Pile Integrity Testing: History, Present Situation and Future Agenda

Amir, J.M.(1)

(1)Piletest.com Ltd., Herzlia, Israel <jmamir@piletest.com>

ABSTRACT. Deep foundations have served humanity for the last few millennia but really fast progress in piling systems and equipment had to wait until the 20th century. Today, piles can be produced in all soil and rock formations, in diameters reaching four meters and depths of 150 m or more. Moreover, it has become apparent that even the most advanced piling technology cannot assure perfect products. As a results, the 1960's triggered the new discipline of integrity testing of deep foundations. Present methods of integrity testing are either not-intrusive (mainly acoustic) or intrusive, the latter requiring the installation of access ducts during casting. Currently, the analysis of test results is invariably an inverse problem, thus, a unique accurate solution is still unavailable. Future research will have to concentrate on the integration of all testing methods with common interface and on advanced digital analysis methods. 1. HISTORY 1.1 Piling Technology Since prehistoric times, humankind has looked for lakeside and riverside dwelling sites that offered both ample water and protection from attack. To support their dwellings above high water level in the muddy soil, driven timber piles were the obvious solution. Modern radiocarbon dating technology established that timber piles recently discovered in London (Figure 1a) were more than 6,000 years old and are still in reasonable shape (Milne et al. 2010). This foundation method was successfully practiced over the millennia in cities like Venice and Amsterdam. A painting by Maximilien Luce (Figure 1b) proves that manual piledrivers were used to drive timber piles as late as the twentieth century even in a developed city like Paris.

a b

Fig. 1: Wood piles - (a) London 4,600 BC, (b) Paris1905.

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The demand for supporting heavier loads led to the use of stone-filled well foundations, such as those supporting the Taj Mahal. The 19th century heralded new construction materials that the piling community was quick to adapt: Portland cement (1824), steel tubes (1825), steel I-beams (1849) and reinforced concrete (1867). The hand-dug Chicago Caisson, filled with reinforced concrete, was introduced in 1893 to successfully support heavy high-rise buildings (White 1962).

During the 20th century, bored piles became prevalent due to the development of larger and stronger drilling rigs. Currently equipment is available for constructing piles of practically any diameter and length in all soil and rock formations, above and below water. Due to the proliferation of piling equipment and dropping prices, piles have largely replaced labor-intensive spread footings (Amir 1983). On the other hand, piles are sensitive to both soil conditions and standard of workmanship. The first published report of defective piles (Hobbs 1957) reported unusual necking of piles in South Africa cast in loose sand under artesian water. Szechy (1961) describes a project in Budapest where piles cast under water were flawed and unable to support the design loads. Since then the issue of pile integrity gradually began to play a growing part in quality assurance programs of construction projects. Pile integrity may be defined as meeting the project requirements, specifically physical dimensions, material properties and verticality. Any failure to meet the above requirements is defined a flaw.

1.2. Flaw Occurrence The process of constructing bored piles is essentially invisible. Therefore, especially when piling in difficult soil conditions and in the presence of ground water, it is reasonable to expect a certain percentage of flawed piles. Fleming et al. (1992) describe many situation that can lead to the creation of flaws in both driven and bored piles. In a survey of 49,000 piles tested in five countries (Amir and Amir 2008) showed that 1.85% of the piles had identified flaws, in addition to 6% that were too short by 20% or more—a total flaw rate of close to 8%. Much higher flaw rates, up to 76%, were found on several sites where workmanship was sub-standard. Such high flaw rates are evidently unacceptable, and can be avoided only by applying an integrity testing program as early as possible. 1.3. Pile Integrity Fiascoes 1.31 Chicago 1966 The John Hancock Center in Chicago, when completed in 1969, was one of the tallest buildings of the world. The construction of its foundations, however, was inflicted with trouble. The tower was designed to rest on 57 concrete caissons bored by a massive drill rig to bedrock, 60 m below grade. The caissons were cased in stages: A length of casing was lowered into the hole and filled with concrete. Once the concrete began to harden, the casing was pulled up until its bottom was barely embedded in concrete. This process was repeated until the caisson was concreted to ground level. Once all the caissons were completed, steel erection for the superstructure began. The alarm was sounded when one of the caissons started to settle excessively under the first floor column—a mere 120-kN load. The engineers immediately stopped the project and embarked on an extensive testing program that included coring and a number of novel non-destructive testing techniques (Baker and Khan 1971). Apparently, some of the concrete adhered to the casing when it was pulled out since 26 of the caissons were found defective, including one with a 4.3 m long inclusion of mud. Around the clock repair work lasted four months at a total cost of 11 million dollars (82 million is present value) before work was resumed.

