Monitoring of negative friction of single pile in collapsible loess area
Release time: 2018-06-23
Li Guangyu
(Wuhan Institute of Rock and Soil Mechanics, Chinese Academy of Sciences      Wuhan 430071 )

Summary   This paper introduces three examples of monitoring the negative friction of a single pile in collapsible loess area. The test pile is placed in a soaking pit, and its diameter or side length is approximately equal to the pile length. During the observation period, water was continuously injected into the pit, until the soil at the end of the pile was saturated, water injection was stopped, and the pile body strain and ground settlement continued to be observed until both were stable. 条桩三种桩型,其中4条桩桩顶无荷载,其余桩在浸水过程中维持设计荷载。 A total of 9 piles were tested in three pile types, of which 4 piles had no load on the top, and the remaining piles maintained the design load during the soaking process. ,仪器分辨率为0.001mm ,即应变分辨率可达1The pile body strain test adopts a “sliding micrometer” made in Switzerland. It is a portable long-gauge high-precision strain gauge that can continuously monitor the relative deformation between two adjacent points. The distance between the two points is 1m . It is 0.001mm , which means that the strain resolution can reach 1 . The monitoring results show that the sag value is generally smaller than the calculated value based on the indoor test data, but the negative friction resistance is larger than the standard value.

Almost   Description:

黄土特征及其分布 1.1 Loess characteristics and distribution

Loess is a Quaternary sediment with all of the following characteristics. When one or more of these characteristics are missing, they are called loess-like soil.

颜色以黄色、褐黄色为主,有时呈灰黄色; (1) The color is mainly yellow, brownish yellow, and sometimes grayish yellow;

颗粒组成以粉粒( 0.05 (2) The composition of particles is powder ( 0.05 0.005mm 以上,几乎没有粒径大于 ), The content is generally above 60% , and almost no particle size is greater than 0.25mm particle;

孔隙比较大,一般在1.0左右; (3) The pores are relatively large, generally around 1.0 ;

富含碳酸钙盐类; (4) rich in calcium carbonate salts;

垂直节理发育; (5) vertical joint development;

一般有肉眼可见的大孔隙。 (6) Generally there are large pores visible to the naked eye.

km 2 ,占陆地总面积的9.3% [1] ,主要分布于南北美洲及欧洲中纬度干旱地区。 Loess and loess-like soils are widely distributed, about 13 million km 2 in the world, accounting for 9.3% of the total land area [1] , and they are mainly distributed in arid regions of North and South America and Europe.km 2 ,占世界黄土总面积的4.9% ,主要分布在北纬3445 °之间,区域内气候干燥,降雨量介于250500mm ,小于250mm的沙漠地区及大于750mm的地区基本上无黄土分布。 The area of loess in China is 635,000 km 2 , accounting for 4.9% of the total area of the world ’s loess. It is mainly distributed between 34 ° and 45 ° north latitude. The region has a dry climate with rainfall between 250 and 500mm , desert areas less than 250mm and 750mm . The area is basically free of loess.

,最高2400mLoess in China is mainly distributed in the middle and lower reaches of the Yellow River, while loess-like soil is mainly distributed in the north and south of the Tianshan Mountains and the Songliao Plain in Xinjiang. The lowest elevation is 200m and the highest is 2400m .

,堆积厚度也最大,黄河中游,特别是洛河和泾河流域中下游,最大厚度达180200m ,而欧洲地区的黄土厚度小于10mAlthough the proportion of China ’s loess area in the world ’s loess area is not large, it is the most typical [2] , and the accumulation thickness is also the largest. The middle reaches of the Yellow River, especially the Luohe and Luohe basins, have a maximum thickness of 180 to 200m . The loess thickness is less than 10m .

