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Application of impact compaction technology in subgrade engineering Author: Yang Shiji, Highway Research Institute of the Ministry of Transport Abstract: By analyzing the mechanism of impact compaction and drawing on engineering examples, it is shown that this technology can reduce post-construction settlement of subgrades, enhance the overall strength of subgrades, and strengthen weak foundations. Keywords: subgrade engineering, impact rolling, impact compactor. 1. Characteristics of impact compaction technology: Engineering practice with vibratory rollers shows that the rolling speed is one of the key factors determining the productivity per unit area of the roller (m3/h); compaction depth and layer thickness are also important parameters affecting the quality of compaction and its productivity. Generally, the optimal rolling speed for a vibratory roller is 3–6 km/h, and the optimal thickness of the compacted layer is 0.3–0.5 m. To improve compaction efficiency and productivity, enhance the density of soil and rock masses, and reduce the compaction settlement deformation caused by their own weight, it is necessary to improve the compaction process, update rolling techniques, change the method of rolling, and increase the rolling speed as well as the thickness of the compacted layer. Impact compaction technology replaces the high frequency and low amplitude of conventional vibration compaction with high amplitude and low frequency, thereby significantly enhancing the compaction efficiency of soil and rock materials during the compaction process. For example, the impact capability of a 25KJ triangular impact compactor is 10 times greater than that of a vibratory compactor; the depth of compaction achieved is 5 meters. The effective compaction thickness increases from 0.20–0.30 meters in the case of vibratory compaction to 1.00–1.50 meters in the case of the impact compactor. Moreover, the rolling speed of the impact compactor is twice as high as that of the vibratory compactor. This has been confirmed through experimental project practices in different regions of the country with various earth and rock fill subgrades. An impact compactor uses triangular or pentagonal \"wheels\" to generate concentrated impact energy in order to compact soil and rock fill materials. The impact compactor can be pulled by a compatible heavy industrial trailer at the front, or it can operate on its own. Figure 1 shows the basic principle of an impact compactor. Impact compactors are rated based on their static energy, which is calculated in kilojoules as follows: E=mgh. Here, E represents the energy in kilojoules, kJ ； m is the mass of the power component, in kg ； g is the gravitational constant (9.81 m/s2) ； h is the difference between the outer radius and the inner radius of the wheel, h = R – r, see Figure 1. The commonly used compactors at present are the 25KJ-T3 triangular type and the 15KJ-T5 pentagonal type. The 25KJ compactor is used for in-situ rolling, rolling of fill materials with a layer thickness of less than 1m, and the inspection of rolling quality. The 15KJ compactor is used for compacting fill with a layer thickness of 50–75 cm; thanks to its pentagonal wheels, it can achieve the desired degree of compaction with fewer passes compared to the 25KJ compactor. In earthwork compaction tasks, the impact compactor overcomes traditional rolling methods; when one of its corners is placed on the ground and it rolls forward, it generates a massive shock wave. As it rolls along the surface in a sequential manner, this allows the soil to be compacted evenly and densely. This machine carries out compaction at a speed of 9–12 km/h; in other words, the impact roller strikes the ground twice per second, which results in low-frequency, high-amplitude impacts on the soil. By striking the ground periodically, it generates strong shock waves that propagate deep into the ground, exhibiting characteristics similar to those of seismic waves. The depth of compaction can increase as the number of passes increases. The high energy of the 25KJ compactor enables deep compaction of the fill material, thereby reducing the permeability of the soil and providing a solid foundation for layered rolling or filling materials. On roads with low traffic volume, deep compaction of the in-situ materials at the construction site can achieve high strength and stability, without the need for material replacement; in most cases, constructing a sub-base and base layer directly yields a high-quality road. Testing compaction is an important part of impact compaction work. The high energy of the 25KJ compactor generates an impact force equivalent to over 250 tons, making it an extremely effective method for testing compaction. With just 10–15 passes using an impact compactor, all weak areas or those with excessive moisture content can be easily identified, and a few more passes will suffice to address them. High-energy impact compactors have been used in impact compaction tests and impact rolling for relevant projects in provinces and cities such as Yunnan, Beijing, Hebei, Fujian, and Hunan. It is preliminarily believed that they are more effective than conventional rolling in reducing post-construction settlement of roadbeds, enhancing the overall strength of roadbeds, and strengthening weak foundations, which holds practical significance for improving the quality of highway construction at present. 