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1.32 Tel Aviv 1996 A high-rise residential tower with three basement levels was built on a lot of 6,000 m2. To protect neighboring streets and properties, a diaphragm wall was constructed around the site. The wall was excavated with the use of polymer slurry that, unknown to the contractor, was of doubtful origin. To save money, no integrity testing was specified for the wall. The true picture (Figure 2) became clear only when the wall was completed and the contractor began to excavate the site. Extensive repairs were necessary, at a cost of several million dollar.

Fig. 2: A flawed diaphragm wall in Tel Aviv.

1.33 Hong Kong 1999 The Hong Kong Housing Department (HD) hired the lowest bidder, Zen Pacific Ltd, to construct the pile foundations for five residential buildings, 41 stories high, in the Yuen Chau Kok site. The large diameter bored piles were supposed to penetrate through unstable layers of fill, marine deposits and decomposed granite, and be belled out on solid granite rock (ICAC 2000). To support the boreholes against caving, HD instructed the contractor to use continuous steel casing. Zen Pacific subcontracted the work to Hui Hon Ltd. that had neither suitable drill rigs nor sufficient casing material. Hui Hon soon confronted massive borehole collapses and decided, without the owner's approval, to replace the casings with Super Mud. This however had no effect and as a result, numerous piles had to be abandoned before reaching bedrock. At this stage, Hui Hon, on the brink of bankruptcy, devised an extensive cover-up program: Working at night when HD staff were absent, falsifying the site records, diverting the excess concrete amounts to other projects, blocking the access tubes installed for cross hole testing and replacing the test with a different test that produced no useful information. In addition, Hui Hon doctored the tape measure used to check the depths and replaced defective cores with good ones taken from other piles. All this failed to save the buildings: by the time the authorities became aware of the situation the two first buildings, already more than 33 stories tall, showed excessive settlements and had to be demolished. The cost? 650 million HK dollars, two Hui Hon directors sentenced to 12 years in jail and the site agent to five.

The serious consequences of such events served as an incentive to the development of pile integrity testing.

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1.4. The development of pile integrity testing Integrity testing aims at detecting zones of reduced cross section (necking) and/or inferior material properties. At an early stage, this was done through direct methods, such as excavation or core drilling. Excavation enables thorough visual inspection of the outer surface of the pile, but is generally limited to the upper few meters. Core drilling, on the other hand, provides full information about concrete quality to large depths, albeit for a small fraction of the pile's cross section. The resulting core hole, however, enables further studies such as caliper and ultrasonic testing or video photography.

The sixties and seventies of the 20th century saw the development of various indirect (imaging) test methods, mostly using analog electronics with recorders or oscilloscopes to plot the data. Once fast microprocessors became available in the late seventies, purpose-built computers and digital signal processing were quickly incorporated into testing apparati. The next logical step came during the nineties, when ruggedized computers became an affordable commodity and were turned into virtual instruments just by hooking them with suitable transducers. Figure 3 is a chronological representation of these developments.

Fig. 3: History of integrity testing techniques.

2. PROPAGATION OF ACOUSTIC WAVES IN PILES After Smith (1960) introduced the one-dimensional wave equation to the analysis of pile driving, it was soon adopted by most integrity testing methods. A basic familiarity with the principles governing acoustic waves is therefore necessary to understand how they function. First, we have to distinguish between the one-dimensional (long wave) case and the three-dimensional (short wave) case. 2.1. One-Dimensional Wave Propagation The mathematics involved are rather rudimentary, provided we first make a few reasonable assumptions:

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• The pile is prismatic rod with a constant cross-section A, elastic with Young’s Modulus, E, and homogeneous with mass density, ρ. • The wavelength is equal to or larger than the pile diameter. • Cross sections remain plane, parallel, and uniformly stressed. • Lateral inertia effects are negligible.

