Roadbed treatment

A question was raised during the road construction drawing design of Ankang Biotechnology Industrial Park. Through geological survey of the area, it was found that the exposed strata in the area are mainly expansive soil. This soil has the properties of swelling by absorbing water, shrinking by losing water and reciprocating deformation. The destructive effect on the roadbed cannot be underestimated, and the damage caused is not easy to repair. In order to ensure the stability of the roadbed and the smoothness of the road surface over a long period of time, and achieve the purpose of safe and comfortable driving, a series of engineering problems caused by expansive soil must be solved. Therefore, the treatment of expansive soil is an important issue in this project. 2. Analysis of the physical and mechanical properties of expansive soil. Expansive soil is classified according to clay minerals and can be summarized into two major categories.: One type is dominated by montmorillonite, and the other type is dominated by illite and kaolin. Montmorillonite clay expands when the water content increases, while illite and kaolin clay expand to a limited extent, which causes the conditions for changes in expansive soil. The analysis is summarized as follows: 1． Expansive soil with water content has a high expansion potential, which is related to the size and change of its water content. If its water content remains constant, there will be no volume change. During construction, structures built on clay whose moisture content remains constant will not suffer damage caused by expansion. When the moisture content of clay changes, volume expansion in both vertical and horizontal directions will occur immediately. Slight changes in moisture content, as little as 1% to 2%, are enough to cause harmful swelling. In Ankang areas, expansive soil is more harmful to people. When the rainy season comes, it is not uncommon for floors built on expansive soil to rise and crack due to the increase in moisture content in the soil. 　　Generally speaking, very dry clay indicates a hazard. This type of clay can absorb large amounts of water, resulting in destructive swelling of the structure. On the other hand, in relatively moist clay, since most of the expansion has already been completed, further expansion will not be very great. However, it should be noted that moist clay may dry out when the water level drops or other conditions change, and the shrinkage displayed cannot be underestimated. 2. Dry bulk density The dry bulk density of clay is closely related to its natural moisture content. Dry bulk density is another important indicator of expansive soil. Clays with γ=18.0KN/m³ usually show high swelling potential. In the Ankang area, people comment on this kind of soil as "hard as stone". This means that clay will inevitably suffer from swelling problems. 3． Mechanical properties In engineering geology, the expansion phenomenon of clay is very common. Through geotechnical experiments, we obtain the mechanical indicators of clay for calculation of soil mechanics. Usually the mechanical analysis of expansive soil is mainly derived from the study of its expansion potential and expansion pressure. Ⅰ. Expansion potential Expansion potential: To put it simply, it is an indoor compaction test according to AASHO standards. After the sample is compacted to the maximum bulk density at the optimal moisture content, the limited sample is subjected to a certain additional load and the expansion percentage is measured after being immersed in water. The expansion rate can be used to predict the maximum potential expansion of a structure. The amount of expansion mainly depends on environmental conditions, such as the degree of wetting, the duration of wetting, and the way water is transferred. Therefore, during engineering construction, transforming the environmental conditions around expansive soil is a starting point for solving problems in expansive soil engineering. Ⅱ. Expansion force Expansion force, that is, expansion pressure. In layman's terms, it is the pressure required until the specimen expands to its maximum limit and then is loaded again until it returns to its original volume. For a given clay, the swelling pressure is constant and varies only with dry bulk weight. Therefore, swelling force can be conveniently used as a measure of the swelling properties of clay. For undisturbed clay soils, dry bulk density is an in situ characteristic of the soil. Therefore, the expansion pressure of the soil at the in-situ dry bulk density can be directly used to discuss the expansion characteristics. 4． Geotechnical Experiment In order to comprehensively understand the physical and mechanical properties of expansive soil and guide the construction design of road subgrade in Ankang Biotechnology Industrial Park, a geological survey was carried out in the area and a series of geotechnical experiments were conducted. From the experimental data, we can find the mechanical changes caused by external and internal factors such as moisture content and dry bulk density on the expansive soil. The following geotechnical experiments were completed by China Nonferrous Metal Industry Xi'an Survey and Design Research Institute. In summary, in addition to the two internal factors of soil expansion and contraction characteristics, changes in expansive soil are two very important external factors, including changes in pressure and moisture content. By accurately understanding the characteristics and changing conditions of expansive soil, it is possible to estimate the deformation of the roadbed and structures built on this foundation, and thus take corresponding foundation treatment measures. Three Expansive Soil Roadbed Treatments In the design of road engineering in Ankang Biotechnology Industrial Park, based on the physical and mechanical properties of expansive soils, based on detailed reports from geological surveys and experience in dealing with expansive soils, the design adopted the idea of ​​comprehensive treatment, conducted targeted research, and proposed the following measures: 　　(1) Embankments with a height of less than 1 meter must be replaced with non-expansive soil and compacted according to regulations. 　　(2) When using expansive soil as filler, in order to increase its stability, lime treatment is used. The dosage range of lime is 10%∽12%. It is required that the total expansion and contraction rate of the expanded soil after lime treatment should be close to zero. 　　(3) The slope parts on both sides of the embankment and the top surface of the embankment should be sealed with non-expandive soil. If necessary, a layer of geotextile must be laid to form a core fill. 　　(4) Do not excavate the cutting slope to the design line at one time. Leave a layer of 30∽50cm thick along the slope. After the cutting is completed, cut off the remaining part and seal it with a mortar grid for slope protection. 　　(5) At the boundary between the embankment and the cutting, that is, at the junction of fill and excavation, the water content in the soil of the two is not necessarily the same, and the original density cannot be the same. During compaction, it should be compacted evenly and tightly to avoid uneven settlement. Therefore, the soil on the surface of the excavation foundation within 2 meters of the junction of fill and excavation should be dug into steps, loosened, and checked to see if its moisture content is similar to that of the fill soil. At the same time, appropriate compaction equipment should be used to compact it to the specified compaction degree. 　　(6) During construction, operations in the rainy season should be avoided and on-site drainage should be strengthened. After excavation of the roadbed, each process should be closely connected and constructed continuously, and the time interval should not be too long. After the embankment and cutting slopes are repaired according to the design, retaining walls and slope protection should be built immediately to prevent direct erosion by rainwater. 　　(7) The strength and compaction standards of road beds in expanded soil areas should be strictly adhered to. * * Relevant regulations and specifications. IV. Conclusion Expansive soil is a special soil quality that affects the construction of roads and other structures. In actual projects, its destructive power is huge. To solve the problem of expansive soil, we should focus on considering the internal and external factors that affect the changes in its physical and mechanical properties, so as to achieve the purpose of treatment by changing the mechanical properties of the soil. A project has its own characteristics and construction conditions. In the road design of Ankang Biotechnology Industrial Park, measures to deal with expansive soil are being used in construction. Some measures have shown good results, and some need further practical proof for their effectiveness. This post was last edited by Lai Lai on 2009-3-24 18:16 ]

