A double-control determination method for safety depth of free section of semi-casing rotary excavating cast-in-place pile

Abstract

The application provides a double-control determination method for the free section safety depth of a semi-casing rotary excavated hole cast-in-place pile, and belongs to the technical field of double-control determination. P s The continuous profile is combined with the critical stability number theory, the defects that the traditional method depends on discrete point drilling tests and cannot reflect the continuous change of strata are overcome, and fine calculation of the free section safety depth is realized. The pile diameter correction coefficient is introduced, and the limitation of the classic Broms stability number on the small-diameter pile hole is broken. The influence of the pile diameter D is explicitly introduced in the formula, the three-dimensional stress arching effect of the small-diameter pile hole is fully tapped, the allowed free section height is higher than the traditional plane strain criterion, and the theoretical possibility for the realization of the semi-casing scheme is provided. The double-control decision-making process of theoretical pre-determination and penetration verification is provided.

Description

Technical Field

[0001] This invention belongs to the field of dual-control determination technology and also relates to the field of geotechnical engineering pile foundation construction technology. Specifically, it relates to a dual-control determination method for the safe depth of the free section of a semi-casing rotary-drilled cast-in-place pile, and more particularly to a quantitative determination method for determining whether the unsupported free section below the bottom of the casing is safe and stable during the construction of a semi-casing driven and rotary-drilled cast-in-place pile. Background Technology

[0002] Rotary drilling cast-in-place piles offer significant environmental and efficiency advantages due to the elimination of the need for mud slurry wall protection, leading to their increasingly widespread application in municipal engineering, construction, and environmentally sensitive areas such as near rivers and coastlines. Rotary drilling cast-in-place piles can be categorized into two types based on the length of the casing: short-casing rotary drilling cast-in-place piles and full-casing rotary drilling cast-in-place piles. Full-casing rotary drilling cast-in-place piles can be further divided into segmented casing follow-up rotary drilling cast-in-place piles and full-length casing driven rotary drilling cast-in-place piles. In sites with good soil conditions, the soil around the pile hole has good self-supporting properties, typically requiring only a short casing to cover the surface loose soil, usually 2-3 meters long, with a maximum depth of 6-8 meters. However, in soft soil layers or loose sand layers, the pile hole's self-supporting ability is difficult, often requiring the full-casing follow-up method. The typical construction process involves using a vibratory hammer to drive a steel casing into the soil layer, then rotary drilling to remove soil within the casing, finally pouring concrete into the casing and extracting the casing. The prior art disclosed in patent publication number CN117779753A is a full casing follow-up process using segmented casings. The upper and lower sections of the casing are connected by guide grooves and pins to transfer shear force. However, the casing splicing time is long, especially the disassembly time after the concrete is poured into the pile hole. This may exceed the initial setting time of the concrete and affect the strength growth of the pile concrete, causing irreversible damage to the pile strength. In 2022, author Pei Yan introduced a construction method using counter-pressure sinking casing, which requires the reaction pile to bear the sinking reaction force of the steel casing. In 2023, author Ding Xue introduced a construction technology of full casing follow-up rotary drilling, in which segmented steel casings are drilled into the soil layer at the pile location, and then holes are drilled to extend the casing and drill it again. This process is repeated until the drilled pile reaches the design depth. Finally, to avoid difficulty in pulling out the casing, a method of segmented concrete pouring and corresponding casing extraction is adopted. Full-casing rotary bored piles have solved the technical challenges encountered by short-casing rotary bored piles in soft soil layers, leading to their rapid promotion and application. In the soft soil foundations of the Yangtze River Delta, for bored piles with a diameter less than 1m and a depth less than 30m, the method of driving full-length casing piles to a specified depth greatly improves construction efficiency, allowing a work team to complete 10-15 piles per day. However, engineering practice and research indicate that the full-length casing method also has the following technical drawbacks.

