In earthwork subgrade construction, the operational quality of rollers directly determines the bearing capacity, stability, and durability of the subgrade. Three core dimensions—matching of construction parameters, adaptation to subgrade conditions, and implementation of operational specifications—require focused attention. The specific considerations can be broken down into the following 6 key areas:
The initial state of the subgrade (moisture content, flatness, and filler properties) is the prerequisite for compaction. If the foundation conditions do not meet standards, subsequent compaction operations will be directly ineffective and may even cause subgrade defects:
Excessively high moisture content: Compaction is prone to causing "spring soil" (the subgrade remains soft and elastic after repeated rolling and cannot be shaped), and it will lead to subsequent subgrade settlement.
Excessively low moisture content: The friction between filler particles is high, making it difficult to compact, and the compaction degree cannot meet the standard.
The moisture content of the filler should be close to its optimal moisture content (determined through indoor compaction tests; usually 10%-15% for sandy soil and 18%-22% for cohesive soil):
Treatment methods: If the moisture content is too high, air-drying or mixing with ash (lime/cement) is required to reduce humidity; if it is too low, uniform sprinkling of water is needed to increase moisture. After sprinkling, the filler should be left to stand for a period to ensure uniform water penetration.
The use of fillers with particle sizes exceeding specification requirements is strictly prohibited (e.g., the maximum particle size of earthwork subgrade fillers is usually ≤100mm, and that of the roadbed surface layer is ≤150mm). Oversized stones will cause local voids in the compacted layer, forming "weak points".
Avoid random mixing of fillers (e.g., arbitrary mixing of sandy soil and cohesive soil). Otherwise, due to differences in compaction characteristics of different soil types, the compaction degree will be uneven, and cracks or settlement are likely to occur later.
The thickness of each filling layer must strictly follow the design requirements (usually, the loose laying thickness of each layer of earthwork subgrade is ≤30cm; specific adjustments should be made based on the roller tonnage and filler type). Excessive thickness will result in "the surface layer meeting compaction standards while the deep layer being uncompacted", while insufficient thickness will reduce construction efficiency.
The surface of the filling layer should be flat to avoid local potholes or protrusions. Otherwise, the steel wheel of the roller will be under uneven force, easily leading to compaction blind spots.
The tonnage, vibration force, amplitude, and frequency of the roller must match the subgrade filler (such as sandy soil, cohesive soil, and soil-rock mixture). Otherwise, "insufficient compaction" or "over-compaction" will occur:
Light rollers (6-10t): Suitable for areas with high compaction requirements but where structural damage must be avoided, such as the roadbed surface layer and the back of bridge abutments.
Medium rollers (12-18t): Suitable for compaction of the middle layer of ordinary earthwork subgrades.
Heavy rollers (over 20t): Suitable for soil-rock mixed subgrades or deep compaction. Care should be taken to avoid "shear damage" to the soft subgrade in the lower layer.
Cohesive soil fillers: Low frequency and large amplitude (frequency: 25-30Hz, amplitude: 1.5-2.5mm) should be selected. The "shear effect" of large amplitude is used to break the bonding force between soil particles and achieve compaction.
Sandy soil fillers: High frequency and small amplitude (frequency: 30-40Hz, amplitude: 0.8-1.5mm) should be selected. High-frequency vibration is used to make sandy soil particles quickly fill gaps and avoid particle "liquefaction" (excessive vibration causes sand instability).
Roller operation must follow the principles of "orderly, uniform speed, and no omissions", with focus on avoiding the following issues:
First compact the edges (30-50cm away from the subgrade slope) to prevent landslides caused by insufficient compaction of the edges later.
Use static pressure (without turning on vibration) for initial compaction to stabilize the filler initially and avoid filler displacement caused by vibration; turn on vibration for re-compaction to achieve deep compaction; use static pressure again for final compaction to eliminate surface wheel marks.
Follow the sequence of "first edge, then center", "first light compaction, then heavy compaction", and "first slow compaction, then fast compaction":
Rolling speed: Usually controlled at 2-4km/h (approximately walking speed). Excessively high speed will result in insufficient contact time between the steel wheel and the filler, leading to inadequate compaction; excessively low speed will reduce efficiency and may cause local over-compaction.
Number of rolling passes: Determined through "trial rolling" (usually 6-8 passes), based on meeting the compaction degree test standards. Do not increase the number of passes blindly (over-compaction will damage the structure of cohesive soil and instead reduce its strength).
"Dead corner areas" such as the back of bridge abutments and the sides of culverts: Conventional rollers cannot reach these areas, so small vibratory rammers (e.g., hand-held vibratory rammers) should be used for layered compaction to avoid the hidden danger of "vehicle jumping at the abutment back".
Subgrade joints (e.g., longitudinal joints in segmented construction): The end of the previous section of the subgrade should be excavated into a "step shape" (step height ≤ compaction layer thickness, width ≥1m). The new filling layer and the old layer should be compacted together to avoid longitudinal cracks.
Construction in rainy weather will cause a sharp increase in the moisture content of the subgrade, making compaction impossible. After rain, the moisture content must be re-tested, and construction can only resume when it meets the standards.
Construction at low temperatures (below 5℃): Cohesive soil is prone to freezing, increasing the friction between particles, making it difficult to meet the compaction degree. Measures such as covering for heat preservation or adding antifreeze (e.g., adding 3%-5% lime) should be adopted.
When working near structures such as bridges, culverts, and pipelines, turn off vibration (use static pressure) or use light rollers to avoid structural cracking or displacement caused by vibration.
Rolling distance: A distance of 50-100cm should be maintained from the edge of the structure, and this area should be re-compacted with small equipment.
Dynamic monitoring is required during the compaction process to avoid "post-construction rework":
After every 2-3 rolling passes, use the "ring knife method" or "sand filling method" (commonly used for earthwork subgrades) to sample and test the compaction degree. If it does not meet the standards, adjust the number of rolling passes or vibration parameters.
Observe the surface state of the subgrade: If cracks, bulges, or spring soil appear, stop rolling immediately, analyze the causes (such as excessive moisture content or uneven filler), and resume construction only after treatment.
After the compaction of each layer is completed, it must be inspected and accepted by the supervision unit (with compaction degree, flatness, and elevation meeting standards) before the next layer of filling can be carried out. "No testing for layers and blind superposition" is strictly prohibited; otherwise, deep-seated hidden dangers will be difficult to detect.
The performance of the roller itself directly affects the compaction effect, so inspections must be done well before construction:
Vibration system: Check if the eccentric blocks and bearings are in good condition to avoid insufficient compaction due to weak vibration.
Steel wheel: Clean the soil adhering to the surface of the steel wheel (especially during cohesive soil construction) to prevent wheel marks on the subgrade surface caused by an uneven steel wheel.
Water sprinkling system: If the subgrade filler is easy to adhere to the steel wheel (e.g., cohesive soil), ensure the water sprinkling system is in normal operation, and use a small amount of water to reduce adhesion.
In summary, the compaction of earthwork subgrades should focus on "qualified fillers, matched parameters, standardized operations, and real-time monitoring". Ultimately, the compaction degree of the subgrade must meet the design requirements (usually, the compaction degree of the roadbed top surface is ≥96%, and that of the subgrade middle layer is ≥94%; specific implementation shall be in accordance with the Technical Specifications for Highway Subgrade Construction JTG/T 3610-2020) to provide a stable foundation for subsequent pavement construction.
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