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Development and Application of Road Impact Rolling Technology Authors: Yang Shiji, Wu Lijian, Liu Shijun, Pang Hongbo Abstract: The development and application of high-energy impact rollers have promoted the innovative development of compaction technology. Engineering practices that make proper use of road impact rolling technology demonstrate clear advantages in effectively reducing settlement deformation of roadbeds, enhancing the overall strength and uniformity of roadbeds, strengthening special types of roadbeds, accelerating the renovation of existing roads, addressing potential problems in roadbeds, and improving the quality of road projects; as a result, this technology has broad application prospects. Keywords: impact roller, impact compaction, subgrade. Practice in road construction shows that the subgrade must be dense, uniform, and stable in order to ensure the proper functioning of the road surface. In highway construction, when the subgrade is adversely affected by slope terrain, the composition of soil and rock fill materials, and the quality of the underlying soil, and as the construction pace accelerates, the subgrade often experiences settlement deformation, which leads to engineering problems. In recent years, impact rolling technology, which makes use of impact rollers for various applications, has seen significant development. It has led to innovations in addressing potential issues related to the quality of roadbed construction, such as effectively reducing post-construction settlement and differential settlement of roadbeds, thereby ensuring the overall stability of embankments ； An additional number of compaction passes was carried out as a test on the subgrade and embankment of the rolled-formed substructure, thereby improving the overall strength and uniformity of the subgrade ； Pre-construction treatment using impact rolling is carried out on collapsible loess foundations or weak foundations to ensure that they meet the requirements regarding bearing capacity and stability ； Applying impact rolling technology to renovate old roads such as gravel roads, asphalt roads, and cement concrete roads not only accelerates the construction progress but also ensures that the project meets quality standards. At present, in addition to the impact rollers produced by companies such as Blue Planet in South Africa and those in the United States, several manufacturers in China also produce impact rollers, with some of them already capable of mass production for market supply. As a result, impact rolling technology has been applied in 26 provinces, autonomous regions, and municipalities across the country. Engineering practice has shown that when used properly, this technology yields significant results; it can effectively eliminate potential problems in road foundations and improve the quality of road construction, offering very promising prospects for future use. 1 Impact rolling technology: Impact rolling represents the latest advancement in compaction techniques for geotechnical engineering. A impact roller is driven by a tractor to make its non-circular wheels roll; the difference in potential energy resulting from the different radii of these polygonal wheels, combined with the kinetic energy generated during movement, enables continuous impact compaction of soil and rock materials by applying static pressure, rubbing, and impacts to the ground surface. This mechanism gives rise to impact compaction characterized by high amplitude and low frequency. Currently, the 25KJ triangular two-wheel impact roller is the most widely used; its two wheels have a weight of 12 tons, the optimal driving speed is 12 km/h, and it exerts a concentrated impact force on the ground of 200–250 tons, which is equivalent to 1111–1543 kPa. According to the **Second Marine Research Institute of the Oceanic Administration**, after 30 passes with a 25KJ triangular two-wheel impact roller at a speed of 12 km/h on the slag and gravel subgrade of the Hangzhou-Jinhua-Qingdao highway, the average vertical dynamic earth pressure at depths of 0.8, 1.5, 2.0, and 2.5 meters was measured to be 1366, 306, 272, and 138 kPa respectively. This high-energy impact force continuously strikes the ground in a periodic manner, generating intense shock waves that propagate downward with the characteristics of seismic waves. The resulting impact compaction effect achieves levels equivalent to ultra-heavy compaction, allowing for a gradual increase in the density of soil and materials at deeper depths. An effective compaction depth sufficient to achieve a