[0003] The excessively high filling coefficient leads to poor economic efficiency and damages pile quality. The concrete filling coefficient of piles constructed with full casing sometimes reaches 1.3-1.4, far exceeding the 1.05-1.15 for underwater cast-in-place piles. The main reason is that during the vibratory driving and extraction of long casings, insufficient effective operating height of the equipment and excessive flexibility of the casing itself cause systematic forced tilting. During vibratory extraction, the tilted casing causes asymmetrical unilateral scraping and compression of the borehole wall, while the vibration energy liquefies the surrounding soft soil, resulting in significant dynamic diameter expansion. This not only leads to severe concrete overfilling but also creates a non-uniform pile cross-section, causing eccentric loading of the pile body. In severe cases, this can lead to cracking of the superstructure, affecting project safety. Furthermore, there is a lack of theoretical guidance on matching the capacity of construction equipment with the casing length. In current construction, the casing length is often designed to follow the entire length of the pile hole depth without considering the actual effective vertical operating height of the pile extraction equipment (such as excavators equipped with vibratory hammers or walking vibratory hammers). When the top of the entire casing exceeds the effective height at the end of the extraction process, the equipment is forced to adopt a forward or backward tilting posture, resulting in an uncompensated height difference gap and forced skew moment, which becomes the fundamental mechanical cause of amplified filling coefficient and pile defects. Furthermore, using a larger excavator would significantly increase equipment costs, leading to substantial waste. Moreover, towards the end of the casing extraction process, the tilted casing would compress the already poured concrete, resulting in insufficient concrete cover thickness in some sections and affecting the reasonable service life of the pile. An excessively large filling coefficient leads to concrete waste and increased carbon emissions. In the process of social development with dual carbon goals, carbon emissions have become a key concern for all industries, especially in the engineering and construction sector, and concrete usage is one of the main sources of carbon emissions in engineering and construction. To reduce total carbon emissions in engineering projects, it is urgent to optimize construction processes and design methods for excessively large and unreasonable filling coefficients. Therefore, this invention proposes an accurate method for quantifying the safe length and safety of the free section under the semi-casing in soft soil layers suitable for the Yangtze River Delta region. By shortening the casing length to accommodate the effective working height of mainstream excavator piling equipment, the filling coefficient is reduced, concrete consumption is decreased, and the green construction technology content of the project is improved.

[0004] The casing length of semi-casing rotary drilling piles can be 10-20m, which can accommodate the effective working height of the piling equipment and fully utilize the extremely high construction efficiency of full-length casing rotary drilling piles. The free section of the pile hole below the bottom of the semi-casing is drilled using a casingless rotary drilling method. However, current technology lacks a design method to quantitatively determine the allowable depth of the free section below the casing. Whether this free section can maintain hole wall stability in the short period between hole formation and grouting is crucial to the viability of the semi-casing scheme. The critical stability number method N=γH / S proposed by Broms and Bennermark in 1967 in classical soil mechanics... uWhile this method provides a basis for evaluating the stability of vertical excavation faces under undrained conditions in saturated cohesive soils, its applicability is based on the plane strain assumption and does not consider the three-dimensional stress arching effect caused by the cylindrical shape of the pile hole; furthermore, this method relies on the undrained strength S obtained from laboratory tests. u It is difficult to reflect the continuous changes in the strata. Although Davis et al. pointed out the three-dimensional effect of the circular excavation face in 1980 and gave the correlation between the stability number and the diameter-to-depth ratio H / D, there is still no practical method that can be combined with continuous field test data to directly guide the decision on the safe depth of the free section of small-diameter pile holes.

[0005] In summary, the existing full-length rotary drilling pile construction has three major drawbacks: large filling coefficient, equipment-method mismatch, and lack of stability assessment for the free section. There is an urgent need for a dual-control method that can quantitatively predict the safe depth of the free section and verify it in real time based on continuous geological data, equipment capacity, and pile hole size, so as to promote the transformation of this technology towards an economical and safe standardized semi-casing construction. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a dual-control determination method for the safe depth of the free section of a semi-casing rotary bored pile. This method is based on continuous data from on-site static cone penetration tests, introduces a three-dimensional critical stability number with pile diameter correction, and combines it with real-time monitoring of vibratory pile penetration. This achieves quantitative prediction of the safe depth of the free section and dynamic verification during the construction process, effectively solving the decision-making challenges in semi-casing schemes.

[0007] The present invention employs the following technical solution.

