A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Load tests are conducted to verify the design assumptions and parameters such as pile friction in soils and sock socket capacity. There are various failure criteria in current construction industry to determine ultimate load resistance of piles in pile load test. For instance in 90% criterion of Brinch Hansen, it is based on the laboratory measured stress-strain relations of soils and a point is identified in which soil fails. This essentially aims at looking for the ultimate bearing capacity and hence the ultimate loads. In fact, this is not intended originally for piles. For some failure criteria, it does not target at finding out the ultimate pile capacity. Instead, it looks for the process for onset of soil yielding at the base of pile toe an allows for controlled displacement. For example, one of these criteria is reproduced as follows: Criterion for maximum movement = (PL/AE + d/120 + 4) mm where P is the load, L is pile length, A is the area of pile and d is pile diameter. This failure criterion was developed based on small diameter driven piles. The term PL/AE refers to elastic shortening of piles. For end-bearing piles, this term is acceptable for usage. However, for friction piles this may not truly simulate the actual shortenings of piles because frictional forces along the pile also come into play. The term (d/120 + 4) represents the amount of soil movement which triggers the yielding of soil beneath pile toe. 3. Can dynamic pile test (PDA test) be used to replace pile load test for testing driven pile capacity? (PDA2) The PDA system consists of two strain transducers and two accelerometers attached to opposite sides of the pile to measure the strain and acceleration in the pile. The force is computed by multiplying the measured signals from a pair of strain transducers attached near the top of the pile by the pile area and modulus. The velocity measurement is obtained by integrating signals from a pair of accelerometers also attached near the top of the pile. PDA test is commonly used for the following purposes: (i) Evaluation of driven pile capacity Soil resistance along the shaft and at the pile toe generates wave reflection that travel to the top of the driven piles. The time the 220
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU reflections arrive at the pile top is a function of their locations along the pile length. The measured force and velocity at the pile top thus provide necessary information to estimate soil resistance and its distribution. (ii) Measurement of pile stress during driving Compressive stress at pile top is measured directly from strain transducers. (iii) Measurement of hammer energy delivered to the piles The hammer energy delivered to the pile is directly computed as the work done on the pile from the integral of force times displacement ( ∫Fdu ) and this can be calculated as force times velocity integrated over time ( ∫Fvdt ). (iv) Determine if pile damage has occurred. Pile integrity can be checked by inspecting the measurements for early tension returns (caused by pile damage) before the reflection from the pile toe. The lack of such reflections assures a pile with no defects. There is a growing trend of using PDA test to check driven pile capacity instead of traditional static load test which is more expensive and time-consuming. Some engineers may have reservation on the use of dynamic formula to evaluate driven pile capacity because of the fear of improper selection of quake values and damping factor. 4. Can both Pile Drive Analyser (PDA) and Pile Integrity Test (PIT) be used for checking pile capacity? (PDA2) Pile Drive Analyser is a high-strain dynamic test to determine the force and velocity response of a pile to an impact force applied axially by a driving hammer at the pile top. It is applicable to driven piles or small diameter bored piles. The operation measures the elastic deformation of a pile after a hammer blow and is mainly used to check the ultimate capacity of piles. However, it may also be adopted to detect damages in pile body and obtain the friction profile along the pile shaft. Pile Integrity Test is a low-strain dynamic test which involves the use of a small vibrator or a light hammer. It is applicable to small diameter driven concrete piles and large diameter bored piles. It can be used to check the following properties: 221
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A Self-Learning Manual - Institution of Engineers Mauritius

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