compaction degree of over 90% according to heavy-duty standards ranges from 1.0 to 1.5 meters, depending on the properties of the soil and materials involved. This method offers better compaction results compared to existing vibration compaction machinery, bringing the compacted soil and materials closer to an elastic state, thereby demonstrating technical advantages in addressing the issues associated with soil and stone roadbeds. The 15KJ pentagonal two-wheel impact roller is used for layered compaction with layer thicknesses of 0.50~0.75m, as well as in the renovation of cement concrete pavements. The technical characteristics of impact rollers determine a different compaction process compared to conventional rollers; instead of using the construction methods of rolling with half a wheel or partial overlap as with existing rollers, a new impact compaction method and construction technique are proposed, taking advantage of the ability of impact force to spread deep into the soil. Each wheel of the impact roller is 0.9 m wide, with an inner distance between the two wheels of 1.17 m. Two passes constitute one cycle, and the rolling width is 4 m. With each impact, the soil pressure is distributed into the soil at an angle of 45°–φ/2 between the edge of the roller’s contact area with the ground and the surface of the ground. In the second pass of each cycle, the roller passes through the center of the gap between the two passes from the first cycle; this results in a theoretical rolling gap of 0.13 m on each side. When the roller moves inward by 0.2 m during the first pass of the second cycle, it fills in the entire gap created by the first pass. On the third pass, return to the position of the first pass and roll again, continuing in this manner until the final number of passes is reached. As the impact roller moves forward across the terrain with peaks and valleys formed by longitudinal compaction, each sequence of one or two passes constitutes a single compaction unit. When an even number of passes are carried out, the turning radius is adjusted so that the peaks and valleys are compacted alternately, thereby reducing the size of these formations and smoothing the surface. Impact rollers generally alternate between moving in a clockwise direction and a counterclockwise direction every five passes. Rolling various soil and rock subgrades 20 to 40 times can result in the formation of a uniform reinforced layer with a thickness of 1.0 to 1.5 meters. 2 Technical effects of road impact rolling 1. Reduction of the post-construction settlement rate of embankments: Through indoor model tests and field observations of embankment settlement, it was found that when the subgrade reaches the compaction degree required by standards, its post-construction settlement rate is around 0.4%. Generally, in roadbed cross-sections on sloping terrain, the differences in settlement increase. If the thickness of the embankment’s compaction layer and the quality of the filler are uneven, if compaction is insufficient or inconsistent, then compression deformation due to the weight of the soil and rocks occurs, resulting in areas of tension and compression strain that further increase the differential settlement. When the settlement gradient between two points exceeds 0.6%, deformation cracks may occur; longitudinal or transverse cracks are common on high-fill roadbeds in mountainous areas. The use of impact rolling technology in the construction of high-fill embankments can reduce the post-construction settlement rate to around 0.1–0.15%, thereby effectively preventing cracks caused by differential deformation. This is an effective technical measure for addressing deformation problems in high-fill embankments made of soil and rock. The fill material for the 34m-high fill subgrade of the Beijing Badaling Expressway is fine-grained soil and gravel composed of weathered granite with boulders; impact rolling is used to compact each layer to a thickness of 1m, with an average compaction degree of 95% according to heavy-duty standards. The width of the subgrade is 10.5m, and settlement observation points are installed on the left, center, and right sides at each cross-section. One year after completion, the settlement amounts were 32, 37, and 32 mm at the K10+260 cross-section. The fill height at the center of the subgrade is 26.4m, resulting in a settlement rate of 0.14% ； At the K10+300 section, the dimensions are 41, 41, 44 mm; the filling height at the center of the subgrade is 33.12 m, with a settlement rate of 0.12%. The gradient of differential settlement is less than 0.1% in all cases. This indicates