[0008] A dual-control method for determining the safe depth of the free section of a semi-casing rotary-drilled cast-in-place pile includes:

[0009] Step 1: Obtain continuous formation parameters along the depth direction of the pile hole;

[0010] Step 2: Determine the initial bottom elevation of the short casing;

[0011] Step 3: Calculate the theoretically permissible maximum safe height H of the free segment. max ;

[0012] Step 4: Calculate the safety factor to determine safety.

[0013] Step 5: Conduct real-time verification of penetration during construction.

[0014] Furthermore, step 1 specifically includes:

[0015] Review the geological survey report and conduct static cone penetration tests (CPT) at or near the pile location to obtain the specific penetration resistance P. s The result of a continuously varying curve along the depth.

[0016] Furthermore, step 2 specifically includes:

[0017] Based on the designed pile length and the proposed casing driving and extraction equipment, the length specification of the short casing was initially selected, and the initial elevation Z of the casing bottom was determined. bottom .

[0018] Furthermore, step 2 specifically includes:

[0019] Based on this, the initial elevation Z of the bottom opening of the casing was determined. bottom The initial height H of the proposed free segment is obtained. free = H pile -H casing H pile To design the pile hole depth, H casing This refers to the depth to which the casing is inserted into the soil.

[0020] Furthermore, step 3 specifically includes:

[0021] Based on the obtained P s Continuous curve, from elevation Z bottom The undrained shear strength S of the soil layer is calculated for each depth point according to the following formula (1). u :

[0022] (1)

[0023] in, This is the conversion factor for static penetration strength, and its value is determined based on regional experience.

[0024] Furthermore, step 3 specifically includes:

[0025] Then, the maximum allowable unsupported height of the free section is calculated using the three-dimensional critical stability number formula (2) which incorporates pile diameter correction. :

[0026] (2)

[0027] In the formula, This is the corrected critical stability number. ;

[0028] This is the critical stability number under plane strain conditions, which is taken as 6.0-8.0 for saturated soft clay;

[0029] D is the design diameter of the pile hole, in meters;

[0030] H is the height of the free segment determined in step S2. free If H is greater than the initial estimate, we can first use H as the reference. freeIterative solution;

[0031] β is the pile diameter correction factor, which is taken as 0.5~0.8 depending on the pile diameter and the geological conditions;

[0032] For P within the depth range of this free segment s The minimum value of the converted undrained shear strength;

[0033] γ is the average unit weight of the soil layer within the free section.

[0034] Furthermore, step 4 specifically includes:

[0035] Calculate the safety factor ,in This refers to the proposed actual height of the free segment.

[0036] Furthermore, step 4 specifically includes:

[0037] like Greater than or equal to the set allowable safety factor If the height of the free section meets the self-stabilizing requirements of the borehole wall, then the elevation of the bottom of the casing needs to be increased, and the height of H needs to be recalculated. free Repeat steps 2 through 4 until the safety factor requirement is met.

[0038] Furthermore, step 5 specifically includes:

[0039] On-site vibration was used to sink the casing to the preset bottom elevation Z. bottom When nearby, the penetration rate of the vibratory hammer into the casing is monitored in real time.

[0040] Furthermore, step 5 specifically includes:

[0041] When the penetration rate of the casing continuously decreases and stabilizes below 15~25 mm / 10s within a continuous depth range of 0.5 meters, it can be determined that the bottom of the casing has entered a stable stratum with significantly superior mechanical properties; otherwise, the casing should continue to be vibrated and driven until the penetration meets the termination criterion, and the actual elevation of the bottom of the casing at this time should be used as the basis for determining the final casing length.

