J. Cent. South Univ. (2020) 27: 2134-2147

DOI: https://doi.org/10.1007/s11771-020-4436-5

Deterioration mechanism and rapid detection of performances of an existing subgrade in southern China

ZHANG Jun-hui(张军辉)¹, DING Le(丁乐)¹, ZHENG Jian-long(郑健龙)¹, GU Fan(顾凡)^{1, 2}

1. National Engineering Laboratory of Highway Maintenance Technology, Changsha University of Science & Technology, Changsha 410004, China;

2. National Center for Asphalt Technology, Auburn University, Auburn 36830, USA

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: To relieve the increasing traffic load, many early built highways need to be widened or reconstructed. The rapid performance detection to existing subgrades is important to their reasonable evaluation and maximized utilization. Based on five kinds of soils taken from an existing highway in southern China, three commonly detecting methods were used to determine their moisture contents, compaction degrees and resilient moduli. The results showed that the measured moisture contents were greater than the design value, and the compaction degrees decreased sharply compared to the original ones. The moisture and heat exchange produced a decrease in the resilient modulus of plate loading test (PLT) from the standard 60 MPa down to 40 MPa. Afterwards, the portable falling weight deflectometer (PFWD) and dynamic cone penetrometer (DCP) were used to evaluate the subgrade performances. The measured PFWD moduli and the DCP penetration rates were correlated with the resilient moduli of PLT, deflections of the Beckman beam test, compaction degrees and moisture contents. The correlation analysis indicates that both of two methods are suitable in rapid detecting subgrade performances, but PFWD method is more recommended for it has higher accuracy and efficiency.

Key words: humid and hot areas; existing subgrade; deterioration mechanism; rapid detection; portable falling weight deflectometer; dynamic cone penetrometer

Cite this article as: ZHANG Jun-hui, DING Le, ZENG Ling, ZHENG Jian-long, GU Fan. Deterioration mechanism and rapid detection of performances of an existing subgrade in southern China [J]. Journal of Central South University, 2020, 27(7): 2134-2147. DOI: https://doi.org/10.1007/s11771-020-4436-5.

1 Introduction

With the rapid development of the economy and society, many highways cannot meet the requirements of the increasing traffic volume in China, traffic jams and accidents take place frequently. Thus, they need to be reconstructed or widened urgently to alleviate this situation. Actually, the annually total length of reconstructed highway has exceeded 7500 km in recently years [1], and most of them were located in southern China, an area with a dense road network distribution. Simultaneously, the climate of this district is hot and humid, which has an obvious impact on subgrades. Several studies have found that the subgrade performances were not stable, for it gradually deteriorated due to the heat and moisture exchange in the soil-atmosphere interaction in most cases [2, 3]. Therefore, the reconstruction or widening of highways in southern China is facing more technical difficulties. In addition, the construction time of reconstruction project is often limited. Therefore, a reasonable and rapid detection of indexes of subgrade performance, including the moisture content, compaction degree, resilient modulus and deflection, is essential for the maximum utilization of the existing subgrades.

Currently, the common methods for detecting the subgrade properties include the core drilling method, cutting ring method, plate loading test (PLT) and Beckman beam test. Core drilling is a half-damaged detecting technique that can test the intensity and internal quality of existing subgrades by special drills. Most of the original state of subgrade soils such as moisture content, dry density, liquid limit and plastic limit can be obtained [4]. Cutting ring method is widely used in determining the density of fine-grained soils in the field and laboratory because of its simpleness and accuracy, described in Chinese standard: Test Methods of Soils for Highway Engineering (JTG E40-2007,China MOT). Plate loading test is an in-situ method for the stress and deformation measurement that can reflect the quality of subgrade fillings, the resilient modulus of plate loading test (PLT) can be calculated from the measured stress and strain by applying loads on a bearing plate with a diameter of 30 cm or 76 cm [5]. As another measurement method of bearing capacity, the Beckman beam is used to detect the deflection of the subgrade top. The length of the beam is 3.6 m or 5.4 m according to Field Test Methods for Subgrade and Pavement for Highway Engineering(JTG E60-2008, China MOT).

