Understanding soils and settlement
by Norman Aceron Garcia
Soil is a naturally occurring mixture of mineral and organic matters with a specific form, configuration, and composition. Soil is composed mainly of minerals that are formed from large material (stones, gravel, and other rock debris), which is broken into small pieces (sand, silt, or clay) by weathering. Typical soil composition is 45 per cent minerals, 25 per cent water, 25 per cent air, and 5 per cent organic matter or humus (both living and dead organisms).
The concern with soil in regard to structure is its capacity to support the building’s weight while remaining stable. Maintaining long-term stability demands suitable soil compaction and consolidation before permanent loads (like foundation footings, walls, concrete floor, driveway slab, etc.) are placed over it. The excavation process agitates soil and loosens it, resulting in larger spaces between soil particles. Thus, engineering specifications often instruct that foundations be constructed on undisturbed soil.
The backfill must be prepared as hard and solid as possible prior to constructing a permanent load over it. This is accomplished by mechanical compaction, wherein soil is placed in layers or lifts and compacted by impact and/or vibration. The compaction procedure and consolidation both force air from the spaces between soil particles, which increases the soil density and capacity to bear a load. Compaction is affected by a number of factors such as soil type (clay, sand, silt, level of organic matter, etc.), soil characteristics (uniformity, gradient, plasticity, etc.), soil thickness, method of compaction, and moisture content at the time of compaction. Since soil is more permeable to air than to water, compacting may take out a large portion of air but a considerable amount of water may be left.
Soil goes through both primary and secondary consolidation. Primary consolidation is short-term and occurs during the mechanical compacting process. On the other hand, secondary consolidation is long-term and happens after the construction phase. Throughout secondary consolidation, the load put on soil slowly expels water out of the void spaces, which will cause soil particles to move closer together and settle.
The source of the weight would be both the structure and the overlying soil. The amount of secondary consolidation is directly proportional to the depth of the affected area. An excavation with backfill five metres deep would have greater secondary consolidation than an excavation with backfill three metres deep. When a house is built partially on undisturbed soil and partially on compacted fill, there will be two areas that will consolidate at different rates. This is “differential settlement.”
Effects of settlement can be seen in any part of house bearing upon the settled soil. In well-compacted soil, settlement will be so minor that evidence will be unnoticeable. However, excessive differential settlement will generate stresses that are abated by cracking. Which materials crack depends on the material properties and the rate of soil settlement. Cracks due to soil settlement are generally seen in interior and exterior wall coverings such as drywall and plaster, and in masonry foundation walls. Most people think concrete is brittle, but it can bend if a load is applied slowly over a long period of time. Cracks will form if pressure is applied before the concrete completely cures.
Fine-grained soils have more interior surface area and can hold more air and water than course-grained soils. To illustrate, drywall is made of courser particles than cement. A kilogram of drywall dust contains about 16,500 square metres of interior surface area, whereas a kilogram of cement dust contains about 165,000 square metres of interior surface area. Greater interior surface area means fine-grained soils can contain more water and have greater surface tension than course-grained soils. To force water out of the void spaces, surface tension must be overcome, which is the tendency of water to adhere to a surface. Surface tension is demonstrated in a glass with water. The water level is a little higher around the edges where water comes into contact with the container surface.
Fine-grained soils also have low permeability, which means water moves through it gradually. This circumstance raise the quantity of time and pressure required for soil consolidation. Soils will keep on settling until it reaches equilibrium with the stresses due from the weight of soil and structure above. The rate of soil settlement depends on the soil composition, levels of moisture saturation, the amount and characteristic of the loads and state of soil’s consolidation. Excessive addition of moisture to the soil can cause a condition in which water is absorbed into the void spaces, thereby decreasing soil density and its capability to carry a load.
Norman is a registered Professional Engineer and a Certified Property Inspector. Please visit www.mrpeg.ca for more information on home inspection and maintenance tips.