Geotechnical Design of Shallow Foundations

Chapter # 03

Engr. Imran Hafeez

GENERAL REQUIREMENTS OF FOUNDATION:- The only requirement is that it should not fail

i.e. it should work efficiently under all conditions of working.

Failures are of two types; Bearing capacity failure. Excessive settlement. So the foundation must be safe against both

the above failures and also it must be properly located. This proper orientation of a foundation can be well explained with the following example;

Case:

Consider a footing ‘A’ say at depth D, and width B. the effective stress envelope is shown in Figure 01. The stresses within the zone are within permissible limits. Latter on a foundation ‘B’ is constructed such that stress envelopes intersect. Now the stress at point ‘X’ will be the sum of the affects of ‘A’ &’B’. This should not increase than the bearing capacity of the soil.

Diagram # 01

AB

X

Example

In Badami Bagh area, many tall buildings collapse. Actually close to these, excavation was carried out and as a result stress pattern under the old buildings changes and ultimately collapses. That is if latter on these changes are to be made, these must be designed already and temporary supports must be given to existing buildings, called as underpinning.

So for design of a footing, both settlement and bearing capacity are checked.

GENERAL REQUIERMENT FOR ANY DESIGN:- By design we always mean that what are the

stresses acting on a particular member and the corresponding size of the members that theses stresses can be carried out efficiently. The general principle of the design is to design the structure into different elements. We are going to discus only one element, i.e. foundation.

Designing is done in two stages; Analysis Sizing.

STRUCTURAL ANALYSIS:-

In first step of every design, we analyze the state of stress and see the strain due to these stresses. In analysis we see the type of loading, type of strain and the modes of failure. In foundation design these stresses are called as bearing capacity and strains as settlements.

So in foundation design, analysis means the determination of bearing capacity and settlement. We have various methods both field tests and empirical methods for finding bearing capacity and settlements.

SIZING:-

Step # 01(Material Selection)Before going to sizing, we decide about the material to be used in the construction of the footing e.g. wood, concrete, steel etc. it depends upon the availability of the material and economy. The cost of project mainly depends on it. Since the foundation system is a very complex system, the construction material is not homogeneous. It consists of soil and other materials (wood, concrete etc.). here we will take concrete only.

Step # 02 (Dimensioning)Now using the data from analysis and the material selected the dimension are chosen (i.e. thickness, width, depth of pad) and the design is completed.

Step # 03 (Documentation)Now the design is represented in the form of drawings and the construction specifications (i.e. procedure, problems and solutions) are also mentioned.

SIZING:-

SPECIFIC DESIGN OF FOUNDATION:-

Design means the determination of; Df =? d =? B =? L =? As =?

P

G.S.L

Df

BXL

B

L

Design Components

Design is divided into two parts; GEOTECHNICAL DESIGN:- The design that takes into account only the properties of

soil is called as Geotechnical Design. SCOPE OF DESIGN:- The scope of geotechnical design is; a) Df =? b) B =? c) L =? GOAL:- The goal of geotechnical design is; Bearing capacity. Settlement should be within permissible limits.

STRUCTURAL DESIGN:-

In design that takes into account the technical aspects related to concrete is called as Structural Design.

GEOTECHNICAL DESIGN OF FOUNDATION:-

For the geotechnical deign of the foundation the following steps are observed;

1. The choice of foundation system between deep and shallow foundation.

2. Fix the vertical location of the foundation i.e. Df in case of shallow and ‘L’ in case of pile foundation.

3. Bearing capacity and settlement analysis and choose an appropriate value of the design pressure “qd”, bearing capacity equation and settlement equation.

4. Using the information of (3) fix the dimensions in the plane i.e. ‘B’ & ‘L’.

5. Evaluate the construction problems such as the problem of excavation, dewatering, water proofing and water tightening, deterioration of concrete and suggest their remedial measures.

STEPS OF GEO-TECHNICAL DESIGN1. Selection of the type of foundation system.

2. Fix the vertical location i.e. Df of the foundation.

3. Bearing Capacity and Settlement Analysis and from this a suitable value of “qd” i.e. the design pressure.

4. The dimensions in plane (B & L)

5. Construction Specification.

Step No. 1:-Selection of the foundation type:- For this the following steps are kept in mind Type of structure and its requirements Sub soil profile at the site. Overall impact on the environment. Relative cost and construction facilities. Broadly speaking the types of foundation are;

Shallow Foundations. Deep Foundations. Floating Foundations.

