Below, we have tried to answer some of the most common questions visitors to this section of our website may have.

If you find that your query is not answered here, please do not hesitate to contact us by telephone or email.

Q1. Some piles at our site had the caps constructed before testing was carried out. Is it possible to obtain meaningful data now that the caps are in place ?

Q2. Can piles be tested from cast level and, if so, what are the merits ?

Q3. Our site has a piled retaining wall. Can this be tested using 'normal' methods ?

Q4. To ensure accurate analysis, do we require a specification for integrity testing ?

Q6. Is there any general information relating to the nature of cracks within concrete piles ?

Q7. For testing, what are the minimum requirements for pile head preparation ?

Q8. Does NDTL have a Works Procedure for undertaking its pile integrity testing services ?

A1. Can piles be evaluated with their pile caps in place ?

Caps which incorporate a single pile can be evaluated, subject to certain criteria being met. If the cap is the same diameter as the underlying pile, there would be no effect on the test data. As the cap size increases, a proportionally larger amount of the downward travelling vibration wave is reflected back to the pile top. This serves to reduce the amplitude of the downward travelling vibration wave which in turn reduces the effective penetration of the tests. A cap / pile area change of about 2:1 (ie. a 600mm diameter cap on a 400mm diameter pile) would result in about half of the downward travelling vibration wave being reflected back to the pile top. A cap / pile area change of 4:1 would result in all of the vibration wave being reflected from the cap base. In this instance, no integrity assessment of the pile shaft below the cap would be possible.

For caps which incorporate multiple piles (and also ground beams), it is not possible to obtain any integrity information relating to the individual piles by undertaking the tests from the top of the cap or beam. This is because the measured data comprises the flexural response of the pile cap only.

Occasionally, piles have been exposed beneath the base of the cap and horizontal notches or ledges have been cut into the pile shaft to facilitate a test. The test data obtained under such circumstances is significantly affected by the presence of the overlying pile cap. Sample tests undertaken on sub-cap piles with notches introduced have been carried out. These indicate that the effective penetration of the tests may be as little as 2-3 pile diameters beneath notch level only.

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A2. Can piles be evaluated from cast level ?

Pile integrity evaluation surveys are often specified to be undertaken from final cut off level. There are, however, some serious disadvantages associated with undertaking the integrity surveys at this late stage. N.D.Technology advocates undertaking the surveys before any excavation or trimming works have been carried out where ever possible. CFA piles, which are necessarily cast to piling platform, are particularly suitable. Pile integrity surveys from cast level have the following advantages:

1) Any pile that is more than 5 days old can be evaluated from cast level provided that the concrete at the pile top is sound. It is necessary only to clean off the pile top and no trimming is usually required. This allows a large number of piles to be tested in any one site visit which ensures that the pile integrity evaluations do not delay the site construction schedule. Further, fewer visits to a particular site can result in a significant cost saving both in terms of mobilization charges and rates per pile.

2) The quality of the test data recorded from the cast level of the pile is often superior to that available after the pile is trimmed. This is because at cast level, only a short length of reinforcing will be exposed. Resonance of the exposed reinforcing bars interfere with the measurement of the pile response.

3) In the event of a pile anomaly being identified from a cast level evaluation, there will be more time to carry out any further investigations necessary to identify the exact cause, and thus structural importance, of the feature. In this way, delays to the construction programme can be avoided.

4) Small diameter piles are susceptible to mechanical damage during excavation and, on average, approximately 3% - 5% of small diameter piles are affected. Mechanical damage usually causes very fine, horizontal fractures, usually within the uppermost 1m-2m of pile shaft and occasionally at the base of the reinforcing cage. Such fractures are rarely visible from inspection of the pile periphery and are of debatable structural importance. Mechanical damage fractures are, however, highly significant in an acoustical sense and readily apparent from integrity tests. Further, their presence precludes any integrity assessment of the underlying pile section. It is therefore necessary to expose and trim the pile to the fracture and carry out a retest.

Piles which are tested from cast level prior to excavation are less likely to have been damaged. The number of piles which require to be trimmed and retested are thus greatly reduced and the piles are evaluated 'as built'. In contrast, piles which are tested from cut off level more frequently require to be trimmed and retested. At this stage, the piles are in the critical path of the site programme and any trimming will require the piles to be rebuilt back to the required cut off level. This inevitably results in delays and associated costs to programme.

