Development of a rainfall test rig as an aid in soil block weathering assessment

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Abstract

Continual changes in earth technology and the search for new trends in soil block weathering practices have resulted in the development of a rainfall test rig (RTR). By positioning the water-spraying nozzle at a fall-height of 2.0 m and water pressure of 0.5 kg/cm2, the device generates a rainfall intensity of 150 mm/h. On application to the standard flour pellet, the RTR discharges spectra of rainfall in which the range of drop sizes, impact velocities and kinetic energy appropriate to natural rainfall conditions. This was applied to soil block samples arranged on a 0.9-m2 adjustable soil block holder (platform). The results of testing two different soil block samples, stabilised with cement, lime and lime/gypsum at varying proportions and cured at both room temperature (RT) and elevated temperature (ET), under controlled and reproducible artificial rainfall conditions, are briefly discussed.

Introduction

In a world of rapidly accelerating technology, great changes are constantly unfolding not only in computer-based, medical, engineering, space exploration technologies, etc. but also in earth technology from which the built environment among others has had tremendous impact. While reliable techniques exist for determining a material's strength, the method for assessing its durability is seldom given the weight it deserves. The high strengths of fired clay and concrete products seem to have diminished the significance of the durability characteristics. While these products are used in a few privileged regions, earth-blocks produced on a small-scale with little capital investment, using simple technology and making maximum use of local labour and raw materials have a greater patronage from the majority of the world's population than the large-scale counterparts, despite their relatively lower strengths, hence the need to evolve a robust test method for assessing their weathering potential. The use of such materials is expected not only to lower the cost of construction, but also to reduce the import content of construction projects and create opportunities for local employment.

Regardless of the production method, the peculiarity of direct measurements of soil block deterioration in wall units has long been recognised by experts in a number of disciplines. Ideally, such measurements should be conducted in situ at field sites under the prevalent environmental conditions. The availability of comparable data from different sites can be a time-consuming, energy sapping and costly exercise requiring long-term observations. If existent at all, the data are available only for a few localities. However, data are available on block samples based on the plethora of laboratory test techniques, but the reliability of such data has been thrown into doubt, due perhaps to the deficiencies of the test methods [1].

Among the weathering elements influencing erosion, rainfall is the most predominant, and requires a lengthy observation period. The processes of soil block erosion can be studied under both natural and simulated rainfall conditions. Whereas it is desirable to study these processes under natural rainfall conditions, the spatial and temporal distribution of natural rainfall characteristics cannot be controlled, and hence, data acquisition is very slow, if not difficult [2]. Rainfall simulators [3], [4] have the ability to create controlled and reproducible artificial rainfall, and to greatly accelerate the speed of research, which in turn expedites data collection. In order to study the effects of rainfall intensity and storm duration, drop size distribution, impact velocity and kinetic energy, and to replicate conditions, many workers have resorted to the use of artificially simulated rainfall. It is of utmost importance, therefore, that any test techniques developed should reflect the diverse nature of the storm occurrences.

The Department of Housing and Urban Development, Washington DC [5] have developed a spray test for measuring the weather resistance of stabilised blocks. The test, however, did not accurately simulate the long-term effects of natural rainfall, as there appears to be little correlation with the actual weathering performance. Dad [6] attempted the development of a modified spray test for assessing the weather resistance of stabilised blocks for periods of simulated natural rainfall up to 25 years. Again, this test failed to derive its own measured parameters, but used estimates based on data from previous workers and without verification and consideration of the conditions under which the tests were performed. However, the test established that spraying samples at an inclination of 90° is unrealistic and results in much greater soil loss than would occur in practice but suggests an inclination of 30° to be a more realistic model. Further comparative tests were carried out to investigate the effects of simulated rainfall on the different faces of the test specimen. It was suggested that a simulation test should be carried out on the side faces of the test block rather than the top face due to the effect of the direction of the compaction pressure and smearing of the soil within the mould.

The significance of the rainfall test rig developed by the authors lies in deriving all the parameter values from the widely used standard flour pellet method [7], a method that has so far not been considered in soil block studies. The RTR attempts to reproduce the effects of real life rain, and to consider the 120-h exposure duration as the worst-case conditions. This approach provides the possibility of detecting materials of poor quality from good quality. The wet/dry cycling test, although it has aggressive effects on block samples, does not precisely mimic realistic natural rainfall conditions and is, therefore, not considered in this paper.

Section snippets

The description of rainfall test rig

The basic framework consists of four principal components: the structural frame, the applicator, the basal reference plane and the control units (Fig. 1). The unit is sheathed on all sides by a polythene screen to curtail drop splashing and to allow observation of samples during experiment.

The structural frame is a rectangular configuration with four vertical aluminium uprights and supports the applicator and the reference plane sections. Each upright (43×27 mm) is bolted to one another both at

Precautionary measures

The reliability of the data anticipated is significantly dependent on the efficacy of the device. The nature of the water, the rate of flow and perhaps the input from the component parts (especially the installations of pressure gauge) constitute the determinants of such efficiency. Although there was a constant supply of water from the mains, the general pressure variations necessitated the use of two pressure regulators that provide flow checks. A regular flow of water with minimal

Approach to simulation

Due to lack of enough information on tropical rainfall data, the task of simulation is often a difficult one. However, in attempting to establish the rainfall characteristics from the simulation process, two alternative methods have been examined:

  • To calibrate the rig using a nozzle (such as 80100 veejet) with pre-determined parameter values; and

  • To derive the rainfall parameters from established methods like the flour pellet method or the use of rainfall distrometer [8].

The use of the flour

Calibration of drop size and rainfall intensity

By combination of the selected nozzle, water pressure, fall-height and careful operation of the system, a wide range of natural rainfall conditions can be replicated. Variable intensities ranging from 75 to 200 mm/h were produced at varying pressure of 0.5–3.5 kg/cm2 at an interval of 0.5 kg/cm2, and a fall-height of 0.8–2.0 m at an interval of 0.4 m. In order to accommodate momentary and localised variations, raindrops were collected at nine points on the basal reference plane at zero

Test rig application

The efficiency of the rig system requires testing under well-defined and controlled conditions. The procedures involved in the testing include block preparation and their setting up on the platform, the exposure test and the determination of the material loss.

To test the efficiency of the RTR, a set of eight blocks was mounted on the platform, inclined at an angle of 30° to the spray direction.

The rig was then adjusted to a pressure of 0.5 kg/cm2 and a fall-height of 2.0 m, corresponding to an

Discussions

Simulated rainfall has become an essential research tool in erosion studies. It is generally more rapid, controlled and adaptable than natural rainfall, which is always unpredictable and frequently perverse. Strictly controlled rainfall reproduction by means of an appropriate device is essential for minimising the shortcomings of the simulation technique. The complexity or otherwise of the simulation technique is always dictated by peculiar circumstances of the research objective.

The size of

Conclusions

A rainfall test rig has been developed for replicating natural rainfall conditions close to reality and for assessing the weathering potential of soil blocks. The generation of rainfall parameters at a pressure of 0.5 kg/cm2 and a fall-height of 2.0 m, using the standard flour pellet method, has proved to adequately describe the physical characteristics of the rainfall event on which the soil blocks weathering depends. The device also offered a degree of advancement to the studies of rainfall

Acknowledgments

The research programme was funded by the Commonwealth Scholarship Association in collaboration with Federal University of Technology, Akure Nigeria and is gratefully acknowledged. Thanks are also due to Mr Vickers at the Silsoe College, UK for providing information on rainfall simulation.

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