Construction method for sand replacement pile in viscous soft soil foundation

Abstract

The invention discloses a construction method for a sand replacement pile in a viscous soft soil foundation. The construction method comprises the following steps: driving a steel casing in the viscous soft soil foundation; soil in the steel casing is taken out through a rotary drilling rig; checking and accepting the hole depth and filling sand with an excavator; the steel casing is pulled out; carrying out vibroflotation on the hole site to dense current; and CPT detection is conducted, specifically, the compacted side friction resistance value is detected, and the compactness and the bearing performance of the sand pile are judged. According to the method, the trial and error cost is reduced, operation simplification is achieved, and the pile body quality and the compaction effect are guaranteed.

Description

Technical Field

[0001] The invention relates to the technical field of sand pile construction, and in particular to a construction method of replacing sand piles in a sticky soft soil foundation. Background Art

[0002] The method of using vibration and water to strengthen soil is called vibroflotation. The vibroflotation method was first used to vibrate and compact loose sand foundations. Later, the vibroflotation method began to be applied to clay foundations. A group of piles composed of loose materials such as stones and gravel were made in the clay soil. These piles, together with the original foundation soil, formed the so-called composite foundation, which increased the bearing capacity and reduced the settlement. For this reason, some people call this method "stone pile method" or "loose particle method". The reinforcement method of using piles to form a composite foundation is an expansion and innovation of the application of vibroflotation. The reinforcement mechanism of the composite foundation is completely different from that of vibroflotation to compact the sand foundation. Simply put, the former uses the vibroflotation method to replace part of the foundation soil with compact pile materials in the foundation, while the latter uses the vibroflotation method to compact the loose sand. Therefore, the vibro-compaction method has evolved into two major branches, one is the "vibro-compaction" method suitable for sand foundations, and the other is the "vibro-replacement" method mainly suitable for clay foundations.

[0003] The traditional vibro-sand pile method can usually be divided into two categories according to the filling method: top filling and bottom filling.

[0004] When constructing the top filling method, in the vibro-replacement project of soft clay soil, it is usually necessary to "protect the wall first, then make piles", which requires that the hole cannot reach the designed depth directly, but first reach the upper range of the soft soil layer, lift the vibrator out of the hole mouth, add a batch of fillers, and then sink the vibrator to squeeze this batch of fillers into the hole wall, so that the hole wall of this section of soft soil layer can be strengthened to prevent the hole from collapsing, and then the vibrator is lowered to the next section of soft soil layer, and the wall is filled and protected in the same way, and this is repeated until the designed depth. After reaching the designed depth, intermittent filling, continuous filling or comprehensive filling is used from the hole mouth for construction.

[0005] The bottom filling method of construction is that there is a special feed pipe for conveying sand and gravel in the vibrator. The sand and gravel are transported to the bottom of the vibrator through the feed pipe, and the pile body is formed after repeated insertion and vibration.

[0006] Both sand pile construction methods are used in China, but they have the following disadvantages: field tests are required to verify the applicability of various processes to ensure the quality of the pile body, and there is a cost risk of trial and error; for complex strata, such as soft interlayers with many and uneven distribution, it is necessary to control parameters such as the compaction current of different strata, which requires a large amount of preliminary test data and greatly increases the difficulty of pile making operations. The diameter of the piles in deep soil cannot be controlled by the two methods. In soft clay soil, when a thin layer of clay is sandwiched in the foundation, the cohesion and damping effect of the clay hinder the movement and rearrangement of soil particles under vibration. The increase in clay content causes the vibration energy transmission of the vibrator to decay rapidly, making it difficult to vibrate and expand the hole. In this soil layer, the diameter of the pile hole suddenly shrinks, and a "neck shrinkage" phenomenon occurs. The narrow pile diameter at the necking point affects the falling and passing of the filler, and the diameter of the place is also very small after the pile is formed, affecting the reinforcement effect. Summary of the invention

[0007] The purpose of the present invention is to provide a vibro-sand replacement pile construction method in a soft clay layer, which reduces the trial and error cost, simplifies the operation, ensures the pile diameter and achieves a better compaction effect.

