Pile foundations (seismic methods)

Marius Gell, winter semester 2011/12

Artikel auf Deutsch

Piles of concrete, steel or occasionally also of wood are used by the building industry for deep foundations. There are several types of piles, such as bored piles, displacement piles, micro-piles and so forth which differ in production method, diameter and material. They are considered for use if the respective ground has such a low load-bearing capacity that a shallow foundation is inadequate. Their quality and minimum length of such piles can be tested using seismic methods.

Application of seismic methods

The length of piles is measured with seismic methods. In addition the pile is tested for quality defects (e.g. cracks, diameter changes. Seismic methods are non-destructive. The foundation of the building under test must not be exposed or destroyed. In the case of new piles these methods are used for quality assurance, in particular for pile integrity testing. A post-construction pile length measurement for existing structures is required because adequate documentation or plans are often not available. But also in the case of disputes and in particular in the case of high precision requirements, indirect testing methods such as seismic methods are used.

Seismic methods in civil engineering

In civil engineering, seismic methods are typically used pile integrity testing though the parallel-seismic method is also additionally used.

Parallel-seismic method

The main area of application of the parallel-seismic method is the length measurement of piles. The travel time of an acoustic signal generated by a hammer blow on the pile is measured. For this measurement, sensors are placed at regular distances within a bore situated parallel to the pile. The waves triggered by the hammer blow run through the pile and soil to the sensors. The most important waves for the measurement are longitudinal waves (p-waves) as they are the fastest and thus the onset time is readily determined. The wave velocity of a p-wave in concrete is c_P = 3500 – 4000 m/s and in the subsoil’s loose material about 300 – 2500 m/s. During testing, precise wave velocities are taken from the measurement data and thus the system is “calibrated”. Geophones or hydrophones are used as sensors.

There are three different evaluation methods used for the test results.

The salient point method

Using the salient point method, the path of the signal from the pile to the sensors in the bore is not evaluated. This means to keep it simple, it is assumed that the sensors are directly attached to the pile. Thus, the length of the pile is consistently over-estimated.

A salient point occurs due to straight line adaptation through the onset times in the areas above and below the pile head (see fig. 2). If as assumed, the sensors are directly attached to the pile, the depth of the salient point roughly corresponds with the length of the pile. The salient point is generated as the signal to the sensors below the pile needs more time because it must travel through the subsoil and the propagation velocity in the latter is significantly shorter than in the pile. The slope of both straight lines in fig. 2 additionally shows the propagation velocity of p-waves in the bored pile (upper straight line) and the subsoil (lower straight line).

Liao’s method

Liao’s method offers a correction of errors made by the salient point method. Liao describes almost the exact pathway of the wave except that he ignores calculation of the pile’s radius.

In comparison with the salient point method, Liao’s method provides the correction value, K.

$//<![CDATA[ \begin{array}{l}L_{Liao}=L_K-K\end{array} //]]>$

$//<![CDATA[ \begin{array}{l}F=\sqrt{D^2+z_{1b}^2}\end{array} //]]>$

$//<![CDATA[ \begin{array}{l}z_{1b}=D*tan\Theta_1\end{array} //]]>$

$//<![CDATA[ \begin{array}{l}\Theta_1=asin\left (sin\alpha_1*\frac{c_{ground}}{c_{pile}}\right )\end{array} //]]>$

The velocity of p-waves is, as in salient point method, evaluated by the straight line adaptation. The method is more precise and the measured length of the pile is somewhat shorter than the exact length of the pile. Thus, one is on the ‘safe side’ when considering that load-bearing capacity increases with pile length.

Niederleithinger method

In the Niederlethinger method, the previous evaluation methods were enhanced. The mathematical/physical bases (wave propagation in the pile) were more carefully considered and an automated evaluation method of the measurement results was introduced. In extensive parameter studies the influence of boundary conditions such as bore and pile inclinations, changes in material and pile diameter, layer boundaries as well as ground parameters (e.g. unsaturated ground areas) are collected. The areas of application were significantly enlarged yet the boundaries of the parallel-seismic method (e.g. foundation in solid rock, determination of flaws in the pile) were also found. Using numeric simulation, the mechanism of wave propagation was determined, so that not only the compression waves used in previous methods but also the shear waves generated in marginal areas are included in the evaluation.