that the post-construction settlement rate of the embankment is reduced, it is densely compacted by rolling, and exhibits good uniformity. A comparative observation was conducted on vibration rolling and ramming for 36m and 34m limestone-filled embankments on the Guangxi Liushui line; during construction, the settlement amount was 76 cm for vibration rolling and 22 cm for ramming, indicating that ramming enhances density. On subgrades that have met the compaction requirements after rolling on domestic highways, the average settlement after 20 additional compaction passes using an impact roller was: 5.4 cm for the Badaling section in Beijing ； Hebei Xuan-Da line: 3.9 cm ； Fujian Fuquan line 5.0~7.0cm ； Hunan Xianglei Line 3.0~7.2cm ； Chongqing-Yuqian line: 4.8~7.3 cm ； Zhejiang Hangjinqu line 2.0~3.4cm ； Jiangxi Liwen Line: 5.0~6.0 cm, etc. A comprehensive analysis shows that after 20 cycles of impact compaction on different soil and rock subgrade surfaces, the settlement amount achieved by the original subgrades, at which the required degree of compaction is reached, is 5.0~7.0 cm. When the settlement is 5.0 cm or less, it indicates excellent compaction quality of the original subgrade; for example, the original compaction degree of the loess subgrade in Xuanhua-Datong, Hebei, was 96%, and the gradation of the slag and gravel subgrades in Hangzhou-Jinhua-Quzhou, Zhejiang, was good, with an original compaction degree of over 97%. Therefore, the settlement after rolling is 4 cm or less. If the settlement exceeds 7.0 cm, it indicates that the original subgrade was not compacted sufficiently, and its compaction degree did not meet the requirements; or when impact compaction is applied in zone 90, the settlement is still greater than 7.0 cm. For embankments under 5m in height, the settlement amount achieved after impact compaction exceeds the post-construction settlement that would occur in normal subgrades, thereby ensuring the stability of the subgrade; this technical advantage is particularly evident in subgrades located on sloped areas. 2. Improving the overall strength and uniformity of the subgrade: Using impact rollers to carry out layered impact compaction on high embankments and supplementary vibration compaction on subgrades that have met the required standards can effectively enhance the overall strength and uniformity of the subgrade. This helps to prevent premature damage to the pavement and extends its service life. The subgrade of the road, which consists of weathered granite containing boulders, fine-grained soil, and gravel in Badaling, Beijing, underwent 20 cycles of compaction. Through testing using a Falling Weight Deflectometer (FWD) at a depth of 1.5 meters below the surface, it was found that the average elastic modulus increased from 180 MPa before compaction to 228 MPa. At a construction site in Quanzhou, Fujian, after 20 passes of impact rolling on the subgrade, the deflection value was measured using a Huanghe-standard vehicle; the average value before additional rolling was 1. =220 (0.01mm), average l after pressure supplementation. =183 (0.01mm), i.e., E before impact rolling. =55.7 MPa, E after pressure replenishment. =63.4MPa。 In the Hunan impact rolling test section, the deflection values before and after 20 rounds of rolling, as measured using a Jiefang vehicle, were 141 (0.01 mm) and 66 (0.01 mm) respectively; when converted to the deflection values using a Yellow River standard vehicle, these figures became 219 (0.01 mm) and 102 (0.01 mm) respectively, which represents the average E value. It increased from 55.9 MPa before rolling to 95.1 MPa. The subgrade of an entire highway in Fujian was re-compacted using 25KJ triangular impact rollers. On the eve of its opening to traffic, the Highway Engineering Testing Center of the Ministry of Transport conducted tests using automatic deflection meters; a total of 596 kilometers of road in 4 lanes were tested, yielding 74,500 data points. K0+000~K66+987 represents an average deflection of 5.80 (0.01 mm), K66+987~K113+028 represents an average deflection of 6.66 (0.01 mm), and K113+028~K154+419 represents a deflection value of 8.53 (0.01 mm). Testing was carried out using a full-vehicle smoothness tester; the testing distance covered a total of 600 kilometers across 4 lanes, resulting in 6,000 data points. The overall IRI average value was 1.21 (σ=0.73). Using the same equipment, 10 kilometers of a connecting road with the same pavement structure and construction conditions were tested, without impact compaction being applied to the subgrade. Test result: The average deflection is 16.27 (0.01 mm) ； The