In contrast, the dynamic modulus test via the portable falling weight deflectometer (PFWD) and the penetration rate test based on the dynamic cone penetrometer (DCP), are two new rapid detection methods developed in the recent decade [6-8]. PFWD is a portable device used to determine the dynamic modulus of soil structures. As shown in Figure 1, it consists of a loading device that can produce defined load pulses, a loading plate, and at least one geophone sensor to detect the deflection of the center position in the loading plate. In the detection process, the weight hit the surface of the loading plate after free falling, and the induced deflection can be detected by the sensor instantaneously. Several studies have recently been conducted to evaluate the PFWD method in detection of the geotechnical engineering. Some researchers demonstrated that the drop weight and the size of loading plate significantly affected the PFWD modulus [9-11]. The stress–strain responses of subgrades and pavements during the PFWD test were reported in previous studies, and the analyses of strain data suggested that the measurement depth of PFWD was 0.9-1.1 times of the plate diameter [12-14]. Many comparative tests revealed that PFWD was an ideal device for quality control during compaction monitoring [15-18]. Some researchers found that the PFWD modulus was consistent with the static resilient modulus of PLT, compaction degree and some other subgrade property indexes, and they set up regression equations to predict the corresponding properties of different soils [19, 20].

Figure 1 Portable falling weight deflectometer:

DCP was originally developed by Scala in 1956, and it has been gradually improved and promoted in geotechnical testing in recent decades [21]. The typical DCP contains an 8 kg hammer. During the test, the hammer drops over a height of 575 mm, driving a cone tip of 20 mm diameter vertically into the pavement or subgrade, the penetration rod gradually penetrates into soil layers (Figure 2). In theory, the softer the soil layers, the deeper the rod penetrates per hammer, and less time the rod takes to penetrate to a certain depth. Therefore, the DCP test results can evaluate the compaction states of the soils. The correlations between the DCP penetration rate and other properties (e.g., resilient modulus, California bearing ratio (CBR) value and infinite compressive strength) were studied, and some empirical correlative formulas were established [22-24]. Simultaneously, DCP method was also used to determine the geological parameters in the areas of special soils [25]. The subgrade and pavement states can also be evaluated by DCP in the highway construction on site [26].

As mentioned above, each method has its own advantages and disadvantages. For the first four traditional methods, they are time-consuming and operation complicated. For the latter two methods, they are the rapid detection methods of none or less destruction. Nevertheless, their results cannot be directly used for subgrade evaluations and need to be correlated with the basic properties obtained by the traditional methods. Therefore, these six methods can be used for different situations. For common purposes, the four traditional methods are qualified. However, when a rapid determination of subgrades properties is required, the latter two methods will be better.

The aim of this study is to reveal the deterioration mechanism of the performances of an existing subgrade in southern China, and provides a rapid detection guidance for the highways widening. To this end, the above six methods were used to detect the soils properties of the selected subgrade sections. Firstly, a series of in situ and laboratory tests were conducted to obtain the properties of different subgrade soils. The variations in the moisture contents and compaction degrees were analyzed, and then the deterioration mechanism of the existing subgrade was revealed. Subsequently, to compare the merits and shortages of the two rapid detecting methods (i.e., PFWD and DCP), the PFWD modulus and the DCP penetration rate were correlated to the resilient modulus of PLT, deflection obtained from the Beckman beam test, compaction degree and moisture content. All the correlation coefficients were summarized and used to determine which method was superior in the view of accuracy and efficiency.

Figure 2 Dynamic cone penetrometer (DCP):

2 Detection of performances of existing subgrade

2.1 Detection site and procedure

The tested subgrades were selected in Lianzhu Highway, which was built in 1999 and widened in 2015 to promote its traffic capacity. The tests were carried out in three sections: the completely weathered red sandstone section from K1094+000 to K1111+720, the completely weathered granite section from K1114+000 to K1126+413 and the completely weathered sandy slate section from K1127+834 to K1142+690. The whole test was divided into three stages. In the first stage, 30 test points in some typical cross section, i.e., the embankment, cutting, cut and fill section, were selected in each section (Figure 3), and then a series of intact soil samples was collected at a depth of 0.8 m below the subgrade surface using a geological drilling machine (Figures 4(a) and (b)). Afterwards, the laboratory tests were carried out to measure some properties of subgrade soils, e.g., moisture content, dry density, liquid limit and plastic limit. On this basis, the soil categories of the samples were identified.