When we hit the pile head with a hammer, we create a compressive wave that travels

downwards along the pile. Βy combining Newton's second law with Hooke's law (Vincke and van Nieuwenburg 1987) we get the one-dimensional wave equation:

𝜕𝜕2𝑢𝑢𝜕𝜕𝑡𝑡2

= 𝑐𝑐2 𝜕𝜕2𝑢𝑢

𝜕𝜕𝑥𝑥2 (1)

Where u is the displacement and 𝑐𝑐 = √(𝐸𝐸/𝜌𝜌) is the wave speed in the rod. In concrete piles, for instance, the harder the concrete the faster the waves. The general relation between concrete compressive strength fc and wave speed is given by Amir (1988): 𝑐𝑐 = 𝐾𝐾𝑓𝑓𝑐𝑐

1/6 (2) Clearly, c is determined by the physical properties of the pile and does not depend on the

strength of the blow. Actually, it is exactly the same for a large pile driver and a small plastic hammer. For concrete grades used in piling it will typically vary between 3,600 and 4,400 m/s. Particle velocity v, on the other hand, is a function of the stress, σ, applied to the head. The ratio between the two, v/c, is equal to σ/E. Therefore, the force P in a given section is given by:

P = σA = (EA/c)v = Zv (3)

The factor Z is defined as the acoustic impedance.

When a compressive wave reaches a flaw (reduced impedance, e.g., a "neck" in the pile), it separates into two parts: One continues to travel down as a compressive wave, while the other is reflected upwards as a tensile wave. A particular instance is that of a stress-free toe (approximated by a toe located in soft soil), where the wave is fully reflected upwards as a tensile wave until it reaches the pile head and "pulls" the head down. In the opposite case, when a compressive wave reaches a bulb (increased impedance) part of it continues downwards and the other part is reflected back. In this case, however, both waves are compressive.

On its way along the pile the wave energy is reduced by skin friction. In homogeneous soil, the total attenuation of the toe reflection is given by the following equation (Paquet 1992):

𝐴𝐴(𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁) = 4𝐿𝐿𝐷𝐷𝜌𝜌𝑠𝑠𝜌𝜌𝑐𝑐

𝑣𝑣𝑠𝑠𝑐𝑐

(4)

Where L is the pile length, D is the pile diameter, 𝜌𝜌𝑠𝑠 and 𝜌𝜌𝑐𝑐 respectively are the soil and pile

densities, vs is the shear wave speed in the soil and c the wave speed in the pile. For a pile with a typical slenderness ratio L/D of 25 in a soil with a shear wave speed of 250 m/s, the total attenuation A is 4.7 Neper or e4.7 = 110. For modern digital instruments, it is a simple matter to amplify the reflections by this amount, but at the same time, any noise present will also be amplified.

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2.2. Three-Dimensional Wave Propagation While in a rod we encounter mainly longitudinal waves (compressive or tensile), in an infinite space, a dynamic impact may create several wave types that radiate from the point of application, mainly: • P-Waves – longitudinal wave in which particle motion coincides with the direction of

propagation • S-Waves – transverse wave in which particle motion is perpendicular to the direction of

propagation. • Rayleigh waves – P-Waves that advance close to a free surface in which the particles move

is both longitudinal and transverse directions. For good-quality concrete, P-waves move at a speed of 4,470 m/s. S-Waves and Rayleigh

Waves are typically slower by 42% and 48%, respectively. In comparison, wave speed in a rod of the same concrete material is 4,080 m/s.

3. INTEGRITY TESTING METHODS Integrity testing methods are either non-intrusive or intrusive. The former need minimal preparation of the pile head while the latter require special access ducts to be installed in the pile during construction. 3.1. Non-intrusive methods Non-intrusive methods are invariably based on the theory of one-dimensional wave propagation. In the following, the Impact Echo Method will be described in some detail since all other non-intrusive methods are actually derived from it. 3.1.1. Impact Echo Method This method, also called Sonic, or Low Strain Impact was the first to be implemented in practice (Beylich 1963) and today is the most widespread method in the world for testing the integrity of all pile types (ASTM 2016a). In this test method, the pile head is hit with a small plastic hammer that sends a compressive acoustic wave down the pile. A suitable transducer (usually accelerometer) that is pressed against the pile head is triggered by the blow and monitors the reflected waves (Figure 4). The accelerometer output is digitized and then integrated to provide the graphic time history of the pile head particle velocity. The system then shifts and rotates this graph to fit the horizontal axis and transforms the horizontal axis from time units t to length units L by the equation L = c.t/2 . The user can then apply exponential amplification to the graph (to compensate for skin friction) and pass it through a suitable digital filter. The resulting reflectogram (Figure 5) can be instantly inspected to provide information about the length of the pile and its continuity. For a reflectogram with anomalies, an iterative signal-matching technique can plot a probable profile of the pile.

