Method for testing foundation bearing capacity characteristic value of underlying layer of pebble foundation after depth correction

Abstract

The invention belongs to the technical field of geotechnical engineering investigation and detection engineering, discloses a method for testing a foundation bearing capacity characteristic value of a pebble foundation underlying layer after depth correction, and aims to solve the problem that difference exists between the calculated foundation bearing capacity characteristic value and an actual value due to the influence of parameter values when formula correction is adopted. The method comprises the following steps: (1) surveying and setting out; (2) a down-the-hole hammer drilling machine is in place, and drilling is conducted; (3) installing test equipment; (4) carrying out a load test according to load test requirements and obtaining test data; and (5) drawing a load and settlement curve according to the obtained test data, and evaluating the foundation bearing capacity characteristic value of the weak underlying layer after depth correction according to the characteristic points Po, Py and Pu on the load and settlement curve. According to the method, the foundation bearing capacity characteristic value of the underlying layer of the pebble foundation after depth correction can be directly obtained, so that the foundation bearing capacity characteristic value is consistent with an actual value.

Description

Technical Field

[0001] The present invention belongs to the technical field of geotechnical engineering investigation and testing engineering, and particularly relates to a method for testing the characteristic value of the bearing capacity of the underlying layer of a pebble foundation after depth correction. Background Art

[0002] At present, in engineering construction, the characteristic value faz of the bearing capacity of the soft underlying layer at the top surface within the stress layer of the foundation is mainly calculated and determined according to Article 5.2.4 of the Code for Design of Building Foundations (GB50007-2011). Due to the influence of parameter values in the calculated bearing capacity characteristic value, there is a certain error between the bearing capacity characteristic value and the actual value. Summary of the Invention

[0003] In order to solve the problem that there is a difference between the calculated bearing capacity characteristic value of the foundation and the actual value due to the influence of parameter values when using formula correction, the present invention provides a method for testing the characteristic value of the bearing capacity of the underlying layer of a pebble foundation after depth correction, which can directly obtain the characteristic value of the bearing capacity of the underlying layer of a pebble foundation after depth correction, making the bearing capacity characteristic value consistent with the actual value. Thus, more accurate data can be provided for the reinforcement of the foundation and the subsequent engineering design and construction to improve the safety of construction and buildings.

[0004] In order to solve the technical problem, the technical solution adopted by the present invention is: A method for testing the characteristic value of the bearing capacity of the underlying layer of a pebble foundation after depth correction, characterized by comprising: (1) Measuring and setting out the lines to determine the elevation of the top surface of the foundation and the depth of the hole formation; (2) Positioning the down-the-hole hammer drill and drilling the hole; (3) Installing the test equipment; the test equipment includes a reaction force mechanism installed on the top surface of the foundation, a reference beam is arranged below the reaction force mechanism, a jack is installed on the reference beam, the upper end of the jack passes through the reference beam and contacts the reaction force mechanism, the lower end of the jack passes through the reference beam and is connected with a support plate, a load transfer rod is connected below the support plate, the lower end of the load transfer rod extends into the hole and is connected with a load test pressure plate, and a dial gauge for measuring the settlement of the load test pressure plate is arranged on the support plate; (4) After the installation of the test equipment is completed, conducting a load test according to the requirements of the load test and obtaining test data; (5)Organize the test data obtained according to step (4), and draw the load-settlement curve and the settlement-time logarithm curve; evaluate the characteristic value of the bearing capacity of the soft subsoil after depth correction based on the characteristic points Po, Py, and Pu on the load-settlement curve; among them, Po is the proportional limit load, Pu is the ultimate load, and Py is usually half of the ultimate load.

[0005] In some embodiments, after step (1) and before step (2), it should be excavated to the top surface of the foundation, and then the down-the-hole hammer drill is positioned and the drilling operation is carried out.