2. Impact compaction reduces post-construction settlement of the subgrade. The subgrade fill material for the Badaling Expressway consists of fine-grained soil and gravel formed from weathered granite, and the subgrade was compacted in layers (20 cm per layer) using VV170 40-ton vibratory rollers. The compaction standard requires a density of 93%; the density of the lower embankment should be 90%. 20 passes of impact compaction were carried out, with an average settlement of S=5.4 cm. The effective compaction depth was calculated to be 1.5 m, resulting in an average density increase to 95%. With a subgrade height of 4.5 m, the settlement rate after impact rolling is 5.4/450 = 1.2%. For the high-fill subgrade with a height of 34 m, which was constructed using impact rolling in layers (with a compaction thickness of 1.0 m per layer), the settlement amount after 10 passes of rolling per layer was 5.5–8.5 cm, while it was 2.4–3.0 cm after 11–20 passes; this value is roughly equivalent to the 2.2 cm settlement observed after 11–20 passes in the subgrade that was compacted using conventional methods. After 20 cycles of impact rolling compaction on roadbeds with different soil types in Fujian and Hunan, when the originally vibrated and compacted roadbeds reached the specified compaction standards for the subgrade, their settlement was 5–7 cm; however, a considerable portion of them had a settlement greater than 7 cm, ranging from 8–12 cm, and in such cases the actual compaction level did not meet the standards. In addition, for the collapsible loess on the Xuan-Da Expressway, after it reached the compaction standard through vibration ramming in layers of 20 cm each, it was further rammed 20 times using an impact compactor, resulting in a settlement of 3.9 cm ； After the stone-filled embankment meets the standards after being vibrated and compacted with 50t force, it is then impacted and rolled 20 times, resulting in a settlement of 4–5 cm ； When a high-fill subgrade is compacted in layers using an impact compactor, with 20 passes per layer, the settlement rate can reach 4%–5%, which effectively addresses the issue of differential settlement after construction in high embankments. 3. Impact compaction enhances the overall strength of the subgrade. The gravel embankments in Lincang, Yunnan were constructed using impact compaction; the dry density was determined by the sand filling method. The average dry density within 80 cm below the surface of the subgrade layer was ρd = 2.136 g/cm3, with an average degree of compaction of kh = 100.5% ； For 80–150 cm, the average dry density ρd = 2.051 g/cm3, and the average compaction degree kh = 96.5%. The compaction degree is 3.5%–5.5% higher than the specified standard, and the settlement amount within 0–150 cm is 6.92 cm. The Badaling granite weathered subgrade, which consists of gravelly soil with boulders, showed an increase in its average elastic modulus after 20 cycles of impact compaction; as measured using a Falling Weight Deflectometer (FWD) at a depth of 1.5 meters below the surface, this modulus increased from 180 MPa before compaction to 228 MPa. At a construction site in Quanzhou, Fujian, after 20 passes of compaction using an impact roller, the deflection was measured using a vehicle compliant with Yellow River standards. The value of ι0 before compaction was 220 (0.01 mm), while it was 183 (0.01 mm) after compaction. Using the formula E0 = 2430ι0 – 0.7, the value of E0 was 55.7 MPa before compaction and 63.4 MPa after compaction. In the impact compaction test section in Hunan, the deflection values before and after 20 cycles of impact compaction using a Jiefang vehicle were 141 (0.01 mm) and 66 (0.01 mm), respectively; when converted to the values corresponding to a Yellow River standard vehicle, these were 218.8 (0.01 mm) and 102.4 (0.01 mm), respectively. Using the formula E0 = 2430×ι0 – 0.7, the average compressive stress increased from 55.9 MPa before compaction to 95.1 MPa after it. After 20 cycles of impact compaction, not only does the strength of different soil and rock subgrades increase, but the subgrades that had already reached the required compaction standard through vibration compaction also settle by 5–7 cm. If the settlement exceeds 7 cm, it indicates that the original degree of compaction was insufficient ； After 20 passes of rolling simultaneously, the compaction degree increased by 3%–5% within a layer thickness of 1.5 m. Since the impact roller carries out uniform compaction over the entire surface of the subgrade, it enables direct inspection of the entire subgrade as well as additional compaction; a continuous, uniform, and dense reinforced layer is formed at a depth of 1.5 meters below the subgrade surface, thereby improving the overall strength and stability of the subgrade and pavement. 