For an isolated foundation on clayey soil, max allowable settlement is 75mm as per IS 1904.For sands & hard clay it is only 50mm. And for rafts founded on sands & hard clay it is limited to 75mm whereas for soft clays it is allowed upto 100mm.
allowable settlement for foundation as per is code
The allowable settlement is defined as the acceptable amount of settlement of the structure and it usually includes a factor of safety.
The allowable settlement depends on many factors, including the following:
The Type of Construction
For example, wood-frame buildings with wood siding would be much more tolerant than unreinforced brick buildings.
Even small cracks in a house might be considered unacceptable, whereas much larger cracks in an industrial building might not even be noticed.The Use of the Structure
The Presence of Sensitive Finishes
Tile or other sensitive finishes are much less tolerant of movements.
The Rigidity of the Structure
If a footing beneath part of a very rigid structure settles more than the others, the structure will transfer some of the load away from the footing.
However, footings beneath flexible structures must settle much more before any significant load transfer occurs. Therefore, a rigid structure will have less differential settlement than a flexible one.
Aesthetic and Serviceability Requirements
The allowable settlement for most structures, especially buildings, will be governed by aesthetic and serviceability requirements, not structural requirements.
Unsightly cracks, jamming doors and windows, and other similar problems will develop long before the integrity of the structure is in danger.
Table below shows the allowable foundation displacement into three categories: total settlement, tilting, and differential settlement.
It indicates that those structures that are more flexible (such as simple steel frame buildings) or have more rigid foundations (such as mat foundations) can sustain larger values of total settlement and differential movement.
| Type of Settlement | Limiting factor | Maximum Settlement |
| Total settlement | Drainage | 15 – 30 cm |
| Access | 30 – 60 cm | |
| Probability of non-uniform settlement: | ||
| 1. Masonry walled structures | 2.5 – 5 cm | |
| 2. Framed structures | 5 – 10 cm | |
| 3. Chimneys, silos, mats | 8 – 30 cm | |
| Tilting | Stability against overturning | Depends on H and L |
| Tilting of chimneys, towers | 0.004L | |
| Rolling of trucks etc. | 0.01L | |
| Stacking of goods | 0.01L | |
| Crane rails | 0.003L | |
| Drainage of floors | 0.01 – 0.02 L | |
| Differential settlement | High continuous brick walls | 0.0005 – 0.001 L |
| One-storey brick mill building, wall cracking | 0.001 – 0.002 L | |
| Plaster cracking | 0.001 L | |
| Reinforced concrete building frame | 0.0025 – 0.004 L | |
| Reinforced concrete building curtain walls | 0.003 L | |
| Steel frame, continuous | 0.002 L | |
| Simple steel frame | 0.005 L | |
Where, L = distance between adjacent columns that settle to different amounts, or between two points that settle differently. Higher values are for regular settlements and more tolerant structures. Lower values are for irregular settlement and critical structures. H = Height and W = width of structure.
allowable settlement of piles
length, L, and diameter, D, ratio is larger than 5. Reduction factor, F1 , was set to 0.75, since point bearing and consideringsome skin resistance.According to Eq. (1) the total value of settlement, S p , was estimated to be equal to 15.6 mm. This value could beconsidered as maximum, obtained from the conservative side, based on end-bearing behavior of the pile.Pile settlement analysis showed that total expected maximum settlement value was 15.6 mm. It includes 2.1 mmsettlement of pile deformation from vertical compressive loads. For such structure, foundation settlement should not bemore than 2% of pile diameter. For the pile of 880 mm diameter, the foundation settlement should not be more than 16 mm.The calculation shows, that for pile of diameter 880 mm, the necessary length was 29 m. Such length is sufficient to endureoverall load.
Static load pile test
On the construction site one test pile and four reaction piles were prepared. The cast-in-place bored piles have beenmanufactured using Bauer equipment by Skanska EMV Ltd in October 2007. The test pile is made on 6th October. Thediameters of the test and reaction piles are 880 mm. The length of test pile is 28.6 m. Concrete type is C30/37. Steelreinforcement cage is made from main bars 10 pcs. Ø20 mm, over full pile length and transverse bars Ø12 mm, step 0.15 m.The outer diameters of the reinforcement cages is 680 mm, the longitudinal bars are distributed eventually along theperimeter. The pile head (1.5 m) is strengthened with a tubular casing.The testing equipment consists of the following parts:1. Two 12 m length steel trusses, having the total bearing capacity 2×3000 = 6000 kN. The trusses are connected withtension piles by welding and bolts2. One 4.5 m length steel beam having the bearing capacity 6000 kN3. Hydraulic jack with electrical oil pump. Maximum load of the jack is 5650 kN4. Dial gauges with accuracy 0.1 mm for measurement of pile top displacements. The displacements of the compressionpile have been measured with four and the displacements of the every tension piles with two gauges. The gauges wereconnected with piles by steel wire5. Reference beams for dial gauges. The length of reference beams was between 6 to 8 m and these were supported on thegroundThe testing procedure was carried out according to Iberdrola’s “Method statement for testing preliminary pile” [10].The static load pile test was performed gradually increasing the load. The test was started at 10:00 in the morning andevery each hour (beginning from 12:00 hour) the load was increased by 250 kN. The basic design load Q (2500 kN) wasreach after 12 hours. The settlement value was 5.2 mm. Fig. 3 shows a shape of the load-settlement curve of static load piletest.
allowable settlement of raft foundation
The maximum differentialsettlementinfoundationon clayey soils and sandy soils should not exceed 40mm and 25mm respectively. The maximumsettlementshould generally be limited to the following values:Raft foundationon clay – 65 to 100 mm.Raft foundationon sand – 40 to 65 mm.
minimum depth of foundation as per is code
The prescriptive pressures forFoundationsin Sands and Clays for aminimum depthof 0.6 m below ground level is given in Tables 6 and 7 respectively in theCode. The recommended design limits for settlement in clay for raftfoundationsshould be 65 – 100 mm, and 50 mm for raftfoundationson sand.
Shallow Foundations
Minimum Thickness
The minimum thickness of a shallowbuilding foundation shall be 250 mm.The prescriptive pressures forFoundations in Sands and Clays for a minimum depth of 0.6 m below ground levelis given in Tables 6 and 7 respectively in the Code.
Settlement
The recommended design limits forsettlement in clay for raft foundations should be 65 – 100 mm, and 50 mm forraft foundations on sand.
Deep Foundations
Timberpile foundations shall be provided where the upper soil strata is of poorstrength and poor compressibility characteristics.
The minimum recommended dimensions for timberpiles shall be:
(i)minimum butt diameter - 300 mm
(ii)minimum tip diameter - 200 mm
(iii)limiting structural defects such as reverse bends, twisted grain etc.
read more:CONSTRUCTION OF WALL FOOTINGS
What are Methods of Driving Piles Over Water?Selection of Pile Foundation Based on Soil Condition
