Study on the bearing capacity of large diameter cast-in-place pile by using fiber optic testing technology

Abstract

The project is located in Yantai, Shandong province, China, and consists of 5 buildings. The height of the north apartment is 122.7∼133.5 m with 41∼43 layers. The height of the south apartment is 127.9∼133.7 m with 40∼42 layers. The height of the hotel is 123.0 m with 32 layers. The height of the building podium is 17.3 m with 3 layers and an underground parking garage. The embedded depth of the foundation is 12.6 m. Natural foundation is adopted for podium and underground garage and pile foundation is adopted for other buildings. According to the geotechnical investigation, the layers below the foundation elevation are Quaternary sedimentary soil layers, Proterozoic schist, quartzite, and gneissic granite. With the pile tip bearing stratum being strongly or moderately-weathered rock, three groups of test piles were arranged in the foundation to determine whether the pile bearing capacity can meet the design requirement of 11500 kN characteristic value. Test group 1 consisted of 3 piles (No. SZ01 to SZ03) with a diameter of 1.2 m. The pile tip of group 1 was into strongly weathered rock for 2.4 m with the pile length of 27.0∼28.5 m. Test group 2 consisted of 3 piles (No. SZ04-SZ06) with a diameter of 1.2 m. The pile tip of group 2 was into moderately-weathered rock for 0.5 m with the pile length of 31.0 m. Test group 3 consisted of 1 pile (No. SZ07) with a diameter of 1.2 m. The pile tip of group 1 was into strongly weathered rock for 0.5 m with pile length of 27.0 m. The test piles are grouted at the bottom and side except No.SZ07. The optical fiber was laid on No. SZ01 and No. SZ05 piles and tied to the reinforcing cage. The side friction resistance and tip resistance of piles under each load level were measured in the load test. The results showed that the pile tip resistance value accounted for only 1.82%∼ 2.32% of the total load when the design bearing capacity characteristic value was 11500 kN load, and only accounted for 4.43% ∼ 9.92% of the total load when the ultimate bearing capacity was 23000 kN load, indicating that the pile tip resistance was not fully utilized. The load test results of 7 piles showed that the bearing capacity of a single pile met the characteristic value of 11500 kN. According to the results of the load test, the design scheme of the pile foundation was optimized. The number of foundation piles was reduced from 274 to 244, and the bearing rock of pile tip was 2.4 m into strongly weathered layer, changing from the initial value of 2.4 m into moderately-weathered layer. The project was completed in November 2008 and is currently running well.