Clay is found in some countries all over the world. It usually has low compressive strength and cannot be used as a bearing material for subgrade soil. In this paper, the influence of basicity on a metakaolin-based polymer binder to improve clay was studied. The effects of the molar concentration of the alkali activator, different concentration of the metakaolin-based geopolymer and curing time on unconfined compressive strength were studied. The alkali activator-to-ash ratio was maintained at 0.7. The percentage of metakaolin added to the soil relative to metakaolin and soil mixture was 6%, 8%, 10% and 12%. The sodium hydroxide concentrations are 2M, 4M, 6M and 8M. Unconfined compressive strength (UCS) was tested on days 3, 7, 14 and 28, respectively. Compared with original clay, the results show that the unconfined compressive strength increases with the increase in metakaolin content and molar concentration of NaOH. The maximum compressive strength of the sample with NaOH concentration of 8M and percentage of 12% was 4109 kN on the 28th day, which is about 112% higher than that of the original clay. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results showed that the cementing compound covered the clay particles due to the reaction of the geopolymer with the clay, resulting in the formation of adhesive particles. The main purpose of this study is to verify the effectiveness and stability of metakaolin-based geopolymer binder polymerization under normal temperature and a strong alkali environment. The results can provide parameters for the application and promotion of metakaolin-based geopolymers in soil improvement engineering.

In order to explore the waterproofing mechanism of F-type socket joint in rectangular pipe jacking tunnel, full-scale tests and finite element numerical calculations were conducted to analyze the sealing performance and failure modes of rubber rings with different heights under different water pressures, revealing the distribution law of contact stress and leakage mechanism of rubber rings. The results show that low-height rubber rings experience leakage at a water pressure of 0.2–0.3 MPa, while the waterproofing ability of medium and high height rubber rings is significantly improved, withstanding 0.7 MPa and 1.0 MPa water pressure, respectively. The 28 mm rubber ring still has no leakage at a 1.6 MPa water pressure. At 0.4 MPa water pressure, the height of the rubber ring increases from 22 mm to 24 mm, the proportion of effective contact stress increases from 35.7% to 78.6%. The waterproofing performance of tunnel joints is jointly determined by the rubber ring height and the proportion of effective contact stress. No leakage occurs at the pipe gallery joint when the proportion of effective stress exceeds 70%. This research provides a theoretical basis for the size selection of waterproofing rubber rings for the rectangular pipe jacking tunnel.