After the Construction Bored Piles Are Completed, What Should Be Done if the Length Is Not Enough?

After the bored piles are poured into piles, occasionally the pile length is shorter than the designed length. How do you deal with this situation?

The problem encountered by this colleague is a bit serious. After testing the bored piles of their project, they found that the length was not enough. Some were 2 meters short, and some were 4 or 5 meters short. The bored piles were of soil foundation and friction pile types. , single pile bearing.

In this regard, everyone has given solutions:

Answer 1: Is it qualified after testing? If qualified, the construction unit’s unfinished work quantity will be deducted and a fine will be imposed on it. If it is unqualified, the detailed information (notes on the short pile diagram and those who fail the test) will be reported to the design institute, and the designer will issue a pile replacement plan or other construction plans to strengthen the foundation.

Answer 2: You need to check the geological report and piling construction records to see whether the pile end has entered the bearing layer. If it enters the bearing layer, it will not have a great impact on the bearing capacity. As long as the quality of the pile body is intact, the engineering pile can be used; Before entering the bearing layer, the vertical bearing capacity of the single pile must be determined based on the static load test, and the design unit shall reinforce it based on the difference between it and the designed vertical bearing capacity of the single pile.

The reinforcement method starts from two aspects: first, from the design load analysis, 1. Review the single pile bearing capacity margin coefficient, generally, there is a certain margin when matching piles; 2. Review the upper load, when the single pile vertical bearing capacity is almost the same When the load is large, can we consider reducing the upper load to solve the problem? Second: Solve the insufficient bearing capacity of the pile body:

1. The simplest way is to repair piles, but the cost is relatively high;

2. When the difference is within 15%, the foundation soil can be considered to participate in the bearing capacity work, that is, the composite foundation reinforcement method;

3. Pile bottom grouting method. This method is more effective when the bottom of the pile is located in a plastic-soft plastic soil layer or a slightly dense silt layer. Pile bottom grouting can increase the bearing capacity of the pile end and increase the internal force of the pile bottom. Passed to the holding layer;

4. Pile side grouting method. When the pile body is long, the foundation soil within a certain range of the length of the pile body can be reinforced to improve the friction of the foundation soil on the pile side. The surroundings of the cap should be reinforced and the static load should be passed. Test acceptance and review.

Answer 3: It is recommended to submit it to the design institute for verification. Maybe there is no need to deal with it because the current design safety factor is relatively large.

Answer 4: Cut corners. 1. The original pile length was surplus. Calculate whether the current pile length meets the requirements. 2. Add piles and set up a capping platform!

If the bored piles on site do not meet the design requirements, the foundation must be reinforced. Dougong has summarized some methods for reinforcing the foundation, as follows:

1. Strong Compaction Method

The dynamic compaction method is a foundation reinforcement method in which a heavy hammer of tens of tons is dropped freely from a height of tens of meters to compact the soil to increase its bearing capacity and reduce its compressibility. This is a new technology developed based on the heavy hammer tamping method.

First, conduct in-situ tests (side pressure test, cross plate test, penetration test) at the construction site to take undisturbed soil samples, measure the water content, and then conduct dynamic consolidation tests in the laboratory or conduct experimental construction on site to obtain a series of Relevant data provide a basis for determining the optimal ramming energy for each pass of formal construction, the optimal number of ramming blows for each point, the distance between each ramming point, and the interval time between the two passes of the formal construction according to the project requirements. The determination of the falling distance h of the heavy M mainly depends on the depth of influence H. Due to various economic and technical reasons, there is currently a trend to increase the drop distance h. The shape and size of the hammer should be adapted to the type of topsoil, and there are now more than ten different forms.

Generally speaking, for sandy soil and gravel filling, it is appropriate to use a hammer bottom area of 4 cubic meters. For silty sand, the hammer bottom area is required to be at least 6 cubic meters. The principle for determining the number of ramming blows at each point is that the volume of soil is compressed to the maximum and the lateral movement of the soil is minimum, generally 5-10 blows. Ramming points are generally arranged in a square grid. The distance between each ramming point is determined based on the shape of the ramming pit and the rise in pore water pressure learned from the test. It is generally 5 to 15 meters. Usually, it is the first pass of ramming. The distance to the hit point is the largest and decreases in subsequent passes. The last pass is continuous ramming with lower ramming energy. In some projects, a layer of sand and gravel is laid before performing forced compaction.

The largest hammer in the world currently weighs 200 tons. It is carried by a special frame equipped with 186 pneumatic wheels. After one pass of ramming according to the specified points and number of blows, the ramming pit is filled with new soil or surrounding soil. , and then perform the next ramming. The interval between passes is usually 2-4 weeks, and for clay or alluvial soil, it is often 2-4 days. If water is found rising into the ramming pit, efforts should be made to remove the water, especially in severe cold seasons, to prevent the formation of ice pits.

2. Rotary Spray Method

The rotary spray method is a new technology developed based on the ordinary chemical grouting method (the so-called static pressure grouting).

The rotary spray method uses a high-pressure pump to pass the slurry through a special nozzle at the drill end and spray it into the soil layer at high speed. When the nozzle sprays the slurry, it rotates slowly while slowly lifting.

