The International Information Center for Geotechnical Engineers

Wednesday, 05 December 2018 14:54

Advances in Drilled Shaft Quality Control

Drilled shaft foundations are widely used to support large axial and lateral loads transmitted from the superstructure to the subsurface.

Dry and wet excavation procedures are two widely used methods for drilled shaft construction. In situations where the stability of the soil material is questionable, and the groundwater table is shallow, the wet method approach is generally preferred over the dry method. In the wet method, slurry is placed into the hole to maintain stability within the excavation. In general, project specifications and guidelines provide details regarding the quality control for the pertinent deep foundation system. In the particular case of drilled foundations, depending on local practice, details are provided regarding base cleanliness, integrity, and geometry of the drilled foundation. There are several testing methods currently available which can be used to assess some of these quality control aspects for drilled shafts. PDI now has developed new methods and devices to better evaluate the quality control considerations. The Thermal Integrity Profiler (TIP) is a non-destructive test method that utilizes the elevated temperature generated by curing cement (hydration energy) to assess the shaft integrity, reinforcing cage alignment, and concrete cover. The Thermal Integrity Profiling (TIP) method uses Thermal Wire® cables that are attached to the reinforcing cage prior to casting the shaft. The thermal cables have temperature sensors spaced every 305 mm (1 foot) along the length of each wire. One thermal cable is installed for each 305 mm (1 foot) of drilled shaft diameter, cables are evenly spaced around the reinforcing cage.

During the hydration process, the heat generated throughout the shaft is directly related to the concrete volume and cement content of the mix design. This cement content is directly related to the quality and volume of the concrete. The elevated temperatures during the hydration process are measured and analyzed with TIP to assess the shaft integrity and concrete quality. The temperature measurements are automatically taken, typically every 15 minutes beginning just after casting and continuing until the concrete reaches its peak temperature, which typically occurs within 12 to 48 hours after casting, depending on the shaft diameter and mix design. These measurements can be downloaded by on-site personnel and sent to the engineer for analysis, or can be sent directly from the site to a cloud server. The temperature measurements, along with placed volume and installation details, are used to model the effective shaft radius, shaft shape, cage alignment and concrete coverage beyond the reinforcing cage. Evaluation of the temperatures prior to the peak temperature assess the presence or absence of defects that affect the structural integrity. Compared to traditional integrity test methods, Thermal Integrity Profiling greatly shortens the time window from shaft construction to shaft acceptance, accelerating the construction process. TIP test procedures are further described in ASTM standard D7949. Drilled shaft bottoms are frequently checked prior to concrete placement to determine debris layer thickness and base cleanliness. Traditional test methods for measuring debris thickness are time consuming and subjective to the viewer, with minimal quantifiable results. The Shaft Quantitative Inspection Device (SQUID) is a device used for measuring the extent of the debris layer at the base of a drilled shaft. The device measures the force independently on each of three instrumented penetrometers as they are advanced through the soil at the shaft base. The displacement is measured using three independent contact plates that remain in contact with the top of the debris layer while the penetrometers move through the debris layer and into the bearing material. This device is quickly deployed by connecting directly to the drilling stem or Kelly bar where it is then lowered to the shaft bottom. The total time typically required to complete the standard base cleanliness evaluation tests at the shaft center and at the four orthogonal sides is on the order of 15 to 30 minutes. The resulting force versus displacement information provides a quantitative measure of the debris thickness at the shaft base.

Depending on the foundation diameter, the verticality plays an important role during the load transfer process and in cases where the foundation is rock socketed, the verticality could significantly impact the foundation performance under eccentric loads at the transition zone between soil and rock. The Shaft Area Profile Evaluator (SHAPE) is a new device used for profiling the sidewalls in wet cast drilled shafts using high frequency ultrasonic pulses. The SHAPE device is attached directly to the drilling stem or the Kelly bar and lowered into the drilled hole. An advancement rate of 305 mm (1 foot) per second allows a drilled hole to be profiled at least twice per 305 mm (1 foot). The SHAPE is fully wireless and simultaneously transmits and receives ultra-sonic signals from eight individual sensors equidistantly mounted every 45° around the perimeter. An integrated self-calibrating feature automatically adjusts for changes in wave speed if the slurry should be denser with depth, greatly improving the accuracy of the computed radii. Once all radii have been determined, the overall shape, volume, and the drilled hole verticality can be determined.

This is part of the latest Newsletter of Pile Dynamics which you can find here.

Source: Pile Dynamics

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