[0042] The beneficial effects of the present invention are as follows, compared with the prior art:

[0043] This invention establishes for the first time a quantitative model of the self-stability of free sections based on continuous formation data. The static cone penetration test P... sBy combining continuous profile analysis with critical stability number theory, this method overcomes the shortcomings of traditional methods that rely on discrete-point borehole tests and cannot reflect continuous changes in the formation, enabling refined calculation of the safe depth of the free section. A pile diameter correction coefficient is introduced, overcoming the overly conservative limitation of the classical Broms stability number for small-diameter pile holes. The formula explicitly incorporates the influence of pile diameter D, fully exploring the three-dimensional stress arching effect of small-diameter pile holes, thus increasing the allowable free section height compared to traditional plane strain criteria, providing a theoretical possibility for the implementation of the semi-casing scheme. A dual-control decision-making process of theoretical prediction and penetration verification is proposed. This not only ensures the theoretical reliability of the design scheme but also dynamically verifies whether the bottom of the casing has indeed entered the stable bearing layer through the penetration value, a real-time and easily obtainable dynamic indicator. This dual-insurance mechanism effectively reduces the risks caused by local lenses or formation undulations. Finally, it provides an operable technical tool for standardized construction of semi-casing systems. This invention can directly output the maximum allowable free section length of a certain specification of casing under given geological conditions or the minimum casing penetration depth required under given pile length conditions. It provides a design basis for the standardization and serialization of casing length, significantly reduces equipment energy consumption, improves construction efficiency, and fundamentally curbs the problems of deviation and excessive filling coefficient caused by excessively long casings. Attached Figure Description

[0044] Figure 1 This is the overall technical roadmap for the dual-control determination method of the safe depth of the free section of a semi-casing rotary bored pile according to the present invention. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, other embodiments obtained by those skilled in the art without creative effort are all within the protection scope of this invention.

[0046] like Figure 1 As shown, this invention proposes a dual-control method for determining the safe depth of the free section of a semi-casing rotary bored pile, comprising the following steps:

[0047] Step 1: Obtain continuous formation parameters along the depth direction of the pile hole;

[0048] In a preferred but non-limiting embodiment of the present invention, step 1 specifically includes:

[0049] Review the geological survey report, especially the static penetration test (CPT) conducted at or near the pile location, to obtain the specific penetration resistance P. s The result of a continuously varying curve along the depth.

[0050] Step 2: Determine the initial bottom elevation of the short casing;

[0051] In a preferred but non-limiting embodiment of the present invention, step 2 specifically includes:

[0052] Based on the designed pile length and the proposed casing driving and extraction equipment, the length specification of the short casing was initially selected, and the initial elevation Z of the casing bottom was determined. bottom .

[0053] In a preferred but non-limiting embodiment of the present invention, step 2 further includes:

[0054] Based on this, the initial elevation Z of the bottom opening of the casing was determined. bottom The initial height H of the proposed free segment is obtained. free = H pile -H casing H pile To design the pile hole depth, H casing This refers to the depth to which the casing is inserted into the soil.

[0055] Step 3: Calculate the theoretically permissible maximum safe height H of the free segment. max ;

[0056] In a preferred but non-limiting embodiment of the present invention, step 3 specifically includes:

[0057] Based on P obtained in step 1 s Continuous curve, from elevation Z bottom The undrained shear strength S of the soil layer is calculated for each depth point according to the following formula (1). u :

[0058] (1)

[0059] in, The conversion factor for static penetration strength is determined based on regional experience. For soft soil areas, the value is 15-18 for silty soil and 18-22 for silty clay.

[0060] In a preferred but non-limiting embodiment of the present invention, step 3 further includes:

[0061] Then, the maximum allowable unsupported height of the free section is calculated using the three-dimensional critical stability number formula (2) which incorporates pile diameter correction. :

[0062] (2)

[0063] In the formula, This is the corrected critical stability number. ;

[0064] This is the critical stability number under plane strain conditions, which is taken as 6.0-8.0 for saturated soft clay;

[0065] D is the design diameter of the pile hole, in meters;

[0066] H is the height of the free segment determined in step S2. free If H is greater than the initial estimate, we can first use H as the reference. free Iterative solution;

[0067] β is the pile diameter correction factor, which is taken as 0.5~0.8 depending on the pile diameter and soil conditions. A smaller value is taken when the pile diameter is small and the soil conditions are good, and a larger value is taken otherwise.

[0068] For P within the depth range of this free segment s The minimum value of the converted undrained shear strength, in kPa;

[0069] γ is the average unit weight of the soil layer within the free section, and the saturated unit weight of 18~20 kN / m³ is taken as a safety margin.