[0006] In some embodiments, when using a down-the-hole hammer drill for the drilling operation in step (2), the drilling depth should be drilled to the top surface of the soft subsoil.

[0007] In some embodiments, the reaction mechanism includes reaction piles or gravity reaction devices.

[0008] In some embodiments, in step (4) when conducting the load test, when any of the following situations occurs, stop loading and take the previous level of load as the ultimate load: (4.1) Obvious lateral extrusion, heaving, or continuous development of radial cracks occur in the side-loaded soil mass at the test point; (4.2) The settlement amount of this level of load is more than 5 times that of the previous level of load; (4.3) The ratio of the total settlement amount to the diameter of the load test loading plate exceeds 0.04; (4.4) The settlement rate cannot reach the relative stability standard within 24 hours under a certain level of load or shows an increasing trend; (4.5) The load pressure cannot be applied or the load pressure is unstable after being applied.

[0009] In some embodiments, the evaluation of the characteristic value of the bearing capacity of the soft subsoil after depth correction based on the characteristic points Po, Py, and Pu on the load-settlement curve includes the following: (5.1) When there is a proportional limit on the load-settlement curve, take the load corresponding to the proportional limit (i.e., the proportional limit load Po) as the characteristic value of the bearing capacity of the foundation; (5.2) When the ultimate load is less than 2 times the proportional limit load value, take half of the ultimate load value (i.e., half of the ultimate load Pu value is Py) as the characteristic value of the bearing capacity of the foundation; (5.3) When the two situations of (5.1) and (5.2) are not satisfied, take the load value corresponding to s / d = 0.01 as the characteristic value of the bearing capacity of the foundation, but this characteristic value of the bearing capacity of the foundation cannot be greater than half of the ultimate load; where s is the settlement amount and d is the diameter of the load test loading plate.

[0010] Compared with the prior art, the present invention has the following beneficial effects: The method for testing the characteristic value of the subgrade bearing capacity of the underlying layer of a pebble foundation after depth correction according to the present invention can directly obtain the characteristic value of the subgrade bearing capacity of the underlying layer of a pebble foundation after depth correction, making the characteristic value of the subgrade bearing capacity consistent with the actual value, thereby providing more accurate data for the reinforcement of the foundation and the subsequent engineering design and construction to improve the safety of construction and buildings. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a schematic diagram when the test equipment of the present invention conducts a load test; Figure 2 It is a p-s curve drawn based on the test data obtained by the invention from the load test; Figure 3 It is an s-lgt curve drawn based on the test data obtained by the invention from the load test; Reference numerals in the drawings: 1, load test loading plate; 2, load transfer rod; 3, PVC pipe; 4, hole formation; 5, support plate; 6, dial gauge; 7, reference beam; 8, jack; 9, gravity type reaction device. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0012] The following further describes the present invention in conjunction with embodiments. The described embodiments are only a part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the protection scope of the present invention.

[0013] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present invention; the terms "first", "second", "third" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance; in addition, unless otherwise clearly defined and limited, the terms "installed", "connected", "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected, or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0014] Combined with the attached Figure 1 drawings, the method for testing the characteristic value of the subgrade bearing capacity of the underlying layer of a pebble foundation after depth correction according to the present invention includes: (1)Measuring and setting out the lines to determine the elevation of the foundation top surface and the depth of the hole formation; during the specific implementation process, a GPS or total station is used to measure the elevation of the test site to determine the elevation of the foundation top surface and the depth of the hole formation.

[0015] (2)The down-the-hole hammer drill is positioned and drilled; the down-the-hole hammer drill is straightened, leveled and stabilized, and the down-the-hole hammer drill drills the hole and the casing and air compressor are followed up throughout the process for hole cleaning. The down-the-hole hammer drill keeps uniform drilling during the drilling process, and the muck at the hole opening is removed in time. After the drilling depth reaches the design depth, the drill is pulled out and a PVC pipe is placed. Among them, the drilling depth should be determined according to the buried depth of the soft subsoil layer (i.e., the depth position in the rock and soil mass).

