This is a very informative video about Low Strain Impact Integrity Test although there are several (5 types to be exact based on ASTM). Out of these five, most time we will utilize two specific types of test which are High Strain Impact or PDA and Crosshole Sonic Logging. I prefer both of these tests than the rest. The followings are addendum about this video which one can benefit when monitoring bored pile or casted pile at site.

Pile Driving Analyzer (PDA) displays the same wave as per low strain in the graph in order to check pile's integrity and/or impedance. For starters, impedance of a pile can be defined as follows. "Change in impedance is related to change in pile cross-sectional area A, as well as pile material quality. Increase in pile impedance or soil resistance forces results in a decrease in measured pile top velocity. Conversely, decrease in pile impedance, results in increased velocity. By observing changes in impedance, pile quality."

In the same manner, we can check necking or bulging/bulb that may occur during boring and casting thru PDA. It is important to check CAPWAP graphs and not rely heavily on summary of the Integrity Class and Acceptability as well as the CAPWAP capacity which is the estimation for both end bearing and skin friction. Nevertheless, it is usually the case when specialist review the classification based on graphs but with some common knowledge about wave(s) propagation, one can crosscheck the work of the specialist signing off that particular test record. I encountered a few cases where the graph is not satisfactorily conform with high integrity rating which I will not discuss here.

The benefit of using low strain compare to high strain is the accessibility and mobility of conducting test on completed sections. Second, it is the only way to have some initial assessments on the shape without the need of tubular pipes for Sonic Logging Test and when Bi-directional Static Load Test (BDSLT) or Static Load Test (SLT) failed the three criteria or a single criterium for working load settlement, twice working load settlement or residual settlement.

The second test would be Sonic Logging (USPIT). This particular test compliment what was discovered in in PDA test. This test is conducted prior to the execution of PDA test where the shape and integrity of the pile is examined through cross sections. The larger the pile, the more sections are required, similar to low strain PIT using echo and receiver. Based on this waterfall wave and spectrum, one can determine the existence of honeycomb and other irregularities such as necking and bulging of pile.

The importance of the shape not only crucial in ensuring the integrity of pile especially when there are possibility of necking. Bulging or bulb is not really crucial as it increases skin friction and overall pile capacity; however additional load may be substantially dangerous for the total load at the bearing and/or skin friction. Additional load from bulging of the casted pile can be detected when conducting BDSLT and not in Static Load Test. The reading of the top side of the O-cell or jack will indicate that when compared to bottom side of the jack. This is one of many ways to vindicate or justify when BDSLT reading may deviate aside from soil strata/stratum change(s). This serves as complimentary observation(s) when checking/diagnosing pile or piles' performance holistically.

There are so many variables when estimating pile's capacity and performance. I reckon that sound regional or local geological understanding plays a very high significance for a site engineer or resident engineer to plan for total depth and the inclusion of proper socketing length. In most cases, I rarely follow recommendations made in the drawing if the soil at the boring point have low performance despite the depth set by the designer. With sound and clear estimation, we rarely find issues with working load settlement and residual settlement. In most cases, the issue will only occur with twice working load. Why? The base of the bored pile was not cleaned totally, hence it requires certain degree of depth to mobilize the end bearing.