Keywords: geotechnical, collapse, packing, Monte Carlo, computer simulation, loess structure.

This paper examines the microstructural fabric of collapsible loess soils using a computer simulation. The subsequent model developed has been validated against both field data from the literature and data from a detailed laboratory programme. Samples from three locations in Europe were used in the laboratory work to provide a good basis for comparison. Samples from Kent in south-east England, west Slovakia and north Bulgaria were used and collapse data are presented in this paper.

The collapse problem can be examined at a macroscopic and at a microscopic level. Whilst the macroscopic behaviour of loess is of direct significance to subsidence, it is also necessary to understand the microscopic nature of the soil to properly utilise the macroscopic information. It is at this single particle level that knowledge is deficient. This paper puts forward a computer simulation to elucidate the microscopic single particle aspects of the collapsible structure of loess soils.

The computer model presented was written in FORTRAN 77 and has been used to examine the initial unstable structure, metastable structure and hydrocollapsed structure that are typically exhibited by natural collapsible loess soils. The program developed generates random numbers using the Monte Carlo method and uses them to positions rectangular blocks representing loess grains being deposited by the wind, at concurrent locations in a two dimensional array to build up an open structure. Blocks of 4:1 width:depth ratio have been chosen as they best represent the typical aspect ratios of the quartz particles (8:5:2) in a two dimensional form for loess[1].

The metastable computer simulation considers the contact point between two rectangular particles of variable widths. A pre-determined value of "slide" is specified. This may be any integer less than the minimum particle width. If the two particles overlap by more than the "slide" value then bonding will occur. If the overlap is less than the "slide" value then the upper particle will move sideways and drop down to come into contact with another particle. Alternatively, a particle may form a bridge between two other particles, in which case bonding will occur. By choosing a suitable value of "slide" void ratios of around 1.0 can be obtained. This is similar to what is observed in loess when it has a metastable form, where void ratios of between 0.9 and 1.1 are found[2].

A comparison of the metastable computer simulation with SEM micrograph of the pre-collapse structure presents some marked similarities. It should be noted that the SEM micrographs present three-dimensional images, compared to the two dimensional computer models. The computer generated structure shows a characteristic staircase of voids. The void ratios produced by metastable simulation equate well with those for the natural loess deposits.

The hydrocollapsed structure forms when the bonding and cohesion mechanisms disintegrate and the system becomes predominately controlled by gravity. As with the metastable structure, the system is a complex one. In order to model the collapse accurately the simulation needs to be simplified in a similar way to that employed for the metastable structure. If the "slide" number is increased gradually from the metastable value, then the void ratio of the structure decreases until the dense, collapsed structure is achieved.

The results from the computer and physical models demonstrate some marked similarities. Void ratios in the range of 1.2 to 1.6 demonstrate the initial, unstable structure; between 0.9 and 1.1 represents a metastable structure; and between 0.5 and 0.7 yield a collapsed structure. A comparison of the theoretical computer simulations with natural loess using scanning electron microscopy also shows clear similarities between the structures obtained in the laboratory with those observed in the field [2,3].

1

C.D.F. Rogers, T.A. Dijkstra, I.J. Smalley, "Hydroconsolidation and Subsidence of Loess: Studies from China, Russia, North America and Europe", Engineering Geology, 37, 83-113, 1994. doi:10.1016/0013-7952(94)90045-0

2

K.J. Northmore, F.G. Bell, M.G. Culshaw, "The Engineering Properties and Behaviour of the Brickearth of South Essex," Quarterly Journal of Engineering Geology, 29, 49-63, 1996. doi:10.1144/GSL.QJEGH.1996.029.P2.04

3

A. Klukanova, J. Frankovska, "Monitoring of collapsible Soils in Slovakia. Slovak", Geological Magazine, 3, 179-190, 1995.

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