,大部分分布于黄河中游地区,如陇东、陕北地区,陇西、关中、河南、山西地区,其厚度一般小于15m ,最大30mNot all loess are collapsible. China's collapsible loess accounts for about 60% of the total loess area. Most of them are distributed in the middle reaches of the Yellow River, such as Longdong, North Shaanxi, Longxi, Guanzhong, Henan, and Shanxi. The thickness is generally less than 15m and the maximum is 30m .

With the development of the western development of China, large-scale projects, such as thermal power stations and water diversion projects, must be constructed in the loess region. The foundation treatment of these projects generally uses pile foundations. Therefore, the negative friction of the pile body due to the loess collapse must be fully considered in the design. 年来,参加了四个工程单桩负摩阻力监测,试验桩位于圆形或长方形浸水坑中,坑径或边长一般等于或大于桩长,测试过程中不间断地向试坑注水至桩端土体饱和度达80%为止,停水后继续观测至水位降至桩端,且地表停止沉降为止。 In the past 15 years, our institute has participated in the monitoring of negative friction of single piles in four projects. The test piles are located in circular or rectangular soaking pits. The diameter or side length of the piles is generally equal to or greater than the length of the piles. Inject water until the saturation of the soil at the end of the pile reaches 80% . After the water is stopped, continue to observe until the water level drops to the end of the pile and the ground surface stops subsiding.

负摩阻力监测对仪器设备的要求 1.2 Requirements for Instruments and Equipment for Negative Friction Monitoring

   Negative friction resistance is due to the settlement of the soil layer is greater than the amount of pile sinking, which causes the drag force to act on the side of the pile, which increases the pile body load and the amount of sinking. If not properly considered, it may sometimes lead to disasters. The settlement of the soil layer can be caused by a variety of reasons such as ground loading, lowered groundwater levels, collapsing of the loess, melting of frozen soil, and the weight of underconsolidated soft soil or hydraulic filling soil. There are many formulas to estimate the negative friction resistance, some consider the compressive strength of the soil, some consider the effective stress and shear strength, some consider the standard penetration number, and the type and surface roughness of the pile. The values calculated by different formulas are quite different, so the actual measurement is of great significance.

   The main purpose of negative friction monitoring is to determine the axial stress distribution of the pile body during ground settlement, so there are special requirements for the testing principle and long-term stability of the instrument. Stress cannot be measured directly, but by measuring changes in certain mechanical or physical parameters under the action of stress, such as strain, acoustic wave propagation speed, etc., and then further estimating stress based on the constitutive relationship of the material, so in addition to accurately measuring the corresponding sensitive amount You also need to understand the constitutive relationship of the material.

   In the pile body friction monitoring, multiple steel bars, concrete “stress gauges” or pressure boxes are usually installed along the axial direction. Because the probe and the medium cannot make an ideal match, and there are more or less zero drifts in the fixed embedded electrical measurement components, the actual measurement results can only be qualitative to a large extent. Take the steel bar as an example, it replaces the average strain of the pile body on the section with the measured strain at a certain point on the steel bar. Then multiply by the average elastic modulus of the pile body to obtain the axial stress of the section.There are a series of problems in this way. First, the strain at the measurement point changes due to the intervention of the probe or resistor (including the moisture-proof layer and the wire), the local stress state changes, and the measured strain does not equal the true strain. Second, the average elastic modulus of the pile body is An estimated value, in fact, it varies greatly along the axial direction, especially for cast-in-situ piles, and the elastic modulus value is also related to the magnitude and rate of loading. Furthermore, the measurement points are limited and the spacing is generally 3 5m , Even larger, the measured values between adjacent points are connected by straight lines, which is insufficient; in addition, the other points drift, and the temperature change of the soil during the water injection cannot be determined. Even if the axial force curve and the position of the neutral point are provided, in fact, it is only the position of the maximum value in the measured value, and does not represent the true neutral point, so it is impossible to determine the negative friction distribution curve.