average IRI value for flatness is 1.55 (σ=0.93). Comparisons show that the technique of impact rolling for the roadbed yields significant benefits, improving the overall strength and uniformity of the subgrade, as well as the service level and service life of the road surface. The settlement amount of the subgrade after impact rolling can be used to calculate the increase in the subgrade density. Calculate using the following formula: S = h(1 – Kh/Kh’), where S is the settlement amount, in cm ； h is the thickness affected by subgrade compaction, in cm ； Kh is the compaction degree of the original subgrade, % ； Kh’ is the increased compaction degree after rolling, in %. After 20 passes of rolling, under normal conditions, the compaction degree within a layer thickness of 1.5 meters increases by 3 to 5 percentage points. Since the impact roller carries out uniform rolling and compaction over the entire surface of the subgrade, it enables direct inspection of the entire subgrade as well as additional compaction; this results in the formation of a continuous, uniform, and dense reinforcement layer 1.0 to 1.5 meters below the subgrade surface, thereby enhancing the overall strength and stability of the subgrade and pavement. 3. Reinforcement treatment of special soil foundations: Dynamic compaction is commonly used to treat collapsible loess foundations. On the collapsible loess foundation of the subbase layer of the Xuanhua-Datong highway, a 25KJ triangular impact roller was used to compact the surface 40 times. After 40 passes of rolling, the average compaction degree of the soil layer within 110 cm below the surface reached Kh=91%; as a result, the dry density of the original loess, which was ρd=1.35 g/cm3, increased to 1.70 g/cm3. Its collapsibility coefficient dropped from 0.0438 to 0.0022, eliminating its tendency to collapse. The average elastic modulus of the soil foundation within 1 m beneath the surface reaches over 80 MPa. Within 1 meter below the base surface of the road, compaction is carried out through rolling to create a continuous, uniform, and dense reinforced layer; its technical parameters fully meet the quality requirements for the reinforcement of loess foundations. Similar reinforcement effects have been achieved by using impact rolling to treat collapsible loess foundations in Gansu, Ningxia, Shanxi, and other regions. The Yutian section of the Jingqin Expressway features a soft soil stretch approximately 16 kilometers long. At K65+800 and K66+100, tests were conducted using impact rollers to reinforce the soft soil foundation through impact compaction for drainage and consolidation purposes; impact compaction has the effect of accelerating settlement while also strengthening the soft soil foundation. On the soft soil sections, a gravel layer 50 cm thick is placed on the surface; plastic drainage boards are inserted to penetrate the soft soil layer and reach the sand layer, with an average length of 15–16 m and a spacing of 1.5–2 m. Fill the gravel layer with soil to a height of 50 cm, and conduct impact rolling 22 times within three days. Monitoring results: ground settlement of 17.4 mm, continuous settlement of 17.8 mm, stratified settlement of 10.1 mm at a depth of 3 m below the surface, and stratified settlement of 5.0 mm at a depth of 7 m below the surface. When the subgrade fill reaches 2.4 m, a second impact rolling test of 40 passes is carried out within six days. Monitoring results: ground settlement of 20.6 mm, cumulative settlement of 21.4 mm; stratified settlement of 12.0 mm at 3 m below the surface, stratified settlement of 5.0 mm at 7 m below the surface, stratified settlement of 2.5 mm at 12.5 m below the surface, and stratified settlement of 0.5 mm at 17 m below the surface. At a depth of 3 m, the pore water pressure increased from 11.274 kPa to 11.677 kPa after 18 passes of rolling; when rolling continued for 33 passes, the pore water pressure rose from 11.274 kPa to 16.766 kPa. The above monitoring results show that the impact roller imparts impact energy to the ground, subjecting the soil to tensile and compressive forces. As the free water in the soft soil is drained to the surface through plastic drainage boards, the density of the soil increases, thereby accelerating the settlement and consolidation of the soft foundation. If a embankment is constructed on soft soil, using an impact roller for layered compaction can accelerate the rate of consolidation of the soft soil during construction, which is beneficial for its settlement and consolidation. The natural consistency of the foundation soil is in a state of 0.5–1.0 over-wetness, requiring reinforcement; this can be achieved through combined use of impact rolling and a cushion layer made of coarse-grained materials with good water stability. The thickness of this cushion layer is determined based on the consistency (wc): when 1.0 > wc ≥ 0.9, the cushion layer thickness is 20 cm ； 0.9＞ wc≥0.75, cushion thickness 30cm ； 0.75＞ wc≥0.5, with a cushion layer thickness of 50 cm. After rolling with an impact roller 20 to 30 times on the coarse-grained material layer, a certain thickness of the soil surface can be compacted, thereby forming a reinforced foundation or subgrade made of coarse-grained materials with good water stability. 