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Fig. 4: Impact echo testing.

The Impact Echo test is fast and inexpensive, with typically less than one minute needed to

test a given pile (Amir & Amir 2008). The main drawbacks of the Impact Echo method are:

• The wavelength produced by the plastic hammer is in the order of 3-4 meters. Therefore, the information it provides regarding the part of the pile close to the head is limited.

• The lengths reported are a function of the assumed wave speed that is usually unknown. • It is influenced by skin friction, thus its effective penetration depth varies between 10

diameters in very hard soils and 60 diameters in very soft soils. • The pile head must be accessible.

Fig. 5: A typical reflectogram.

The testing methods described in the following sections have tried to address some of these

shortcomings.

3.1.2. Frequency response The Steady-State Frequency Response test was developed in France by Paquet (1968), specifically for handling relatively shallow discontinuities. Paquet placed an electrodynamic shaker on top of the pile, gradually increasing the excitation frequency while keeping the force constant. For each frequency, the measure pile head velocity v and the applied force F were recorded. Paquet plotted the ratio v/F, defined as the Mobility M, versus the frequency. The difference, ∆f, between successive peaks (resonant frequencies) indicated the depth, z, of the uppermost discontinuity: z = c/(2∆f) (6)

In addition, the inverse of the slope of the initial part of the mobility plot is equal to the

apparent low strain rigidity of the pile head. This method did not gain popularity because of the bulky vibrator and the elaborate preparations it required. This situation has changed in the 1970's

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when compact force transducers and Fast Fourier Transform became available. The Impact Frequency Response method (Higgs and Robertson 1979) replaced the vibrator with an instrumented handheld hammer incorporating a force transducer. Both force and velocity records were transformed into the frequency domain and the mobility presented against the frequency.

In spite of its sophistication, the frequency response may be beneficial only in testing the upper part of the pile; it is more difficult to interpret than the impact echo method and less capable of dealing with slender piles in hard soils. Paquet (1992) realized that plotting the pile geometry is viable only if the head excitation data is combined with the surrounding soil data, namely shear wave speeds in the various soil layers. The methodology he developed, the Impedance Log method, showed some success in controlled tests.

The PileInspect project was launched in 2013 by the European Union in order to improve and standardize the procedures for pile integrity testing and evaluation. The consortium formed employs no less than ten partners: Universities, research institutes, professional organizations and private laboratories. The PileInspect hardware comprises a controllable vibrator, capable of exciting the pile in the axial direction, an accelerometer mounted on the vibrator, additional accelerometers mounted on the pile head and a control and processing unit. The whole setup is almost identical to that produced by Paquet (1968), albeit with improved electronics. The aim this time is to employ sophisticated signal processing techniques to power an autonomous system that will decide if the pile is “damaged" or "undamaged” pile and estimate the damage severity. At the time this paper was written there were still no positive results reported from this project.

3.1.3. Multiple Sensors The main obstacle to accurate determination of the pile length is the imperfect knowledge of the wave speed. (Paquet 1968) suggested embedding a second sensor at the bottom of the pile during concreting. Although this approach provides the mean wave speed in the pile, it is impractical for routine testing. Johnson and Rausche (1996) analyzed a model pile with two accelerometers located 3.0 and 4.6 m, respectively, below pile head. They claim that the wave speed can be calculated by dividing the sensor spacing by the travel time between them. This, however, can be in error since the wave speed in the pile is not necessary uniform (Amir et al. 2014). Niederleithinger et al. (2015) experimented with a pile equipped with five sensors attached to the pile sidewall exposed by excavation. Evidently, this setup is unsuitable for standard quality assurance of piles. 3.1.4. Parallel Seismic test The Parallel Seismic test (Hurtado 1979), that can replace the impact echo method where there is no access to the pile head, is based on wave transmission rather than on wave reflection. Its primary purpose is to establish the depth of foundations supporting existing structures where no records exist. In preparation to the test, a water-filled vertical access duct is installed in the ground, at a typical distance of 0.5-1.5 m from the foundation element and to a depth exceeding the assumed depth of the tested element by at least five meters. Where the ground is soft and water table shallow, the duct can be replaced by inserting a suitable piezo cone into the ground. Above the ground water table, the access duct must be grouted in the hole to assure proper wave transmission.

