Before embarking on in-situ CBR tests, careful thought should be given to how relevant these are to the design method to be used and whether the condition of equilibrium water content is likely to pertain.
disadvantages of cbr method of pavement design
The design curve only gives the value for the total thickness of pavement for different traffic intensity and CBR values of subgrade. These curves do not specify the thickness of subbase, base and surfacing separately which are needed to evolve most economic design pavement.
The soil runway is a special type of runway that can be constructed quickly and is both flexible and easy to conceal and camouflage. With increasingly complex requirements of modern high-tech warfare logistics supply, strategic airdrop support capabilities, and combat and special missions, it has become the primary task of soil runways to ensure the effective take-off and landing of large transport planes. This requires complicated high load-bearing landing structures, thereby presenting new requirements for soil pavement design.
Under the current backdrop, this study analyzes the structure and damage modes of soil pavement and the limitations of existing design methods. According to cumulative fatigue theory, a new design method for soil pavement was proposed, which adopted the rut depth as the only criterion to evaluate the damage of soil pavement and used the resilient modulus as the control variable. Reliability of the proposed design method was verified by onsite testing, with the objective of improving soil pavement design so that it is more suitable for real-world situations and can better meet the requirements of various types of aircraft, in particular, large heavy aircrafts.
Development of emergency airfields requires fast construction; however, these airfields usually have short usage periods, several weeks to years. Therefore, repetitive loading is less important in the design of temporary airfield pavements compared to those of permanent airfields. Moreover, the long-term effects on the natural environment can be ignored. Durability is not essential; therefore, as much as possible, the design of soil pavement structures should allow easy material selection and construction and the structural layer should be as thin as possible. Furthermore, if possible, locally available materials should be selected as the main components to reduce bulk material transportation requirements. Therefore, the most ideal runway is simple and can be developed using a simple construction onsite method, while meeting the requirements for a predefined number of aircraft take-offs and landings. The commonly used soil pavement structure consists of the reinforced base layer and pressed subgrade soil. If the base layer does not meet requirements, it can be paved or an additional layers can be added, which can take several forms such as an additional base layer, functional material, or assembled road panels, as shown in Figure 1.
The main limitations of the current soil pavement design methods are reflected in two aspects. More important aspect is that the CBR method has large disadvantages when applied to the design for soil pavement:
(1) The CBR design method was developed from design practices used for road pavement and improved upon by the multi wheel heavy gear load full-scale test to the airfield pavement design method which can be applied to the aircraft traffic load and wheel configuration. The results obtained by the empirical method are related to the observed pavement as well as the materials, environment, and loading conditions of the full-scale test. If the conditions for the specially designed pavement are similar to the conditions for which the method was originally developed, a satisfactory design can be obtained. However, due to differences in environmental conditions, rapid development of traffic loads, and emergence of new materials, it is necessary to continuously extend the empirical relationship. Thus, applicability of the CBR method is limited because it was established for the design of permanent airfields [13, 14]. Although strict standard controls, a sufficient construction period, and suitable remedies can limit the design error to be within an acceptable range for permanent airfields, the situations for the use of soil pavement are completely different. The material used in the construction of soil pavement is often obtained locally, the construction period is shorter, and the materials and environments can differ significantly. As result, application of CBR design methods to soil pavement can produce huge errors that cannot be ignored.
(2) The CBR value is only an empirical index and not a direct measurement of the load bearing capacity of a material. Therefore, it has little relationship with the elastic deformation of the subgrade soil of airfield pavement. However, the actual pavement structure design requires the CBR value to work under the elastic stress. Under these working conditions, the shear strength of subgrade soil characterized by the CBR value is not important in the pavement design, but resilient properties (resilient modulus) of subgrade soil and plastic deformation under repeated loading [15].