[0008] The technical solution to achieve the above purpose is:

[0009] A method for constructing sand pile replacement in a clay soft soil foundation, comprising:

[0010] Step S1, laying a steel casing in a clay foundation;

[0011] Step S2, taking out the soil in the steel casing by using a rotary drilling rig;

[0012] Step S3, inspecting the hole depth and filling it with sand using an excavator;

[0013] Step S4, removing the steel casing;

[0014] Step S5, vibrating the hole to a compacting current;

[0015] Step S6, CPT testing, testing the side friction value after compaction;

[0016] Preferably, in step 1, the steel casing is driven according to the designed point and depth to ensure that the steel casing has the same diameter as the sand pile and to control the center coordinates and verticality of the pile position.

[0017] Preferably, the step S1 of installing a steel casing in a clay foundation comprises:

[0018] Step S11, pre-construction preparation, including: ensuring the site is solid and flat, preparing steel casing and installation equipment that meet the design requirements;

[0019] Step S12, measurement and positioning, includes: setting measurement control points to provide a reference for steel casing positioning, and accurately measuring and placing the pile position of the steel casing according to the control points;

[0020] Step S13, the steel casing installation includes: lifting into place, vertical adjustment and burial depth control;

[0021] Step S14, quality inspection, includes: inspecting the quality, position and verticality of the steel casing.

[0022] Preferably, in step S2, the soil in the steel casing is completely removed by using a rotary drilling rig, using a bucket tooth spiral drill bit, and attention should be paid to controlling the rotation speed, drilling pressure and drilling speed.

[0023] Preferably, in step S3, the hole depth is inspected by measuring multiple points and taking the average value.

[0024] Preferably, in step S3, the process of filling the sand material with an excavator should try to avoid excessive disturbance to the clay soft soil foundation.

[0025] Preferably, in step S4, the steel casing is removed by using a crawler crane + vibratory hammer.

[0026] Preferably, since the cohesive force of the soft clay foundation is relatively large, a vibrating hammer with relatively large exciting force needs to be selected.

[0027] Preferably, in step S5, the hole position vibration punching needs to control the compaction current well and leave a certain margin, and the vibration punching is stopped after the compaction current value is reached and stabilized.

[0028] Preferably, the step S6CPT detection includes: penetrating the probe into the sand pile, detecting the friction resistance generated by the sand pile soil to the side of the probe, controlling the penetration rate, and performing data correction after the detection is completed.

[0029] The beneficial effects of the present invention are: using a steel casing with the same diameter as the designed sand pile to protect the wall, and then completely taking the soil, avoiding the influence of complex geological conditions such as hole collapse on the pile formation. After taking the soil, the sand material is directly filled into the steel casing and then the steel casing is removed, which can fully ensure that the minimum diameter within all design depths is not less than the designed sand pile diameter, reducing the trial and error cost and ensuring the quality of the pile formation and the compaction effect. The later stage of vibro-impacting is simplified to vibro-impacting in a uniform sandy stratum, and the construction parameter only needs to control the compaction current, and the difficulty of operation is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a flow chart of the construction method of replacing sand piles in a clay soft soil foundation of the present invention; DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings.

[0032] See also Figure 1 The method for constructing sand piles in a clay soft soil foundation of the present invention comprises the following steps:

[0033] Step S1, placing a steel casing in a clay foundation, comprises:

[0034] Step S11, pre-construction preparation, including: ensuring that the site is solid and flat, providing a stable foundation for the parking and operation of subsequent construction equipment. Prepare steel casings and pile hammers, vibratory hammers, cranes and other driving equipment that meet the design requirements. For example, the wall thickness of the steel casing is generally determined according to the pile diameter and the depth of burial, generally between 6-12mm;

[0035] Step S12, measurement and positioning, including: according to the design drawings and the benchmark points on site, use a total station or other measuring instruments to set measurement control points at the construction site to provide a benchmark for the positioning of the steel casing, and accurately measure and release the pile position of the steel casing according to the control points. Generally, the deviation of the position of a single pile does not exceed 50m;