Construction procedures of rotary spraying method:

(1) The drilling rig is in place and the injection performance test is carried out.

(2) At the beginning of drilling, the drill pipe is drilled with low-pressure water injection.

(3) After drilling, inject slurry at high pressure.

(4) Carry out rotary spray rods while rotating and lifting at the same time.

(5) After the pile is formed, lift the drill pipe, flush the pipe with low pressure, move the machine to the place where the second pile needs to be rotary sprayed, and then repeat the above process.

The horizontal jet of high-pressure slurry continuously cuts the soil and forces the soil to stir and mix. Finally, within the effective range of the jet force, a cylindrical solidified body composed of a continuous accumulation of disk-shaped mixture is formed, that is, a rotating jet pile. The pile diameter is generally 0.5-1.0 meters, and the maximum is 1.5 meters. The maximum pile length has reached 40 meters. It has higher strength and a smaller permeability coefficient than the original soil and can be used for foundation reinforcement water interception, and seepage prevention. During jet spraying, there are generally two methods for upward lifting. One is the continuous lifting method, that is, while jet spraying, the drill pipe is continuously lifted upward at a certain speed until the entire pile is sprayed. The other is the stage lifting method, that is, spraying three times continuously at a formation elevation and then raising it by a distance. During the lifting process, the drill pipe is still rotating and the slurry is still sprayed. After reaching the lifting distance, it is sprayed continuously at the new formation elevation. Repeat three rounds to increase the distance by one, to complete the spraying work of the entire pile from bottom to top.

3. Grouting Method

The grouting method is a foundation treatment that uses pressure-feeding equipment to inject slurry into the pores between soil particles through grouting pipes to reduce the permeability of the stratum, enhance the strength of the stratum, and cement the original loose soil particles into a whole to prevent stratum deformation. method. Among them, slurry is injected into the formation through penetration, filling, and squeezing. In short, the grouting method is a reinforcement treatment method that improves the physical and mechanical properties of the grouting object and uses air pressure, electrochemical principles, or hydraulic pressure to inject curable slurry into the cracks and gaps of the medium to adapt to various needs of civil engineering. The segmented grouting method and the gradual densification method are the two main densification laws of grouting construction. Limiting the grout within the grouting range and orderly and gradually dense the grouting holes are the main principles that should be followed in grouting construction. The main agent, solvent, and various admixtures are mixed to form the grouting material used in grouting projects. The main agent is usually called the grouting material. Grouting materials are divided into two basic types: 1) chemical grouting materials such as epoxy resin, propylene glycol, and formaldehyde; 2) solid particle grouting materials such as fly ash, sand, cement, and clay.

Construction technology: Tie rod holes should be reserved for anchor plates and rib columns. The connections between anchor plates, rib columns, and screw end rods should be filled with asphalt mortar before filling, and asphalt hemp bars should be used to plug the joints. The exposed end rods and After the components are stabilized by the fill soil, they are then sealed with cement mortar. When the tie rods and anchor plates are buried, the fill should be rammed to 20 cm above the height of the tie rods before digging trenches into place. The over-excavated portion in front of the anchor plate should be backfilled and compacted with concrete or lime soil. When digging the trench, the anchor plate should be raised 3 mm to 5 mm higher than the designed position, and the tie rod or anchor plate must not be directly rolled over. To prevent the wall from tilting outward or avoid the insecurity caused by parallax, a certain degree of backsliding is reserved strictly by the design requirements during the construction of the rib columns, that is, the rib columns are tilted 5% toward the filling side. angle. The anchor plate retaining wall should be filled with 0.3 m thick permeable material from the bottom of the back of the wall to 0.5 m below the top of the wall, or use sand-free concrete slabs or geotextiles as the filter layer, and a drainage system on the surface. Water holes are used as drainage measures.

4. High-Pressure Jet Grouting Method

The high-pressure jet grouting method is a new foundation reinforcement method developed by applying high-pressure jet technology based on the grouting method. Compared with other foundation treatment methods, it has the characteristics of a wide application range, simple construction, and good durability. It can be regarded as a good foundation treatment method among many foundation treatment methods.

Mechanism of high-pressure jet grouting The high-pressure jet grouting method uses high-pressure water or slurry jets to cut and stir the formation, and at the same time inject cement slurry or composite slurry to form new condensation, changing the structure of the original formation or completely replacing it with new composite materials. Structure, improve the bearing capacity or the anti-seepage ability of the original foundation, and achieve the purpose of strengthening the foundation and preventing seepage. The process is to use a drilling rig or other hole-making equipment to create holes, then lower the grouting pipe with a nozzle to a predetermined depth of the soil layer, and use a high-pressure water pump or high-pressure mud pump to spray the slurry into the nozzle with a high-pressure jet of 10 to 25Mpa. Projected to impact and destroy the soil at a predetermined depth. The jet energy is large and the speed is fast. When the pulsating jet pressure intensity is greater than the strength of the soil, the soil particles will peel off from the original soil. Some of the fine soil particles will emerge from the ground along with the slurry, and the rest will be thicker. Under the influence of the impact force, centrifugal force, and gravity of the jet flow, the soil particles are mixed with the injected cement slurry and other slurries and then rearranged regularly according to a certain slurry-soil ratio and mass size, forming in the soil. Condensation. When spraying, if it is lifted and rotated at the same time, a columnar body, that is, a jet spray pile will be formed. If it is lifted and swung in a certain direction and angle, a wall-shaped body will be formed. High-pressure jet grouting construction equipment mainly consists of two parts: a high-pressure mud pump and a drilling rig. Due to the different injection methods, the types and quantities of equipment used in single-pipe, double-pipe, and triple-pipe jet spraying operations are different, mainly including drilling rigs, high-pressure pumps, mud pumps, air compressors, special grouting drill pipes, grouting Pipes, nozzles, high-pressure hoses, slurry pipes, flow meters, slurry mixers, etc.

