In-situ and numerical investigation on the dynamic response of unbounded granular material in permeable pavement

Abstract

Permeable pavements have been widely used as an effective means to improve hydrological characteristics and the ecology of the urban environment. This study aims to investigate the response of fully permeable pavement (FPP) subjected to dynamic loading under dry and saturated conditions. A full-scale test track topped with polyurethane bound permeable material (PUPM) was built to obtain the stress response with an accelerated pavement test (APT) system. In addition, comprehensive analyses were performed based on the coupled Stress-dependent Moisture-sensitive Cross-anisotropic Elastoplastic (SMAEP) model in FEM. The APT test showed that the worst state was observed when the pavement structure was fully saturated, and that and brittle failure of the pavement surface occurred when the critical load level was achieved. The prediction of vertical stress predicted by Stress-dependent Cross-anisotropic Elastic (SAE) and SMAEP were both validated with the field data. The horizontal stress predicted by SAE gave a very high and unreasonable tensile stress prediction at the bottom of the unbounded granular base (UGB) layer when subjected to the high load level. With the consideration of moisture effect and the plastic properties of the material, the prediction made by SMAEP is effective to estimate the dynamic response of the UGB layer. Based on the sensitivity analysis, the optimized designs for FPP based on PUPM were suggested.

Introduction

Conventional dense graded asphalt concrete blocks stormwater runoff from penetrating into the ground, which may result in various environmental issues including, but not limited to, urban flooding and the falling of the underground water table [1]. In recent years, permeable pavement materials have been widely used as an effective means to improve hydrological characteristics and the ecology of the urban environment [2]. By replacing traditional dense pavements, this pavement has higher permeability, which allows rainwater to penetrate through the pavement structure, reducing surface runoff and urban drainage pressure. According to the depth of permeable water, the permeable pavement can be mainly divided into the following three permeable pavement structures: (a) Fully Permeable Pavement (FPP), where water can infiltrate to subgrade; (b) Half Permeable Pavement, wherein some water infiltrates the subgrade, and most is stored in the pavement structure; and (c) Permeable Pavement Surface (PPS), which does not allow water to infiltrate into the subgrade but stores it in the pavement structures for slow release. However, pavement with high permeability is subject to higher susceptibility to damage resulting from moisture and traffic loadings [3], [4], especially in a FPP system.

Enormous studies have been carried out to gain insight into the service behavior of FPP. It was reported that moisture, oxidization, and traffic loading are of critical importance for the performance and service life of FPP [5], [6], [7]. Among these, moisture plays a dominant role, as it causes premature and severe distress [8], [9]. Despite great efforts to extend the service life of FPP, the average service life of FPP is still shorter than that of dense graded asphalt pavement [10], [11]. With the continuous innovation of polymer materials in the engineering field, polymer composite materials are also increasingly applied in road materials. Among them, the polyurethane bound permeable material (PUPM) was developed to replace a traditional asphalt binder [12], [13], [14]. A huge number of studies have proved that PUPM as a pavement material not only has good functionality but also has a particularly high mechanical strength. The success of the development and application of PUPM has made it possible to widely use FPP. As an alternative to asphalt binder, polyurethane has been evaluated in the laboratory due to its excellent functional performance and distress resistance properties. Based on laboratory experimentation, it was reported that the durability and functional performance of porous polyurethane mixture is better than the conventional Open Graded Friction Course (OGFC) [15]. Similar results were also reported by other researchers [16]. Wang et al. verified the feasibility of using polyurethane as the binder of a pore-elastic road surface (PERS) [17], [18].

FPP systems including a permeable surface, unbounded granular base (UGB) layer, and subsoil must function under the stress of both water and traffic loading. Due to the high porosity and permeability of the pervious surface, water can quickly infiltrate through the surface layer and accumulate in the UGB layer. Consequently, the UGB layer is more sensitive to moisture, which in addition to the significant plastic behavior and anisotropic behavior, is one of the serious problems affecting the performance of the whole FPP structure [19], [20]. In particular, hydro-mechanical interaction can significantly change the modulus of UGB, which in turn affects the service life of FPP, and results in completely different mechanical distributions in the pavement layers [21], [22]. Previous studies have developed models to simulate the stress-dependent anisotropic and plastic behaviors of the UGB material [19]. However, few studies can be found having coupled stress-dependence, moisture sensitive, and cross anisotropic elastoplastic (SMAEP) models for this material. The currently available standards and evaluation methods on FPP, nevertheless, mainly focus on the performance of the pervious pavement surface. Thus, it becomes a fundamental but challenging task to understand the dynamic response of the FPP system response based on the reasonable UGB constituents.

Previous studies contributed greatly to understanding the service behavior of permeable pavement by means of developing innovative experimental methods and performing comprehensive laboratory work, whereas only a few studies have examined the whole FPP structure behavior with high water content and subjection to dynamic loads [12], [13], [14]. Given this, the main objective of this study is to investigate the response of FPP subjected to dynamic loading. To achieve this objective, a full-scale test track topped with PUPM was purposely designed and built at RWTH Aachen University. An accelerated pavement test (APT) system, namely MLS 30 provided by Federal Highway Research Institute in Germany (BASt), was selected to perform the accelerated loading under different water saturation conditions on the test track. Instruments, including total pressure sensor and volumetric water content sensor, were embedded into the wearing course, UGB base course, and subgrade, to enable the recording of the stress response of the permeable pavement under loading. Based on the collected data, comprehensive analyses were performed based on the coupled Stress-dependent Moisture-sensitive Cross-anisotropic Elastoplastic (SMAEP) model in FEM.