[0016] When the air compressor is used for hole cleaning, it should be ensured that the bottom of the hole is clean without sediment.

[0017] Install the test equipment; the test includes a reaction force mechanism installed on the top surface of the foundation. A reference beam is arranged below the reaction force mechanism, and a jack is installed on the reference beam. The upper end of the jack passes through the reference beam and contacts the reaction force mechanism. The lower end of the jack passes through the reference beam and is connected with a support plate. A load transfer rod is connected below the support plate. The lower end of the load transfer rod extends into the hole and is connected with a load test pressure plate. A dial gauge for measuring the settlement amount (i.e., the settlement) of the load test pressure plate is arranged on the support plate. During the specific implementation process, the load transfer rod transfers the load to the load test pressure plate, and then the settlement amount of the load test pressure plate is obtained by using the dial gauge. During the specific implementation process, the diameter of the load test pressure plate is mutually adapted to the diameter of the hole formation to ensure that the pressure plate fits the hole wall of the hole formation.

[0018] (4)After the installation of the test equipment is completed, a load test is carried out according to the load test requirements and test data are obtained.

[0019] During the specific implementation process, the loading limit of the load test is taken as 2 times the value and is loaded in 10 levels. After each level of loading, in the first hour, readings are taken at intervals of 5 min, 5 min, 10 min, 10 min, 15 min, 15 min, and then every 30 min for the settlement amount. When the settlement amount is less than 0.1 mm / h within 2 hours, the next level of load can be added. Each level of unloading is 2 times the value of each level of loading. If the actual number of loading times is odd, the first unloading is 3 times the value of the loading. After each level of unloading, readings are taken once every 10 min, and after three readings, the next level of load is unloaded. When the rebound amount is less than 0.01 mm / 30 min after all unloading, the readings are stopped.

[0020] Among them, there is also a static load test in the prior art. However, the existing static load test is used to detect the foundation soil. Affected by the buried depth of the soft layer and groundwater, it is impossible to detect the soft underlying layer with a relatively large buried depth. Therefore, depth correction is required according to the buried depth of the underlying layer after detection. As mentioned in the background art of the present invention, during the correction process, due to the influence of parameter values, the characteristic value of the foundation bearing capacity obtained by calculation is not accurate. The method of the present invention is not affected by the buried depth of the underlying layer and groundwater, and can directly obtain a more accurate and real characteristic value of the foundation bearing capacity.

[0021] (5) Organize the test data obtained according to step (4), and draw the load-settlement curve (i.e., p-s curve) and the settlement-time logarithm curve (i.e., s-lgt curve); evaluate the characteristic value of the foundation bearing capacity (i.e., the value of faz) of the soft underlying layer after depth correction based on the characteristic points Po, Py, and Pu on the load-settlement curve.

[0022] Among them, Po is the proportional limit load, that is, the load when the soil starts to produce plastic deformation. Under this load, the ratio of the shear stress to the stress of the soil remains unchanged, that is, the soil is in an elastic state.

[0023] Pu is the ultimate load, that is, the load when the soil reaches the ultimate plastic deformation. Under this load, the soil produces the maximum plastic deformation, the shear stress is equal to the shear strength of the soil, and the soil may be damaged.

[0024] Py is usually half of the ultimate load. When the proportional limit load and the ultimate load are very close, if the ultimate load Pu is less than twice the proportional limit load value, then Py takes half of the ultimate load value.

[0025] In the specific implementation process, when conducting the load test, the load-settlement curve (i.e., p-s curve) and the settlement-time logarithm curve (i.e., s-lgt curve) can be drawn synchronously. Among them, the s-lgt curve is mainly used to check the authenticity of the data and assist in determining the bearing capacity. The s-lgt curve can help analyze the settlement characteristics of the foundation under different loads, including the settlement amount, settlement rate, etc., so as to evaluate the bearing capacity and stability of the foundation.