[0070] Step 4: Calculate the safety factor to determine safety.

[0071] In a preferred but non-limiting embodiment of the present invention, step 4 specifically includes:

[0072] Calculate the safety factor ,in The actual free section height is the planned height of the pile hole without steel casing support, which is the actual height of the pile hole from the bottom of the casing to the designed pile bottom after the initial selection of the casing length.

[0073] In a preferred but non-limiting embodiment of the present invention, step 4 further includes:

[0074] like Greater than or equal to the set allowable safety factor ](suggestion[ If the height of the free section is 1.2~1.5, then the proposed height of the free section meets the self-stabilizing requirements of the borehole wall and can be implemented accordingly; otherwise, the elevation of the bottom of the casing needs to be increased and the height of H needs to be re-proposed. free Repeat steps 2 through 4 until the safety factor requirement is met.

[0075] Step 5: Conduct real-time verification of penetration during construction.

[0076] In a preferred but non-limiting embodiment of the present invention, step 5 specifically includes:

[0077] On-site vibration was used to sink the casing to the preset bottom elevation Z. bottom When nearby, the penetration rate of the vibratory hammer into the casing is monitored in real time.

[0078] In a preferred but non-limiting embodiment of the present invention, step 5 further includes:

[0079] When the penetration rate of the casing continuously decreases and stabilizes below 15~25 mm / 10s within a continuous depth range of 0.5 meters, it can be determined that the bottom of the casing has entered a stable stratum with significantly superior mechanical properties; otherwise, the casing should continue to be vibrated and driven until the penetration meets the termination criterion, and the actual elevation of the bottom of the casing at this time should be used as the basis for determining the final casing length.

[0080] Through the above steps, the dual-control determination of the safe depth of the free section of the semi-casing rotary bored pile is completed, realizing the geological exploration prediction based on theoretical calculation (primary control) and the construction verification based on the field penetration (secondary control).

[0081] A specific embodiment of the present invention is shown below:

[0082] Example 1 (A structural pile foundation project)

[0083] A project located on the riverbank employs rotary drilling grouting pile technology with full casing. The pile diameter D=0.6m, and the designed effective pile length is 24.0m. Typical site strata are: ① miscellaneous fill (approximately 1.5m thick), ② a layer of brownish-yellow silty clay (hard crust layer, approximately 5-8m thick, P...). s ≈0.8MPa, undrained strength S of the vane plate u =41.8kPa), ③ layer of gray silty clay, ④ layer of gray silty clay. Of the original geological survey data, only layer ②1 possessed good self-stabilizing capacity. The original plan adopted a 25.5m full casing for follow-up construction, with a measured filling coefficient of 1.32~1.4 for a single pile, resulting in severe over-cubic-meter concrete accumulation and a non-uniform characteristic of a sudden increase in pile diameter at the top.

[0084] The method of the present invention is now used to determine the optimization of the semi-casing, and the specific implementation steps are as follows:

[0085] Step S1: Conduct a supplementary static cone penetration test near the test pile location to obtain P. s Continuous profile along depth. Data shows that Ps remains stable at 0.7~0.9MPa within layer ②1, with a bottom depth of approximately 10m.

[0086] Step S2: Considering the effective working height of the pile extraction equipment (a 220-type excavator paired with a ZX6901CH-5A walking hydraulic vibratory hammer) is approximately 22m, a standardized short casing length of 12m is initially selected, with 0.5m remaining above ground and a casing embedment depth of 11.5m. Therefore, the proposed free section height H is... free =24.0m-11.5m=12.5m.

[0087] Step S3: Obtain soil parameters within the free section (hole depth 11.5m~24.0m). This section is mostly located below layer ②1 and above layer ③, so a conservative estimate of P within this depth range is taken. s The smallest S converted u ② Lower part of layer 1, P s =0.75MPa, N k Take 20 (silty clay) to obtain S u,min =0.75×1000 / 20=37.5kPa. Soil weight γ is taken as 18.5kN / m³. Critical plane strain number N. c,∞ Take 6.0. Pile diameter D = 0.6m, H is taken as a proposed value of 12.5m, and β is conservatively taken as 0.5.

