Composite pile foundation construction method for controlling negative friction influence

Abstract

A composite pile foundation construction method for controlling the negative friction influence comprises the following steps that firstly, pile position hole forming is conducted, and a formed pile hole penetrates through a negative friction stratum and a non-negative friction stratum; when a hole is formed to a non-negative friction stratum, high-pressure rotary jet grouting stirring is started, high-pressure water is firstly fed, then cement paste and compressed air are fed, spraying is conducted while rotation is conducted till a bearing stratum is reached, then re-spraying is conducted on a cement soil area, and silicate cement paste is selected as spraying slurry; a center pile body is implanted, the lower end of the pile penetrates through the cement soil solidification layer, and pile implanting is stopped when the pile reaches the bearing layer; and after the composite pile foundation meets the design age requirement and reaches the design bearing capacity, a complete pile foundation is formed. According to the composite pile formed by implementing cement soil reinforcement on the non-negative friction stratum section of the pile body, the proportion of negative friction to the bearing capacity of the pile foundation is reduced, so that the bearing performance of the pile foundation is effectively improved, and the construction task of the composite pile foundation can be reliably completed through the construction method.

Description

Technical Field

[0001] The invention belongs to the field of pile foundations in geotechnical engineering, and particularly relates to a construction method for a composite pile foundation for controlling the influence of negative skin friction. Background Art

[0002] With the rapid advancement of urbanization construction, the shortage of land resources has become an important factor restricting urban development. More and more engineering projects are constructed in special soil areas, which has become the main way to solve the problem of land shortage at present, such as thick loose fill, self-weight collapsible loess, under-consolidated soil, and under-compacted soft clay. In these areas, the application of pile foundations is also increasing day by day. When the pile body penetrates the formation that can generate negative skin friction and the pile tip is supported on a hard bearing layer, the upper soil around the pile body generates deformation greater than the settlement of the pile body due to its own consolidation or ground surcharge, resulting in downward negative skin friction on the upper side of the pile body, causing additional downward load on the pile body and seriously weakening the bearing capacity of the pile foundation.

[0003] In order to reduce the negative skin friction around the pile, currently, coating is often applied on the pile side or isolation is carried out around the pile body to reduce the influence of negative skin friction. However, during the construction of pile driving, on the one hand, due to the direct scraping of the surface coating by the soil, it is easy to cause damage and peeling of the coating, greatly reducing its effect of reducing negative skin friction; on the other hand, the modified pile body structure is complex and the construction difficulty increases, resulting in limited feasibility of the above methods. Therefore, how to effectively and conveniently control the influence of negative skin friction is an urgent problem to be solved. Summary of the Invention

[0004] The purpose of the invention is to provide a construction method for a composite pile foundation for controlling the influence of negative skin friction based on the deficiencies in the existing process methods for reducing negative skin friction, so as to more effectively exert the bearing performance of the pile foundation.

[0005] The purpose of the invention is achieved in the following way: A construction method for a composite pile foundation for controlling the influence of negative skin friction has a central pile body. The central pile body is a precast pile with one or more sections. The upper and lower sections of the multi-section precast pile are connected by welding or mechanical connection. The lower end of the central pile body is supported on the bearing layer. The part of the central pile body above the neutral point is the negative skin friction formation, and the part of the central pile body below the neutral point is the non-negative skin friction formation. A cement-soil area formed by high-pressure cement slurry jet mixing is arranged on the outer periphery of the central pile body in the non-negative skin friction formation. The cement-soil area and the central pile body are combined to form a composite pile foundation. The construction method of the composite pile foundation includes the following steps: Step 1: Drill a pile hole at the designed pile position. The formed pile hole penetrates the negative skin friction formation and the non-negative skin friction formation. Step 2: When the hole is drilled to the non - negative skin friction stratum, start the high - pressure jet grouting and mixing equipment. First, supply high - pressure water, then supply cement slurry and compressed air. The compressed air is supplied 30 s later. Rotate and jet simultaneously until reaching the bearing stratum. Then lift the drill bit and re - jet the cement - soil area formed by the high - pressure cement slurry jet grouting and mixing. The jet - grouted slurry uses ordinary Portland cement slurry. Step 3: Use a static press or hydraulic hammer to implant the central pile body. When the pile length is greater than that of a single precast pile, use welding or mechanical connection for pile splicing; stop pile implantation when the lower end of the pile pierces the cement - soil solidified layer and reaches the bearing stratum. Step 4: After pile implantation is completed, when the composite pile foundation meets the design age requirements and reaches the design bearing capacity, a complete pile foundation is formed.