The information comes from the Internet and is for educational purposes only* Use! ! ! 1 Introduction Due to the large water content and high compressibility of soft soil, the strength of soft soil foundation is low, which leads to the failure of embankments due to uneven settlement or excessive residual settlement. Therefore, to ensure the stability of the roadbed, the soft base must first be fixed. 　　The soft foundation of the K119+170~K119+348 section of Guixin Highway has the characteristics of deep soft soil thickness and large area. In response to this situation, vibration-pressure sinking gravel piles were used to fix it. The following is a brief introduction to the handling situation. 2 Basic overview of soft foundation The K119+170~K119+348 section of the Guixin Highway is located in a mountain valley basin. Due to the flat terrain, poor drainage, and the development of groundwater (there are two springs on the left), long-term siltation has formed a large area of ​​soft soil. Geological drilling revealed that the soft foundation in this section is covered by a flow-plastic to soft-plastic silt soil layer, which is gray-black and has a fishy smell. It is spread over the entire area and is 1.5 to 2.5 meters thick. ； Below it is a silt soil layer, containing gravel, in a soft to plastic state, saturated with water, low in strength, and 6.0 to 14.8 meters thick. ； The underlying bedrock is a thin layer of siliceous limestone intercalated with claystone in the Wujiaping Formation of the Upper Permian System, and its top is highly weathered. Experiments were conducted on four groups of soft soil samples and their physical and mechanical properties are shown in the table. Physical properties Mechanical properties Moisture content w (%) 48.9~63.6 Consolidation test compression coefficient a100~200 (Mpa-1) 0.990~1.520 Density P (g/cm3) 1.54~1.66 Compression modulus Es100~200 (Mpa) 1.522~2.243 Specific gravity Gs 2.56~2.60 Direct shear test cohesion C (Kpa) 6~24 Porosity ratio e 1.332~1.741 Internal friction angle Ф (degree) 7.2~13.5 Liquid limit WL (%) 65.70~84.6 Loss on ignition (%) 9.43~11.35 Plastic limit WP (%) 34.90~43.90 Plasticity index Ip 29.90~43.00 Soil sample category: Organic matter, high liquid limit silt, gray black clay, containing humus grass roots and gravel. Natural consistency Wc 0.19~0.56 It can be seen from the table that the soft soil in this section has high natural water content, large void ratio, high compressibility, and low shear strength. 3 Soft foundation treatment measures and calculation basis 3.1 Treatment plan The K119+170~K119+348 section of Guixin Highway has a maximum fill volume of 9.46 meters, a minimum fill volume of 3.66 meters, and a fill area of ​​8302 square meters. Due to the large depth of the soft foundation (7 to 17 meters), the flat terrain, and the large area, it is not easy to use soil replacement and rock filling or riprap to squeeze out silt. After joint review and argumentation, it was decided to use vibrating-pressure sinking gravel piles for consolidation. The gravel piles are arranged in a square shape with the same pile diameter and pile spacing on the entire plane. According to the "Specifications" issued by the ministry and previous construction experience, the pile spacing was selected to be 1.2 meters, the pile diameter was set to 32.5 meters based on the outer diameter of the vibrator, and the gravel piles were required to pass through weathered rock formations. After the gravel pile construction is completed, a 20 cm thick gravel cushion shall be laid on the foundation to enhance roadbed drainage. 3.2 The calculation is based on the fact that the embankment filler is clay, and the values ​​of its main calculation parameters are: γ soil=18kN/m3, C soil=25Kpa, Ψ soil=15. ； Values ​​of main calculation parameters for soft foundation: γ soft=16kN/m3, C soft=15Kpa, Ψ soft=7. , Consolidation coefficient Cv=1.5x10-7m2/s. 　　