Effects of dry density and thickness of sandy soil on impact response due to rockfall

doi:10.1016/j.jterra.2005.05.009

Journal of Terramechanics

Volume 43, Issue 3, July 2006, Pages 329-340

https://doi.org/10.1016/j.jterra.2005.05.009 Get rights and content

Abstract

Sand cushions against impact force are widely used for rockfall prevention covers. The objective of this paper is to investigate the effects of the dry density and thickness of a sand cushion on an impact response due to a falling weight likened to a rockfall. A series of laboratory experiments for a decomposed granite soil was executed in the combination of the mass and drop height of the weight. As a result, the impact pressure applied to the soil surface increases with the dry density, but it does not depend on the thickness of the soil. The earth pressure at the bottom of the mold increases with the dry density, and it decreases with the thickness of the soil. Therefore, the transmissibility of the impact pressure decreases rapidly with the thickness of the soil.

Introduction

Roads constructed in mountainous districts like Japan are exposed to the danger of rockfalls owing to local downpours, weathering and earthquakes. The estimates of impact force caused by a rockfall and the falling point are very important for the design of rockfall prevention covers. Sand cushions are widely used to cover, for example, a road facility in order to keep it safe against a rockfall.

For the design of rockfall prevention covers in Japan, impact force on a sand cushion caused by a rockfall is estimated as follows (Japan Road Association [1]): $P = 2.108 \cdot (m \cdot g)^{2 / 3} \cdot λ^{2 / 5} \cdot H^{3 / 5},$ where P is impact force [kN], m is the mass of a rockfall [t], g is a gravity acceleration of 9.8 m/s², λ is Lamé’s constant of a sand cushion [kN/m²] and H is the height of a drop [m]. The above equation is derived from an elastic contact theory by Hertz, assuming that the rockfall is a rigid sphere having a specific gravity of 2.65 and the sand cushion is a plane. The general value of λ used is 1000 kN/m². However, the actual values of λ range from 1000 to 10,000 kN/m² according to the density of the sand cushion.

The objective of this paper is to investigate the effects of the dry density and thickness of a sand cushion on an impact response due to a falling weight likened to a rockfall. A series of laboratory experiments for a decomposed granite soil was executed in the combination of the mass and drop height of the weight. The quantitative effects of the dry density and thickness of the sand cushion on the impact acceleration, the impact duration, and the earth pressure at the bottom were investigated. Furthermore, the transmission ratio of the impact pressure was discussed.

Section snippets

Experimental methods

Soil used was a decomposed granite soil, having a grain size not exceeding 4.75 mm. The soil was adjusted to the optimum water content because the settlement was expected to become large. The soil properties are shown in Table 1.

An apparatus shown in Fig. 1 consisted of three main parts: a falling weight likened to a rockfall, a mold made of steel having an inside diameter of 200 mm and a height of 300 mm, and a guide. A piezoelectric accelerometer was built into the weight at the upper-side

Settlement

Fig. 2a shows the relationship between the settlement of the falling weight S and the potential energy mgH. Fig. 2b shows the relationship between the settlement of the falling weight S and the momentum mv₀, where the impact velocity v₀ can be calculated as $\sqrt{2 gH}$ according to the law of energy conservation. The settlement of the falling weight S is be approximately expressed as an exponential function of the potential energy mgH rather than the momentum mv₀, comparing the correlation coefficients

Conclusion

To investigate the effects of the dry density and thickness of a sand cushion on an impact response due to a falling weight likened to a rockfall, a series of laboratory experiments for a decomposed granite soil was executed in the combination of the mass and drop height of the weight. As a result, the following conclusions were obtained.

•
The settlement of the falling weight and the impact duration decreases with the dry density of the soil; however, it is independent of the thickness of the

Acknowledgement

This study is partly supported by the Grant-in-Aid for Scientific Research from Mext (Ministry of Education, Culture, Sports, Science and Technology) of Japan.

References (3)

Compilation by Japan Road Association. Handbook for rockfall measures; 2000. p. 20–3 [in...

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Citation Excerpt :
For the sand cushion, the impact force curve rises rapidly, reaching the first peak (also its maximum value) and then falls, rising slightly to the second peak on the way down, and finally falling to zero, where the block and the cushion are out of contact. In this scenario, the first peak is attributed to the material compaction caused by projectile penetration; the second is attributed to the compaction resulting from the confining effect of the steel frame with certain deformability (Kawahara and Muro, 2006; Lambert et al., 2009). At a low impact energy, the confining effect of the steel frame does not play an important role in the degree of compaction of the sand.
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