[0088] Calculate the corrected critical stability number :

[0089] =6.0×(1+0.5×0.6 / 12.5)≈6.0×(1+0.024)=6.144

[0090] Calculate the maximum allowable free segment height H max :

[0091] H max =(6.144×37.5) / 18.5≈12.45m.

[0092] Safety factor =H max / H free =12.45 / 12.5≈0.996, slightly less than the allowable safety factor. The value is 1.2, indicating a risk of instability.

[0093] Step S4: Based on the assessment results, increase the casing length to 18m (17.5m buried). Revise the H... free =24.0-17.5=6.5m. At this time, the entire free section is located within floor ②1, S u，min Take 41.8 kPa (peak strength of the vane). Recalculate:

[0094] =6.0×(1+0.5×0.6 / 6.5)≈6.0×(1+0.046)=6.276

[0095] H max =(6.276×41.8) / 18.5≈14.18m

[0096] The safety factor F_s = 14.18 / 6.5 ≈ 2.18, which is much greater than 1.2, so the stability requirement is met.

[0097] Step S5: During on-site construction using an 18m casing, vibratory pile driving was performed to a depth of approximately 17.0m, and the penetration rate was recorded. As the casing bottom sank from 17.0m to 17.6m (0.6m), the penetration rate rapidly decreased from an initial 40mm / 10s to a stable 12mm / 10s, and remained below 20mm / 10s for 0.5m consecutively, indicating reliable penetration into the stable stiff silty clay layer. This point of sudden drop in penetration was ultimately taken as the casing termination depth; the actual casing penetration was approximately 17.6m, consistent with the theoretically predicted height.

[0098] Result verification:

[0099] Multiple test piles were constructed using the 18m short casing scheme determined by this invention. During construction, the verticality remained stable, and the disturbance during casing extraction was significantly reduced. After pile completion, through back-calculation of segmented grouting records and total volume verification, the average equivalent pile diameter was approximately 0.67m, and the filling coefficient was reduced to 1.08~1.12, a reduction of about 20% compared to the original full casing scheme. Furthermore, the pile cross-section was uniform, with no abrupt diameter expansion in the middle section, strongly verifying the correctness and reliability of the method of this invention.

[0100] It should be recognized that embodiments of the present invention may be implemented or carried out by computer hardware, a combination of hardware and software, or by computer instructions stored in a non-transitory computer-readable storage medium.

[0101] The method can be implemented using standard programming techniques, including a non-transitory computer-readable storage medium configured with a computer program in the computer program, wherein the storage medium is configured such that the computer operates in a specific and predefined manner.

[0102] Each program can be implemented in a high-level procedural or object-oriented programming language to communicate with the computer system; however, if required, the program can be implemented in assembly or machine language.

[0103] In any case, the language can be either compiled or interpreted.

[0104] Furthermore, for this purpose, the program can run on programmed application-specific integrated circuits.

[0105] The processes described herein (or variations and / or combinations thereof) can be executed under the control of one or more computer systems configured with executable instructions, and can be implemented by hardware or a combination thereof as code (e.g., executable instructions, one or more computer programs, or one or more applications) that commonly executes on one or more processors. The computer program includes a plurality of instructions executable by one or more processors.

[0106] Furthermore, the method can be implemented in any suitable type of computing platform, including but not limited to personal computers, minicomputers, mainframes, workstations, networked or distributed computing environments, standalone or integrated computer platforms, or in communication with charged particle tools or other imaging devices.

[0107] Various aspects of the present invention can be implemented in machine-readable code stored on a non-transitory storage medium or device, whether portable or integrated into a computing platform, such as a hard disk, optical read and / or write storage medium, RAM, ROM, etc., such that it can be read by a programmable computer, and when the storage medium or device is read by the computer, it can be used to configure and operate the computer to perform the processes described herein.

[0108] Furthermore, machine-readable code, or parts thereof, can be transmitted via wired or wireless networks.

[0109] When such media includes instructions or programs that combine with a microprocessor or other data processor to implement the steps described above, the invention described herein includes these and other different types of non-transitory computer-readable storage media.