4. Impact compaction for strengthening weak foundations: The dynamic compaction method is commonly used to treat collapsible loess foundations. On the collapsible loess foundation of the sub-base layer of the Xuanhua-Datong highway, a 25KJ—T3 impact compactor was used to compact the surface 40 times. It also measures indicators such as foundation settlement, dry density, collapsibility coefficient, and elastic modulus. After 40 passes of rolling, the average compaction degree of the soil foundation within 110 cm below the surface reached kh=91%; as a result, the dry density ρd of the original loess increased from 1.35 g/cm3 to 1.70 g/cm3, and its settlement coefficient dropped from 0.0438 to 0.0022, thereby eliminating its tendency to settle. 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. The Yutian section of the Jingqin Expressway features a soft soil stretch approximately 16 km long. From April to August 1997, tests were conducted using impact compactors at K65+800 and K66+100 to reinforce the soft soil foundation through impact compaction and drainage consolidation; 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 through the soft soil layer into 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 22 passes of impact rolling from April 30 to May 2. 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 reached 2.4 m, a second test of 40 passes of impact rolling was carried out from August 18 to 24. Monitoring results: ground settlement of 20.6 mm, cumulative settlement of 21.4 mm; stratified settlement of 12.0 mm at a depth of 3 m below the surface, 5.0 mm at a depth of 7 m below the surface, 2.5 mm at a depth of 12.5 m below the surface, and 0.5 mm at a depth of 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. After rain stopped and rolling was halted for two days, the pore water pressure dropped from 11.677 kPa to 11.274 kPa. When rolling continued for another 33 passes, the pore water pressure rose from 11.274 kPa to 16.766 kPa, while the pore water pressure at depths below 7 m remained unchanged. The above monitoring results show that the impact compactor 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 compactor with a layered compaction method can accelerate the rate of consolidation of the soft soil during construction, which is beneficial for its settlement and consolidation. 5. Improvement of the construction technique for stone-filled embankments: The properties of the materials used for filling vary greatly; to minimize post-construction settlement of the embankment, it is necessary to achieve optimal compaction. Generally, the better the grading of the stone material, the higher the density and elastic modulus that can be achieved through compaction. The settlement of the fill material is determined by the material properties (type of stone, gradation, maximum stone size, and shape) and the compaction methods (type of roller, paving thickness, and number of passes). The properties of the stone material and the compaction process are interrelated and must be considered together; this unity is particularly important in areas where the fill layer is thick or where there are sharp changes in the fill thickness. The gravel embankment is compacted in layers using vibratory compactors of 50 tons or more, with the number of compaction passes determined when the amount of settlement resulting from compaction reaches zero and remains stable. Due to the varying content of gravel, the maximum dry density obtained from compaction tests also varies; the maximum dry density of gravel fill must be determined using the content of gravel particles larger than 5 mm and the corresponding maximum dry density curve. The sand filling method and surface wave compaction density meter are also used to determine whether the compaction degree reaches the specified value. In the case of block stone embankments, due to the large size of the block stones, after layered compaction using vibratory compactors with a capacity of over 50 tons until the settlement value reaches zero, each layer is 1.5–2.0 meters thick; thereafter, a 25KJ–T3 impact compactor is used to carry out 20 additional compaction passes as a verification step. If the settlement amount is between 5–7 cm, it indicates that the initial compaction was successful. The block stone embankment is constructed directly using a 25KJ—T3 type impact compactor, which offers high efficiency and rapid speed. Its triangular dual-wheel design enables it to generate a shock load of 250 tons at a speed of 12 km/h, thereby