反射波法检测水泥搅拌桩的可行性和方法研究

STUDY ON TECHNIQUES OF REFLECTION WAVE FOR EXAMINING CEMENT MIXED IN PLACE PILES

水泥搅拌桩作为桩基础的一种特殊形式 ,在软土地基地区已被广泛应用。由于其桩身纵波波速远低于混凝土桩的波速 ,且长径比一般较小及桩身均质性等问题 ,所以应用于混凝土桩的小应变反射波检测方法是否可以应用于对水泥搅拌桩进行检测 ,在测桩界尚存在分歧意见。作者从理论上分析了反射波法的本质 ,指出波阻抗差异、分辨率和波长的关系是反射波法的要害。本文讨论了长径比的实际意义及其与一维波动理论的关系 ,提出了水泥搅拌桩“桩身质量”的新观点。从理论分析和实际检测效果资料的讨论出发 ,肯定了反射波完全可以对水泥搅拌桩进行检测 ,并总结了实际检测过程中的方法技术。

As a special form of pile foundation, the cement mixed in place piles are widely applied in weak soil region. These piles are formed with cement and soil mixed in place with a ratio of cement to soil about 12%-15%. They usually have a diameter about 500-600mm and a length from 8-18m. Their density is about 2.0T/m 3 and the velocity of longitudinal wave is generally from 1?000 m/s to 2?200 m/s. Because the velocity of longitudinal wave in cement mixed in place piles is far lower than the velocity in the concrete piles, the ratio of length to diameter is generally smaller and their bodies are relatively inhomogeneous, whether the reflection wave method can be used to examine the quality of cement mixed in place piles is still open to question though the method has been commonly applied to check the concrete piles. In this paper, we will analyze the nature of reflection wave method and suggest that the difference of wave impedance between pile and soil and the relation between resolution and the wave length are the keys to the application of this method. The wave impedance in cement mixed in place piles is much higher than that in the soil around and under the pile. The wave length in cement mixed in place piles is much smaller than length of the pile. Because the wave theory has demonstrated that the reflection wave can distinguish the object with a size bigger than 1/4 wave length, the reflection wave method therefore can be used to examine cement mixed in place piles. We will also discuss the meaning of L/D and its relation to 1 D wave theory and put forward some new point of view about pile integrity in this paper. Based on theoretical study and practical experiments, we believe that the reflection wave method is effective in examining the cement mixed in place piles. However, comparing with examining the concrete piles, the following peculiar techniques should be emphasized in practice: a.The obvious level and extent of reflection signal of pile bottom are vital features that are related to the quality of cement mixed in place piles. If pile body is relatively homogenous and the strength of pile body is relatively high, the reflection signal of pile bottom is clear.The obvious level and extent of reflection signal of pile bottom are vital features that are related to the quality of cement mixed in place piles. If pile body is relatively homogenous and the strength of pile body is relatively high, the reflection signal of pile bottom is clear. b.When the piles are examined after 28 days since their formation, the wave velocity of 1?000 m/s or higher could be considered as the standard for examining the strength of piles even if the reflection signal of pile bottoms are not very obvious. c.Between the signal of pile top and the reflection signal of pile bottom, the curve shape may change because the pile body is inhomogeneous and the wave impedance of pile body is different. Even so, if the reflection signal of pile bottom is very clear, the wave velocity can reach the standard and the pile body is not very inhomogeneous, the pile is also passable.