[0036] Step S13, installation of steel casing, for example: after determining the center of the pile position, the vibration method can be used to connect the vibrating hammer to the top of the steel casing, turn on the vibrating hammer, and use the high-frequency vibration generated by the vibrating hammer to liquefy the soil around the steel casing, reduce the friction between the steel casing and the soil, so that the steel casing can sink smoothly. The vibration frequency is usually between 10-30Hz. For example, a vibrating hammer can be used to drive a 120mm diameter steel casing from an elevation of +3mmCD to -11mmCD. During the lifting process, the steel casing must be kept stable, and attention must be paid to ensuring verticality and controlling the depth of penetration during the driving process;

[0037] Step S14, quality inspection and acceptance, includes: inspecting the quality of the steel casing, including whether the appearance of the steel casing has any defects such as deformation, cracks, holes, etc., and inspecting the position and verticality.

[0038] Step S2, using a rotary drilling rig to completely remove the soil in the steel casing, using a bucket-tooth spiral drill bit, the bucket teeth of this drill bit can effectively cut into clay soil, the spiral blades are convenient for transporting soil to the ground, the optimized bucket tooth shape can reduce drilling resistance, and the appropriate spacing can ensure the balance of the amount of soil cut each time, thereby improving drilling efficiency, and at the same time, attention should be paid to controlling the rotation speed, drilling pressure and drilling speed. When drilling begins, low-speed drilling is adopted, and the rotation speed is generally around 10-20r / min. As drilling proceeds, the drill bit cuts the soil and loads it into the drill bucket. When the drill bucket is full of soil, the drill rod is lifted out of the steel casing, and the lifting speed also needs to be controlled to prevent soil blocks from falling. The general lifting speed is around 0.5-1m / s.

[0039] Step S3, use the method of multi-point measurement and averaging to measure the hole depth and then check the value, and use an excavator to backfill the sand into the above-mentioned steel casing. The selection of sand generally gives priority to medium-coarse sand with uniform particles and hard texture. Since the water content of the clay soft soil itself is relatively high, the water content of the sand should be appropriately adjusted by drying or sprinkling water, usually controlled between 7% and 10%. During the backfilling process, the layered filling method can make the sand more dense and uniform in the casing, and the filling thickness of each layer should be 30-50cm. At the same time, try to avoid excessive disturbance to the clay soft soil foundation, control the movement amplitude of the excavator, prevent damage to the structure of the foundation soil, affect the friction between the sand pile and the foundation soil, and make the bucket completely filled with sand.

[0040] Step S4, remove the steel casing, and use a crawler crane + vibratory hammer to remove the steel casing. The crawler crane is selected according to the length, diameter, weight of the steel casing and the actual working conditions of the construction site. Since the cohesion of the sticky soft soil foundation is relatively large, it is necessary to select a vibratory hammer with a larger exciting force. The larger the size of the steel casing, the larger the required exciting force is usually. For small steel casings with a diameter of less than 1m and a length of less than 10m, in a sticky soft soil foundation with a cohesion of less than 20kPa and a water content of more than 50%, it is preliminarily estimated that an exciting force of 50-100kN may be required; for large steel casings with a diameter of more than 2m and a length of more than 20m, in a foundation with a cohesion of more than 30kPa, an exciting force of 300-500kN or more is required. Before use, the crawler crane and vibratory hammer must be fully inspected and debugged, and the steel casing must be cleaned, and the soil and debris on the surface of the steel casing must be removed to reduce the friction between the steel casing and the vibratory hammer clamp, and ensure that the clamp can firmly clamp the steel casing. During the removal process, attention must be paid to the control of the pulling force and vibration parameters, while ensuring the verticality of the steel casing. After removal, the steel casing must be cleaned and inspected, and the holes left on the sticky soft soil foundation must be treated to prevent the soil around the holes from collapsing and affecting the stability of the foundation.

