Construction method for accurately determining pile length of cast-in-place pile in karst cave area

Abstract

The invention relates to a construction method for accurately determining the pile length of a cast-in-place pile in a karst cave area, which comprises the following steps of: drilling a pile body of the cast-in-place pile penetrating through the karst cave area, and enabling the depth of advanced drilling into a complete rock to be equal to the value of a designed engineering pile into the rock + 5m; advanced drilling data are analyzed, and the pile length of the cast-in-place pile is preliminarily determined according to the design drawing, the normative rock entering requirement and the adjacent pile bottom height difference requirement, the rock entering is not smaller than one time of the pile diameter D, and the height difference of the adjacent pile bottoms in the drawing is not larger than one time of the pile center distance. And according to the preliminarily determined pile length of the cast-in-place pile, when the depth of the short pile entering the complete rock is larger than 2D, drilling holes are added. And according to newly-added drilling data, the pile length judgment standard of the short piles without karst cave distribution in the drilling result is adjusted, the pile length of the short piles is determined again, and if karst cave distribution exists in the drilling result, construction is conducted according to the pile length of the standard judgment standard of an original design drawing. And according to the final pile length, cast-in-place pile construction is conducted, and after construction is completed, low-strain dynamic detection, ultrasonic transmission detection and a low-speed load maintaining method static load test are conducted. According to the method, the pile length of the cast-in-place pile can be accurately determined, pile length redundancy caused by conservative design is avoided, time cost, personnel cost and material cost can be effectively reduced, usability is high, and the method is suitable for application and popularization.

Description

Technical Field

[0001] The invention belongs to the technical field of foundation engineering, and relates to an advance drilling construction technology, and specifically to a construction method for accurately determining the length of cast-in-place piles in a karst cave area. Background Art

[0002] With the acceleration of my country's urbanization process, urban space is expanding rapidly, the scale of construction land is increasing, and projects with complex geological conditions are becoming more common, especially for karst geology. In karst geology, the strata are uneven, the distribution of karst caves is random, and the size, depth, and filling properties are different. This makes it difficult to accurately predict the position and conditions of the bearing layer at the pile end during the design stage, increasing the uncertainty of pile length determination; some information can be obtained through detailed surveys, but due to the concealment and irregularity of karst caves, detailed survey results are often difficult to fully reflect the actual situation, and may omit or underestimate the characteristics of certain karst caves, affecting the accurate judgment of pile length; due to the uncertainty of karst caves, a more conservative strategy is often adopted during design, increasing the pile length to ensure the stability of the pile foundation, which increases unnecessary costs and construction difficulties. Therefore, it is crucial to accurately determine the pile length in the karst area. If the pile length is determined improperly, it may lead to serious pile foundation quality problems and affect the stability of the building.

[0003] Therefore, a construction method for accurately determining the length of cast-in-place piles in the cave area is proposed. Summary of the invention

[0004] The key points of the existing bored pile construction technology are mainly to determine the pile length through detailed survey reports. Although this solves the bored pile construction under general geological conditions, the detailed survey points are spaced far apart, and the rock strata in the karst cave area are undulating, making it impossible to accurately determine the distribution of the rock strata at the location of the bored piles, and thus it is impossible to accurately determine the length of the bored piles. The purpose of the present invention is to propose a construction method for accurately determining the length of bored piles in the karst cave area, so as to avoid conservative design and increase unnecessary costs and construction difficulty.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] The construction method for accurately determining the length of cast-in-place piles in the cave area is as follows:

[0007] Step 1: For the bored piles passing through the karst cave area, the pile body is drilled three times. The holes are arranged in an equilateral triangle. The distance between the holes and the pile core is 3 / 8 times the pile diameter D. The depth of the advanced drilling into the complete rock is at least the designed engineering pile rock penetration value + 5 meters.

[0008] The additional drilling further clarifies the distribution of the bearing layer at the pile end. The rock penetration is carried out according to the deepest value of the complete rock surface obtained from the three drilling results. The rock penetration depth is greater than or equal to the diameter D of the bored pile.

[0009] Step 2: Analyze the advance drilling data and preliminarily determine the length of the cast-in-place piles based on the design drawings and rock penetration requirements and the height difference requirements of adjacent pile bottoms.

