Identification of Surface Failure Hazard Locations Using Soil Strength Probe

1. Introduction

  Many slope failures occur as a result of a phenomenon called surface failure, which is a slide of a thin surface of the slope (of a few meters). Formerly, it was generally understood that surface failure can occur at any location, but the research conducted so far has revealed that hazard locations can be narrowed down by a detailed survey of the thickness, strength, or gradient of topsoil.

  Since the distribution of hazard locations is highly characterized by non-homogeneity, it is necessary to survey many points on actual slopes to narrow down hazard locations. However, due to the high cost of boring surveys, survey locations tend to be limited. General simple ground survey methods including simple dynamic penetration tests have their own problems, including the inability to obtain sufficient physicochemical values or difficulty in clarifying the constituent materials of the ground.

  As a solution, the Public Works Research Institute (PWRI) developed a soil strength probe, which is a rod-type device capable of examining the thickness or strength of topsoil inexpensively and quickly compared with conventional methods (Photo 1).

Photo 1: Soil strength probe
(Left: overall configuration; top right: vane cone; bottom right: conical cone)

2. Overview of Soil Strength Probe

  A soil strength probe is a device that can be applied to soil of up to 5 m in thickness and is capable of measuring the thickness, penetration strength, or shear strength of the topsoil. Weighing about 5 kg, the device is about one fourth lighter than the conventional simple dynamic penetration tester and it facilitates measurement on mountain slopes, where measurement was previously difficult (Photo 2).

  Attach a conical cone to the front end of the rod and manually press the rod into the ground. In only a few minutes, the probe can measure the depth of the topsoil, which can be a cause of failure. When the road is pressed with a load meter, it can also convert the penetration strength of the topsoil to measure to the equivalent N value of a Swedish sounding tester or the Nd value of a simple dynamic penetration test (Fig. 1). Also, when you attach a vane cone (a cone fitted with a vane-like plate) to the rod and press the rod into the ground by rotating it with a torque wrench to shear the soil, then the measurement of the shearing allows you to easily estimate the shear strength of the soil (cohesive force C and internal friction angle).

Photo 2: Soil strength test and combinations of devices for each type of measurement


Fig. 1: Example comparison between soil strength probe and existing tests

  A manual giving details of the method of use and notes on use (Technical Memorandum of PWRI No. 4176, "Manual for Survey of Soil Layers on Slope with Soil Strength Probe" (Draft)) is available on the Geology Research Team website ((https://www.pwri.go.jp/team/tishitsu/topics_dokenbo.htm) [Japanese only]), as are related videos. In addition, PWRI has established a research consortium with private companies called the Soil Strength Probe Research Committee, and it works jointly on soil strength probe R&D, including diffusion and precision improvement.

(Contact: Geology Research Team)

Design and Construction of Concrete Pavement in Cold, Snowy RegionsIntroduction to the Manual (Draft)

1. Issues concerning concrete pavement in cold, snowy regions

  Amid the recent social demand for reducing infrastructure development/maintenance costs, road pavement today is required to have improved durability and service life and reduced life cycle costs. With this trend in the background, concrete pavement-which has greater durability and longer service life compared to asphalt pavement-is drawing increasing attention. However, the concrete pavement coverage of national roads in Hokkaido, a prefecture in a cold, snowy region, is only 3%, whereas the national average is 5%, due to concerns about concrete pavement's structural failures caused by frost heaving and the weakening of subgrades' bearing capacity during the thaw period.

  To address this problem, we clarified the issues concerning concrete pavement in cold, snowy regions and proposed countermeasures for failures caused by frost heaving. Figure 1 shows the mechanism of a concrete pavement failure caused by a frost heave. When the freezing temperature has penetrated into subgrade soil layer under the pavement, water in the soil forms ice lenses (ice layers), which sometimes cause upward heaving of the soil under the concrete pavement. The vehicle loads applied on such pavement may result in earlier occurrence of pavement cracks than those applied on a pavement without frost heave. Based on the results of site investigations and numerical analyses (Figure 2), our team pointed out that even small unevenness could reduce the service life of concrete pavement. Based on this finding, we suggested that a subgrade above the freezing front should be designed to be made of non-frost-susceptible materials.

2. Manual concerning design and construction (draft)

  The Pavement Research Committee, Association for Civil Engineering Technology of Hokkaido, which consists of entities from the industrial, academic and government sectors, including CERI, conducted studies on concrete pavements and issued the Manual for Design and Construction of Concrete Pavement in Cold, Snowy Regions (Draft) in September 2017, summarizing matters to be noted concerning pavement design and construction in cold, snowy regions. This draft manual also reflected the results of our studies on frost heaving. The draft manual can be downloaded from the Committee's or our website. We hope this manual will serve you well.

(Contact: Road Maintenance Research Team, CERI)