[0026] In the p-s curve, when the p-s curve has an obvious straight line segment and turning point, generally, the pressure (Po) corresponding to the end point (turning point) of the straight line segment is defined as the proportional limit value, and the intersection point of the asymptote of the steep drop segment of the curve and the horizontal axis representing the pressure is defined as the ultimate limit value (i.e., the ultimate load Pu).

[0027] In some embodiments, after step (1) and before step (2), it should be excavated to the top surface of the foundation, and then the DTH hammer drill is positioned and the drilling operation is carried out. That is to say, the elevation of the hole opening is the same as the elevation of the top surface of the foundation.

[0028] In some embodiments, when using a down-the-hole hammer drill for drilling operations in step (2), the drilling depth should reach the top surface of the soft underlying layer. In the specific implementation process, the bottom of the hole enters the top surface of the soft underlying layer by no more than 50 cm.

[0029] In some embodiments, the reaction force mechanism includes reaction piles or gravity reaction devices. The reaction force mechanism is a commonly used structure in pile foundation static load tests. Whether using reaction piles or gravity reaction devices to provide reverse acting forces is clear and understandable to those skilled in the art. Those skilled in the art can set a suitable reaction force mechanism according to the site conditions, which will not be elaborated here. In the appendix Figure 1 The reaction force mechanism shown is a gravity reaction device 9. Among them, both the gravity reaction device 9 and the reference beam 7 are firmly above the top surface of the foundation.

[0030] In some embodiments, during the load test in step (4), when any of the following situations occurs, stop loading and take the previous level of load as the ultimate load: (4.1) Obvious lateral extrusion, heaving, or continuous development of radial cracks occur in the side load soil of the test point; (4.2) The settlement of this level of load is more than 5 times the settlement of the previous level of load, and the p-s curve plotted shows an obvious steep drop.

[0031] (4.3) The ratio of the total settlement to the diameter of the load test pressure plate exceeds 0.04; (4.4) The settlement rate cannot reach the relative stability standard or shows an increasing trend within 24 hours under a certain level of load; (4.5) The load pressure cannot be applied or becomes unstable after being applied.

[0032] In some embodiments, the evaluation of the characteristic value of the bearing capacity of the soft underlying layer after depth correction based on the characteristic points Po, Py, and Pu on the load-settlement curve includes the following: (5.1) When there is a proportional limit on the load-settlement curve, take the load corresponding to the proportional limit (i.e., the proportional limit load Po) as the characteristic value of the bearing capacity of the foundation. The principle is: Under the action of the proportional limit load, that is, the load when the soil starts to produce plastic deformation. Under this load, the ratio of the shear stress to the stress of the soil remains unchanged, that is, the soil is in an elastic state.

[0033] (5.2) When the ultimate load (the load at the previous level before the termination of loading) is less than twice the value of the proportional limit load, take half of the ultimate load value (i.e., half of the ultimate load Pu value is Py) as the characteristic value of the foundation bearing capacity. The principle is: the load when the soil reaches the ultimate plastic deformation. At this load, the soil produces the maximum plastic deformation, the shear stress is equal to the shear strength of the soil, and the soil may fail. Considering the safety reserve, take 1 / 2 of this value as the characteristic value of the bearing capacity.

[0034] (5.3) When the two situations in (5.1) and (5.2) are not satisfied, take the load value corresponding to s / d = 0.01 as the characteristic value of the foundation bearing capacity, but this characteristic value of the foundation bearing capacity cannot be greater than half of the ultimate load; where s is the settlement and d is the diameter of the load test plate.

[0035] The method for testing the characteristic value of the foundation bearing capacity after depth correction of the underlying layer of the pebble foundation in the present invention can directly obtain the characteristic value of the foundation bearing capacity after depth correction of the underlying layer of the pebble foundation, making the characteristic value of the foundation bearing capacity consistent with the actual value, so as to provide more accurate data for the reinforcement of the foundation and the subsequent engineering design and construction, and improve the safety of construction and buildings.

