash pile, each place is parked about 2 to 3mn, and then inserted in another position, until it is inserted throughout the entire ash pile, and enough water should be used to fully dissolve the ash.
(3) For a small amount of unsolved parts and moderated quicklime blocks, a sieve with a 1cm sieve hole should be passed.
(6) Soil materials are not sieved
1． Phenomenon: The soil contains large dirt blocks, large bricks, large stones or other debris.
2． Cause analysis:
(1) The soil is cohesive, clumps, and unbroken.
(2) The construction muck contained in the soil material is not screened.
3． Hazards: The strength and water stability of the plain soil are much lower than that of the limestone, if the gray soil contains large soil blocks, it is equivalent to containing weak parts in the solid plate body; the large bricks and large stones in the gray soil cannot be condensed with the limestone into a whole, just like the "boils" on the wooden board, which damages the integrity of the board body, which is the weak link that causes damage to the plate body.
4． Treatment method: all soil should be broken in advance, when manually mixed, it is necessary to pass through the sieve of 2cm sieve hole; when mechanical mixing, it can not be sieved, but large bricks, large stones, etc. must be removed, and the content of more than 2cm soil blocks shall not be greater than 3%.
(7) The ash soil is too much or too wet to crush
1． Phenomenon: Mixing and paving the ash soil is too dry or too wet, and the optimal moisture content is larger; often when it is too dry, after rolling, it is sprinkled on the surface, so that only the surface layer is wet, and the water cannot penetrate into the entire gray soil layer. When it is too wet, the crushing appears to flutter and crack.
2． Cause analysis:
(1) In the process of excavation, transportation or in-situ sieving and mixing, a large amount of water evaporates in the feeding process, and no water is added during the mixing process.
(2) The soil taken is too wet or not crushed in the rain or after the dust is mixed. It is not dried and is rolled in a state that greatly exceeds the optimal moisture content.
3． Hazards: The dusty soil is rolled in a state of too dry or too wet, and the optimal compactness cannot be achieved. Too wet soil or too wet lime can not be stirred evenly; the overly dry ash soil layer, only sprinkled on the surface, can only make the surface layer reach a higher density, the entire gray soil layer will not reach a consistent optimal density. This will lead to a reduction in the carrying capacity of the clay layer, endangering the life of the entire structure.
4． Governance Approach:
(1) Limestone mixing must have sprinkler equipment, if the water is lost in the process of soil extraction, transportation and mixing, it should be mixed with the splash during the mixing process. until the optimal moisture content is reached. At the same time after crushing into life. If the superstructure is not paved, it should be continuously sprinkled with water to maintain health and keep it moist (because the ash soil often maintains a certain humidity in the early stage, which can accelerate the formation of the hardening process); during the formation of the strength of the ash soil, a series of interactions are inseparable from water.
(2) The incoming taxi material is too wet or too wet after rain, it should be dried, so that it reaches or is close to the optimal moisture content and then add ash and mix. If the dust after mixing is rainy, it should also be dried. Roll when optimal moisture content is achieved. If the ash soil is left for too long, it must be tested, and if the lime fails, it should also be mixed with ash and rolled.