(3) The CBR design method cannot accurately characterize the damage mode of soil pavement. The most important point is that the CBR design method considers rutting damage on the pavement surface to be the result of excessive stress on the soil base layer and the in-layer deformation generated within each pavement layer can be ignored. In fact, due to the low strength of the surface layer of soil pavement, rutting on the pavement surface is attributed more so to the in-layer deformation within the pavement structure. Moreover, the fatigue cracking and low-temperature cracking of the surface layer and other damage modes of the pavement structure, which can be the main damage modes of the soil pavement, cannot be characterized using CBR.
According to literature [23], the slip width is not actually a fixed value but changes with the aircraft taxiing distance on the runway. The maximum slip width consists of the distance from the initial take-off/landing positions to the runway center line and the taxiing distance on the runway. Factors affecting the taxiing distance include the sideslip angle on landing, wind speed, and sideslip coefficient. The sideslip coefficient is related to the frictional resistance and the longitudinal and transverse slopes of the pavement. The calculation method for slip width based on reliability was also given. According to the method reported in literature [12], the sideslip distance of an A-type transport on a soil runway under different friction coefficients could be calculated, as presented in Table 1.
To improve the empirical method, the formula for conversion between the CBR and resilient modulus can be represented [20] as Eq. (12). Substituting (11) and (12) into (10), a relationship between the number of repetitions on the soil runway and the rut depth can be obtained. For WES [9], the rut depth on the soil runway should not exceed 76 mm (3 in); therefore, an equation for the allowable number of repetitions for soil pavement can be defined as (13)
The soil pavement design method adopts the FAA ordinary concrete pavement design method based on cumulative damage. According to the Miner principle, the extent of fatigue damage due to different aircraft is linearly accumulated and a cumulative fatigue damage factor (CDF) can be obtained. Based on whether the CDF is close to 1, we can determine if the pavement has reached fatigue destruction [24, 25]:
In soil pavement design, the expected usage time of the soil pavement should first be determined. Then, according to the characteristics defined to guarantee the task and use requirements of the soil pavement emergency airfield, the load parameters of every aircraft model using the runway are acquired. Moreover, the increase in traffic volume owing to take-off and landing modes, as well as the sortie and the follow-up tasks, is predicted. Based on the surrounding environment, soil quality, and material conditions of the proposed location, the pavement combination and form are preliminary fitted to determine the thickness and parameters of each structural layer and of the soil-based materials. Subsequently, the predicted traffic volume and pavement structure are used to calculate the pass-to-coverage ratio of each aircraft as well as the expected coverages during the design period. Furthermore, based on the fitted pavement combination, the maximum compressive stress under each aircraft load can be calculated and the soil pavement fatigue equation can be adopted to calculate the allowable number of actions of each aircraft. Finally the cumulative damage factor for each aircraft is calculated and linearly superimposed and compared with 1 to regulate the pavement thickness. A flowchart of the design process is presented in Figure 6.
In this study, the reliability of the new design method was validated by onsite traffic testing. The test section was constructed at an Air Force test center in Jining, China. First, theoretical calculations were carried out. A theoretical soil runway with the parameters identical to those of the test section was fitted to guarantee take-off and landing of A-type transport aircraft. Some of the fitted parameters are listed in Table 2. Using the relevant test section parameters and the test aircraft model, the theoretical maximum allowable repeated actions on the soil runway were calculated employing the currently used CBR method, the fatigue equation, and the proposed method. Then, a traffic test was carried out on the test section to obtain the theoretical maximum allowable repeated actions for the A-type transport aircraft on the soil pavement. Finally, the theoretical calculation results and experimental results were compared.
(1) The theoretical results calculated by the CBR method are much larger than the actual values, which is consistent with the performance of the soil runways constructed by the US military. The reasons are outlined in Section 2.2. The CBR method uses empirical parameters and is unable to characterize the most common damage mode of soil runways, which is rut damage. Furthermore, the traffic volume conversion between the aircraft required in the design magnifies the error; thus the method should be avoided in future soil runway design as much as possible. 2ff7e9595c
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