线法监测原理及滑动测微计 1.3 Principle of Line Monitoring and Sliding Micrometer

教授等提出了线法监测( Linewise observation )及相应的测试技术,如滑动测微计、三向位移计等[34567]In the early 1980s, Professor K. Kovari of the Swiss Federal Institute of Technology in Zurich proposed Linewise observation and corresponding testing techniques, such as sliding micrometers, three-way displacement meters, etc. [3 , 4 , 5 , 6 , 6 , 7] . ),点法监测充其量只能获得测点处的信息,测点之间如有某种不连续面或空洞( E值降低),则无法分辨出来。 As mentioned above, the steel bar gauge, pressure box, etc. are point observations . Point monitoring can only obtain the information at the measurement points at best. If there is some discontinuity or void between the measurement points (the E value decreases) ), You ca n’t tell. The line method monitoring principle is to continuously measure the relative displacement between two adjacent points on a line (straight line or curve). In this way, not only the axial force and friction resistance can be reasonably calculated, but also all defects of the pile body can be exposed.

,两端带有球状测头的位移传感器,内装一个线性电感位移计( LVDT )和一个NTC温度计。 The main body of the sliding micrometer is a displacement sensor with a standard length of 1m and a spherical probe at both ends. A linear inductive displacement meter ( LVDT ) and an NTC thermometer are built in. 安置一个具有特殊定位功能的环形标,其间用硬塑料管相连,滑动测微计可依次地测量两个环形标之间的相对位移,可用于多条测线,是一种便携式高精度应变仪。 In order to measure the strain and temperature distribution on the survey line, a ring mark with special positioning function is placed every 1m on the survey line, and a hard plastic tube is connected between them. Displacement, which can be used for multiple measuring lines, is a portable high-precision strain gauge. )。 (Figure 1 ).


滑动测微计 Figure 1 Sliding micrometer

Sliding Micromeiler ISETH


Figure 2   Line method (sliding micrometer) and point method

   ("Rebar Gauge") Basic Differences in Monitoring



Compared with traditional methods, the new method has the following main advantages:

它连续地测定标距 (1) It continuously measures the gauge distance 1m The average strain of the pile body, the slight deformation of any part of the pile body is reflected in the measured value, while the traditional method can only measure the strain of a few points, the deformation between the two points can only be inferred, and the strain at the measuring point is due to the probe The local stress distortion caused by intervention will deviate from the true value.

传统方法是将被测元件予埋在桩身内部,不仅测点有限,而且易于损坏,更主要的是零点飘移无法避免,无法修正,新方法只在桩内埋设套管和测环,用一个探头测量,简单可靠,不易损坏;而且探头可随时在铟钢标定筒内进行标定,可有效地修正零点飘移,特别适用于长期观测。 (2) The traditional method is to bury the tested component inside the pile body. Not only the measurement points are limited, but also easy to damage. The main point is that the zero point shift cannot be avoided and cannot be corrected. The new method only buryes the casing and the measurement ring in the pile. It is simple and reliable to measure with a probe, and it is not easy to be damaged; moreover, the probe can be calibrated in the indium steel calibration barrel at any time, which can effectively correct the zero drift, especially suitable for long-term observation.

新方法所用探头具有温度自补偿功能。 (3) The probe used in the new method has a temperature self-compensation function. Temperature coefficient is less than 0.002mm ° C ,而且附有一分辨率为 / m / ° C with a resolution of ° C 0.1 ° C The thermometer can monitor the temperature of the measurement section at any time, and is especially suitable for on-site monitoring with temperature changes during the observation period to distinguish the temperature strain and the strain caused by the stress, which cannot be achieved by traditional methods. During the soaking period, the ground temperature will drop several degrees after soaking due to the great difference between the original ground temperature and the water temperature (such as winter).

