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Application and Development of Impact Compactors Authors: Wang Yujin, Cheng Heqian, Chen Xiliang, Tai’an Highway Bureau, Shandong Province. Impact compaction is a technique that uses non-circular, high-power devices for continuous rolling impact compaction of road surfaces and subgrades. It was introduced in the 1950s by Aubrey Berrange Company in South Africa, but it wasn’t until the 1970s and 1980s that non-circular rolling impact compactors became mature enough for practical use; their adoption on a global scale began in the 1990s. In 1995, this compaction equipment was introduced to China by the South African company Blue Team. It boasts advantages such as fast operation speed, fewer construction steps, shorter project duration, lower costs, and a wide range of applications. It can enhance the strength, stability, and uniformity of roadbeds, thereby preventing road surface damage caused by uneven settlement. During impact compaction, the potential and kinetic energy of the compaction wheel are periodically converted into concentrated impact energy that acts on the ground, thereby achieving continuous crushing and compaction of the road surface. The crushed debris can also be directly pressed into the foundation, which helps to shorten the time required for road repairs and significantly reduce construction costs. 1 Basic structure of impact compactors: To date, more than a dozen types of impact compactors have been developed. The compaction wheels come in various designs such as 3-, 4-, 5-, or 6-sided shapes, as well as solid, hollow, and fillable types. The 5YCT20 impact compactor of Xiamen Gong Sanying Heavy Machinery Co., Ltd. is shown in Figure 1. Figure 1 5YCT20 Impact Compactor: 1. Tractor; 2. Cross-shaped buffer connection assembly; 3. Compactor frame; 4. Compaction wheel lifting and buffering assembly; 5. Compaction wheel; 6. Tire. 1.1 Tractor: The tractor for the 5YCT20 impact compactor was designed by Sanming Heavy Machinery Company. The front and rear frames are hinged together, with the engine, hydraulic torque converter, front axle, and cab located on the front frame. 1.2 Crushing and Compacting Unit The crushing and compacting unit (i.e., the impact device) is mainly composed of a compacting wheel assembly, frame, linkage frame, swing rod, traveling wheels, connection heads, anti-rotation device, and hydraulic cylinders. 2 Applications of impact compactors: The compaction achieved by impact compactors relies on the combined action of impact force, vibration force, and the weight pressure from rolling (see Figure 2). Figure 2 Working principle of the impact compactor (1) The impulse generated by the fall when the centroid of the compactor is at its highest point ； (2) Vibration caused by the rotation and bouncing of the compaction wheel at a certain speed ； (3) The weight of the compaction wheel does work while compacting the soil foundation during rolling. Impact compactors are calibrated based on their potential energy when at rest; this potential energy is converted into momentum during movement, thereby generating a large impact force. For example, in the 25T3-25KJ type impact compactor, the mass is 15 tons; its potential energy E is 25 KJ, and the impact force is 250 tons, which is 16 times the mass of the compaction wheel. 2.1 Application in subgrade construction: The vibration amplitude of the impact compactor is as high as 0.22 m, with a frequency of 2 Hz at normal speeds; the amplitude is generally 2 mm. Large-amplitude vibrations increase the mass of the compacted soil or other fill materials that participate in the vibration, thereby increasing the depth of compaction and the degree of compaction. The effective depth affected by an impact compactor is 2-3 times that of a conventional heavy-duty vibratory roller. The traveling speed of the impact compactor is 12–15 km/h, and the number of passes varies from 10 to 40 or more. The production rate of each impact compactor per shift is 10,000 m2, and its efficiency is 5 times that of a vibratory roller. The cost per square meter for impact compactors (with the number of passes ranging from 20 to 40) is between 4 and 10 yuan, which is approximately 77% of the cost of vibratory rollers. Impact compactors play a role in kneading and draining during subgrade construction, thereby relaxing the requirements regarding moisture content. The measures include: first, laying a layer of sand and gravel; second, using a small impact roller such as one with 5 wheels for compaction; third, stopping compaction from time to time and allowing the area to dry for a few days before continuing; fourth, using a plow to loosen the surface layer and expose it to the sun. It has been proven in practice that this can be increased to 7 percentage points in some soils with high viscosity. After each compression cycle with the impact compactor, the surface becomes wavy; after 3 to 5 cycles of compaction, it is necessary to level the surface using a grader or bulldozer, otherwise the compaction speed will decrease and this will affect the quality of compaction. 2.2 Application in the crushing of cement pavements In China, the total length of cement concrete pavements constructed in the highway and municipal sectors has reached over 220,000 kilometers, making it the country with the largest number of such pavements in the world. However, due to inadequate quality control, the quality of many cement pavement subbases and slabs fails to meet the design requirements. Moreover, traffic volume has surged and overloading is severe, so many road surfaces have suffered serious damage. The subgrade is insufficiently compacted, resulting in significant settlement and instability; the cement pavement structure lacks permeable drainage facilities, which consequently reduces the service life of the road by a factor of several. When a cement concrete pavement is damaged, the repair method of applying a overlay is used; first, the original pavement must be broken up, and an impact compactor can effectively carry out this task. The impact not only eliminates the possibility of reflective cracks forming, but also creates interlocking between the broken fragments; as a result, the integrity and strength of the old pavement layer and the subgrade are significantly improved, fully meeting the requirements that a cover layer should have for the base layer. Fracturing, stabilizing, and overlaying with asphalt are commonly used methods for repairing old concrete pavements, offering advantages such as short construction time, low cost, and minimal disruption to traffic. Use an impact compactor to crack stable cement pavements, with a maximum force of 100–250 tons. Depending on the road surface structure and degree of damage, cracks generally start after 3–5 passes; settlement occurs during 5–15 passes, and the technical requirements can be met after 20 passes. Its efficiency is 4 times that of traditional compaction methods. 3 Development of impact compactors The use of impact compactors for fracturing and stabilization represents a new direction in the development of this technology. Due to the ability of impact compactors to carry out work on cracking and stabilizing cement pavements without disrupting traffic, as well as their low cost, fast speed, and good results, they have been widely used in China in recent years. Our unit has achieved good results in its application during the construction of the Taihua section of the Beijing-Shanghai Expressway; it was also used in the renovation of Panghe Street in Tai’an City, and tests showed that it fully met the technical requirements. In terms of usage, compacted fill results in greater road surface settlement in soft ground areas, while settlement is less severe in areas with excavation. With the continuous improvement of impact compactors, they will inevitably play an even more important role in subgrade and pavement construction. References: 1. Yang Shiji. Application of impact compaction technology in subgrade engineering. Highway, July 1999. 2. Han Jian, Chang Ruijun. The 5YCT20 type impact compactor. Construction Machinery, June 2003