SETTLEMENT ANALYSIS By: Engr. Shujaat Abbas

Contents 1. Definition 2. Types & Modes of settlements 3. Primary and secondary consolidation settlements 4. Normally & Pre-consolidated soils 5. Mechanics of consolidation 6. Theory of one dimensional consolidation, assumptions and validity 7. problems Settlement Analysis

 Settlement : Vertical downward movement or displacement caused by a change in stress Settlement Analysis Magnitude of settlement can be computed if we know 1- Magnitude of the change in stress 2- Stress strain properties of soil For other materials like steel & concrete calculation is simple problem (Young’s modulus & Poisson’s ratio)  Soil has a much complicated stress-strain behavior that reflects multiple physical processes some of them are time dependent  Types of settlement w.r.t. 1.Mode of occurrence 2.Permanency 3.Uniformity

1- Mode of occurrence 1. Immediate settlement or constant volume settlement or un-drained settlement, s i : Completion time within 7 days 2. Primary consolidation settlement, s c : Time dependent settlement, or long term settlement, completion time 1 to 5 years or more. 3. Secondary consolidation settlement, s s : Settlement under constant effective stress with no drainage, predominant in organic soils, also known as creep. Settlement Analysis

2. Permanency 1- Permanent or Irreversible: Settlement due to sliding and rolling of soil particles and due to change of material size because of crushing (e.g. settlement due to consolidation) 2-Temporary or Recoverable or Reversible Settlement: Settlement due to elastic deformation, usually small in soils. Settlement Analysis

3. Uniformity 1- Uniform settlement: Possible under rigid foundations loaded with uniform pressure founded on uniform soil deposit (a rare possibility). No risk to structural stability, Risk to serviceability and utility lines. 2-Differential settlement: Risk to structural stability, angular distortion,  = 1/250 noticeable to naked eye. 2/3 of maximum settlement in sands and 1/3 in clays. Settlement Analysis

Types of settlement Distortion Settlement Consolidation Settlement Secondary Compression Settlement Settlement Analysis

1- Distortion Settlement :  d  Results from lateral movements of soil in response to changes in vertical effective stress  No volume change  Poison's effect where an object loaded in vertical direction expands laterally Distortion settlement primarily occurs when the load is confined to a small area e.g. near the edges of large loaded areas such as embankments Settlement Analysis

2- Consolidation Settlement:  c (primary consolidation settlement)  Most important source of settlement.  This phenomenon happens in all soils as they are subjected to increase in vertical effective stress  Volume of voids decreases due to expulsion of water from voids (as water is incompressible) in saturated soil  Re-arrangement of soil particles in tighter packing This process occurs quickly as load is applied. Fast in granular soils Slow in fine soils Rate of consolidation depend upon  Degree of saturation of soil  Hydraulic properties of soil Settlement Analysis

Secondary compression Settlement :  s As primary consolidation completes it starts. It is due to Particles re-orientation Creep Decomposition of organic materials It is not due to change in vertical effective stress It occurs at a constant vertical effective stress It is time dependent process and can be significant in highly plastic clays, organic soils and sanitary land fills. It is negligible in sands and gravels. Settlement Analysis

The settlements (  ) at the ground surface is the sum of these components  =  d +  c +  s Because soil settlement can have both time dependent and non-time dependent components, it is often categorized in terms of short term (immediate) settlement and long term settlement Settlement Analysis

Total50 mm (but limiting value of 25 mm is usually adopted for isolated footings and 50 mm for rafts) Differential20 mm Relative rotation1/500 Relative rotation (also known as angular distortion) between two points under a structure is equal to the differential settlement between the points divided by the distance between them. This is critical for framed buildings and reinforced load bearing walls. Deflection ratio (ratio of deflection to the length of the deflected part) is the criterion for unreinforced load bearing walls which fail by sagging or hogging. Settlement Analysis

Classification based on structural damage CategoryDegreeDescriptionCrack width (mm) 1Very slightEasily treated by decoration < 1.0 2SlightFill cracks< 5.0 3ModerateRepointing & repair5 to 15 4SevereReplace sections15 to 25 5Very severeRebuild> 25 Settlement Analysis