Application of sliding micrometer in single pile static load test

Li Guangyu Chen Dong

(Yantai High-tech Engineering Consulting Co., Ltd.)

**Summary:** Solexperts 公司滑动测微计（ Sliding Micvometer ）的原理及其在单桩静载试验中的应用。 This paper introduces the principle of Sliding Micvometer from Solexperts, Switzerland , and its application in single pile static load test. The types of test piles include steel pipe piles, cast-in-situ piles, square precast piles, prestressed pipe piles, branch and pile piles, etc. The instrument can continuously measure the average strain in each meter of the pile body, and can accurately calculate the pile side friction resistance, end resistance, maximum bending moment point, intermediary horizontal load, ultimate horizontal load, deflection curve, negative The main parameters of pile foundation design such as friction resistance can also comprehensively evaluate the pile body quality. Several engineering examples show that this instrument is a high precision, easy to use, and has broad application prospects in the field of geotechnical engineering.

滑动测微计 、单桩静载试验、正负摩阻力、端阻力 **Keywords:** sliding micrometer , single pile static load test, positive and negative friction resistance, end resistance

**、概述** **1.Overview**

Pointwise observation ）方法。 In the past, the strain measurement of piles generally used steel bars, concrete strain gauges, or pressure boxes. Because the probe and the medium could not be ideally matched and the zero point of the electrical measuring element drifted, the actual measurement results had large errors. Representing the strain value at the measurement point is a typical pointwise observation method.

1(a) 计算模型 Figure 1 (a) Calculation model 1(b) 三种直径钢筋计的实测力及应变值 Figure 1 (b) Measured force and strain values of three diameter rebar gauges

1 所示， 20 、 30 、 40mm 钢筋计的力增长率为 27.8 —— 21.1% ，应变增长率为 30.6 —— 18.2% 。 The calculation of the axisymmetric finite element method shows that since the diameter of the rebar gauge is larger than the diameter of the rebar and the protruding portion at the two ends, its measured stress and strain in the pile are greater than the actual stress or strain, as shown in Figure 1 , 20 , 30 , The force growth rate of the 40mm steel bar gauge is 27.8-21.1% , and the strain growth rate is 30.6-18.2 % .

2 —— 55GPa 时，实测应变增长 27 —— 11% ，一般弦式应变计弹模约为 0.5 —— 0.8GPa ，因此误差更大，如图 2 、 3 所示。 Since the elastic modulus of the strain gauge is not equal to the elastic modulus of the concrete, the same problem exists for the concrete strain gauge. When the elastic modulus of the strain gauge is assumed to be between 2 and 55 GPa , the measured strain increases by 27 to 11% , and the general string strain is The elastic modulus is about 0.5-0.8GPa , so the error is greater, as shown in Figures 2 and 3 .

String strain gauge concrete

2 计算模型 Figure 2 Calculation model 3 应变计实测应变与计算应变对比 Figure 3 Comparison of measured strain and calculated strain of strain gauge

年代初期，瑞士联邦苏黎世综合科技大学 Kovari 教授等人提出了线法监测原理（ Linewise observation ）及相应的测试技术 —— 滑动测微计（ Sliding Micrometer——ISETH ） ^{[1]} ^{、} ^{[2]} ^{、} ^{[3]} 。 In the early 1980s, Professor Kovari of the Swiss Federal University of Technology Zurich and others proposed the principle of linewise observation ( Linewise observation ) and the corresponding testing technology - Sliding Micrometer (ISETH ) ^{[1]} ^{,} ^{[2]} ^{,} ^{[3]} .

1m ，两端带有球状测头的位移探头，内装一个线性电感位移计和一个 NTC 测度计。 The main body of the sliding micrometer is a displacement probe with a gage length of 1m and a spherical probe at both ends. A linear inductive displacement meter and an NTC gauge are installed. 1m 安置一个具有特殊定位功能的环形标，其间用硬塑料（ HPVC ）管相连，滑动测微计可依次测量两个环形标之间的相对位移，并可用于多条测线（图 4 、 5 ）。 In order to measure the strain and temperature distribution on the line, a ring mark with special positioning function is placed every 1m on the measurement line , and a hard plastic ( HPVC ) tube is connected between them. A sliding micrometer can measure the distance between the two ring marks in order. Relative displacement, and can be used for multiple survey lines (Figure 4 , 5 ).

4 滑动测微计 Figure 4 Sliding Micrometer

5 滑动测微计测试原理 Figure 5 Testing principle of sliding micrometer

滑动测微计 具有如下优点： Compared with the fixed method of point method such as bar gauge and pressure box in test pile, sliding micrometer has the following advantages:

(1) 1m 的测段平均应变，分辨率高（ 1 με），任何部位微小变形都反映在测值中，可评估构件质量，计算弹性模量。 Continuously measure the average strain of the segment with a gauge distance of 1m , high resolution ( 1 με), and small deformations at any part are reflected in the measured value. The component mass can be evaluated and the elastic modulus can be calculated. The traditional method can only measure the strain at a few points, and the deformation between the two points can only be inferred. Moreover, the strain at the measuring point will cause local stress distortion due to the probe's intervention, and the measured value will deviate from the true value.