4. Accelerate the renovation of old roads: When road upgrades require the modification of existing roads, it is necessary to improve the quality of the roadbed to meet the compaction standards required for the new road grade. This is typically achieved by excavating the pavement, subgrade, and embankments, then backfilling them in layers and compacting them to achieve the specified degree of compaction. Asphalt or cement pavements need to be crushed, removed, and cleared as well. By using impact rolling technology, there is no need to excavate the road surface or subgrade; the impact roller can be used directly on the existing road surface to carry out rolling operations, ensuring that the subgrade meets the required quality standards while allowing the old road surface to be reused. Using this new process can save road construction materials, contribute to environmental protection, ensure project quality, and accelerate the progress of highway renovation. The renovation of the Yian-Xing highway in Ningxia involves upgrading the original gravel road from a low-grade road to a second-class road. 25KJ triangular double-wheel impact rollers are used to compact the old road surface; once the test section is completed, guidance is provided for the construction of the entire route. After 50 passes of ramming in the low liquid limit silt section: the average compaction degree increased from 84.8% to 97.1% at depths of 0–30 cm ； It increased from 85.0% to 95.3% at 30–80 cm ； It increased from 82.5% to 94.1% at 80~150 cm. The average settlement is 24.5 cm. The technical parameters of the subgrade meet the standard requirements; for the subgrade made of low liquid limit silt in the constructed section, it was constructed by ramming 50 times. After 30 passes of rolling on the siltey sand section: the average compaction degree in the 0–30 cm layer increased from 91.4% to 97.6% ； It increased from 90.7% to 95.6% at 30–80 cm ； It increased from 93.5% to 96.9% at 80–150 cm. At 180 cm, it increased from 91.8% to 93.9%. The average settlement is 11 cm. The technical parameters of the subgrade meet the standard requirements; for the siltey sand subgrade in the constructed section, it was constructed by ramming 30 times. Upon comparison, the cost per kilometer for impact rolling is reduced from 150,000 yuan to 70,000 yuan compared with conventional construction methods, representing a cost reduction of 46.7%. The original 5 cm asphalt concrete surface layer, 15 cm manually placed rubble layer, and 10 cm gravel sublayer on the Baodong line in Inner Mongolia were renovated; a 25 KJ triangular two-wheel impact roller was used to directly impact and compact the surface layer in order to strengthen the roadbed. After 50 passes of impact rolling: the average compaction degree in the 0–80 cm depth range increased from 86.8% to 96.4% ； It increased from 87.3% to 93.3% at 80~150 cm ； At 150~200 cm, it increased from 86.3% to 90.3%. The compaction degree of the subgrade meets all the specification requirements. The cement concrete pavement on Highway 205 in Tianchang, Anhui Province has been damaged and requires reconstruction. A 15KJ pentagonal twin-wheel impact roller is used to crush cement pavements and reinforce the subgrade. For impact-rolled pavements with fewer than 20 passes, the pavement surface exhibits network-like damage and experiences a certain amount of settlement. The subgrade has been reinforced, and the semi-rigid base together with the original pavement structure still functions as before. In cases where certain parts of the subgrade are weak, replacement and reinforcement measures are required. After testing and implementing the technical treatment measures, the surface layer of the highway is laid. The construction of the upper and lower lanes has been completed in sequence. In addition, impact rolling tests and construction for the renovation of cement concrete pavements using the same pentagonal impact roller were carried out