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A3. Integrity tests on retaining wall piles.

The integrity evaluation of piles within retaining wall structures is undertaken in an identical manner to piles which are in complete isolation. This requires that a downward travelling vibration is introduced into the pile at head level and, if reflecting interfaces are encountered by the wave, measurement of the resulting reflections at the pile top.

If, for instance, a wall pile is of uniform section, completely isolated from adjacent piles and the toe of the pile is within the depth penetration of the test, then it would be expected to see a single reflection corresponding to the pile base within the test data. If, as is often the case, a pile is attached to its adjacent pile(s), then this attachment would represent a change (increase) in the pile properties. This in turn would cause reflection of the downward travelling vibration wave back to the pile top. The magnitude of this reflection would be dependent on the magnitude of the attachment. For example, a ‘small’ attachment over a very short distance would result in a relatively small / partial reflection of the test wave. This allows propagation of the remainder of the test wave below the attachment which in turn allows assessment of the underlying pile shaft. As the attachment becomes greater / more rigid, then a greater proportion of the test wave is reflected back to the pile top. This allows less resolution of features at deeper depths. In a worst case, complete attachment of adjacent piles prevents any assessment of the piles below the level of the attachment.

Attachment of wall piles tends to occur within soils where overbreak occurs more readily i.e. granular / fill materials. These materials tend to be at their ‘loosest’ towards piling platform level and piling contractors will ensure that greater concrete oversupply occurs within these strata. It is thus not uncommon for piles on close centres to be connected close to the pile heads. This may restrict acoustic assessment of the piles to the uppermost 1m-2m of pile shaft only. In these circumstances, visual inspection of the wall after excavation may offer more integrity information than the tests themselves.

It is sometimes commented that it may be possible to undertake evaluations prior to installation of adjacent piles. It is, however, normal practice that neighbouring piles are installed typically 1-2 days apart. Integrity evaluations should not be scheduled until the piles have achieved reasonable maturity and are at least, say, 3 days old. This dictates that adjoining piles are often installed before integrity tests can be undertaken.

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A4. Should we use a specification ?

Pile integrity ‘tests’ are now required on most piling contracts. The test methods and procedures are, however, rarely specified. Consequently, integrity surveys are frequently undertaken using inappropriate testing systems which are capable only of providing minimal information regarding the integrity of the piles. Reporting can also be misleading. Facts, arising directly from the test data, and inferences are often indistinguishable and reports rarely provide the Specifier with any indication of the limitations of its conclusions or the level of assurance afforded by the pile integrity survey.

In response to requests from local authorities and several leading Consulting Engineers, NDTL has produced a model specification for the evaluation of cast in situ piles. The Specification is generic and is designed to ensure that integrity evaluation works are undertaken to a worthwhile standard.

A copy of this specification is available upon request.

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A5. Why don't integrity tests always penetrate throughout the entire piled length ?

All pile integrity evaluations are subject to a finite depth penetration. This is a physical limitation.

As a vibration wave propagates along the pile shaft, it is progressively attenuated by the action of dynamic load shedding to the surrounding soil. The rate of attenuation is logarithmic with depth and the amplitude of the propagating vibration wave very soon becomes only a small proportion of its initial value at the pile head.

The ability of test systems to resolve faint reflections returning from depth depends upon the dynamic range of the testing system and the signal-to-noise ratio of the measurement. The dynamic range of testing systems is usually 40dB-60dB. This means they can resolve a signal of 1 part in 100 or 1 part in 1000, respectively, depending upon the quality of the measuring transducers and system electronics.

Testing systems which utilise signal processing techniques such as ensemble averaging, auto correlation or cross spectral averaging to improve the signal-to-noise ratio of the measurement can resolve fainter reflections than systems which rely on inspection of the raw signal data only.

All systems, however, have finite capabilities which limit the effective penetration depth of the test. In loose sands, alluvium etc, the penetration depth of the most sophisticated systems is of the order 40-50 pile diameters. In London clay, this reduces to 25-30 pile diameters. In hard chalks, mudstone etc, the penetration of the tests may be less than 10 pile diameters.

The condition of the pile head at the time of testing is crucial in ensuring that depth penetration is maximised. Poorly trimmed pile heads, laitence and heavy exposed reinforcing cages all introduce unwanted ‘noise’ into the measurement system which can significantly reduce the effective penetration of the tests.