[0006] The cement - soil in the cement - soil area uses ordinary Portland cement with a grade not lower than 32.5, and the water - cement ratio is 0.8 - 1.5; the cement dosage per unit volume of the mixed soil: for cohesive soil and silt, ≥360 kg / m 3 ; for sandy soil, ≥325 kg / m 3 ; for gravelly soil, ≥290 kg / m 3 . See Table 1: Table 1 Cement Dosage and Water - Cement Ratio of Cement Slurry Soil types <![CDATA[Cement dosage in the soil mass to be stirred (kg / m 3 )]]> Water-cement ratio Clay, silt ≥360 0.8~1.5 Sand ≥325 0.8~1.5 Gravelly soil ≥290 0.8~1.5 The initial setting time of the slurry is not less than 2 h, and the curing time is 12 h - 48 h.

[0007] The jet - grouting pressure and grouting volume of the cement slurry are determined according to different soil types. For silt and silty clay, the jet - grouting pressure is 12 - 25 MPa and the grouting volume is 0.2 - 0.4 m 3 / m 2 ; for cohesive soil and silt, the jet - grouting pressure is 12 - 25 MPa and the grouting volume is 0.2 - 0.4 m 3 / m 2 ; for completely weathered rock and strongly weathered rock, the jet - grouting pressure is 12 - 20 MPa and the grouting volume is 0.2 - 0.4 m 3 / m 2 ; for fine - grained sand, the jet - grouting pressure is 18 - 30 MPa and the grouting volume is 0.4 - 0.6 m 3 / m 2 ; for medium sand, the jet - grouting pressure is 18 - 30 MPa and the grouting volume is 0.4 - 0.6 m 3 / m 2 ; for coarse sand and gravelly sand, the jet - grouting pressure is 18 - 30 MPa and the grouting volume is 0.4 - 0.6 m 3 / m 2 ; for cobbles, angular gravels, gravels, and pebbles, the jet - grouting pressure is 18 - 30 MPa and the grouting volume is 0.4 - 0.6 m 3 / m 2;During shotcreting, the flow rate is greater than 30 L / min; the shotcreting volume is the volume of the grout sprayed per square meter of the side area. See Table 2: Table 2 Shotcreting Pressure and Shotcreting Volume of Cement Grout Parameter Silt, silty clay Clay, silt Completely weathered rock, strongly weathered rock Fine sand Medium sand Coarse sand, gravelly sand Round gravel, angular gravel, crushed stone, cobblestone Shotcrete pressure (MPa) 12~25 12~25 12~20 18~30 18~30 18~30 18~30 <![CDATA[Spraying volume (m 3 / m 2 )]]> 0.2~0.4 0.2~0.4 0.2~0.4 0.4~0.6 0.4~0.6 0.4~0.6 0.4~0.6 The outer diameter of the cement soil area is 1.6 - 2.4 times the diameter of the central pile body; the height of the cement soil area is 0.4 - 0.6 times the height of the central pile body.

[0008] The construction method process of the composite pile foundation is as follows: leveling the site → lofting the pile positions → the drill rig is in place after equipment assembly → spraying test → rotary drilling → drilling to the designed depth → starting the grout spraying after ensuring the grout supply, continuing rotary drilling while stirring → stopping the spraying after reaching the bearing stratum, forming the solidified area → lifting the drill and re-spraying the grout to the top elevation of the solidified area → pressing in the precast pile → penetrating the solidified area, with the pile tip reaching the bearing stratum → the pile formation is completed.

[0009] The beneficial effects of the present invention compared with the prior art are as follows: (1) It avoids many technical quality problems of setting the negative skin friction elimination coating above the neutral point; (2) The composite pile formed by implementing cement soil reinforcement in the non-negative skin friction stratum section of the pile body forms a pile body structure with a small upper cross-section and a large lower cross-section, reducing the proportion of negative skin friction in the pile foundation bearing capacity, thus effectively improving the bearing performance of the pile foundation; (3) The construction method is reliable and easy to implement, and can effectively complete the composite pile foundation for controlling the influence of negative skin friction. Description of the Drawings

[0010] Figure 1 It is a schematic diagram of a composite pile foundation applied to a negative skin friction stratum; Figure 2 It is a flow chart of the construction method of a composite pile foundation applied to a negative skin friction stratum.

