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Pavements and embankments fail for different reasons; poor design, poor materials and poor construction methods are the most common. The pavement foundation (subgrade) represents one of the key elements in pavement design and its behavior shall influence the overall pavement performance. It was found necessary improving gypseous subgrade stiffness characteristics in order to prolong pavement design life. We also believe that too much emphasis in finding out untraditional methods in treating weak gypseous soils and then using them in subgrade layers taking into consideration type and values of loading effects in the employed design models. This has lead to erroneous predictions. In general, the more resistant to deformation the subgrade is the more loads it can support before reaching a critical deformation value. Gypseous soils cover about (35%) of the total area of Iraq and therefore the study of the properties of these soils to be used as subgrade embankments becomes very important due to the problems caused by these soils during leaching which lead to what is called collapsibility. Many trials were conducted in order to study the possibility of improving the properties of gypseous soils using chemical and natural materials. Results showed that some natural and chemical materials could improve the behavior of gypseous soils, but these results are based on routine laboratory tests. In this study, two types of local gypseous soils are selected to be improved by three chemical stabilizing materials, namely, 5% hydrated lime, and 2.5% hydrated calcium chloride and 6% kaolin. The soils employed are classified as (SP-SM) and (SP) with gypsum content of (48.3%) and (35%) for Baiji and Al-Thurthar area soils, respectively. An engineering model is adopted to carry out tests on these soils and to analyze their behavior before and after improvement especially their collapsibility property. The results show that all employed chemical and natural additives can be used to reduce the collapsibility property of gypseous soils, but with different degrees. Using lime reduces the collapsibility of gypseous soils to a small degree compared with calcium chloride and kaolin.

Laboratory model tests have been carried out to explore the behavior of piles subjected to lateral soil movement in the sand. The results of a single pile test and ten tests on of group piles arranged in a row (perpendicular to the direction of soil movement) are presented. Different parameters were utilized in these tests, (e.g., pile spacing, number of piles within a group and pile head condition). Group effect was calculated using group factor (F m ) which is defined in terms of the measured maximum bending moments of an individual with the group and that of the single pile. The results showed that the maximum bending moment of group piles decreases from the single pile with decreasing pile spacing and the pile behaves essentially the same as a single pile when the pile spacing is 7d or more. The maximum reduction of the maximum bending moments of about (17%) and (28%) as compared with that for the standard single pile may be observed for a pile spacing of 3d for the free and capped headed condition, respectively. The number of piles does not appear to have a significant effect on the group factor F m of piles, except for the free headed inner piles who's the group factors appear to decrease with increasing the number of piles within a group.

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