Successful projects all start with planning and design, end with construction, and include quality programs for every step. For successful deep foundation projects, that means quality control and assurance to validate design assumptions and determine if the deep foundation was installed as planned. To do this we perform integrity testing, which is a set of nondestructive tests that assist engineers as they evaluate the in-place condition of a deep foundation.
What are the most used test methods for deep foundation integrity testing?
Braun Intertec performs integrity tests most often on drilled, deep foundations since the act of driving a pile into the ground is a form of integrity testing itself. The most common integrity tests we use with deep foundations are cross-hole sonic logging, thermal integrity profiling and low-strain pulse echo tests.
Cross-hole sonic logging
Cross-hole sonic logging (CSL) requires installing access tubes within the shafts. After the grout or concrete has reached sufficient strength, a transducer and receiver are placed inside the tubes. The time required for a sonic wave to travel from the transducer to the receiver is measured as the transducer and receiver are raised together from the bottom of the tubes. The process is repeated until all possible combinations of transducer and receiver pairings between the tubes are tested. The data is evaluated to determine if there are significant differences in arrival time or energy of the sonic wave that indicate an anomaly within the deep foundation.
Thermal integrity profiling
Thermal integrity profiling (TIP) is a test method that evaluates the amount of heat generated by concrete or grout curing to measure the integrity and quality of the foundations. This test can detect necking, bulging, cage eccentricities, pile shape and variations in cover by the relative cool or warm regions of the foundation as it cures. The temperature is measured by attaching thermal cables to the vertical elements of the foundation cage with cable ties, or by using a thermal probe and the same access tubes used with CSL.
Low-strain integrity tests
Low-strain integrity tests are performed by attaching an accelerometer to the top of a deep foundation and hitting the top with a lightweight (1- to 8-pound) hammer with a special cap. The cap on the hammer lengthens the impact pulse so that it is suitable for integrity testing. The accelerometer then measures the pile response to the induced stress wave.
The engineer then evaluates the velocity of the pile response with time. A good test will show a clear toe reflection at the anticipated length. If the stress wave is reflected early, it can indicate necking, bulging, a void, or soil inclusion.
Quality assurance programs: Essential to a deep foundation project
The only “real way” we can know what a contractor builds below the ground is to exhume it, which defeats the purpose of putting it there to begin with. A comprehensive, construction quality program that includes detailed observations, load tests and applicable integrity tests, can provide us with enough information to apply our engineering judgment on the overall ability of the deep foundation system to meet project requirements.
Interested in learning more about the role of integrity testing for deep foundations? Click the link below to sign up to for our webinar featuring deep foundations expert Matt Glisson, PE and John Focht, PE.