Foundation Settlement Foundation settlement is the shifting of

Foundation Settlement Foundation settlement is the shifting of the foundation (and the structure built upon it) into the soil. This can cause damage to the structure. Whether the soil is moist or dry is central to predicting the amount of settlement to expect in a given foundation. Areas with moist soils will have more foundation settlement than dry areas. The idea is that as water is squeezed out from the soil, the structure will shift according to the empty spaces the water left. The more water, the more shift.

Settlement Immediate or Elastic Settlement: Occurs immediately after the construction. This is computed using elasticity theory (Important for Granular soil) Primary Consolidation: Due to gradual dissipation of pore pressure induced by external loading and consequently expulsion of water from the soil mass, hence volume change. (Important for Inorganic clays) Secondary Consolidation: Occurs at constant effective stress with volume change due to rearrangement of particles. (Important for Organic soils) ________________________________ For any of the above mentioned settlement calculations, we first need vertical stress increase in soil mass due to net load applied on the 2 foundation

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Elastic Settlement 5

Types of Foundation Settlement 1 - Immediate Settlement or elastic Settlement Immediate settlement concerns the initial pressure on the soil under and surrounding the foundation. It is "immediate" because it occurs during and right after construction. It has nothing to do with water displacement, but is merely caused by the weight of the structure. In terms of building foundations, immediate settlement is relatively easy to predict and measure. In many cases, given the nature of the soil, foundations are constructed with the ability to withstand a certain amount of shift without damage. Damage usually occurs only in the long term, as the shift slowly continues over time. • Immediate settlement takes place as the load is applied or within a time period of about 7 days. • Predominates in cohesion less soils and unsaturated clay • Immediate settlement analysis are used for all fine-grained soils including silts and clays with a degree of saturation < 90% and for all coarse grained soils with large co-efficient of permeability (say above 10. 2 m/s)

2 - Consolidation • Consolidation settlement is distinguished from immediate settlement both by the duration of the settlement and by displacement of water. Consolidation is the more worrisome form of settlement because it is difficult to predict over months or years. Consolidation settlement is the settling of a foundation, over time, due to pressure exerted by the structure and squeezes out the water content of the soil, thus compressing it. Expulsion of moisture from the soil usually is a longterm process. • Consolidation settlements are time dependent and take months to years to develop. The leaning tower of Pisa in Italy has been undergoing consolidation settlement for over 700 years. The lean is caused by consolidation settlement being greater on one side. This, however, is an extreme case. The principal settlements for most projects occur in 3 to 10 years. • Dominates in saturated/nearly saturated fine grained soils where consolidation theory applies. Here we are interested to estimate both consolidation settlement and how long a time it will take or most of the settlement to occur.

The Leaning Tower of Pisa is the bell tower of the Cathedral. Its construction was commenced in 1173 and contiued haltingly over a period of 200 years! The tower began “leaning” soon after construction began in 1173. The inclination of the tower is attributed to the non-uniform, sponge-like saturated clay soil on which the foundation of the tower rests. The softer area within this strata has settled more causing the tilt. Several engineers have proposed plans to “straighten” the tower. However, with its 800+ years of “leaning” history, locals do not want the tower to be straightened. Every few years some form of restoration is performed to ensure that the tower does not become unstable or collapse.

3 - Primary and Secondary Consolidation settlement has two components, primary and secondary. The former deals explicitly with the settlement caused by soil moisture displacement, and the latter deals with the elastic settlement after all movable water has been squeezed out of the soil. Primary consolidation is the most significant and potentially harmful of the two. Primary consolidation takes quite a bit of time, from weeks to years. Secondary consolidation is the quicker result of primary consolidation. Once primary has been completed, and all movable water has been moved, secondary kicks in. Secondary consolidation occurs immediately after primary, and takes far less time to complete.

After secondary consolidation is complete, the structure remains in its permanent position. As a result, many builders advise residents in new homes to avoid repairing any settlement damage until secondary consolidation is complete, which is normally after two years at most. • Occurs under constant effective stress due to continuous rearrangement of clay particles into a more stable configuration. • Predominates in highly plastic clays and organic clays.

Uniform Settlement Total settlement refers to the uniform settlement of the entire structure and occurs due to weight of the structure and imposed loads. When all points settle with equal amount, the settlement is called uniforms

Definition of Differential Settlement • • Differential settlement refers to the unequal settling of a building's piers or foundation that can result in damage to the structure. The damage occurs when the foundation sinks in different areas at different times. Differential or uneven settlement can occur if the loads on the structure are unevenly distrbuted, variations in the soil properties or due to construction related variations.

Settlement Limits Total settlement is the magnitude of downward movement. Differential settlement is non-uniform settlement. It is "the difference of settlement between various locations of the structure. Angular distortion between two points under a structure is equal, to the differential settlement between the points divided by the distance between them. Theoretically speaking, no damage will be done to a structure if it settles uniformly as a whole regardless of how large the settlement may be. The only damage would be to the connections of the underground utility lines. However, when the settlement is non-uniform (differential), as is always the case, damage may be caused to the structure.