Based on the fill height, fill bulk density, additional stress, soft soil depth and compression coefficient, the settlement of the subgrade before reinforcement can be calculated as S = 82cm. The total settlement reduction of the roadbed after reinforcement S minus = × S in the previous formula n__ _ _ _ _ Pile-soil stress ratio, for cohesive soil n=2~4, take n=2 η__ _ _ _ Area replacement rate η=d2/de2 d__ _ _ Pile diameter__ _ _ Equivalent circle diameter, since the pile holes are arranged in a square shape, de=1.13 times the pile spacing η=0.325/(1.13×1.2)2 =0.05744 S minus= pile=20×C soft/KC soft __ _ _ Cohesion, C soft = 15KPa K__ _ _ Safety factor, K=1.25 Pile=20×15/1.25 =240Kpa Composite foundation bearing capacity = β soil × η__ _ _ _ Area replacement rate, η=0.0574 soil __ _ _ _ The bearing capacity of natural foundation soil between piles is n= pile/soil, which is called pile-soil stress ratio. According to relevant data, its value varies between 2-12. The value in this article is 2 soil=pile/2=240/2=120Kpa β__ _ _ The reduction coefficient of soil bearing capacity between piles is 1.0 because the pile-soil stress ratio has already taken this factor into account. Complex = 1×120× =126.9KPa The above calculation proves that the bearing capacity of the reinforced foundation can be greatly improved, the settlement is controlled within a reliable range, and the calculation results of the bearing capacity provide a reliable numerical basis for the later bearing test. 4. Construction technology and precautions Before construction, plastic silt should be removed, a 0.5-meter-thick temporary cushion of stone chips should be laid, and the site should be leveled to facilitate the entry of construction machinery. The particle size of the cushion should be strictly controlled to prevent pore formation from being impossible. Then lay out the wire to determine the hole location. The construction sequence should be from left to right, first the edges and then the middle, so as to facilitate the deepening of the soil consolidation degree of the composite foundation over time. During construction, attention should be paid to the control of water, electricity, and materials. There should be enough water, but not too much water to prevent the filler from flowing back out. ； Electricity mainly controls the compaction current during the vibration compaction process. ； When adding ingredients, be careful not to add too much. In principle, you need to add ingredients frequently, but you should not add too much per batch. The particle size of gravel should not be larger than 5cm. After completion, a 20cm thick gravel cushion will be added to facilitate drainage. Dredging, drainage and leveling site, setting out and positioning equipment, materials, entering the site, hole-forming test, load-bearing test, checking strength, laying cushion and rolling gravel pile construction 5. Load-bearing test, detection and conclusion After one month of intense construction, a total of 73,122 meters of gravel pile footage was completed, 5,916 holes were formed, and about 11,000 cubic meters of gravel was poured. Based on the foundation bearing capacity calculation data, three sets of five-point static load tests were conducted on the load-bearing plate, two points on the piles and three through the soil between the piles. The circular load-bearing plate used in the test is equal to the diameter of the equivalent circle of influence, and the total load is at least twice the design value of the composite foundation. The load is loaded in one level at 25Kpa. After each level of loading, the settlement value is observed in time until the deformation value per hour is less than 0.1mm. It is considered that the bottom plate is relatively stable and the observation is terminated. Then the next level of loading is carried out until the end of the test. The conditions for ending the test are: The cumulative loading value reaches 300Kpa, or the foundation will be significantly deformed before this load, and cracks and bulges will appear around the bottom plate. When unloading, measure the amount of springback until the deformation is complete. 　　After sorting out the observation data and drawing the relationship curve between settlement S, load P and time T, it can be seen that the curve is composed of two approximate straight lines. The curvature changes little, and the graded settlement value does not increase significantly. The cumulative observed settlement at each point is less than the design value 4cm. When the load is within 300Kpa, there is no sign of damage to the slab bottom composite foundation. Therefore, the reinforced composite foundation is in the compaction stage, the bearing capacity is greatly improved, and the soft foundation treatment is successful. Moreover, the consolidation effect should increase with time, so the filling construction should not be too fast in the future. The filling should be carried out in strict accordance with the specifications. The stacking and embankment settlement should be observed, and the settlement value should be used to control the filling construction rate. This post was last edited by Lai Lai on 2009-3-24 18:18 ]