The superstructure above the tested element is hit repetitively with a hammer equipped with an impact switch while a hydrophone is continuously lowered into the duct. The recorded pulses are plotted vs. depth and, if the test is performed properly, a break in the first arrival plot denotes

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the level of the pile tip. If the access duct is indeed parallel to the tested pile, the slope of the upper branch is equal to the wave speed in the pile and indicates the pile material.

The Parallel Seismic test has proved its effectiveness in determining the pile length. However, attempts to use it to detect flaws in piles (Niederleithinger 2012, DeGroot 2014) were inconclusive.

3.2. Intrusive methods The main weakness of all non-intrusive methods lies in the fact that the input (hammer blow) is applied to the head, while potential flaws may be situated many meters below. The intrusive methods described below, while more expensive, offer improved flaw detectability by embedding access tubes or sensors in the pile during the concreting stage. 3.2.1. Ultrasonic testing The Ultrasonic Cross Hole method (Levy 1970) was developed in France in the late 1960's. In this method, several equally spaced access ducts, typically 50 mm in diameter, are attached to the inside of the reinforcement cage. The ducts, either steel or plastic, are filled with water to facilitate wave propagation. Once the concrete has hardened, two ultrasonic transducers, an emitter and a receiver, are in turn lowered to the bottom of each pair of ducts (profile). As the transducers are raised in unison, the emitter sends short duration pulses at predetermined intervals. The First Arrival Time (FAT) and Relative Energy (RE) of all pulses intercepted by the receiver are recorded and plotted on a computer screen. An anomaly in either plot can point to a zone of inferior material. This test has been widely accepted and standardized (ASTM 2016b) and is most probably the leading method in the world for testing large-diameter piles.

The test as described above provides only one-dimensional information: the respective levels of the bottom and top of a flaw. The tomographic option (Paquet and Briard 1976) has been developed for establishing also the lateral dimensions of a flaw. Wherever a flaw is suspected, the zone is investigated with one transducer stationary at a time and the other moving. The oblique pulses are combined with the horizontal ones to draw the shape of the flaw. When this procedure is repeated for all profiles the results can further be analyzed to produce a full three-dimensional representation of the pile, including zones with different wave speeds (Figure 6). On the downside, the method provides little, if any, information about the pile outside the area bounded by the access tubes.

Another variation on the ultrasonic method is the single-hole test that is especially useful for small-diameter piles where there is no room for multiple access ducts. In this case, the duct must be of plastic material (Amir 2002) and both transducers are lowered into it, typically 600 mm apart. This method can also be used in holes produced by core drilling to extend the investigated range (Baker and Khan 1971).

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a b c

Fig. 6: Results of three-dimensional ultrasonic testing: 3D interactive model (a), vertical cross-section (b) and horizontal cross-section (c).

3.2.2. Gamma-gamma (radioactive) logging In the radioactive method (Preiss 1971) the cylindrical probe consists of a weak radioactive source (usually Cesium 137) and a photon counter, separated by a lead shield. The source emits gamma radiation in all directions, and the photons are partly absorbed by the surrounding concrete and partly backscattered and recorded by the counter. A high photon count means low concrete density and vice versa. Since concrete density changes little with time, the radioactive method can be applied soon after casting. With suitable calibration of the instrument, the photon count readings can be readily transformed to density. On the downside, the readings are strongly dependent on the proximity of rebars and the typical range of the probe is less than 100 mm. the method is presently rarely used, mainly because of regulatory restrictions on the handling of radioactive material. 3.2.3. Thermal Logging Thermal Logging (Mullins and Kranc 2004) is based on the phenomenon that concrete hardening is exothermic. Shortly after casting, the temperature of the fresh concrete begins to rise, the rate mainly depending on the amount and composition of the cement, the W/C ratio and the aggregates used. 24 to 48 hrs. after casting, maximum temperatures may exceed 700C, but at the same time the outer surface of the pile (air or soil) starts to cool down according to Newton's Law of Cooling. A temperature gradient develops and the heat flows out. The access ducts for this test must be dry, and the temperatures measured by an infrared thermometer at regular intervals. If the access ducts are equidistant from the center, all the temperature readings are supposed to be equal. A lower

