Developer Junfa is building Kunming’s tallest skyscraper right in the middle of the city center. Lingye Foundations contracted all the foundation work. The Chunzhiyan twin tower project, with a footprint of 38000 sqm was delayed, and the design was revised several times resulting in very high urgency once construction finally started. Lingye built the diaphragm wall and bored pile foundation.

The CEO of Lingye, Mr. Ling, explained the site had one of the highest equipment density ever seen in Yunnan province. Lingye is a leading contractor in Yunnan and an early adopter of construction equipment, including the first Resonance Free vibratory hammer sold in China. Cranes, bore pile rigs, welders, concrete trucks and pumps, all butted each other. A tricky hard gravel layer at -8 meters complicated the work further, making strenuous demands on the vibratory hammers.

The longest foundation piles bearing the weight of the 456m tall main tower are 98m long with a diameter of 1000mm.  The 28RF was used to drive 20m casings without resonance in the more sensitive areas of the jobsite close to the Catholic cathedral, while the 1412C worked on the more difficult areas further away from the sensitive areas. A gravel layer at 8 meters below ground level posed quite tough for even the 1412C to overcome.  The resistance was so high that the hammer easily goes into re-bound.  Rebound is the most important cause of damage to vibratory hammers. It occurs when the pile stops penetrating by hitting hard soil layers and the vibrator is kept vibrating. During rebound the hammer is bouncing on the pile as nearly all the energy delivered by the vibrator is reflected from the soil to the hammer and gives cause to damaging forces. When rebound occurs the vibratory hammer must be stopped immediately. 

Strategies to overcome rebound are to use pull-down of the piling rig, or to add extra weight if the power pack’s operating pressure is not at maximum yet. 

Some users also report better results with casings that have been cut at an angle at the tip to better slice through the soil.

Because the gravel layer was quite close to the surface the casing alignment is also easily affected. To counter this the client installed the 28RF hammer on a Sany piling rig SR280, and the 1412C on a SR360; the power pack were installed as counter weight. This set-up keeps the pile within the required verticality. Previously Lingye had hoisted their vibro’s mostly with crawler cranes, but with so much equipment vying for space and verticality challenges, the rigs were a good choice. A next step is to use a dedicated ICE Rig. The ICE Rigs are really piling machines that do much more than just holding the hammer. They have the advantage of more stability, 360 turning of the tracks, extendable counterweight for more stability and powerful pull-down force.

Lingye was the first bore pile construction company to understand that the hydraulic vibratory hammer is not just an auxiliary tool for boring under difficult conditions, but an essential part of the process to increase quality and efficiency for every job. This understanding has made them into a leader in their field and an example to the industry. ICE has references across all provinces of China. Get in touch with us for your questions on bore pile applications.