Because dry jet mixing pile construction involves a large volume and wide area and constitutes concealed work, existing construction equipment cannot automatically and accurately control construction quality. Therefore, how to test the construction quality of dry jet mixing piles and effectively control quality during construction is an urgent problem in soft foundation treatment that remains unsolved and is also a problem that engineering and technical personnel have long been exploring and studying. At present, the main quality testing methods for dry jet mixing piles are as follows:
(1) The pile excavation inspection method is the method specified in current soft foundation design codes. It requires excavation inspection of pile formation at a sampling frequency of 2% of the total number of piles, and then full-scale pile heads are cut from locations such as 50 cm and 150 cm below the pile top for unconfined compressive strength tests.
Obviously, this method can only relatively accurately test the quality of the upper part of the pile body (2–3 m), and this part is precisely the part where dry jet mixing pile construction machinery can most easily ensure quality during powder injection and remixing. Moreover, this part is often located within the hard crust layer rather than the soft soil layer, so pile formation and pile strength generally will not have problems. However, the lower part of the dry jet mixing pile, where problems are likely to occur, cannot be tested. Therefore, the test results of this method are entirely unrepresentative. It not only involves a large amount of work in excavating piles and cutting pile heads, but also damages the natural strata and makes backfilling difficult. Accordingly, this method has more disadvantages than advantages and should not be recommended as a code method.
(2) The light penetrometer method is also a method specified in the code. Light penetration testing must be carried out at an early age, generally no more than 5–7 d, and the detection depth of light penetration is generally no more than 4 m. Therefore, the deep quality of dry jet mixing piles cannot be determined, and, like the first method, the results are unrepresentative. Thus, as a code-specified method, it obviously cannot meet the requirements of engineering quality control.
(3) The borehole coring method uses a geological drilling rig to drill and core the full length of dry jet mixing piles (generally at an age of 28 d). This is currently a commonly used method in quality testing of dry jet mixing piles, and the test results can reflect the overall quality of the piles relatively well. However, this method also has problems such as long testing time and high drilling costs. Borehole coring generally needs to be carried out after 28 d, making it difficult to implement dynamic control of dry jet mixing pile quality. Therefore, considering cost and time factors, only a small number of piles can be selected for borehole coring testing.
(4) The static load test method can qualitatively determine pile body quality according to pile bearing capacity. However, because the testing cost is relatively high, only a very small number of piles can be spot-checked in each project, so the test results are also unrepresentative.
(5) The dynamic testing method mainly refers to the low-strain dynamic testing method. It is based on one-dimensional wave theory and uses the propagation laws of elastic waves to analyze pile integrity. Obviously, dynamic testing is fast and simple. However, a large amount of domestic data shows that the relationship between pile body strength and wave velocity in dry jet mixing piles is scattered, there is no obvious change between pile end impedance and the surrounding medium, and bottom reflection is not obvious. Therefore, it is difficult to use dynamic testing to evaluate pile body quality.
The above methods each have advantages and disadvantages, but none of them can quickly, effectively, and economically test the quality of dry jet mixing piles and ensure construction quality. In this paper, the static cone penetration test (CPT) method and the standard penetration test (SPT) method were used to conduct a large number of tests on dry jet mixing piles of the Xulian Expressway. The test results show that the CPT method combined with the SPT method can better evaluate the construction quality of dry jet mixing piles and implement dynamic quality control during the construction process.