[0010] The specification requirements mentioned are "For rock-embedded piles, the rock-embedded depth shall be determined by comprehensively considering factors such as load, overlying soil layer, bedrock, pile diameter and pile length; the full-section depth of the pile embedded in inclined intact and relatively intact rock shall not be less than 0.4d and not less than 0.5m; for moderately weathered rock with an inclination greater than 30%, the rock-embedded depth shall be appropriately increased according to the inclination and rock integrity; the depth of the pile embedded in flat, intact hard rock and relatively hard rock shall not be less than 0.2d and shall not be less than 0.2m."

[0011] Step 3: Based on the preliminarily determined cast-in-place pile length, when the depth of the short pile into the intact rock is greater than 2D, add more holes, and the location of the newly added holes is 600mm to 1000mm from the edge of the short pile on the line connecting the centers of the adjacent long and short piles;

[0012] Step 4: According to the newly added drilling data, the pile length determination standard of the short piles without cave distribution in the drilling results is adjusted, and the pile length of the short piles is re-determined. If the drilling results show cave distribution, the pile length determined by the original design drawing is used for construction;

[0013] Furthermore, the adjacent piles are preliminarily determined by drilling results and drawing requirements according to the following formula:

[0014] H1=|h1-h0|≤S, and the depth of penetration into the rock ≥D

[0015] Among them, h0 is the bottom elevation of the long pile; h1 is the bottom elevation of the short pile; H1: the height difference between the bottoms of adjacent piles; S: the distance between the pile centers; D: the diameter of the cast-in-place pile.

[0016] Furthermore, if no cave is found in the additional drilling, the height difference between the bottoms of adjacent piles is within the Technical Specifications for Building Pile Foundations.

[0017] (JGJ94-2008), combined with the design unit to consider a certain degree of redundancy, the pile length is accurately determined according to the following adjustment:

[0018] H1=|h2-h0|≤2S and the rock penetration depth ≥D

[0019] h2: pile bottom elevation determined by the present invention;

[0020] Reduce the redundant design length H to: H = |h2-h1|.

[0021] Step 5: Carry out cast-in-place pile construction according to the final pile length. After the construction is completed, conduct low-strain dynamic testing, ultrasonic transmission testing, and slow-maintained load method static load testing.

[0022] Beneficial effects:

[0023] (1) The present invention adopts advanced drilling in complex karst cave areas and then continues to add drilling holes to better determine the geological conditions of the rock strata where the pile body is located, which can solve the problem of inaccurate prediction of the distribution of the bearing layer at the pile end during the design stage.

[0024] (2) The present invention can detect multiple piles in complex areas and accurately determine the geological conditions of the rock layer where the pile body is located, which can solve the problem of redundant design of adjacent pile lengths, while improving construction efficiency, reducing construction costs, and lowering construction difficulty. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram of single pile drilling;

[0026] Figure 2 Schematic diagram of the pile length determination process of the present invention; 1- steel casing; 2- first layer rock surface; 3- cave; 4- complete rock surface; 5- add advance drilling and supplementary holes;

[0027] FIG3 is a flowchart of the implementation of the present invention; FIG3 (1) preliminarily determines the length of the cast-in-place pile; FIG3 (2) adds a schematic diagram of drilling;

[0028] Figure 4 1 is a construction process diagram of Example 1, in which: (1) preliminary pile length is determined; (2) additional drilling layout is added; (3) pile length is determined; S: pile center distance; D: cast-in-place pile diameter; h0: long pile bottom elevation; h1: short pile bottom elevation determined by drawing requirements; h2: short pile bottom elevation determined by this patent; H1: height difference between adjacent pile bottoms; H: reducing redundant design length. DETAILED DESCRIPTION

[0029] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] Example 1

[0031] A construction method for accurately determining the length of cast-in-place piles in a cave area comprises the following steps:

[0032] Step 1: If Figure 1 As shown, in this embodiment, there are 6 cast-in-place piles, and the pile bodies of the 6 piles are drilled three times, the boreholes are arranged in an equilateral triangle, and the distance between the boreholes and the pile core is 3 / 8 times the pile diameter D (in this embodiment, D=800mm), and the remaining piles are drilled once, and the advance drilling depth into the complete rock is at least the designed engineering pile rock value + 5 meters (as shown in Table 1);

[0033] Table 1 Advanced drilling data table

[0034]

[0035]

[0036] Step 2: Analyze the advance drilling data and preliminarily determine the length of the cast-in-place pile according to the design drawings and the rock penetration requirements and the height difference requirements of the adjacent pile bottoms, as shown in Table 2 and Figure 3 (1). Among them, the preliminarily determined standard for the length of the cast-in-place pile based on the advance drilling data is the drawing requirements (the rock penetration is not less than one pile diameter and the height difference of the adjacent pile bottoms in the drawing is not greater than one pile center distance).