Settlement Limits • The tolerable, settlements of different structures, vary considerably. Simple-span frames can take considerably greater distortion than rigid frames. A fixed-end arch would suffer greatly if the abutments settle or rotate. For road embankments, storage silos and tanks a settlement of 300 mm - 600 mm may be acceptable, but for machine foundations the settlement may be limited to 5 mm 30 mm. Different types of construction materials can withstand different degrees of distortion. For example, sheet metal wall panels do not show distress as readily as brick masonry. • To reduce differential settlement, the designer may limit the total settlement and use the following equation for the calculation of the differential settlement: • (ΔHdiff) max = ½ ΔHtotal

• • Guidelines to limiting values are suggested by a number of sources, but following routine limits appear to be conventionally acceptable (Skempton and Mac Donald, 1956) Sands Maximum total settlement = 40 mm for isolated footings = 40 to 65 mm for rafts Maximum differential settlement between adjacent columns = 25 mm Clays Maximum total settlement = 65 mm for isolated footings = 65 to 100 mm for rafts Maximum differential settlement between adjacent columns = 40 mm. The differential settlement may also be evaluated in terms of the angular distortion given by: (ΔHdiff) = Δ/L Where Δ = relative settlement between the two points and L = Horizontal distance between the two points. Based on a large number of settlement observations and performance of structures, the suggested limits for tolerable differential settlements are show in table below.

Angular distortion 1/150 Type of limit and structure Structure damage of general buildings expected 1/250 1/300 1/500 1/600 1/750 Tilting of high rigid buildings may be visible Cracking in panel walls expected Difficulties with overhead cranes Limit for buildings in which cracking is not permissible Overstressing of structural frames with diagonals Difficulty with machinery sensitive to settlement

Causes Differential settlement is primarily due to the condition of the soil upon which the structure sits. Soil has the capacity to expand or contract based upon the temperature or weather conditions. It can also shift or wash away due to poor drainage, heavy rainfall, soil drying unevenly, or changes in the water table.

Effects The settlement causes cracks in a structure's foundation, slab or supporting piers. These cracks lead to cracks in the building's interior walls and uneven settling of the building's doors, windows and trim.

Prevention and Solution The best way to prevent damage from differential settlement is to thoroughly analyze the soil and make necessary amendments before construction begins. It may be necessary to reinforce the structure's piers or foundations if a problem occurs after the building has been constructed.

Signs of Foundation Settlement When the foundation to your home or office building has settled, it means that there has been movement of the building below the position in which it was built. The most common reasons for changes in the foundation include shifting soil, changes in the soil or even changes in the moisture. Most houses experience some settling after construction, but these are normally not significant. Knowing signs of foundation settlement can help you address the issue in your home or avoid purchasing a home with severe foundation movement. Read more:

Cracks Settlement cracks often occur in the foundation or the house slab or on the ceilings and walls. Small cracks are often because of minor foundation settlement or even because of expansion and contraction of the settlement. Large cracks, however, can represent a more significant concern. Cracks can warrant concern if they exceed onequarter of an inch in width. Windows, Doors and Plumbing Foundation settlement may make it difficult to open or close doors and windows because the door frame does not fit the door and causes some sticking. Gaps between the windows and doors with the frame may also occur. A door or window may not close all the way. Plumbing lines can also be affected by a poor foundation as well as mechanical equipment.

Examples of prediction of settlement 1 - Tilted Chimney A tilted chimney is a sign that the foundation of a building or home has settled; however, it can be difficult to recognize unless it is severe. A slight shift in the chimney may not be as noticeable, but it is a sign of a shifting or settling foundation.

2 - Loose Nails and Wallpaper When nails begin to loosen or wallpaper starts to separate from the walls, it can be a sign of a foundation problem. Nails and wallpaper shift if the walls move and they do not bind or attach to the wall any more. Bending or bowing walls can be a sign of the age of the home, settling of the foundation, deterioration of the home or shrinkage.

3 -Slanted flooring Slanted or sloped flooring may be a result of foundation settlement in a specific area of the building's foundation or other structural issues, such as rotting or termite damage. In some cases, the foundation causing the sloping floor may need to be jacked up so that new footers can be installed.

How to Repair Cracks in Concrete Pavements It is not possible to pour concrete that will not develop cracks or incur damage during use. Many factors that contribute to cracks and damage can be avoided when the concrete is prepared and poured. Proper subgrade preparation and support, providing adequate expansion joints and correctly consolidating, finishing and curing the concrete can prevent most damage. Even so, cracks can occur and they need to be repaired before they become worse. Nonprofessionals can repair concrete cracks with a reasonable amount of effort and time

Instructions For Shallow Cracks 1 -Gauge the depth of the crack to determine whether it is less than a 1/2 -inch deep. If it is, wear safety goggles, work gloves and a dust mask to clean the crack using a stiff wire brush and whisk broom, then clean the area around the crack. 2 -Paint the crack with a latex bonding agent, available at home improvement stores, to improve the bond when the mortar mix is applied. 3 -Fill the crack with concrete repair mortar mix, available at home improvement stores, while the latex bonding agent is still wet. Mix and apply the mortar according to the label directions on the mix. 4 -Allow the patch to dry and settle for 24 hours. Apply more mortar mix if necessary and trowel smooth. 5 -Paint the patch and the area immediately surrounding it with a waterbased polyurethane sealer, available at home improvement stores, to protect the patch from water seepage and staining.

Instructions For Deep Cracks 6 - Wear safety goggles, work gloves and a dust mask to clean the crack. If it is more than 1/2 -inch deep and 1 inch wide, use a hammer and chisel to remove loose chunks of concrete. Be sure to remove all weak areas around the crack that are crumbling or in danger of breaking off. 7 - Use a stiff wire brush to abrade the crack and sweep it out thoroughly to remove all debris. Paint the crack with a water-based polyurethane sealer, available at home improvement stores. 8 - Fill the crack with concrete repair mortar mix, according to label directions, while the sealer is still wet. Smooth with a trowel and let it set for 24 hours. Add more filler, if needed. 9 - Paint the patch area and the area immediately surrounding it with a water-based polyurethane sealer.

Example

Example: - A plan of a foundation 1 mx 2 m is shown in Figure 3. 30. Estimate the consolidation settlement of the foundation, taking into account primary consolidation settlement.