compacting the stones in the deeper layers of the embankment continuously. This high-amplitude, low-frequency impact energy, after uniform compaction of the embankment layers, enables the stones to interlock tightly with one another. As a result, the settlement deformation caused by impact loads on the stone-filled structure is much greater than the deformation caused by the weight of the structure itself and external loads after the embankment is completed, thereby preventing differential deformation that could lead to cracks in the road surface. Based on the characteristics of rock filling works and construction, impact compaction equipment yields the best compaction results. The construction requirements for impact compaction are as follows. (1) Stone filling construction control. The particle size and gradation of the fill stones are controlled at the quarry; the construction party uses methods such as loose blasting, smooth blasting, or small-scale blasting depending on the conditions on site. The fill stones are required to meet the following specifications: maximum particle size < 500 mm, coefficient of irregularity Cu > 5 (Cu = d60/d10), and curvature coefficient Cc = 1–3. (2) Control of the fill stone layer thickness. When the stone-filled embankment is located on a horizontal terrain, the thickness of the compacted layer is 100 cm ； In slope areas, the thickness of the compacted layer for stone-filled embankments is 80 cm. The loose laying coefficient is generally 1.15–1.20. (3) Control of compaction settlement value. After several passes of impact rolling, the value of compaction settlement stabilizes. By combining this with the measurements taken using a drop-weight deflection tester, the required number of passes for rolling, as well as the corresponding control values for compaction settlement, can be determined. (4) Stone filling construction. The key to stone filling is to achieve the required gradation distribution. This requires the end-point method. This method ensures that the largest stones are at the bottom of each layer, while the finer particles are at the top. It ensures optimal interlocking and transfer of compressive force. At the same time, it provides a surface that will not cause damage to the compaction rollers and rubber tires of the tractor during movement. This method requires dumping the material using trucks at the top of the end of the fill layer, and then using a bulldozer to push the filler from that end into the layer below, so that it reaches the same height as the layer being filled. 6. Compaction standards for stone-filled embankments: It is specified that the use of vibratory rollers or impact compactors with a capacity of over 50 tons is required to achieve higher compaction standards for such embankments; the specific requirements are shown in Table 1. Table 1 Compaction Standards. Depth below the embankment crest/cm: For depths up to 150 cm, the compaction percentage required is 95%; for depths below 150 cm, it is 93%. By raising the compaction standard from 93% to 95% for depths between 80 cm and 150 cm, and from 90% to 93% for depths below 150 cm, post-construction settlement of the embankment can be significantly reduced. Calculate using the following formula: S = h(1 – kh/kh′), where S is the amount of settlement, in cm ； h is the road height, in cm ； kh is the compaction degree of the original standard, % ； kh′ is the improved compaction degree, %. Taking a 10m stone-filled embankment as an example, after raising the compaction standards, the additional settlement S achieved during construction is: S=70×(1-93/95)+850×(1-90/93)=28.9 cm. When impact compaction is used for layered rolling, the degree of compaction can be increased by another 3%–4%; assuming an increase of 3%, the additional settlement of the 10m embankment after impact compaction is 31.2 cm. If impact compaction is applied 20 times only on the surface of the subgrade, the average increase in settlement is 6 cm. By filling it in this way, the post-construction settlement of the stone embankment that has been compacted has **exceeded the original post-construction settlement amount, thereby preventing differential deformation and settlement of the subgrade. For gravel embankments, the maximum dry density as per the compaction standard is determined using heavy compaction tests, based on the curve relating the gravel content of >5 mm to the maximum dry density. For block stone embankments, the maximum dry density of the block stone filler is determined using the surface vibration compactor method specified in the \"Highway Geotechnical Test Procedures\" (JTJ051—93) T0133. Reference 1: Technical Specifications for Highway Subgrade Construction (JTJ 033-95). 2 Specifications for Geotechnical Tests on Roads (JTJ 051-93). 3 Compaction standards and testing methods for road stone fill embankments (research report). Road Research Institute of the Ministry of Transport, 1998.