[0110] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A dual-control method for determining the safe depth of the free section of a semi-casing rotary-drilled cast-in-place pile, characterized in that, include: Step 1: Obtain continuous formation parameters along the depth direction of the pile hole; Step 2: Determine the initial bottom elevation of the short casing; Step 3: Calculate the theoretically permissible maximum safe height H of the free segment. max ; Step 4: Calculate the safety factor to determine safety. Step 5: Conduct real-time verification of penetration during construction.

2. The dual-control determination method for the safe depth of the free section of a semi-casing rotary bored pile according to claim 1, characterized in that, Step 1 specifically includes: Review the geological survey report and conduct static cone penetration tests (CPT) at or near the pile location to obtain the specific penetration resistance P. s The result of a continuously varying curve along the depth.

3. The dual-control determination method for the safe depth of the free section of a semi-casing rotary bored pile according to claim 2, characterized in that, Step 2 specifically includes: Based on the designed pile length and the proposed casing driving and extraction equipment, the length specification of the short casing was initially selected, and the initial elevation Z of the casing bottom was determined. bottom .

4. The dual-control determination method for the safe depth of the free section of a semi-casing rotary bored pile according to claim 3, characterized in that, Step 2 also includes: Based on this, the initial elevation Z of the bottom opening of the casing was determined. bottom The initial height H of the proposed free segment is obtained. free = H pile -H casing H pile To design the pile hole depth, H casing This refers to the depth to which the casing is inserted into the soil.

5. The dual-control determination method for the safe depth of the free section of a semi-casing rotary bored pile according to claim 4, characterized in that, Step 3 specifically includes: Based on the obtained P s Continuous curve, from elevation Z bottom The undrained shear strength S of the soil layer is calculated for each depth point according to the following formula (1). u : (1) in, This is the conversion factor for static penetration strength, and its value is determined based on regional experience.

6. The dual-control determination method for the safe depth of the free section of a semi-casing rotary bored pile according to claim 5, characterized in that, Step 3 also includes: Then, the maximum allowable unsupported height of the free section is calculated using the three-dimensional critical stability number formula (2) which incorporates pile diameter correction. : （2） In the formula, This is the corrected critical stability number. ; This is the critical stability number under plane strain conditions, which is taken as 6.0-8.0 for saturated soft clay; D is the design diameter of the pile hole, in meters; H is the height of the free segment determined in step S2. free If H is greater than the initial estimate, we can first use H as the reference. free Iterative solution; β is the pile diameter correction factor, which is taken as 0.5~0.8 depending on the pile diameter and the geological conditions; For P within the depth range of this free segment s The minimum value of the converted undrained shear strength; γ is the average unit weight of the soil layer within the free section.

7. The dual-control determination method for the safe depth of the free section of a semi-casing rotary bored pile according to claim 6, characterized in that, Step 4 specifically includes: Calculate the safety factor ,in This refers to the proposed actual height of the free segment.

8. The dual-control determination method for the safe depth of the free section of a semi-casing rotary bored pile according to claim 7, characterized in that, Step 4 also includes: like Greater than or equal to the set allowable safety factor If the height of the free section meets the self-stabilizing requirements of the borehole wall, then the elevation of the bottom of the casing needs to be increased, and the height of H needs to be recalculated. free Repeat steps 2 through 4 until the safety factor requirement is met.

9. The dual-control determination method for the safe depth of the free section of a semi-casing rotary bored pile according to claim 8, characterized in that, Step 5 specifically includes: On-site vibration was used to sink the casing to the preset bottom elevation Z. bottom When nearby, the penetration rate of the vibratory hammer into the casing is monitored in real time.

10. The dual-control determination method for the safe depth of the free section of a semi-casing rotary bored pile according to claim 9, characterized in that, Step 5 also includes: When the penetration rate of the casing continuously decreases and stabilizes below 15~25 mm / 10s within a continuous depth range of 0.5 meters, it can be determined that the bottom of the casing has entered a stable stratum with significantly superior mechanical properties; otherwise, the casing should continue to be vibrated and driven until the penetration meets the termination criterion, and the actual elevation of the bottom of the casing at this time should be used as the basis for determining the final casing length.

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