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temperature measured in any access duct may mean reduced concrete cover near that duct or an adjacent inclusion of foreign material that does not produce heat. The main advantage of the method is the ability to test the pile shortly after casting and to provide indication about the amount of concrete cover. At the same time, analysis of thermal logging results is still an inverse problem in which mass distribution is inferred from discrete temperature readings close to the boundary. Some more shortcomings are: • The method is sensitive to inhomogeneity in the surrounding soil, such as saturated sand

or ground water flow. • The method is sensitive to variations in the amount of retarder used, especially in large

piles cast by several truck mixers. • The method may totally miss a discontinuity of small vertical dimension. • The test must be performed once the pile is hot enough, and before it had time to cool

down too much. Once this time windows was missed for any reason, there is no second chance. This limitation can be overcome by using embedded strings of thermistors connected to an automatic data logger, but this option is obviously more costly.

3.2.4. Optical means Trying to actually look into six experimental piles led Sarhan et al. (2002) to construct them with transparent polycarbonate tubes with inner diameters of 15.9 mm, enough for lowering a compact video camera. The results, however, were not reported.

Habel and Krebber (2011) inserted fiber-optic sensors in two model piles, and managed to monitor the strain paths in both models under low stress pulses. The flaw in one of the piles showed very clearly.

3.2.5. Pile Verticality If we define pile integrity as meeting the project requirements, it certainly includes pile verticality. Practically all piling specifications limit the allowable deviation from the vertical, as excessive deviation may lead to serious consequences: overloading of foundation piles, loss of water-tightness in piled retaining walls, loss of parking space in deep basements etc.

Conscientious contractors check the verticality of the drill mast before they start drilling and repeat this check during the operation. This, however, may not be enough since in deep holes, the Kelly bar becomes flexible and the holes may deviate from the vertical. Several systems on the market designed to measure the profile of open holes using ultrasonic waves can also check their verticality, but their use is limited to slurry-filled holes. Systems based on inclinometers combined with gyros (Amir and Amir, 2012) can measure the deviations of open holes using the drilling bucket as a centralizer. With suitable adaptors, they can measure the deviation of finished piles and diaphragm walls inside 50 mm access ducts. 4. PRESENT SITUATION (CONCLUSIONS) After more than fifty years of evolution, integrity testing of deep foundations is now universally accepted. At the same time, the industry is still searching for the Holy Grail: a practical, cost-effective testing method that is able to accurately plot both internal and external geometry of the pile and determine its material characteristics. Unfortunately, pile integrity testing is not rising to

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the challenge, as evidenced by the statistics of nine Stress-Wave conferences (Figure 7). These events – arguably the main forum on pile testing - show a consistent increase in the number of papers dealing with integrity testing until 1996, with a marked output slowdown afterwards – clearly a symptom of a stagnant discipline.

The analysis of all present non-intrusive methods is still a classic inverse problem, with no unique solution. To achieve a probable solution, one must start with some reasonable assumptions and check them with a suitable forward model. Given a reflectogram obtained by the impact echo method, for example, the four basic assumptions we commonly make are:

• The tested element is a pile • The one-dimensional wave equation is valid • The wave speed in the pile is X m/s • The skin friction distribution is known (or neglected).

Fig. 7: Numer of papers presented to stress-wave conferences.

The first assumption is trivial, but the second one is not. While for driven steel piles the one-dimensional wave equation may be applicable, for the purpose of integrity testing, all the assumptions on which it is based are more or less wrong. A pile with flaws, for instance, is neither prismatic nor homogeneous, and the soil profile for a specific pile is known only approximately. Under the circumstances, the best we can sometimes offer is a reasonable estimate of the pile length. Evidently, the wave equation approach does not deliver and the most sophisticated analyses will produce little more than an axisymmetric pile profile with no direct bearing on the structural properties of the pile (Figure 8).

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Fig. 8: Assumed pile profile from Impedance Log testing. By definition, intrusive methods are much more informative than non-intrusive ones but they

too have their deficiencies. The engineer specifying the test has therefore to make a compromise based on type of the piles, equipment availability and budget. In case of uncertainty, the prudent engineer will muster some redundancy: hire another laboratory to repeat the test (a practice adopted in Hong Kong after the 1999 fraud) or test with other methods. The cross-hole ultrasonic test, for example, can be supplemented by single-hole testing to investigate the external part of the pile. Fig. 9 illustrates the case of a pile with a 15% flaw at 4 m depth tested by impact echo with no anomaly and by single hole ultrasonic clearly showing increased FAT and attenuation. Of course, redundancy may produce conflicting results, where sound engineering judgement is unavoidable.