Step No. 2:- DEPTH OF FOUNDAION;1. For depth of foundation, the following two

considerations are kept in mind,2. Mechanical Consideration.3. Physical Consideration.4. In mechanical consideration we check Bearing

Capacity and Settlement.5. For Bearing Capacity Terzaghi’s equation (for

general share failure) is applied i.e.6. qult = ScCNc + γ Df Nq + Sγ0.5γBNγ7. From this equation it is clear that with the same

soil, the properties remain the same and if ‘b’ is kept constant, then the Bearing Capacity goes on increasing by increasing the depth, but economy is also given due regards.

For pure clay;

(qult )Net =ScCNc + γDf (Nq-1) + Sγ 0.5γBNγ

C = qu/2

Nc = 5.7

i.e. for clays, the depth effect is zero. But for sands there is effect of depth.

Similarly in the case of Settlements, it goes on decreasing by increasing the depth.

S = Cc/ (1 + eo) (H) log (σ0 +Δ σ)/(σ0)

Physical Requirements:-

Following are the different physical requirements; Footing should be below Top organic soil. Susceptible zone.

Surface erosion zone. Frost line. Scour depth.

There should be the specified edge distance i.e. Level Difference

Step No. 3:- BEARING CAPACITY ANALYSIS:-

FOR BEARING CAPACITY:- qult = CNc + γDfNq + 0.5γBNγ qult / F.O.S = safe gross B.C or safe B.C (qult )Net = CNc+ γDf (Nq-1) + 0.5γBNγ (qult )Net / F.O.S = safe net B.C For square footing and circular footing; qult = 1.3CNc + γDfNq + 0.5γBNγ For pure clay Nc =5.7, Nq=1 & Nγ=0 B.C calculated by these equations is called B.C w.r.t. shear.

BEARING CAPACITY WITH S.P.T ‘N’ VALUES:- In F.P.S system; For square footing, qult =2N2BRw + 6(N2 + 100)DfRw’ For very long footing, qult =3N2BRw + 5(N2 + 100)DfRw’ Where, qult =Net ultimate bearing pressure,(PSF)

=Pressure at bottom of footing in excess of the pressure at the same level due to the weight of soil immediately surrounding the footing.

N =Standard Penetration Test. B =Width of footing. Df =Depth of footing. If the ground levels on both sides of footing are not equal, D should be

measured from the lowest ground level. If D > B, use D + B for computation.

Correction factors for position of water levels Rw & Rw’ =correction factors for position of water levels.1. If water table is at a depth ‘B’ or greater from the bottom of footing then Rw = Rw’ = 1 If water table is at base of footing Rw’ = 1 Rw = 0.5 If water table is at top Rw & Rw’ = 0.5 And in between the linear variation is made. Is S.I system, For square footing; qs = 0.105N2BRw + 0.314(N2 + 100)DfRw’ with F.O.S = 3 Here B &Df are in “m” and q = KPa.

Net allowable bearing pressure in psf for maximum settlement of 1” is

qa = 720(N-3) (B = 1 / 2B)2 Rw Kd

Where, Kd = 1 + D / B ≤ 2 If N value is given and there is no information about

Ø & γ then from Mayerhoff’s Ø =28 + N / 10 And for the minimum density samd unit, Weight γ = 115 – 120pcf.

FOR SETTLEMENT ANALYSIS:- δ = [Cc / (1 + lo)] (H) log (бo +Δ б) / бo Where, Cc =compression index, for normally consolidated clays. Cc =0.009(L.L - 10) Δ =initial overburden pressure at that level.(above mid-

height of consolidating layer) бo =γ(Df + level). Δ б =additional pressure at that level. =P / (B + Z) H =length of strata (layer). If the soil is drained on top and

bottom as in the consolidation test, half thickness should be used

e0 =natural void ratio of the soil in place.

Settlement from S.P.T ‘N’ value:- δ = 2 / N [B / (B + 0.3)]2 Cd Cw qa δ = Settlement in “mm” Cd = Depth factor =1 / Kd Kd = 1.033D / B ≤ 1.33 Cw = Water correction factor. = 1 (always) It is reflected in ‘N’ values. qa = ∆ б’ So, δ = 2 / N [B / (B + 0.3)]2 Cd(∆бo) Here, Cd = 1 / Kd & Kd = 1 + Df / B (.33) ≤ 1.33 The Df used in the layers other than the first one is at the level of the layer under

consolidation. Similarly in ∆ б = P / (B + Z)2 Where, Z is the distance up to the c/l of layer from the bottom of footing