[0037] Table 2 Preliminary determination of cast-in-place pile length (unit: meter)

[0038]

[0039] Step 3: Based on the initial cast-in-place pile length, add additional holes for short piles that are more than twice the rock penetration depth in the design drawings. The additional holes are located at a distance of 600 mm to 1000 mm from the edge of the short pile on the line connecting the centers of the adjacent long and short piles, as shown in Table 3 and Figure 3(2);

[0040] Table 3 Data of newly added holes in advance drilling

[0041] Add drilling number Orifice elevation First rock surface elevation Cave top elevation Cave height Complete rock surface elevation 1′ 7.10 -13.80 -15.50 2.30 -17.80 2′ 7.12 -15.68 -15.68 3.40 -19.08 3′ 6.19 -14.41 -14.41 3.10 -17.51 5′ 6.30 -13.30 -14.10 3.20 -17.30 6′ 6.22 -18.08 -18.88 4.40 -23.28

[0042] Step 4: According to the newly added drilling data, the pile length judgment standard of the short piles without cave distribution in the drilling results is adjusted, and the pile length of the short pile is re-determined. If the drilling results show cave distribution, the pile length determined by the original design drawing is used for construction (as shown in Table 4); among which, the adjusted pile length judgment standard is that the rock penetration is not less than one pile diameter and the height difference of adjacent pile bottoms in the drawing is not more than two pile center distances;

[0043] Table 4 Determination of cast-in-place pile length

[0044]

[0045] Step 5: Carry out cast-in-place pile construction according to the final pile length, and carry out low-strain dynamic testing, ultrasonic transmission testing, and slow-maintained load method static load testing after the construction is completed. Among them, the qualified rate of each test is 100%.

[0046] The above embodiments are only used to illustrate the technical method of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical method of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical method of the present invention.

Claims

1. A construction method for accurately determining the length of cast-in-place piles in a cave area, characterized in that: The following steps are involved: Step 1: Drill the pile body of the cast-in-place pile that passes through the cave area, and drill into the complete rock in advance to the designed engineering pile rock penetration depth + 5 meters; Step 2: Analyze the advance drilling data, and preliminarily determine the length of the cast-in-place pile according to the rock penetration requirements and the height difference requirements of the adjacent pile bottoms in the design drawings and specifications, with the rock penetration not less than one pile diameter D and the height difference of the adjacent pile bottoms in the drawings not greater than 1 pile center distance; Step 3: Based on the initially determined cast-in-place pile length, when the short pile enters the intact rock depth > 2D, increase the drilling hole; Step 4: According to the newly added drilling data, the pile length determination standard of the short piles without cave distribution in the drilling results is adjusted, and the pile length of the short piles is re-determined. If the drilling results show cave distribution, the pile length determined by the original design drawing is used for construction; Step 5: Carry out cast-in-place pile construction according to the final pile length. After the construction is completed, conduct low-strain dynamic testing, ultrasonic transmission testing, and slow-maintained load method static load testing.

2. The method for accurately determining the length of cast-in-place piles in a karst cave area according to claim 1, characterized in that: In step 1, the drilling times are 3 times, the boreholes are arranged in an equilateral triangle, and the distance between the boreholes and the pile core is 3 / 8 of the pile diameter.

3. The method for accurately determining the length of cast-in-place piles in a karst cave area according to claim 1, characterized in that: In step 3, the newly added drilling position is 600mm~1000mm away from the edge of the short pile on the line connecting the centers of adjacent long and short piles.

4. The method for accurately determining the length of cast-in-place piles in a karst cave area according to claim 1, characterized in that: In step 2, the length of the short pile is preliminarily determined according to the following formula: H1=|h1-h0|≤S, and the rock penetration depth ≥D Among them, h0 is the bottom elevation of the long pile; h1 is the bottom elevation of the short pile; H1: the height difference between the bottoms of adjacent piles; S: the distance between the pile centers; D: the diameter of the cast-in-place pile.

5. The method for accurately determining the length of cast-in-place piles in a karst cave area according to claim 1, characterized in that: In step 4, if no cave is found in the additional drilling, the height difference between the bottoms of adjacent piles is accurately determined based on the Technical Specifications for Building Pile Foundations (JGJ94-2008) according to the following formula: H1=|h2-h0|≤2S and the rock penetration depth ≥D h2: pile bottom elevation determined by the present invention; Reduce the redundant design length H to: H=│h2- h1│.

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