当用于垂直试桩时,不仅可提供摩阻力负摩阻力、端阻力等参数,还可评估桩身质量,缺陷部位,提供弹性模量等。 (4) When used for vertical test piles, it can not only provide parameters such as friction resistance, negative friction resistance, end resistance, etc., but also evaluate the pile body quality, defect location, and provide elastic modulus. 及极限承载力Hu外,还可提供挠度曲线,其精度可达10 -5 ,比常用钻孔测斜仪高一个量级。 When used for horizontal test piles, in addition to the temporary bearing capacity Hcr and the ultimate bearing capacity Hu , the deflection curve can also be provided, and its accuracy can reach 10 -5 , which is an order of magnitude higher than the commonly used drilling inclinometer.


2   Test of negative friction resistance in warm subsidence loess area

年从瑞士购入滑动测微计以来,约20个试桩工程采用了该仪器,其中有五个工程监测负摩阻力,除一个工程在沿海淤泥地基外,其余四个均处于黄土地区。 Since our institute purchased a sliding micrometer from Switzerland in 1983, the instrument has been used in about 20 test pile projects, of which five projects monitor negative friction resistance. Except for one project on the coastal silt foundation, the remaining four are in Loess area.

蒲城电厂负摩阻力测试 2.1 Negative friction resistance test of Pucheng Power Plant

为马兰黄土( Q 3 )下部为离石黄土( Q 2 ),总厚60m ,属大厚度黄土地基,地下水稳定水位为-62.8mThe Pucheng Power Plant is located on the Weibei Loess Plain in Shaanxi Province. The stratum is layered and interactively distributed between the loess and the ancient soil. The upper 6m is the Malan Loess ( Q 3 ) and the lower is Lishi Loess ( Q 2 ), with a total thickness of 60m . The stable groundwater level is -62.8m .

型为φ 1.2m ,长40m ,扩大端φ 2.5mB型为φ 1.0m ,长32m ,扩大端φ 2.2mThe test piles are bored cast-in-situ piles, two sizes, each of two A -types is φ1.2m , 40m in length, with an enlarged end of φ2.5m ; B -types are φ1.0m , 32m in length, and an enlarged end of φ2.2m .

1m的试坑中,坑底0.3m小卵石。 The test pile and anchor pile are placed in a test pit with a diameter of 40m and a depth of 1m , and a pebble of 0.3m at the bottom of the pit. ,深32-40m ,间距3m的注水孔140个,孔中填粒径为28cm卵石。 There are 140 water injection holes with a diameter of 157mm , a depth of 32-40m , and a spacing of 3m . The holes are filled with pebble with a particle size of 2-8cm .

1115日开始注水,共注水79860m 3 ,历时40天,于1225日停水。 In order to saturate the pile foundation soil layer, water injection was started from November 15 , 1990, and a total of 79860m 3 of water was injected for 40 days. The water was stopped on December 25 . 以上地层饱和度达90%The saturation of formations above 40m reaches 90% .129日,当最后5天平均湿陷量小于1mm时,符合《黄土规范》稳定标准,即停止观测,最大湿陷量为65mm ,小于70mm ,评定为非自重湿陷性黄土。 The settlement observation continued until January 29 , 91. When the average wet subsidence in the last 5 days was less than 1 mm , the stability criteria of the Loess Code were met, that is, the observation was stopped. The maximum wet subsidence was 65 mm and less than 70 mm , which was assessed as non-gravity wet. Recessive loess. It is particularly worth noting that during the water injection, the stratum not only subsided, but lifted up. The collapse occurred after the water stopped, and the ground layer consolidated as the groundwater level dropped.

四桩进行了全面的试验,包括天然状态下的垂直和水平静载试验,浸水期负摩阻力监测,饱和状态下垂直和水平静载试验。 During the test of the pile, comprehensive tests were performed on the four A 1 A 2 B 1 B 2 piles, including vertical and horizontal static load tests in the natural state, monitoring of negative friction during the flooding period, and vertical and horizontal static load tests in the saturated state.The test was hosted by the Institute of Foundation of the Chinese Academy of Sciences. The following only introduces the monitoring of negative friction during flooding [8] .