[0011] In the figure: 1 - central pile body; 2 - negative skin friction stratum; 3 - non-negative skin friction stratum; 4 - bearing stratum; 5 - cement soil area Detailed Embodiments

[0012] Referring to Figure 1 、 Figure 2 For a construction method of a composite pile foundation for controlling the influence of negative skin friction, there is a central pile body 1, and the central pile body 1 is a precast pile of one or more sections. The upper and lower sections of the multi-section precast pile are connected by welding or mechanical connection. The lower end of the central pile body 1 supports on the bearing stratum 4. The part of the central pile body 1 above the neutral point is the negative skin friction stratum 2, and the part of the central pile body 1 below the neutral point is the non-negative skin friction stratum 3. Around the central pile body 1 in the non-negative skin friction stratum 3 is a cement soil area 5 formed by high-pressure cement grout rotary jet mixing. The cement soil area 5 and the central pile body 1 together form a composite pile foundation; the construction method of the composite pile foundation includes the following steps: Step 1: Drill a hole at the designed pile position, and the formed pile hole penetrates through the negative skin friction stratum 2 and the non - negative skin friction stratum 3. Step 2: When drilling to the non - negative skin friction stratum 3, start the high - pressure jet grouting mixing equipment. First, send high - pressure water, then send cement slurry and compressed air. The compressed air is sent 30 s later. Rotate and jet simultaneously until reaching the bearing stratum 4, and then lift the drill bit and re - jet the cement - soil area 5 formed by high - pressure cement slurry jet grouting mixing. The jet - grouted slurry uses ordinary Portland cement. Step 3: Use a static press or hydraulic hammer to implant the central pile body 1. When the pile length is greater than that of a single precast pile, use welding or mechanical connection for pile splicing; stop pile implanting when the lower end of the pile pierces through the cement - soil solidified layer and reaches the bearing stratum 4. Step 4: After pile implanting is completed, when the composite pile foundation meets the requirements of the designed age and reaches the designed bearing capacity, a complete pile foundation is formed.

[0013] The cement - soil in the cement - soil area 5 uses ordinary Portland cement with a grade not lower than 32.5, and the water - cement ratio is 0.8 - 1.5; the cement dosage per unit volume of the mixed soil: for cohesive soil and silt, ≥ 360 kg / m 3 ; for sand, ≥ 325 kg / m 3 ; for gravelly soil, ≥ 290 kg / m 3 ; the initial setting time of the slurry is not less than 2 h, and the curing time is 12 h - 48 h.

[0014] During the construction of the cement - soil area 5, the jet pressure and jet volume of the cement slurry are determined according to different soil types. For silt and silty clay, the jet pressure is 12 - 25 MPa and the jet volume is 0.2 - 0.4 m 3 / m 2 ; for cohesive soil and silt, the jet pressure is 12 - 25 MPa and the jet volume is 0.2 - 0.4 m 3 / m 2 ; for completely weathered rock and strongly weathered rock, the jet pressure is 12 - 20 MPa and the jet volume is 0.2 - 0.4 m 3 / m 2 ; for fine sand, the jet pressure is 18 - 30 MPa and the jet volume is 0.4 - 0.6 m 3 / m 2 ; for medium sand, the jet pressure is 18 - 30 MPa and the jet volume is 0.4 - 0.6 m 3 / m 2 ; for coarse sand and gravelly sand, the jet pressure is 18 - 30 MPa and the jet volume is 0.4 - 0.6 m 3 / m 2 ; for cobble, angular gravel, gravel, and pebble, the jet pressure is 18 - 30 MPa and the jet volume is 0.4 - 0.6 m 3 / m 2During shotcreting, the flow rate is greater than 30 L / min; the shotcreting volume is the volume of the grout sprayed per square meter of the side area.

[0015] The outer diameter of the cement-soil area is 1.6 to 2.4 times the diameter of the central pile body; the height of the cement-soil area is 0.4 to 0.6 times the height of the central pile body.

[0016] The construction method process of the composite pile foundation is as follows: leveling the site → setting out the pile positions → the drill rig is in place after equipment assembly → spraying test → rotary drilling → drilling to the design depth → starting the grout spraying after ensuring the grout supply, continuing rotary drilling while stirring → stopping spraying after reaching the bearing stratum, forming the solidification area → lifting the drill and re-spraying the grout to the top elevation of the solidification area → pressing in the precast pile → penetrating the solidification area, with the pile tip reaching the bearing stratum → the pile formation is completed.

