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This article compares the behavior of spread footing on rammed aggregate piers and spread footing on cast in place concrete piers as two types of semi-deep foundation with small-scale on-site. In this study, the area replacement ratio (pier on spread footing), composite (spread footing-pier) stiffness modulus and the bearing capacity of spread footing-pier are three main factors of the aim. For this purpose, 18 compressive load test in three groups with 6 elements composed of the following circular spread footings, spread footings on rammed aggregate piers, spread footings on concrete piers with constant diameter and length of piers and variable diameter of spread footing were built and tested in the coastal area of Bushehr port, Iran. In all three groups, the diameter of spread footings were 200, 250, 300, 350, 400 and 450 mm and in two groups of spread footings on piers, nominal diameter and length of concrete piers and rammed aggregate piers were 135 and 600 mm, respectively. The trial site was composed of a wet soil layer of low plasticity silt (ML). Underground conditions at the site were evaluated using in-situ and prevalent laboratory tests. Measurements performed in the site were composed of applied load and top settlement in spread footings on piers and pier's bottom settlement. The results show that, for the area replacement ratios (pier/spread footing) greater than 20%, the average of design limit stress and stiffness modulus of spread footings on concrete piers are 1.1 and 1.3 times of the corresponding values of spread footings on rammed aggregate piers, respectively. Also, for the area replacement ratios (pier/spread footing) less than 20%, the use of cast-in-placed concrete piers or rammed aggregate piers haven't any effect on the stiffness modulus and bearing capacity of spread footing on piers. In general, comparison of the results of load tests on spread footings on concrete piers and rammed aggregate piers and two types of piers shows that the performance of them is desirable in practice. Based on the results obtained in this study and previous research by the author in 2015 at the same site testing, the bearing capacity and stiffness modulus of concrete piers and spread footing on concrete piers are relatively greater than of rammed aggregate piers type. However, it is unclear in other local soil type, layering and other technical characteristics of such a result occur. On the other hand, as for the diameter and length of the trial piers, tip stress (against bulging) was governed on two types of piers and spread footing on piers. While, it is not clear with the occurrence of bulging deformation on the top of pier and effect of spread footing on it, such an outcome to be repeated. However, very little research has been done in this field, and as long as more research is done in this regard, for the decision to choose any one of these systems in action, case study should be done in this field. In this field, conducting a case study with a fewer loading tests and numerical modeling with validation of results can significant help to reduce the cost and duration of initial studies, prediction of results and to choose any of the systems.

Keywords: rammed aggregate piers, soil improvement, bearing capacity, stiffness modulus, the area replacement ratio

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