With the new method useful results can also be achieved in cases of large distances (2-3 m) between pile and bore. Using the combination of different inversion methods (calculation of material parameters and base geometry of geophysical measurement data) and the classical evaluation method (system response of previous geometry) an automated, iterative evaluation method was developed. Thus, results are produced faster and are also much more precise.

Pile integrity testing

Pile integrity tests provide quality assurance of concrete piles. As in the parallel-seismic method, a hammer blow on the pile head also serves here as pulse generator. The shock wave travels down the pile and is reflected at the pile base and at flaws. The reflected waves are registered by a sensor (often a piezo-electric acceleration sensor).

The oscillation velocity at the pile’s head is determined by integration of the measured accelerations. A strike on a high quality pile introduces a deflection amplitude-time diagram, and if the signal is reflected at the pile base, it reaches the pile head again.

In the case of additional deflections between the two main deflections, it indicates a change in impedance (e.g. cross-section declination) at these areas in the pile.

With the travel-time measurement of the signal $//<![CDATA[ \begin{array}{l}T_{measured}\end{array} //]]>$ and the length of the pile according to the plan $//<![CDATA[ \begin{array}{l}L_{plan}\end{array} //]]>$ the velocity of the wave $//<![CDATA[ \begin{array}{l}c\end{array} //]]>$ can be determined.

$//<![CDATA[ \begin{array}{l}c=\frac{2\times L_{plan}}{T_{measured}}\end{array} //]]>$

The velocity of waves in concrete is mostly between 3500 and 4200 m/s. If the measured velocity is slower than 3500 m/s, it indicates inferior concrete quality. If the measured velocity is over 4200 m/s, it could be that the piles are shorter than according to the plan. Pile integrity testing is a very cost efficient method. Implementing and evaluation of the measurements however need expertise and should be carried out by skilled personnel.

Physical fundamentals of the propagation of elastic waves

Sound waves can be divided into two different wave forms: Indirect waves and surface waves. Indirect waves consist of p-waves (compression or longitudinal waves) and s-waves (shear or transverse waves). In the case of a p-wave, the oscillation direction of the wave is similar to the propagation direction. In the case of s-waves, the directions are perpendicular to one another. A hammer blow on a pile of concrete generates both p and s-waves. As p-waves are the fastest, they are used for the evaluation using seismic methods. As there is no overlapping with other kinds of waves, with these waves it is the easiest to determine the point in time when the wave reaches the sensor.

A p-wave in a homogeneous, isotropic, infinitely expanded hemi-anechoic room has a propagation velocity of $//<![CDATA[ \begin{array}{l}v_P=\sqrt{\frac{E*(1-\nu)}{\rho*(1-2\nu)*(1+\nu)}}\end{array} //]]>$ [m/s] and a s-wave $//<![CDATA[ \begin{array}{l}v_S=\sqrt{\frac{E}{2*\rho*(1+\nu)}}\end{array} //]]>$ [m/s].

With: $//<![CDATA[ \begin{array}{l}E\end{array} //]]>$: Young’s module [N/m²], $//<![CDATA[ \begin{array}{l}ν\end{array} //]]>$: Poisson’s ratio [-], $//<![CDATA[ \begin{array}{l}ρ\end{array} //]]>$: Density [kg/m³]

Web links

http://www.bam.de/de/kompetenzen/fachabteilungen/abteilung_8/fg82/fachgruppe_82j.htm

Literature

• Niederleithinger, E., 2011: Optimierung und Erweiterung der Parallel-Seismik-Methode zur Bestimmung der Länge von Fundamentpfählen. Dissertation Universität Potsdam/BAM Berlin
• DGGT AK2.1, 2012: Empfehlungen des Arbeitskreises Pfähle (EA-Pfähle). Ernst & Sohn, Berlin

References

1. Niederleithinger, E., 2011: Optimierung und Erweiterung der Parallel-Seismik-Methode zur Bestimmung der Länge von Fundamentpfählen. Dissertation Universität Potsdam/BAM Berlin