5. Foundation Treatment and Reinforcement Mechanism Using Cement-Soil Mixing Method

The cement-soil mixing method uses cement and other materials as the curing agent and uses a special mixing machine to forcefully mix the soft soil and the curing agent on site so that the soft soil hardens into cement-reinforced soil with integrity, water stability, and a certain strength. Thereby improving the strength of the foundation soil and increasing the deformation modulus.

6. Composite Material Piles

Composite material piles are a new method of reinforcing soft soil foundations developed based on gravel piles, deep mixing piles, and CFG piles. It uses industrial waste fly ash as the main component and uses lime, sulfates (industrial Glauber’s salt or phosphogypsum) industrial waste as the excitation component, and low-strength plain concrete piles prepared with sand, gravel, or other granular industrial slag as aggregates to reinforce the soft soil foundation to form a high bearing capacity. Composite foundation. Usually, small diameter and shallow processing are used. The general pile diameter is 15-25cm and the reinforcement depth is 8.0m. Use plum blossom-shaped or square cloth piles on the plane.

(1) The reinforcement mechanism of composite material piles is: first vibrate to form holes and compact the foundation soil, then add composite materials and vibrate the pipes, vibrate the composite material of the pile body and further vibrate the soil between the piles, and then move the pile driver to drive. A pile. After the above effects, a composite foundation with composite material piles as reinforcements is formed in the reinforcement area.

(2) The advantages of composite pile composite foundations are the miniaturization of construction machinery, fast construction speed, and high placement accuracy. Compared with other composite foundations, its construction technology is more reasonable. Composite material pile composite foundation can not only effectively improve the physical and mechanical properties of the soil between piles, but also form an ideal pile-soil stress relationship between piles and soil due to its moderate strength, thereby fully exerting the bearing capacity of the soil between piles. The project cost of composite piles and composite foundations is relatively low. Generally, when the design strength of the pile body is the same, the material price of the same volume is about 60% of that of the CFG piles; when the design bearing capacity of the composite foundation is the same, the total project cost is about 85% of that of the CFG piles.

7. Lime Pile

A lime pile refers to a water-absorbing column made by drilling holes in the foundation and pouring quicklime to accelerate the consolidation of a weak foundation.

Through mechanical stirring, the soft soil is reshaped and an appropriate amount of lime is added. The lime reacts chemically with the soft soil minerals to form a complex water-insoluble calcium silicate gel that binds the soil particles together. Calcium silicate gel plays the role of wrapping and connecting, forming a network structure that intersperses between soil particles, making the soil particles firmly connected, improving the physical and mechanical properties of the soil, and exerting the strengthening effect of the lime curing agent. Specifically, the basic effects of lime on soft soil are as follows:

(1) Quicklime and foundation soft clay are evenly mixed by force, and hydration will occur quickly to form Ca(OH)2. During the process of turning quick lime into slaked lime, the heat generated promotes the evaporation of water, reducing the moisture content of the soft soil foundation. At the same time, the volume of lime expands. At this time, the work done by the expansion force is converted into the deformation potential energy of the surrounding soil. For example, there is a 4.5m cover culvert on the dedicated line of Yunfu Pyrite Mine in Guangdong Province. The foundation uses lime powder deep stirring to treat the soft foundation. The diameter of the drill bit is 500ram. After the lime pile is formed, the diameter of the fine sand layer increases to 520mm. The inner diameter of the soft soil layer increases to 600-700mm, and the pile volume increases, which plays a compacting role in the surrounding soil.

(2) The Ca2+ ions of hydrated lime react with the soft soil particles under the action of water to produce a flocculation reaction. This reaction process thins the thickness of the water film combined with the soft soil particles, reduces the plasticity of the soil, and increases the bonding force between soil particles. Soil strength and water stability are improved. The above two chemical reaction processes mainly occur within a few hours after the forced stirring and mixing of quicklime and soft soil. They are the early basic effects of lime on soft clay.

(3) The slaked lime reacts slowly with the active silica-aluminum minerals in the clay particles. In the process, it absorbs the moisture in the slaked lime slurry, forming crystals and generating aluminates and hydrated calcium silicate, which changes the structure of the clay. This reaction process will last for several years and is the later effect of lime on soft clay.

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