(2) The traditional method is to embed the tested component in the component, which not only has limited measurement points, but also is easy to be damaged. The main point is that the zero point shift cannot be avoided and cannot be corrected. 1.5 × 10 ^{-6} /°C, 可有效地修正零点飘移，特别适用于长期观测。 In the new method, only the casing and the measuring ring are embedded in the component, and the measurement is performed with a probe, which is simple and reliable, and is not easy to be damaged. The probe can be calibrated at any time in an indium steel calibration cylinder. Can effectively correct the zero shift, especially suitable for long-term observation.

(3) 2 × 10 ^{-6} /°C ，而且附有一支分辨率为 0.1°C 的 NTC 温度计 , 可随时监测构件温度 , 特别适用于长期监测，例如：桩身负摩阻力监测，岩土工程、钢或混凝土等大型构件长期监测等，以区分温度应变及应力导致的应变，这是传统方法无法做到的。 The probe used in the new method has a temperature self-compensation function, the temperature coefficient is less than 2 × 10 ^{-6} / ° C , and an NTC thermometer with a resolution of 0.1 ° C is attached , which can monitor the temperature of the component at any time , especially suitable for long-term monitoring, such as : Monitoring of pile body negative friction resistance, long-term monitoring of large components such as geotechnical engineering, steel or concrete, etc., to distinguish temperature strain and strain caused by stress, which cannot be done by traditional methods.

(4) 1 × 10 ^{-5} ，而且可用于任何方向钻孔中。 For large components subjected to lateral forces, such as piles, underground continuous beams, dams, etc., two survey lines are buried in parallel, and the lateral displacement is calculated using the strain difference. Up to 1 × 10 ^{-5} and can be used for drilling in any direction. ^{[4]}

**．** **2** **.** **Applications**

1987 年开始，本项测试技术首先在我国北仑电厂试桩工程中应用，随后普遍应用于陕西蒲城电厂、山西河津电厂、阳泉第二电厂、陕西宝鸡第二电厂试桩，它适用于各种土层及各种桩型。 Starting from 1987 , this test technology was first applied to the test pile project of Beilun Power Plant in China, and then widely used in the test piles of Shaanxi Pucheng Power Plant, Shanxi Hejin Power Plant, Yangquan Second Power Plant and Shaanxi Baoji Second Power Plant. Soil layer and various pile types. 10 年来，欧美大地仪器公司及岩泰高新技术公司利用瑞士滑动测微计 (Sliding Micrometer) 测定了几十个工程、几百条试桩，均取得了满意的结果。 In the past 10 years, the European and American Dadi Instrument Company and Yantai High-tech Company have used the Swiss Sliding Micrometer to measure dozens of projects and hundreds of test piles, and have achieved satisfactory results.

**．** **2** **.** **垂直静载试桩** **1** **Vertical static load test pile**

The test adopts the form of anchor pile reaction force, and the pressure is maintained by the slow speed maintaining load method. The initial readings in each test tube are measured twice from top to bottom and bottom to top before loading to ensure test accuracy. The corresponding readings are measured after each level of load is stable, and the difference is the value of each measurement section under each level of load. Strain value. m 安装一个锥形合金测环。 Two measuring tubes were buried in parallel in the symmetrical part of the reinforcement cage, and a conical alloy measuring ring was installed every 1 m in the measuring tube . The average strain of the corresponding measurement sections of the two tubes is used for analysis and calculation, thereby avoiding the eccentric effect during loading. When used for horizontal test piles, the connection direction of the two test tubes must be the same as the thrust.

Due to the difference in bore diameter and pile concrete quality, the measured data inevitably has some errors. The measured values cannot be used to calculate the axial force and friction. Otherwise, the error will be magnified, and even positive and negative friction will appear alternately Reasonable phenomenon. Therefore, the measured strain curve must first be polished by the fitting method. ^{[5]}

E _{i} =AB × ε _{i} (GPa), 计算轴向力和摩阻力时采用不同的弹模值，如下式所示： According to the strain at the top of the pile or the zero strain after regression treatment, the change law of the elastic modulus with the strain level can be calculated. Generally, it can be expressed by a one-time linear equation, such as E _{i} = AB × ε _{i} (GPa), and calculate the axial direction. Different elastic modulus values are used for force and friction, as shown in the following formula:

The axial force calculation formula is: _{i} =A _{i} × E _{i} × ε _{i} (kN) Q _{i} = A _{i} × E _{i} × ε _{i} (kN)

: The formula for calculating unit friction is : _{i} =( Q _{i} -Q _{i+1} )/(πD)(kPa) f _{i} = (Q _{i} -Q _{i + 1} ) / (πD) (kPa)

**摩擦桩** **2.1.1** **Friction pile**

6 为二条测管在各级荷载下的实测应变，由于桩顶扩大头，故应变较小，此外东测管上部应变较大，西测管上部应变较小，这是由于加载偏心影响，平均后就正常了。 Figure 6 shows the measured strains of two test tubes under various loads. Due to the enlarged head of the pile, the strain is small. In addition, the upper test tube has a larger strain and the western test tube has a smaller strain. This is due to the eccentric loading. It is normal after averaging. 、 31 、 41m 处的应变较高，表明该处混凝土质量较差或孔径较小 Higher strains at 26 , 31 , and 41m indicate poor concrete quality or smaller pores

7 、 8 分别为回归处理后的应变曲线及摩阻力曲线。 Figures 7 and 8 show the strain curve and friction curve after the regression treatment , respectively.

6 （ a ）东测管实测应变 Figure 6 ( a ) Measured strain of the east test tube |

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