Types of Deep Foundations used in Construction and Their Uses

What is the maximum settlement for isolated foundations on clayey soils?

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.

The Use of the Structure

Even small cracks in a house might be considered unacceptable, whereas much larger cracks in an industrial building might not even be noticed.

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 considering some 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 be considered 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 mm settlement of pile deformation from vertical compressive loads. For such structure, foundation settlement should not be more 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 endure overall 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 been manufactured using Bauer equipment by Skanska EMV Ltd in October 2007. The test pile is made on 6th October. The diameters of the test and reaction piles are 880 mm. The length of test pile is 28.6 m. Concrete type is C30/37. Steel reinforcement 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 the perimeter. 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 with tension piles by welding and bolts 2. One 4.5 m length steel beam having the bearing capacity 6000 kN 3. Hydraulic jack with electrical oil pump. Maximum load of the jack is 5650 kN 4. Dial gauges with accuracy 0.1 mm for measurement of pile top displacements. The displacements of the compression pile have been measured with four and the displacements of the every tension piles with two gauges. The gauges were connected with piles by steel wire 5. Reference beams for dial gauges. The length of reference beams was between 6 to 8 m and these were supported on the ground The 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 and every each hour (beginning from 12:00 hour) the load was increased by 250 kN. The basic design load Q (2500 kN) was reach after 12 hours. The settlement value was 5.2 mm. Fig. 3 shows a shape of the load-settlement curve of static load pile test. 

allowable settlement of raft foundation

The maximum differential settlement in foundation on clayey soils and sandy soils should not exceed 40mm and 25mm respectively. The maximum settlement should generally be limited to the following values: Raft foundation on clay – 65 to 100 mm.Raft foundation on sand – 40 to 65 mm.

minimum depth of foundation as per is code

The prescriptive pressures for Foundations in Sands and Clays for a minimum depth of 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 raft foundationsshould be 65 – 100 mm, and 50 mm for raft foundations on sand.

Shallow  Foundations

Minimum  Thickness  

       The minimum thickness of a shallow building foundation shall be 250 mm. The prescriptive pressures for Foundations in Sands and Clays for a minimum depth of 0.6 m below ground level is given in Tables 6 and 7 respectively in the Code.

Settlement

     The recommended design limits for settlement in clay for raft foundations should be 65 – 100 mm, and 50 mm for raft foundations on sand.

Deep  Foundations

—Timber pile foundations shall be provided where the upper soil strata is of poor strength and poor compressibility characteristics.   

—The  minimum recommended dimensions for timber piles 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

Types of Deep Foundations used in Construction and Their Uses