At the beginning of the second stage, 10 test points were positioned in the diseased locations of each section. Subsequently, test pits with an area about 2.0 m² were excavated at the selected points, and then plate loading tests were performed to measure the resilient modulus, as shown in Figure 4(c).

In the last stage, 90 soil samples were collected from the subgrade slopes corresponded to the same cross sections of selected 90 points in the first stage. Afterwards, compaction tests were performed to identify their maximum dry density and optimum moisture content, see Figure 4(d).

2.2 Test results and discussion

Various properties reflecting the performances of the tested existing subgrade were obtained to study their change law. The results showed that there were mainly five soil categories, i.e., clayey sand, low-liquid-limit sandy clay, low-liquid-limit sandy silt, high-liquid-limit-silt and clayey gravel. The histograms of the optimum moisture content, moisture content and plastic limit of five soil samples were presented in Figures 5(a)-(e).

According the calculation, the average moisture content of all the samples was 22.7%,much higher than the average optimum moisture content of 15.4% and close to the average plastic limit of 25.6%. Most of the moisture contents were close to the plastic limit, and some were even greater than the corresponding plastic limits. Thus, it could be inferred that the tested subgrade was seriously humidified in the long operation period.

Figure 3 Location of test points on subgrade

Figure 4 In situ and laboratory test methods:

For the average moisture content seen from Figure 5(f), the soil with smaller particle size has a higher moisture content under the same condition, presenting a better water-holding capacity. Because the soil particles of smaller radii have larger surface contacts with water molecules [27]. By contrast, the soil consisted of large size particles is easier to form an interlocking structure with a larger porosity, and thus a minor moisture content is obtained in the test.

After twenty years operation, the measured compaction degree of tested subgrade soils decreased significantly shown in Figure 6. Since the design compaction degree was 95%, all the soil samples were unqualified in this index. The change amplitudes in compaction degree of the high-liquid- limit silt was smaller than other four kinds of soils. The average compaction degree of all the soil samples was 83.5%, less than 96% that specified in Specifications for Design of Subgrades (JTG D-2015, China MOT).

The test results of 30 typical samples at the diseased locations were listed in Table 1. Among them, the dry densities were measured by the cutting ring method, and the variations in dry density and moisture content were presented in Figure 7. The samples were ordered from 1 to 30 according to their dry densities. It can be noted that, as the dry density gradually increased, the moisture contents tend to be decreased. Because the measured moisture content was much greater than the optimum moisture, the changing trend of the moisture content and density of the soils was consistent with the compaction curve[28].

The resilient modulus is a representative property that can reflect the performance of the subgrade [29, 30]. The deterioration mechanism of the tested existing subgrade in southern China is further analyzed based on the changes of resilient modulus of PLT, as illustrated in Figure 8. 30 samples were reordered according to their soil types.

Figure 5 Histograms of moisture contents of different soil samples:

The average resilient modulus of these samples approximates to 40 MPa with a minor change, much less than the standard value of 60 MPa in the Specifications for Design of Highway Asphalt Pavement (JTG D50-2017, China MOT). It indicated that the performance of the subgrade deteriorated greatly compared to its original state, and the bearing capacity of the tested subgrade decreased significantly under the influence of long- term vehicle loads and environmental factors.

3 Comparison between two rapid detection methods

There are many rapid detection methods for the subgrade in-situ test. The determination of the most suitable one is a key problem that needs to be solved in the road reconstruction and widening engineering. In this section, additional tests were conducted to address this issue.

