With the development of the economy, the traffic volume of expressways has increased rapidly, and expressway expansion projects in economically developed areas are imperative. In expressway expansion, the roadbed expansion methods mainly include 2-side widening, single-side widening, and staggered 3-side widening. Among these methods, the 2-side combined width has the highest requirements on the stability of the roadbed. Due to the need to maintain the original traffic flow, the construction must bemanagement organization,Safe construction. For roadbed treatment of expansion projects, the safety of soft foundation treatment is particularly important due to the requirements of maintenance work. This article combines engineering examples to introduce the design method and construction technology of soft foundation treatment for pine pile retaining walls.
1 Project Overview
There is a soft foundation at K310 plus 140 of the PA2 section of a certain expressway expansion project. The roadbed width of this section of the expressway is to be widened on one side on the right side. At the same time, it is required to keep the old road open to traffic. Therefore, the stability and safety of the old roadbed must be ensured during the expansion. There is an access bridge over the river on the right side of the highway. In order to connect the mechanized farming road with the access bridge, a section of shoulder retaining wall is set up on the right side of the highway during the expansion.
The geological longitudinal section of this section is: 2m of plain fill on the surface, 3m of silt underneath, followed by residual silty clay, residual sandy clay soil, and residual sandy clay soil. If the filling method is used, the excavation process There may be silt flow in the old road subgrade, which will affect the stability of the old road subgrade; if steel sheet piles are used for replacement, the cost will be even higher. The use of concrete pipe piles is not only expensive, but also difficult to operate on site. Therefore, pine piles are used It is most economical and practical to treat the soft soil foundation of this section of retaining wall.
2pine wood pileGround treatment principle
There are many methods to treat soft foundations, such as powder spray piles, replacement filling, riprap and cast-in-place piles. In actual operation, you can choose according to the situation. Pine wood is rich in rosin, and rosin can well prevent groundwater and bacteria from damaging it. Corrosion, so pine piles are suitable for working below the groundwater level. However, pine piles are not suitable for areas where the groundwater level changes greatly or the groundwater is highly corrosive.
　　pine wood pileThe treatment is divided into two types. If the bearing layer is shallow, end-bearing piles can be used; while in subgrades with thick soft foundations and relatively low bearing capacity requirements, short wooden piles with smaller spacing (usually The distance between piles is less than 3 times the pile diameter). Due to the compaction of soft soil between piles, a composite foundation is formed, thereby achieving the design bearing capacity.
　　pine wood pileIt also has good advantages in terms of material prices, transportation, construction machinery, etc. In addition, pine wood is widely distributed, which conforms to the highway construction principle of adapting measures to local conditions and using local materials. Therefore, the use of pine piles to treat the soft foundation of highways has become a method of soft foundation treatment of highways. Economical and applicable method. Pine piles are often used in soft foundation sections of highways, bridge pier foundations, culverts, and retaining wall foundations where the groundwater level is high and the soft soil layer is thick and less than 5m to improve the foundation bearing capacity to meet the design. Require.
3pine wood pileRetaining wall foundation treatment design
3.1 Bearing capacity calculation
According to the geological profile, the third layer of the retaining wall design section is residual silty clay. According to geotechnical experiments, the bearing capacity is 120kPa. The distance from the retaining wall foundation to this layer is about 3m, and the bearing layer is shallowly buried, so end-bearing piles are used. After carrying out design calculations, the single pile bearing capacity of pine piles is calculated as follows:
Pa=ΨσA (1)
Pa - single pile bearing capacity;
Ψ - Longitudinal bending coefficient, related to the soil quality between piles, generally takes 1;
——The stress reduction coefficient of the pile material, for wooden piles, it is 0.5;
σ——allowable pressure of pile material, kPa.
According to the calculation results of the retaining wall, the retaining wall is 33.5m high, and the maximum base stress of the retaining wall foundation is 124kN. According to formula (1), when pine wood is used as the material and the pile diameter is 15cm, σ is 2773.4kPa, which is calculated as The pile bearing capacity is Pa=24.5kN\/pile, then the number of piles required per square meter is n=5.06\/m2, rounded to 5\/m2.
3.2pine wood pilePlane and retaining wall facade layout
The layout of pile holes should minimize the uncrowded blank area. Therefore, the pile holes should be arranged in equilateral triangular lines to form a plum blossom layout. It is calculated that a pile spacing of 0.5m can meet the bearing capacity requirements. The plan layout is shown in Figure 1 shown.
In order to make the base of the retaining wall evenly stressed and to coordinate the deformation, an 80cm thick graded gravel sand reinforced cushion is laid on the top of the pile, and geogrid is used as the reinforcing material. The width of the pile treatment is 20cm wider than the bottom width of the retaining wall. Then calculate based on the diffusion angle of 35 degrees. The cross section and elevation layout of the pine pile retaining wall are shown in Figures 2 and 3.
　　4 Construction of wooden piles　
　First of all, the site must be cleaned before construction. The old road bed must be stable during excavation. The cleaning height should not be too low, and vertical excavation along the edge of the old roadbed is not allowed. Secondly, temporary drainage work must be done for the pine pile retaining wall foundation, and trenches must be dug to reach the piles. Below the top.
Before piling, a test pile should be carried out to adjust the mechanical parameters and determine the construction pile length, provide relevant data for large-area construction, ensure project quality, and save materials. The test pile can be carried out with an excavator. Before construction, you should refer to the geological profile. Then verify the thickness of the soft soil layer. At certain distances (usually 23m) at the periphery and center of the base, test piles to verify the thickness of the soft soil layer so that the length of the construction pile can be selected. Generally, it should not be shorter than 2.5m.
During piling construction, use an ax to sharpen the pine pile head and turn the excavator's bucket upside down to press the pile, which can achieve higher work efficiency. Before pressing the pile, the staff will hold the pile in place and gently press the bucket until the pile is inserted into the soil. After the pile is stabilized, the staff will leave and press the pile under the excavator. After the piling is completed, the pile head will be sawed off to make the pile top of each section of pile basically horizontal. The mud squeezed out during piling will be excavated, and then backfilled with graded gravel. , the general thickness is 80cm, ensuring that the foundation does not cover the pile head, so that the force is evenly distributed.
In order to achieve good results during construction, piling must be driven from the 4th perimeter of the base to the inner ring to achieve a good compaction effect. For areas with complex geology, detection measures should be taken for stable monitoring.
Finally, take soil samples for geotechnical testing, or use a long rod penetrometer to verify whether the actual bearing capacity of the treated foundation meets the design requirements.
　5 Conclusion
　　pine wood pileIt also has good advantages in terms of material prices, transportation, construction machinery, etc., and practice has proven that pine pile processing on soft foundations is not affected by rainy days, has fast progress, small workload, and is suitable for projects with tight construction schedules. In certain geological and construction environments Among them, pine piles are an effective means to treat soft foundations.