Claims

1. A method for testing the characteristic value of the bearing capacity of the underlying layer of a pebble foundation after depth correction, characterized in that Including: (1) Measuring and setting out the lines to determine the elevation of the foundation top surface and the depth of the hole formation; (2) Positioning and drilling with a down-the-hole hammer drill; (3) Installing test equipment; the test includes a reaction force mechanism installed on the foundation top surface, a datum beam is arranged below the reaction force mechanism, a jack is installed on the datum beam, the upper end of the jack passes through the datum beam and contacts the reaction force mechanism, the lower end of the jack passes through the datum beam and is connected with a support plate, a load transfer rod is connected below the support plate, the lower end of the load transfer rod extends into the hole and is connected with a load test pressure plate, and a dial gauge for measuring the settlement of the load test pressure plate is arranged on the support plate; (4) After the test equipment is installed, conduct a load test according to the load test requirements and obtain test data; (5) Sort out the test data obtained in step (4), and draw a load-settlement curve and a settlement-time logarithm curve; evaluate the characteristic value of the bearing capacity of the soft subsoil after depth correction based on the characteristic points Po, Py, and Pu on the load-settlement curve; where, Po is the proportional limit load, Pu is the ultimate load, and Py is usually half of the ultimate load.

2. The method for testing the characteristic value of the foundation bearing capacity after depth correction of the underlying layer of pebble foundation according to claim 1, characterized in that, After step (1) and before step (2), it shall be excavated to the foundation top surface, and then the down-the-hole hammer drill shall be positioned and the drilling operation shall be carried out.

3. The method for testing the characteristic value of the foundation bearing capacity after depth correction of the underlying layer of pebble foundation according to claim 1, wherein In step (2), when using a down-the-hole hammer drill for drilling operation, the drilling depth shall be formed to the top surface of the soft subsoil.

4. The method for testing the characteristic value of the bearing capacity of the subsoil under a pebble foundation after depth correction according to claim 1, characterized in that, The reaction force mechanism includes a reaction force pile or a gravity reaction force device.

5. The method for testing the characteristic value of the bearing capacity of the subsoil under a pebble foundation after depth correction according to claim 1, characterized in that, In step (4), when conducting a load test, when any of the following situations occurs, stop loading and take the previous level of load as the ultimate load: (4.1) Obvious lateral extrusion, heaving, or continuous development of radial cracks occur in the side load soil of the test point; (4.2) The settlement of the current level of load is more than 5 times that of the previous level of load; (4.3) The ratio of the total settlement to the diameter of the load test pressure plate exceeds 0.04; (4.4) The settlement rate cannot reach the relative stability standard or shows an increasing trend within 24 hours under a certain level of load; (4.5) The load pressure cannot be applied or the load pressure is unstable after being applied.

6. The method for testing the characteristic value of the bearing capacity of the subsoil under a pebble foundation after depth correction according to claim 1, wherein The evaluation of the characteristic value of the bearing capacity of the soft subsoil after depth correction based on the characteristic points Po, Py, and Pu on the load-settlement curve includes the following: (5.1) When there is a proportional limit on the load-settlement curve, take the load corresponding to the proportional limit (i.e., the proportional limit load Po) as the characteristic value of the bearing capacity of the foundation; (5.2) When the ultimate load is less than 2 times the proportional limit load value, take half of the ultimate load value as the characteristic value of the bearing capacity of the foundation; (5.3) When the conditions in (5.1) and (5.2) are not met, take the load value corresponding to s / d = 0.01 as the characteristic value of the bearing capacity of the foundation, but this characteristic value of the bearing capacity of the foundation cannot be greater than half of the ultimate load; where, s is the settlement, and d is the diameter of the load test pressure plate.

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