[0041] Step S5, vibrate the hole to the compacting current. For the cohesive soft soil foundation, due to the soft soil, the vibration frequency of the vibrator is generally selected to be 1450-1500 times / minute, and the exciting force is between 30-80kN, which can effectively compact the sand pile. At the same time, in the cohesive soft soil foundation, the compacting current is generally between 40-60A, which is specifically adjusted according to the design requirements of the sand pile, the properties of the sand material and the actual situation of the foundation soil, and considering the cohesion and friction characteristics of the cohesive soft soil, a certain margin should be reserved when setting the compacting current. After starting the vibrator, the current gradually rises, and the ammeter should be closely watched. When the current is close to the compacting current value required by the design, it should be carefully monitored. When the current reaches the compacting current value, keep the vibrator working for a period of time, generally 30 to 60 seconds, to ensure that the filler is fully compacted.

[0042] Step S6, perform CPT detection on sand piles. Consider the characteristics of soft clay soil, which has a high water content and will affect the strength and stability of the soil around the sand piles. Insert the probe into the sand pile to detect the friction resistance of the sand pile soil to the side of the probe. The clay soil around the sand pile will produce a large viscous resistance to the probe penetration. During the penetration process, the viscous resistance and the side friction resistance of the sand pile itself must be carefully distinguished to control the penetration rate. After the detection is completed, data correction and calibration are required. Comparing the side friction resistance value after compaction with the design requirements can determine whether the sand pile has reached the expected density and bearing capacity.

[0043] The above embodiments are only used to illustrate the present invention, rather than to limit the present invention. Those skilled in the relevant technical field may make various changes or modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions should also belong to the scope of the present invention and should be defined by the claims.

Claims

1. A method for constructing sand piles in a clay soft soil foundation, characterized in that: include: Step S1, laying a steel casing in a clay foundation; Step S2, using a rotary drilling rig to completely remove the soil in the steel casing; Step S3, inspecting the hole depth and filling it with sand using an excavator; Step S4, removing the steel casing; Step S5, vibrating the hole to a compacting current; Step S6, CPT detection, detecting the side friction value after compaction.

2. The method for constructing sand piles in a viscous soft soil foundation according to claim 1, characterized in that: In step S1, the steel casing is driven according to the designed point and depth to ensure that the diameter of the steel casing is the same as that of the sand pile and to control the center coordinates and verticality of the pile position.

3. The method for constructing sand piles in a clay soft soil foundation according to claim 1, characterized in that: In the step S1, a steel casing is placed in a clay foundation, comprising: Step S11, pre-construction preparation, including: ensuring the site is solid and flat, preparing steel casing and installation equipment that meet the design requirements; Step S12, measurement and positioning, includes: setting measurement control points to provide a reference for steel casing positioning, and accurately measuring and placing the pile position of the steel casing according to the control points; Step S13, the steel casing installation includes: lifting into place, vertical adjustment and burial depth control; Step S14, quality inspection, includes: inspecting the quality, position and verticality of the steel casing.

4. The method for constructing sand piles in a clay soft soil foundation according to claim 1, characterized in that: In step S2, the soil in the steel casing is completely removed by using a rotary drilling rig with a bucket tooth spiral drill bit, and attention should be paid to controlling the rotation speed, drilling pressure and drilling speed.

5. The method for constructing sand piles in a clay soft soil foundation according to claim 1, characterized in that: In step S3, the hole depth is inspected by measuring multiple points and taking the average value.

6. The method for constructing sand piles in a clay soft soil foundation according to claim 1, characterized in that: In step S3, when filling the sand material with an excavator, excessive disturbance of the soft clay foundation should be avoided.

7. The method for constructing sand piles in a clay soft soil foundation according to claim 1, characterized in that: In step S4, the steel casing is removed by using a crawler crane + vibratory hammer.

8. The method for constructing sand piles in a clay soft soil foundation according to claim 1, characterized in that: In step S5, the compaction current used in the hole vibrating is reserved with a preset margin, and the vibrating is stopped after the compaction current value is reached and stabilized.

9. The method for constructing sand piles in a clay soft soil foundation according to claim 1, characterized in that: In step S6, the CPT detection includes: the probe penetrates the sand pile, detects the friction resistance of the sand pile soil to the side of the probe, controls the penetration rate, and performs data correction after the detection is completed.

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