How do I know that my piles are cast right?

We have done the piling. Now what? Can we immediately start constructing over the piles?

No! Wait…Before doing that, first we should ensure that the piles we have just cast are really capable enough to take that big responsibility…. This process is called testing of piles.

There are different types of tests commonly practiced in completed piles. Let us understand what they are. The tests done on piles are broadly classified into two based on the purpose of the tests: Please remember – what we are mentioning below is specific for cast-in-situ concrete piles only.

               

    A. Tests to make sure that the piles are cast correctly and are homogeneous in nature. Here we make sure that the piles;

a.       Are cast without any breakage or discontinuity

b.      Are straight without any bend or inclination

c.       Have reached till the expected depth

d.      Does not have any defects such as bulging / caving / necking / soil inclusion or voids.

·    B. Tests to ensure that the piles can carry the expected load when put into action. These tests are called load tests.

In this lesson let us see the first case. Let us see how to check whether the piles are cast correctly.

Piles are buried structure - that means, once done, it is not possible to visually inspect them to verify whether they are cast correctly. Hence we need to resort to some other techniques to gather information about the correctness of the piles. The tests used for this purpose are generally termed as Pile Integrity Test.

Pile Integrity Test is also known as low strain dynamic test. The principle is very simple. When we apply a small impact at the top of the pile, it produces a compression wave. This wave travels down through the shaft of the pile and reflects at points where there are any discontinuity or change is strata due to any defects of pile such as caving, necking etc. we measure the time taken by the wave to reflect back, and analyse the whole process with help of computer to determine the quality of the pile. What we should keep in mind is that this method is NOT used to assess the strength of the concrete or load carrying capacity of piles. This is purely a substitute for visual inspection.

Now, let us go a little more deep into the process and methodology for these tests.

There are no guidelines specified for pile integrity test in Indian Standards (IS) – hence we follow ASTM D5882 for conducting and monitoring the tests. 

There are two commonly used methods for testing the pile integrity.

1. Pulse Echo Method (PEM): Here, the pile head motion is measured as a function of time. This record is then evaluated for pile integrity.
2. Transient Dynamic Response (TDR) method: In this method, the pile head motion and force is measured as a function of time. These data are then evaluated for pile integrity.

Before going into the process in depth, let us understand the apparatus that are used for the integrity tests.

• Apparatus for applying impact: A hand held hammer. This would ideally be with hard plastic tip so that it can induce short force pulse without damaging the pile surface.
• Apparatus for velocity measurement: Accelerometer. It is placed near pile head with its sensitive axis parallel to the pile axis using a suitable thin layer of bonding materials such as wax, vaseline etc to correctly measure axial pile motion.
• Recording unit: The signals from the sensors are transmitted to an apparatus for recording analysing and displaying data.
• Force measurement: in TDR method, the hammer (impact devise) shall be capable of measuring the impact force as a function of time.

Let us now see how the test works:

In the case of cast in situ concrete piles, the tests are generally conducted after 7^th day or when the concrete achieve 75% of the design strength. The pile head surface is made accessible, clean from debris or loose particles. Proper pile top preparation is essential for successful application of this method. The motion sensors are fixed at the selected locations away from the edge of the pile head. The location to fix motion sensors (transducers) may be prepared smooth by grinding using a hand grinder. For piles with diameters more than 500mm, the accelerometers shall be attached at more than three places so that the integrity evaluation is possible from each localized sections of the pile. The accelerometers are attached at the top of the pile (not at sides). The impact is then applied at top of the pile with the hammer at a location which is not more than 300mm from the accelerometer.

Generally, the tests are done using a Pile Integrity Tester, which is a small and compact testing unit that is used for testing and analysing data. When the impact is applied to the pile top, a low stress wave is transmitted to the pile. And this will result in generation of a very low strain to the pile. The acceleration generated by the impact is recorded in the accelerometer and converted in to velocity form. Reflections from pile toe, pile discontinuities, cross-sectional changes, soil resistance changes, the wave speed through the pile etc., are graphically displayed on the recording device screen.

The test involves collection of several blows during the stage of testing. All such similar blows are averaged before display. If there are any random signals, they will get cancelled by this averaging technique. The signals are also exponentially amplified. Low strain signals generated due to hammer impact are often damped by skin friction. For long piles with high skin friction, reflections from pile toe may be small. By amplifying the records exponentially with time, this will enhance the identification of relevant reflections that have low energy.

Discontinuities, cross-sectional changes, material property changes etc cause reflections of the generated waves. Such reflections are measured and checked for each pile. Thus, any defect in the pile shaft can be easily detected. By measuring the travel time of any such reflection, the location of defect in the pile can also be located. The wave speed is determined from the response of the pile toe. This measured wave speed by itself is an important test result, from which the general condition of concrete can be estimated.

 

The result is generally displayed in the form of a graph with depth in one axis and amplitude in other. Let us assume that the speed of the compression wave is V m/s, and it takes ‘t’ seconds to reflect back from the toe or any irregularities within the pile. It means, the time taken to travel from the origin to the point of irregularity is t/2. With this information the depth of pile can be determined using the formula d = t/2 X V. Thus, by measuring the response time, the pile depth or / and the depth at which the anomalies occurred can be judged correctly.

Next, let us look at the load tests done on concrete piles.