Additional penetration limits are imposed where piles are of irregular form. For instance, a pile containing a ‘bulb’ in pile section will result in a reflection (and, in turn, a reduction in the amplitude) of the downward travelling vibration wave. A ‘bulb’ which represents, say, a doubling of the effective area of the pile will result in a partial reflection (about 50%) of the downward travelling vibration wave. A bulb which represents an area change of 4 or greater will result in all of the vibration wave being reflected back to the pile top. In this case, the vibration wave cannot penetrate past the bulb and no integrity assessment of the underlying pile shaft would be possible.

The same rules apply for piles which contain relative reductions in their properties.

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A6. Is there any general information regarding the nature of cracks within concrete piles ?

Cracks within small diameter piles are a common occurrence. Typically 5% of all piles evaluated contain cracks / fractures. Normal causes of fractures are:

• Lateral impact damage from site plant. This is by far the most common cause and can be exacerbated where the piles occur in an area of deep excavation.
• Lateral loading from soil surcharge where the piles are used as temporary support for an
• excavation can also result in cracking.
• Ground heave.
• Thermally induced shrinkage.
• Inappropriate trimming methods (e.g. machine mounted concrete pecker).

Cracks are particularly common within CFA piles because these are often lightly reinforced and constructed from concrete with a small aggregate size and thus low tensile strength.

Cracks which have arisen from mechanical impact when the pile concrete has cured for at least 48 hours tend to be horizontal in orientation and very fine. This is because the reinforcing cage prevents the formation of a wide, open crack. Cracks which have arisen when the pile has not cured fully are more likely to have an orientation of about 45 degrees and are open and wedge shaped. This is because the younger concrete is strained when impacted and cannot reform when the impact load is removed. Acoustically, cracks of this type almost always represent complete discontinuities.

Cracks caused by impact damage usually occur within 3 to 4 pile diameters of the pile head or, within soils of relatively low shear modulii, at the base of or below the reinforcing cages.

Soil surcharge normally results in cracking 1m-2m below the base of the excavation and often coincident with the base of the reinforcing, if short cages have been used. Heave and thermal cracks occur at depths typically 40%-50% of the full pile depth.

Cracks represent very large changes of acoustical impedance. They are, therefore, highly significant in an acoustical sense and are readily apparent from integrity tests. In a structural sense, however, they are of debatable importance. This is because they tend to be fine and horizontal and do not detract from the pile’s ability to carry axial loads. Tests on old piles which have served as foundations reveal that many contain cracks. Many large diameter piles within London contain heave and/or thermal cracks with no detriment to their service. It is probable that cracks have always been a common facet of piled foundations but it is only recently, with the widespread use of acoustical integrity evaluations, that the frequency of their occurrence is being realised.

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A7. What are the minimum requirements for the preparation of the pile heads ?

All piles to be surveyed in any one site visit shall be prepared fully in advance in accordance with the following sub sections.

The piles shall have attained the following ages before scheduling the PIE survey. The pile ages depend upon the concrete mix:

• C30 MPa or greater with not more than 25% cement replacement - 3 days.
• All other mixes - 5 days.

The piles may be surveyed from cast level or cut off level. For piles cast to ground level (e.g. CFA and driven cast in-situ piles) the former is recommended.

The pile tops shall be left clean and free from debris and surface water. Unrestricted vertical access shall be given to the pile heads. Reinforcing cages for ground beams / pile caps must not be placed until the survey has been completed.

If blinding is to be placed, the blinding concrete in contact with the pile shall not exceed a thickness of 10% of the pile diameter.

The pile heads shall be prepared (if necessary) to provide an approximately level surface, perpendicular to the pile axis, over at least 75% of the pile cross section. The concrete within this surface shall be hard and free from laitence and fractured, loose material. This may be checked by tapping the surface with a small hammer and checking that it rebounds cleanly and that no material breaks off. If the piles are trimmed, the final trimming of the piles shall be with small air tools, CP9 or similar.

If the piles have been exposed, the length of the exposed section above the reduced ground level shall not exceed 3 pile diameters.

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A8. Does NDTL have a Works Procedure / Method Statement for undertaking the tests ?

Yes, a copy of which is available upon request.

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