Figure 6 Compaction degree of five types of soil:

3.1 Test procedures

In the beginning, three typical homogeneous subgrade sections were selected as illustrated in Figure 9. The soil category within each section was identical. 20 test points were selected in each section with a distribution shown in Figure 9. They were arranged at an equal distance of 5 m along the centerline of the carriageway or the overtaking lane. Afterwards, some in situ tests were carried out to obtain the properties of the subgrade including the PFWD modulus, resilient modulus of PLT, the Beckman beam deflection, moisture content, compaction degree and DCP penetration rate (Figure 10).

3.2 Results and discussion

The in-situ test results of six indexes of subgrade performances can be obtained. Table 2 lists the detailed values of section 1. There were some points that differed significantly (K1105+020, K1105+025, K1105+050, K1105+060, K1105+065, K1105+070, K1105+095, K1105+100), which is consistent with the deterioration mechanism previously mentioned: as the moisture content increased, the compaction degrees decreased, the subgrade structure became loose, resulting the measured deflection of Beckman beam test and penetration rates of DCP increased. The measured PFWD modulus and resilient modulus of PLT are also found to be smaller than those of the adjacent points. After sorting out the test data of the three test sections, the results of all the points were labelled in Figures 11 and 12 to analyze the correlations between the indexes of the traditional and rapid detection methods.

Table 1 Test results of subgrade at 30 typical diseased locations

Figure 7 Moisture content and dry density of 30 typical soil samples

Figure 8 Resilient modulus of PLT of 30 samples

There are several equations used in the previous study to fit the test data. It was revealed that a relatively higher correlation can be obtained between these properties when the power- exponential function was adopted [31-33]. In this study, the exponential equation was used to correlate the PFWD modulus (E_p) and the DCP penetration rate (PR) with the resilient modulus (E_b) of PLT, the deflection (L) of the Beckman beam test, the moisture content (w) and the compaction degree (K). As the w and K were the two basic physical indexes, E_p or PR can be correlated with w and K at the same time in the following equations.

Figure 9 Distribution of test points on three subgrade sections

Figure 10 Field tests:

For PFWD modulus:

 (1)

For DCP penetration rate:

 (2)

When the measured E_b, L, K, ω were selected to correlated with E_p and PR, respectively, there were eight correlation cases for each soil (E_b-E_p, L-E_p, K-E_p, ω-E_p, E_b-PR, L-PR, K-PR, ω-PR) and a regression curve that takes into account all the soils was added for each case. The following figures summarize fitting results of all the indexes. As the data fitting quality and the correlation between the indexes can be reflected by determination coefficient R². The superior detection method can be determined according to the size of R². In the Eqs. (1) and (2), the R² associated with PFWD modulus were greater than those associated with DCP penetration rate.

Table 3 summarizes all the determination coefficients in Figures 11 and 12. When considering the type of soils, the determination coefficients R² varies from 0.855 to 0.968 (E_b-E_p), 0.950 to 0.957 (L-E_p), 0.789 to 0.849 (ω-E_p)_, 0.792 to 0.932 (K-E_p), and changes from 0.713 to 0.893 (E_b-PR), 0.631 to 0.834 (L-PR), 0.755 to 0.884 (ω-PR), 0.765 to 0.821 (K-PR). Therefore, it can be seen that the determination coefficients involved with E_p were generally greater than those involved with PR. In the condition of not distinguishing the type of soils, the R² related with E_p are still greater than those related with PR except in the correlation analysis of moisture content. Simultaneously, the determination coefficients in ω-E_p, K-E_p and ω-PR, K-PR are very small. This is because ω and K are two physical indexes mainly affected by the particles size and composition of the soil, and they are independent of the test methods. Thus, it is not suggested to correlate the rapid detection indexes (E_p or ω) with the compaction degree or moisture content without classifying the soil types.

Table 2 Test results of sections from K1105+005 to K1105+100

For a given type of soil, the determination coefficients between E_p and other indexes including E_b, L, ω and K exceed 0.78, which indicated that the PFWD rapid detection could reflect each subgrade performance well. The determination coefficients in ω-PR and K-PR vary from 0.75 to 0.88, less than that of 0.78 to 0.93 in ω-E_p and K-E_p, revealing that the PFWD method has a higher accuracy than the DCP method in the precondition of classification of soils. The possible reason for this result is that: the DCP probe head only measures the soil body in a small peripheral range, leading to a large variability. The bearing plate of PFWD can measure the stress and strain of the whole subgrade structure layer. In contrast, PFWD is used to measure the loading and deformation of the entire soil structure layer, it can effectively eliminate the error of singular point in a small space.