Claims

1. A construction method of a composite pile foundation for controlling the influence of negative skin friction, having a central pile body, the central pile body being a precast pile of one or more sections, the upper and lower sections of the multi-section precast pile being connected by welding or mechanical connection, the lower end of the central pile body being supported on a bearing stratum, characterized in that: The part of the central pile body (1) above the neutral point is the negative skin friction stratum (2), and the part of the central pile body (1) below the neutral point is the non-negative skin friction stratum (3). The peripheral part of the central pile body (1) in the non-negative skin friction stratum (3) is the cement-soil area (5) formed by high-pressure cement slurry rotary jet mixing. The cement-soil area (5) and the central pile body (1) are combined to form a composite pile foundation. The construction method of the composite pile foundation includes the following steps: Step 1: Drill a hole at the designed pile position, and the drilled hole penetrates through the negative skin friction stratum (2) and the non-negative skin friction stratum (3). Step 2: When the hole is drilled to the non-negative skin friction stratum (3), start the high-pressure rotary jet mixing equipment. First, send high-pressure water, then send cement slurry and compressed air. The compressed air is sent 30 s later. Rotate and jet simultaneously until reaching the bearing stratum (4), and then lift the drill bit and re-jet the cement-soil area (5) formed by high-pressure cement slurry rotary jet mixing. The jetting slurry is ordinary Portland cement slurry. Step 3: Use a static press or a hydraulic hammer to implant the central pile body (1). When the pile length is greater than that of a single precast pile, welding or mechanical connection is used for pile splicing. The lower end of the pile pierces through the solidified layer of the cement-soil area and stops pile implanting when reaching the bearing stratum (4). Step 4: After pile implanting is completed, when the composite pile foundation meets the requirements of the designed age and reaches the designed bearing capacity, a complete pile foundation is formed.

2. The construction method of the composite pile foundation for controlling the influence of negative skin friction according to claim 1, characterized in that: The cement soil in the cement soil area (5) is selected as ordinary Portland cement with a grade not lower than 32.5, and the water-cement ratio is 0.8 - 1.5; the cement dosage in the soil mass to be stirred per unit: for cohesive soil and silt ≥ 360 kg / m 3 ; for sandy soil ≥ 325 kg / m 3 ; for gravel soil ≥ 290 kg / m 3 ; the initial setting time of the slurry is not less than 2h, and the curing time is 12h - 48h.

3. The construction method of the composite pile foundation for controlling the influence of negative skin friction according to claim 1, characterized in that: During the construction of the cement-soil area (5), the jetting pressure and jetting volume of the cement slurry are determined according to different soil conditions of different soil layers. For silt and silty clay, the jetting pressure is 12 - 25 MPa and the jetting volume is 0.2 - 0.4 m 3 / m 2 ; for cohesive soil and silty soil, the jetting pressure is 12 - 25 MPa and the jetting volume is 0.2 - 0.4 m 3 / m 2 ; for completely weathered rock and strongly weathered rock, the jetting pressure is 12 - 20 MPa and the jetting volume is 0.2 - 0.4 m 3 / m 2 ; for fine sand, the jetting pressure is 18 - 30 MPa and the jetting volume is 0.4 - 0.6 m 3 / m 2 ; for medium sand, the jetting pressure is 18 - 30 MPa and the jetting volume is 0.4 - 0.6 m 3 / m 2 ; for coarse sand and gravelly sand, the jetting pressure is 18 - 30 MPa and the jetting volume is 0.4 - 0.6 m 3 / m 2 ; for cobble, angular gravel, crushed stone, and pebble, the jetting pressure is 18 - 30 MPa and the jetting volume is 0.4 - 0.6 m 3 / m 2 ; during jetting, the flow rate is greater than 30 L / min; the jetting volume is the volume of the slurry jetted per square meter of the side area.

4. The construction method of the composite pile foundation for controlling the influence of negative skin friction according to claim 1, characterized in that: The outer diameter of the cement-soil area is 1.6 - 2.4 times the diameter of the central pile body; the height of the cement-soil area is 0.4 - 0.6 times the height of the central pile body.

5. The construction method of the composite pile foundation for controlling the influence of negative skin friction according to claim 1, characterized in that The construction method process of the composite pile foundation is: site leveling → pile position lofting → rig in place after equipment assembly → spraying test → rotary drilling → drilling to the designed depth → start slurry jetting after ensuring slurry supply, continue rotary drilling while mixing → stop jetting after reaching the bearing stratum, formation of the solidified area → lift the drill bit and re-jet the slurry to the top elevation of the solidified area → press in the precast pile → penetrate the solidified area, pile tip reaches the bearing stratum → pile forming completed.

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