in Guangdong, Henan, Zhejiang and other regions, achieving good technical results. Road upgrades and renovations require widening and constructing new roadbeds; especially when secondary roads are upgraded to highways, the use of impact rolling technology can effectively address the deformation and cracking issues caused by the connection between the old and new roads. Within the foundation area 1.0 m outside the widened subgrade and the foot of the slope, the foundation shall be reinforced by impact rolling. For the aforementioned special soil foundations, technical measures are employed in combination with impact rolling for reinforcement. After the newly widened subgrade was compacted in layers to form the roadbed, impact rolling was used for supplementary compaction at the junction between the old and new sections as well as in the new roadbed. Depending on the actual condition of the completed subgrade, geogrids were added to the subgrade at the junction if necessary. By handling it in this way, settlement and deformation cracks caused by the combination of the old and new roads can be effectively avoided. 3 Precautions for using impact rolling technology 1. Select the appropriate model appropriately. Currently, there are 20 models produced by 12 manufacturers in China for impact rollers; with such a wide variety of options, improper use makes it difficult to achieve the desired results. For the test compaction of embankments and subgrades, as well as the layered compaction of rock-fill and soil-rock mixed embankments, national engineering practices have shown that a 25KJ triangular double-wheel impact roller is suitable for use. For the renovation of cement pavements and the layered compaction of soil embankments, a 25KJ pentagonal twin-wheel impact roller is recommended. 2. Use the impact rolling construction technique correctly. For two-wheel impact rollers, one pass should be considered as two passes, with a compaction width of 4m as the calculation unit, and the construction process described above should be followed. A single-wheel impact roller uses the width of one pass of its wheel as the unit for compaction calculation. 3. Correctly understand that impact rolling has a wide range of water content. Due to the high-energy compaction capability of impact rollers, which is equivalent to the compaction energy required by ultra-heavy compaction standards, the moisture content at which heavy compaction is achieved only increases within the range below the optimal moisture content; it does not increase in the range above the optimal moisture content. Therefore, depending on the plastic index of the soil, the moisture content should be controlled so that the consistency is not less than 1.1~1.2. Otherwise, compacting a soil layer 80–100 cm thick will result in springy soil that cannot be compacted. 4. Control the safety distance of structural elements. There should be a 1m safety distance between the wheel edges of the impact roller and the structures. The fill thickness on bridge and culvert structures shall be no less than 2.5 m. Currently, impact rollers in China are mainly used in areas where significant engineering benefits can be achieved, such as the layered ramming compaction of earth and rock embankments on highways, corrective compaction of embankments and subgrades, and foundation reinforcement. The Standardization Committee for Construction Machinery has begun to consider formulating technical specifications for the impact roller product series. The Ministry of Transport has issued a mandate to develop technical specifications for the use of road impact rolling, and work is currently in progress on this. In the future, further efforts will be made to promote the development of impact rolling technology. Given the technical characteristics of impact rollers, a systematic study is still needed on the compaction mechanism of different soil and rock materials under impact compaction ； The impact rolling technology can also be utilized to study the theoretical principles and methods for the overall design of subgrades and pavements under different natural conditions ； Currently, due to the fact that impact rollers can only move forward, it is necessary to develop corresponding stamping equipment. Furthermore, impact rolling technology also has broad application prospects in agriculture, water conservancy, environmental engineering, and other fields. References: Development Trends of Impact Rollers, He Jie et al., Construction Machinery, 2002(02), 2002, No. 2; Application of Impact Compaction Technology in Subgrade Engineering, Yang Shiji, Highways, 1999, No. 7