The traditional test methods of subgrade properties, e.g., plate loading test, the Beckman beam deflection test and the cutting ring method, they cost more labour power and test time. However, the PFWD and DCP methods are relatively more convenient and particularly timesaving.

Figure 11 Correlation between E_p and other indexes:

Figure 12 Correlation between PR and other parameters:

Table 3 Statistical results of correlation coefficients in all tests

Both of two rapid detection methods just need three testers with no more than 5 min for each test point, much less than those of the traditional test methods. Therefore, two rapid methods should be given priority to recommend in the site detection, for their significant benefits come from the saved time and the accurate detection. With all the factors comprehensively considered, it is recommended to use PFWD in rapid detection and evaluation of the performances of the existing subgrades.

4 Conclusions

To reveal the deterioration mechanism of performance of existing subgrades, the moisture content, compaction degree and resilient modulus of PLT of five kinds of soil samples taken from the construction site were measured. It was found that the moisture content of the tested existing subgrade gradually increases from the design value up to an equilibrium one due to the soil-atmosphere interaction. It causes a decrease in the dry density, compaction degree and resilient modulus of the subgrade soil, leading to the deterioration of subgrade performances.

In the field testing, two rapid detecting methods, i.e., DCP and PFWD, were compared. Based on the data analysis of 60 test points in three subgrade section composed of different soils, it showed that the PFWD modulus and the DCP penetration rate can be correlated with the resilient modulus of PLT, the Beckman beam deflection, moisture content and compaction degree. Because of the high accuracy and efficiency, the PFWD detection is the recommended method in evaluating of the performances of the existing subgrades.

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(Edited by HE Yun-bin)

中文导读

南方地区既有路基性能劣化机理及其快速检测

摘要：为了缓解日益增长的交通压力，许多早期修建的道路都需进行加宽。对既有路基的快速检测有助于准确评估其性能状态，实现其最大程度的利用。在华南地区的某一老路基试验段选取了五种土，采用3种常规检测方法，分别测定其含水量、压实度和弹性模量。结果表明，经过多年运营后，路基实测含水率大于设计值，实测压实度较设计值显著降低。路基和大气的水热交换使得其弹性模量从标准的60 MPa下降至40 MPa。随后，采用便携式落锤式弯沉仪(PFWD)和动力锥贯入仪(DCP)对路基进行性能检测。将PFWD模量和DCP贯入度与回弹模量、贝克曼梁弯沉、压实度和含水量进行了关联分析，证实这两种方法都适用于路基性能的快速检测，但PFWD方法具有更高的精度和效率。

关键词：湿热地区；既有路基；劣化机理；快速检测；落锤弯沉仪；动力锥贯入仪

Foundation item: Project(2017YFC0805307) supported by the National Key Research and Development Program of China; Projects(51878078, 51927814, 51911530215) supported by the National Natural Science Foundation of China; Project(2018-025) supported by the Training Program for High-level Technical Personnel in Transportation Industry, China; Project (2018JJ1026) supported by the Excellent Youth Foundation of Natural Science Foundation of Hunan Province, China; Project(17A008) supported by the Key Project of Education Department of Hunan Province, China; Projects(kfj150103, kfj170104) supported by the Open Research Fund of State Engineering Laboratory of Highway Maintenance Technology, Changsha University of Science & Technology, China; Project(CX20190644) supported by the Postgraduate Scientific Research Innovation Project of Hunan Province, China

Received date: 2020-03-29; Accepted date: 2020-04-28

Corresponding author: ZHANG Jun-hui, PhD, Professor; Tel: +86-731-85258255; E-mail: zjhseu@csutst.edu.cn; ORCID: 0000-0003- 4199-4884