settlement of shallow foundations

Supervisor: Dr.Ghazavi

Researcher: Amirmasoud Taghavi

Settlement of Shallow foundations

KNTU university of technology

Definitions

Settlement is the vertically downward movement of structure due to

the compression of underlying soil because of increased load.

• Maximum Settlement : It is the

absolute maximum downward

movement of any part of buildi

ng element.

• Maximum Settlement = Smax

• Differential Settlement : It is the

maximum difference between two

points in a building element.

• Differential Settlement = Smax - S

min

Weak Bearing Soils

Poor Compaction

Changes in Moisture Content

Maturing Trees & Vegetation

Soil Consolidation

The causes of foundation settlement

Settlement cracks tend to widen and worsen over time. As they

do, they will allow outside influences into your home, such as

cold air, humidity, rainwater, termites, rodents, and more. The

damage will have an enormous impact on the value and resale

value of your home.

Several things cause a foundation to fail

1.Evaporation: Hot dry wind and intense heat will often

cause the soil to shrink beneath the foundation.

This settlement may cause cracks to appear throughout

the structure.

2.Transpiration: Tree roots may desiccate the soil beneath a

home causing the soil to shrink and the home to settle.

3.Plumbing Leaks: Water from plumbing leaks is often a

cause to foundation problems.

4.Drainage: Improper drainage is one of the leading causes to

foundation failure. Excess moisture will erode or consolidate

soils and cause settlement.

5.Inferior Foundation Construction: Insufficient steel and

inferior concrete will contribute to movement in the slab.

6.Inferior Ground Preparation: Soft, low density soils and/or

improperly compacted soil beneath a home is the leading

cause of foundation failure. Cut and fill methods are a leading cause of

foundation settlement.

7.Poor Soil conditions: Poor soil and its expansion and/or contraction

contribute to foundation failure.

The most common cause of foundation settlement is expansive

soils such as clay. When these highly permeable soils get wet,

they expand. Likewise, when they dry out, they contract. This

causes the soil under your home or property to settle and shift

at an uneven rate, compromising the stability of your structure

Signs of foundation failure

When all parts of a building rest on the same kind of soil, and

the loads on the building and the design of its structural system

are uniform throughout, differential settlement is normally not a

concern. However where soils, loads, or structural systems differ

between parts of a building, different parts of the building structure

may settle by substantially different amounts, the frame of the

building may become distorted, floors may slope, walls and glass

may crack, and doors and windows may not work properly.

Types of Settlement

Immediate Settlement or Elastic settlement

Occurs immediately during or after the construction of structure.

Also known as ‘Distortion Settlement’, it occurs due to distortion in

foundation soil.

Although its not truly elastic, it is computed using elasticity theory. It

is denoted by 𝑆𝑖 . It occurs in all types of soil due to elastic compression.

It occurs immediately after the application of load

It depends on the elastic properties of foundation soil, rigidity, size

and shape of foundation.

Typical Range of Poisson’s Ratio for

different soils

Typical Range of Soil Modulus in

Undrained state

Primary Consolidation OR Consolidation

settlement

Occurs 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. This component is determined using

Terzaghi’s theory of consolidation.

It is denoted as Sc.

1It occurs due to the process of consolidation.

Clay and Organic soil are most prone to consolidation settlement.

Consolidation is the process of reduction in volume due to

expulsion of water under an increased load.

It is a time related process occurring in saturated soil by draining

water from void.

It is often confused with Compaction.

Consolidation theory is required to predict both rate and magnitude

of settlement.

Types of movement for different kind of soil’s

𝑆𝑟 > 90% → − 𝑆𝑟 < 90% → 𝑌𝑒𝑠

Stress Increase in Soil Due to Applied

Load

For any of the mentioned settlement calculations, we first need

vertical stress increase in soil mass due to net load applied on

the foundation.

The total settlement (S)

Total foundation settlement can be divided into three different

components, namely Immediate or elastic settlement, consolidation

settlement and secondary or creep settlement as given below.

The total settlement is given as;

𝑆 = 𝑆𝑖 + 𝑆𝑐 + 𝑆𝑠 + 𝑆𝜃

IMMEDIATE SETTLEMENT COMPUTATIONS

𝑆𝑖 = 𝑞0𝐵′1 − 𝜇2

𝐸𝑠𝑚𝐼𝑠𝐼𝐹

• In practice, most foundations are flexible. Even very thick ones

deflect when loaded by the superstructure loads. Some theory

indicates that if the base is rigid the settlement will be uniform (but

may tilt), and the settlement factor Is will be about 7 percent less

than computed by Eq.

• On this basis if your base is "rigid" you should reduce the Is factor

by about 7 percent (that is, Isr=0.931Is)

𝐵 = 𝐵2

𝐿′ = 𝐿2

m=4

𝐵 = 𝐵2

𝐿′ = 𝐿

m=2

𝐵 = min(𝐿 2 , 𝐵)

𝐿′ = 𝐿2

m=4

𝐵 = 𝐵

𝐿′ = 𝐿

m=1

• least lateral dimension of contributing base ar

ea in units of ∆𝐻

B’

𝐼𝑖(𝐼𝑠, 𝐼𝑓) • influence factors, which depend on(

𝐿′

𝐵′) thickness of stratum ∆𝐻,

Poisson's ratio(𝜇), and base embedment Depth (𝐷𝑓)

𝐼𝑠 = 𝐼1 +1−2𝜇

1−𝜇𝐼2

𝑁 =𝐻

𝐵′ 𝑀 =

𝐿′

𝐵′

• Use linear interpolation when it needed.

𝐼𝐹

• Use linear interpolation when it needed.

𝑆𝑖 = 𝑞0𝐵′1 − 𝜇2

𝐸𝑠𝑚𝐼𝑠𝐼𝐹

The equation should be used as follows

(Bowles method

1. Make your best estimate of base contact pressure (𝑞0) 2. For round bases, convert to an equivalent square.

3. Determine the point where the settlement is to be computed and divide

the base.(find B’, L’ and m)

4. Find depth actually causing settlement

𝐻 = min(5𝐵,Depth to where a hard stratum is encountered)

5. Enter Table 5-2, obtain 𝐼1 and 𝐼2 with your best estimate for 𝜇, compute

𝐼𝑠and obtain 𝐼𝐹 from the figure.

6. Obtain the weighted average 𝐸𝑠 in the depth z = H

𝐸𝑠𝑎𝑣𝑒 =𝐻1𝐸1 +𝐻2𝐸2 +𝐻3𝐸3 +⋯ .+𝐻𝑛𝐸𝑛

𝐻

7. Immediate Settlement→ 𝑆𝑖 = 𝑞0𝐵′1−𝜇2

𝐸𝑠𝑚𝐼𝑠𝐼𝐹

THANKS

FOR YOUR

ATTENTION