Geosynthetic materials (such as geogrids) have the advantages of good quality, good overall continuity, high tensile strength, good corrosion resistance and anti-microbial erosion, and convenient construction.； Non-woven geofibers have the properties of small equivalent pore diameter, good permeability, soft texture, and good combination with soil. The laying construction of geopolymers requires many construction methods and machines according to the engineering application and specific conditions. However, attention should be paid to the effectiveness of the fibers and whether the construction methods are appropriate. The construction must be careful, paying attention to uniformity, flatness, and the position and anchoring of the ends. preload: In order to improve the bearing capacity of the soft foundation, reduce settlement, and improve the degree of foundation consolidation, a certain static load is applied to the soft soil foundation in advance to make the foundation soil dense. Preloading is the most commonly used soft soil foundation treatment method. The purpose is to solve the problem of allowable post-construction settlement. It is suitable for general road sections with low allowable post-construction settlement standards or low embankment height. When the construction period is strictly limited and the preloading time is short, the overload preloading method can also be used to speed up the settlement during the preloading period. As preloading or overload preloading, the embankment material is suitable. Due to the subsidence of the foundation during the preloading process, the actual filling height of the embankment is greater than the design height of the embankment. The actual filling height of the embankment should be equal to the sum of the design height of the embankment and the settlement during the preloading period. The above are all construction methods for soft soil foundation embankments. It is recommended to refer to books on this subject. References: 〈Technical Specifications for the Design and Construction of Highway Soft Soil Foundation Embankments〉 PS: The above information comes from the Internet!

