2.3. Test Method

This experiment refers to the standard of the “geotechnical test standard (GB/T 50123-2019)” and uses a microcomputer-controlled electronic universal testing machine of Naier Co., Ltd, Jinan, China. for testing. In the experiment, the axial strain rate was set to 1% per minute. Through pre-testing, the curing ages were set to 1, 3, 5, 7 and 14 days, which were more appropriate. The samples with curing ages of 1, 3, 5, 7 and 14 days were used to study the effect of the curing age on their strength. Additionally, the PU dosages of 2, 4 and 6% (the ratio of PU weight to dry clay weight) and chromium ion amount of 1000, 2000 and 3000 mg/kg were considered. In addition, the samples with the PU dosages of 0, 2, 4, 6 and 8% were used to study the effect of the PU dosage on the strength, while the chromium ion content was 2000 mg/kg and the curing ages were 1, 3, 5 and 7 days (see second part of Table 3). Furthermore, the samples with the chromium ion content of 1000, 2000 and 3000 mg/kg were prepared to study the influence of the clay with different chromium ion contents, while the PU dosages were 2, 4 and 6% and the curing age was seven days (see third part of Table 3).

Samples in unconfined compression experiment.

d = day.

According to the standard of the ASTM Method 1311-TCLP, the clay samples in the unconfined experiment were crushed, passed through a 9.5 mm sieve, dried at 50 °C and tested at pH ≥ 5. Glacial acetic acid was selected as a binder to prepare the samples according to the standard. The prepared samples were placed in a flip oscillation device. After 18 h of flip oscillation at a speed of 30 r/min, 15 mL of supernatant was extracted and stored in the refrigerator. After preparing all the samples, the leaching agent was placed in the refrigerator. The chromium ion content was measured using a SHIMADZU AA-6880 flame atomic absorption spectrometer. The sample details are listed in Table 4.

TCLP samples setting chromium ion 2000 mg/kg.

d = day.

The distribution of soil pores is an important factor affecting the strength of the soil. To investigate the effect of the PU on the soil pore distribution and to explain the mechanism of soil strength change, an NMR analyzer (Newmark Co., Ltd, Suzhou, China.) was used to study the change in soil pore distribution. The same method as described in Section 2.2 was used to prepare the artificial chromium-contaminated soil. The prepared contaminated soil was placed in a cylindrical mold with a diameter of 18 mm and a height of 30 mm. After curing for seven days and vacuum drying for 48 h, the samples were placed in a nuclear magnetic resonance device for analysis.

The influence of the PU on the structure of the solidified soil can be directly observed by the morphology of the soil particles. In addition, the influence of the PU solidified chromium ions in the soil can be preliminarily judged by observing the binding morphology of the PU and the soil particles. A high-resolution field-emission scanning electron microscope was used to perform the SEM tests. Before use, the samples were placed in a freeze dryer, vacuum-dried for 48 h and then sprayed with gold. Finally, the samples were placed in the instrument for analysis. The images were magnified 1000 times.