Assessing Data Quality Assessing Data Quality

8.1 Assessing Data Quality

The PDA-S has available ‘expert advice’ to inspect data quality. We strongly suggest reviewing the warnings displayed in the Data Quality bar above the graphs (highlighted area in Fig ure 8.1 ): signal clipping, proportionality, excessive bending, velocity sensor ratio, whether displacements or velocity integration is stable from impact to impact, and Beta (BTA) damage indicators. If anything exceeds the defined limits, the warning box is highlighted.

Figure 8.1:

Figure 8.1: Data warnings displayed in the upper window Data warnings displayed in the upper window and output quantitiesand output quantities

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Warning s based on th e data li mi ts (See “Da ta Limit s” on pa ge 52. ) are also integrated into the quantities list, with the quantity highlighted or the font color changed as defined in the graph setup ( See “Cust omized Col or Schemes ” on page 89. ).

The limits for many of these warnings can be user adjusted by selecting the FN button on the operational toolbar followed by the LIMITS button on the Function Sub-Menu.

Adj usti ng values ar e descri bed in “Data Limits” on pag e 52 .

8.1.1 Signal Quality 8.1.1 Signal Quality

Figure 8.2:

Figure 8.2: Note the V1 Channel shows instability in the middle to later portion of theNote the V1 Channel shows instability in the middle to later portion of the record (Example 5) Note the

record (Example 5) Note the Vend warning in the data quality barVend warning in the data quality bar In order to confirm quality data, set the timescale (TS) to full scale and check for consistency. Set the display to show individual force and velocity traces. Forces may be different due to bending but should show similar frequency content and no spikes. For similar hammer blows, the velocity from each accelerometer should be similar to each other from blow to blow. If one accelerometer is more consistent blow to blow, it is better to use only that accelerometer than average a bad signal. To help evaluate velocity quality, review the displacement graph. Better data has correct final displacement (compared with observed set per blow); questionable data or negative (upward) final set will require more data adjustment in CAPWAP and may over predict capacity and underestimate transferred energy. It is vitally important for diesel hammers to have a sufficient pretrigger buffer (minimum 35 ms).

In the field, begin with all connected sensors active. By watching the data display, the engineer can select the sensor for inclusion into the average signal. If velocit y is unstable from blow to blow, turn off the u nsta ble v eloc ity c hannel. Most of th e u ser’s time during

the hammer operation should be spent observing the PDA graphic screen to view data

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friction distri bution, da ma ge, wa ve sp eed, ca pacity me thod s, et c. The energy tran sfe r should be in line with energy transfers of similar hammer pile combinations, the displacement curve should be reasonable, compression stresses should probably be similar from pile to pile. If some result looks suspicious, it is up to the user to at least investigate and see if something is in error and needs to be corrected.

8.1.2 Proportionality 8.1.2 Proportionality

Figure 8.3:

Figure 8.3: Bad proportionality at the first time peak based on poor WS selection. This isBad proportionality at the first time peak based on poor WS selection. This is evident both in the FV curve and the WU curve. (Example Ex-1)

evident both in the FV curve and the WU curve. (Example Ex-1) It is very important to check for proportionality of force to velocity for the major input rise (peaks don't necessarily have to match exactly in amplitude). The relative change of the force rise to the velocity rise is important. It is often easier to see this effect in the wave up (WU) curve, wave up should be smooth and monotonically increasing through the impact and be free from obvious steps or valleys, like the incorrect valley at the impact time in Figure 8.3.

For diesel hammers, the slow compression build-up can result in the force being higher than the velocity prior to impact in harder driving; this is acceptable as the slow pressure build-up time is greater than 2L/c and thus causes soil resistance reflections from the shaft and even from the pile toe. Slow rise times, impedance increases just below the transducer location, or transducers near the ground with high friction resistances in the upper soil layers can also cause the force to exceed the velocity at the first peak. In very low end bearing and shaft resistance situations (e.g. easy driving), the tensile reflection from the to e due to th e precom pres sion ma y al so cause vel ocity to slig htly exceed th e for ce. Fo r im pedance reductio ns just belo w the sensor s, th e vel oc ity ca n ex ceed th e for ce.

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The sensors should never never be attached near (either just above or below or worse still, straddling) an impedance or cross section area change as the stress path can rapidly change through this area. If you have a cho ice, select a location to atta ch your sensors to the pile that is at least one diameter below any cross section change and with as long a uniform section below the sensor location as possible.