This issue is considered in several points: 1. For the road bed, please refer to the "Highway Subgrade Design Code" (JTG D30-2004). For the definition of "road bed", please refer to Article 2.0.2 of the specification. For the strength and compaction of the road bed, please refer to Article 3.2 of the specification. 2. For the definition of "roadbed", please refer to the "Highway Subgrade Design Specification" 2.0.1. The roadbed treatment of highway projects follows the "Technical Specification for Construction Foundation Treatment" JGJ 79-2002, but when located in the port area, the "Port Engineering Foundation Specifications" (JTJ 250-98) should be followed. The foundation treatment methods and principles used by the two are the same, but the regulations are different according to the nature of the port project. It is necessary to determine the specifications used. 3. If the construction period is not tight, first consider inserting plastic drainage plates for preloading. See the specifications for specific applicable conditions.； If the deadline is tight, consider other methods such as: Stirring piles, jet grouting piles, plastic drainage boards combined with dynamic compaction, etc. PS: It depends on the type of pavement. There are corresponding specifications for asphalt and concrete. This post was last edited by tpjj on 2009-3-25 00:21 ]

For foundations with thick soft soil, the post-construction settlement will be large, and the presence or absence of a transition section will not be reflected in the changes in the road surface alignment. However, for cases where the processing depth can penetrate the soft soil layer and the post-construction settlement is small, it is necessary to set up a transition section.

The soil layers are mixed fill soil - silty fine sand - silt - silty fine sand - silty clay - medium coarse sand. How to deal with it more economically?

geological aspects: It is necessary to look specifically at the subgrade aspects such as the hierarchical description of the geological report, parameters of each soil layer, groundwater level and supply situation, etc.: It depends on the current ground elevation and the economical requirements for designing the ground elevation. Generally speaking, when the silt is not thick and the depth is ≤3m, dig it out and replace it with filling. ； The silt is not thick but relatively deep, and the roadbed needs to be filled, so let’s preload it with loads. ； If the silt is deep, insert plastic drainage boards + stack preloading. This cost is less but takes a long time in practice. Because of the silt, it will take a long time to deal with it, at least a few months. Near the coast, the silty-fine sand layer is likely to be terrestrial and marine alluvial sand, which is water-rich and loose, and needs to be dealt with. Take a look at the geological report. If there is not much groundwater recharge, the construction period is not tight, and the roadbed filling load is not very large, stacking preloading is the first choice. Less money and a short construction period is the best choice. This post was last edited by tpjj on 2009-3-25 10:19 ]

We also have a soft soil foundation here, which is collapsible loess, and the water level changes with the seasons. The so-called rubber soil is formed after repeated rolling by heavy vehicles. The method used is to over-excavate the roadbed by about 1,000 meters, then dewater and fill it with super-sized pebbles (specification is 150-150). 200) After 500, fill with 500 natural graded gravel and compact it in 2 layers. After the compaction coefficient reaches the requirements of the roadbed, make a waterproof layer and build drainage ditches on both sides of the road. This may be expensive, but according to local conditions, the raw material has more natural graded gravel and is reasonably priced. I wonder if it will be helpful to the original poster? It can be regarded as a reference.

Is it feasible to use gravel cushion-lime sand-concrete surface layer? The key is durability, so that cracks and breakage don't occur over time.

If the following methods are used to meet the durability requirements, it is best not to cause breakage or damage. 1. 100 thick gravel cushion 2. Natural gravel (5% cement and 95% sand) 200 thick 3. Cement lime sand cushion 180 thick 4. C30 concrete floor 200 thick

The stacking preloading effect is not good! Replacement depends on the thickness of the soft soil layer! Designing pile foundations is not economical!