B 1由于桩顶分别施加了60004800kN轴向力,浸水期间桩顶沉降较大(分别为 (1) As A 1 and B 1 exert axial forces of 6000 and 4800 kN respectively at the top of the pile, the settlement of the top of the pile during flooding is relatively large ( respectively 37.7mm , 15.6mm ,因而中性点位置较高(分别为 ) , So the neutral position is higher (respectively 18m , 11m B 2二桩由于顶部荷载为零,沉降只是由于浸水后土层下曳力引起,因而较小,中性点位置较低,而且随水位下降而下移, A 2), Because the top load of A 2 and B 2 is zero, the settlement is only due to the drag of the soil layer after soaking, so it is smaller, the neutral point is lower, and it moves down as the water level drops. 15m decrease to 25m Place, B 2 by 15m decrease to 21m .B 2中性点处的轴向力只在停水后才随水位降低而增长。 The axial forces at the neutral points of A 2 and B 2 only increase with the water level after the water is stopped. As the water level remained at the last observation 25m , So it has not yet reached its maximum.

浸水初期, A 2B 2不但未出现负摩阻力,反而产生拉应变,特别是A 2桩,在172529 (2) In the initial stage of water immersion, A 2 and B 2 did not show negative friction resistance, but instead produced tensile strain, especially for A 2 piles, at 17 , 25 , 29 , 35m με,这一现象可用浸水初期地表抬升来解释,浸水后,土颗粒结构尚未破坏,而有效应力降低到(γ -1H ,桩身自重由于浮力也降低了,而桩端因扩大头的镶嵌作用,相当于一固定端座,这些因素导致桩身产生拉应力。 Around, it is as high as 1285 με. This phenomenon can be explained by the surface uplift during the initial soaking period. After soaking, the soil particle structure has not been damaged, and the effective stress has been reduced to (γ -1 ) H. The self-weight of the pile body is also reduced due to buoyancy, and the end Due to the embedded effect of the enlarged head, it is equivalent to a fixed end seat. These factors cause the pile body to generate tensile stress. 桩由于原有裂缝的存在,水渗入后产生“楔”作用,使裂纹扩张。 Due to the existence of the original cracks in the A 2 pile, a “wedge” effect is generated after water infiltration, which causes the cracks to expand. After the water was stopped, the water level dropped and the cracks gradually closed from top to bottom.(Figure 3 )

4表明各种工况条件下B1桩实测应变,回 (3) Figure 4 shows the measured strain of the B1 pile under various working conditions.

Change curve of normal strain and positive and negative friction. The data processing method will be described in the next section.(Figure 4 )

浸水期间,桩身正负摩阻力及端阻力分布如表 (4) During immersion, the distribution of positive and negative friction resistances and end resistances of the pile body are as shown in the table.

所示, As shown in 1 ,   桩由于下沉量大大超过其他三根桩,因而 Because the sinking amount of the B 1 pile greatly exceeds the other three piles,


Figure 3   桩浸水期实测应变 Measured strain of A 2 pile during immersion


The neutral point is the highest, and the negative friction resistance and unit negative friction resistance are smaller.B 2桩而言,单位正摩阻力略高于负摩阻力。 For A 2 and B 2 piles without a pile top load, the unit positive friction resistance is slightly higher than the negative friction resistance.


Figure 4a     桩实测应变曲线 Measured strain curve of B1 pile             Figure 4b   桩回归应变曲线 B1 pile return strain curve            Figure 4c     桩正负摩阻力曲线 Positive and negative frictional resistance curve of B1 pile

Table 1   Positive and negative friction resistance and end resistance during immersion in Pucheng Power Plant



Pile top load


Neutral point


Negative friction


f N


Forward friction resistance


f p


End resistance


End resistance

Total load

A 1









B 1









A 2









B 2


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