Fig. 9: A pile with a 15% flaw at 4 m – impact echo (left) and single hole ultrasonic (right).

5. FUTURE AGENDA (RECOMMENDATIONS) Forecasting the future is inherently risky. However, even with all the advancement in piling technology we may safely assume that flaws in piles will not disappear, ensuring continuing demand for advanced integrity testing. Research and development will have to adopt revolutionary thinking to meet this demand. The main effort should concentrate on integrating all the existing testing and analysis methods into a comprehensive system with a common interface.

We shall start with integrating the intrusive and the non-intrusive methods by embedding optical fibers in the piles during casting. This means will add a wealth of internal data (such as temperature, stress and strain distribution) to the results of external mechanical excitation. At the

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analysis stage the exterior pile profile will be obtained from methods such as CFA monitoring or bored pile calipering. This will be augmented with interior mapping by ultrasonic tomography and optical fiber readings to serve as the first iteration of pile geometry in a standard CAD format. If the pile supports a superstructure such as pile cap it can be added to the model, together with data from the closest borehole log. The software will discretize the system into elements and apply the input load at the appropriate point. The calculated displacement time-history will be compared to the measured one at the same point and the pile geometry adjusted to obtain the next iteration. A suitable evolutionary algorithm will serve to find the pile geometry that will provide the best fit.

A pre-requisite for the success of the integrated system is to revamp our analytical approach. Although intrinsically inapplicable to integrity testing, the one-dimensional wave equation (Smith 1960) has always been the backbone of the industry since it managed with the modest computing resources available in the 1960's. However, assuming that Moore's Law is valid, computer power has since increased more than1010 fold so it is not too early to divorce the time-honored wave equation and move over to the Finite Element Method (FEM). Following is a short list of the advantages of FEM:

• It can model piles with any shape and physical properties (Fig. 10) • It can accurately model the surrounding soil profile, even if irregular, with proper

geotechnical parameters • It can model the superstructure in cases where there is no access to the pile head • The input force can be static or dynamic, transient or steady state, axial or lateral, concentrated

or distributed. • The output displacements can be monitored at any convenient location.

By a rough estimate, this project may take at least ten years and several millions of Dollars to complete. Initially the system could be developed in two dimensions (axisymmetry) and later in full three dimensions. Once complete, the system will be capable of positively detecting at least 90% of important flaws. This will certainly lead piling technology towards better, safer and more economical foundations.

Fig. 10: Piles with bulges (left) and finite element simulation (right).

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6. REFERENCES Amir, J.M., 1983. Piling in rock - construction aspects. Proc. 6th Asian Conf. SMFE, Haifa,

Vol. 1, pp. 231-234. Amir, J.M., 1988. Wave velocity in young concrete, Proc. 3rd Conf. Application of Stress-Wave

to Piles, Ottawa, pp. 911-912. Amir, J. M., 2002. Single-Tube Ultrasonic Testing of Pile Integrity, ASCE Deep Foundation

Congress, Orlando, Vol. 1, pp. 836-850. Amir, E.I. and Amir, J.M., 2008. Statistical Analysis of a Large Number of PEM Tests on Piles,

Proc. 3rd Conf. Application of Stress-Wave to Piling, Lisbon, pp. 671-675. Amir, J.M. and Amir, E.I, 2012. Testing of Bored Pile Inclination. Proc. 9th Intl. Testing and

Design Methods Deep Foundations, Kanazawa, pp. 233-236. ASTM, 2016. Standard test method for low strain impact integrity testing of deep foundations

D5882-16, W. Conshohocken, Vol. 4.09. Baker, C.N. and Khan, F., 1971. Caisson Construction Problems and Correction in Chicago,

ASCE Journal for Soil Mechanics and Foundation Engineering, 97(2) 417-440. Beylich, M., 1963. Types des Fondations Classiques - Controle des Pieux. Compte Rendu des

Journees des Fondations. LCPC. De Groot, P.H., 2014. Evaluation of the Parallel Seismic detection of defects in pile foundations,

M.Sc. Thesis, Delft Technical University, 192 p. Fleming, W.G.K., Weltman, A.J., Randolph, M.F. and Elson, W.K., (1992). Piling Engineering

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