# Intelligent Compaction Measurement Value in Variability Control of Subgrade Compaction Quality
## 1 Introduction  
Continuous compaction control (CCC) technology has a promising application prospect into quality control practices of subgrade digital construction projects in Europe and the United States [1]. Successful quality control of subgrade soil guarantees the long-term performance of pavements. However, infiltration and non-uniform settlement easily occur in a subgrade with insufficient strength, which is an important cause of early pavement failure, such as cracking and subsidence [2]. Thus, CCC can achieve the sufficient strength and stability of the subgrade and then extend the service life of pavements. There are several drawbacks in the manual measurements of the existing compaction assessment (ring knife method, sand filling method, settlement control method) can be summarized into two aspects: (1) the testing results acquired from limited points cannot find weak areas of local compaction, and (2) the above measurements are time-consuming, and it is impossible to obtain the variation law of real-time compaction information [3]. Due to many defects of manual measurements, a new continuous compaction control technology named intelligent compaction (IC) has been gradually developed to solve the problems [4]. In the 1970s, [PERSON] and [PERSON] put forward the concept of continuous compaction control and then analyzed the vibration characteristics of a vibration roller so as to achieve comprehensive detection of subgrade compaction quality [5; 6; 7; 8]. After 40 years of rapid development of continuous compaction control technology, many compaction control indicators were developed based on different principles [9; 10; 11]. Because the influence depth of the vibratory roller is high, the index value of compaction control actually reflectsthe overall compaction quality of the subgrade within the range of the influence depth. However, the strength of the underlying layer is very high in special conditions such as the excavation area at the junction of filling and excavation. Meanwhile, the control indexes of continuous compaction become larger because of the strength influence of the underlying layer, and it is difficult to reflect the actual compaction quality of the current filling layer [12]. Actually, the vibration modulus \(E_{vib}\) is a mechanical index derived from the principles of dynamics and material mechanics. Theoretically, it is unreasonable to establish the correlation between \(E_{vib}\) and compactness of the physical index with the influence of the high-strength underlying layer [13; 14]. In addition, the influence of the underlying layer's strength and filling height on the change of control indexes is not clear. However, the regression correlations between ICMVs are complicated as the correlations are not only affected by roller parameters but also by subgrade parameters [15; 16; 17]. In terms of soil properties, it mainly involves the moisture content and underlying stiffness [18]. In addition, investigating the influence of above factors on the correlation between ICMVs will contribute to the further development of IC technology. In fact, ICMVs can reduce laborious efforts and enable real-time control of subgrade quality. In addition, the dynamic solution of steel wheels and subgrade models has become simple because of the development of computer simulation technology, and related software can independently determine whether the steel wheel has bouncing behavior [19]. In terms of model refinement, considering the overall performance of the mechanical cab, wheel frames, and other components, and the impact of results is analyzed instead of a single rigid cylindrical roller for calculation [20; 21; 22]. Therefore, it takes into account the harmonic phenomenon was caused by an increase in stiffness in the calculation of ICMVs [23; 24; 25]. High-order harmonics can interfere with the detection results, and the actual improvement effect of detection accuracy may not be very ideal [26; 27; 28].