Presented by: Asst. Lecturer Israa Abdul-Jalil Majeed, Department of Civil Engineering, Al-Mustaqbal University Abstract Gypsiferous soil is considered a problematic soil in arid and semi-arid regions, because gypsum is relatively soluble in water, so the soil’s structure changes and the soil collapses and settles when exposed to moisture, and it also attacks concrete through sulfates. This article reviews the characteristics of this soil and the foundation types suitable for it (raft, isolated and strip footings, piers and piles, and floating foundations), together with supporting measures and selection criteria. Keywords: gypsiferous soil, gypsum dissolution, collapse, raft foundation, piles, sulfate attack. 1. Gypsiferous Soil and Its Engineering Problems Gypsum is the mineral hydrated calcium sulfate (CaSO₄·2H₂O), and its solubility in water is about 2 g/L, a high value compared with most soil minerals. A soil is called gypsiferous when it contains a notable proportion of gypsum (problems often begin at around 10–15% and above), and such soils are widespread in Iraq and other arid and semi-arid regions. When dry, they are usually characterized by high strength and little settlement, because the gypsum grains act as a binding material between the soil particles. Problems arise when water enters the soil: Collapse: Water dissolves the gypsum binder, the particles separate, and the soil settles suddenly under its load. Dissolution and cavities: Flowing water causes continuous dissolution and the formation of voids and caves, whose roofs may suddenly collapse, similar to karst phenomena. Differential settlement: Gypsum content and moisture vary from one location to another, so settlement beneath the foundation differs. Sulfate attack on concrete, through the formation of ettringite and secondary gypsum, causing expansion and cracking of the concrete, in addition to corrosion of reinforcing steel. Change in properties with water: Shear strength decreases and compressibility increases as soon as the soil becomes saturated, even though the strength of the dry soil was good. Collapsibility is measured in the laboratory by the one-dimensional consolidation test with saturation (ASTM D4546), and gypsum content is determined by drying at low temperatures that avoid loss of water of crystallization. 2. Suitable Foundation Types The design philosophy follows three paths: reducing the access of water to the soil, distributing the load with a rigidity that tolerates local settlement, or bypassing the gypsiferous layer to reach a sound layer. The foundation is always designed for the strength of the soil in its saturated state (the post-collapse condition), not its dry strength. A. Shallow foundations (isolated and strip footings). Suitable when the gypsum content is low and water can be kept away from the site. Their footings are widened to reduce the contact stress on the soil and are connected with tie beams to reduce differential settlement, and their depth is set below the surface zone subject to seasonal wetting. Bearing capacity and settlement are checked using saturated soil values. B. Raft foundation. This is the most common choice when gypsum content is high or local cavities are possible, since it distributes loads over a wide area, reduces stress, tolerates loss of local support, and limits differential settlement. It is strengthened with ribs or as a box (cellular raft) for sensitive structures, and it is preferably placed over an improved layer. C. Foundations on an improved or replacement cushion. The surface gypsiferous layer is removed (often 0.5–1.5 m thick, depending on the study) and replaced with compacted non-gypsiferous gravel or sand, separated from the soil by a geotextile and covered with a low-permeability layer. The cushion distributes the load and reduces local collapse. D. Piles and drilled piers. Used when the gypsiferous soil is thick or there is a risk of cavities, transferring the load to a sound layer or rock beneath. The following are considered: Neglecting side friction resistance within the gypsiferous layer and relying on end-bearing resistance. Drilling dry, or using gypsum-saturated water or polymer, because ordinary water dissolves the gypsum and enlarges the hole. Using steel casings and sulfate-resistant concrete. E. Floating (compensated) foundations. Net loads are reduced by excavating a weight approximately equal to the weight of the building, thereby reducing the additional stress on the soil; they are often combined with a raft foundation in structures with moderate loads. F. Strengthening of voids. When cavities are discovered, they are filled by grouting with dissolution-resistant materials before the foundation is built above them. 3. Supporting Measures Water control: Surface slopes that direct water away from the building, tightly sealed drainage channels, impermeable surrounding pavements not less than 1.5 m wide, inspection of water and sewage networks and prevention of leakage, and avoiding irrigation near foundations. Concrete protection: Using sulfate-resistant cement (Type V) and a water/cement ratio not exceeding 0.45 under severe exposure (classes S2 and S3 in ACI 318, i.e., 0.20% or more water-soluble sulfate in the soil), increasing the concrete cover, and coating contact surfaces with bitumen. Foundation insulation: A polyethylene layer or cut-off membranes beneath the concrete and on the sides of the foundation. Soil improvement: Controlled pre-wetting to induce collapse before construction where neighboring structures are not affected, and grouting to seal pores and voids. Stabilization with lime or cement is treated with caution in high-sulfate soil because it may form ettringite and thaumasite, which cause swelling, and so it is adopted only after laboratory tests. Monitoring: Settlement and tilt gauges in sensitive buildings and monitoring of the water level for years after construction. 4. Selection Criteria and Conclusion The choice of foundation depends on the gypsum content and the thickness of the gypsiferous layer, the possibility of water reaching the site (groundwater level, leakage, and rain), the type of structure and its sensitivity to settlement, and cost. As a guide: Low gypsum content with good water control: widened shallow foundations with tie beams. Medium content or sensitive structures: reinforced raft foundation over an improved cushion. High content, large thickness, or cavities: piles or piers reaching a sound layer, with grouting of voids. Selection does not replace a geotechnical investigation that includes test pits, determination of gypsum content and soluble sulfates, collapse testing, and estimation of the water level. Conclusion. The safety of foundations in gypsiferous soil is determined by the degree of control over water more than by the strength of the dry soil. The most effective solutions combine a foundation type suited to the gypsum content, strict water control, sulfate-resistant concrete, and post-construction monitoring. The percentages and thicknesses mentioned are indicative and vary by site; local codes and the site-specific geotechnical study should be consulted. References Barzanji, A. F. (1973). Gypsiferous soils of Iraq. PhD thesis, University of Ghent. Fookes, P. G., & Collis, L. (1975). Problems in the Middle East. Concrete, 9(7), 12–17. Cooper, A. H. (1998). Subsidence hazards caused by the dissolution of Permian gypsum in England. Geological Society Engineering Geology Special Publication 15, 265–275. Das, B. M. Principles of Foundation Engineering. Cengage Learning. ACI 318. Building Code Requirements for Structural Concrete. American Concrete Institute. ASTM D4546. Standard Test Methods for One-Dimensional Swell or Collapse of Soils.