Development Area Soil Profile Layout

Shallow Foundation
Design Procedure

The basic bearing capacity equation developed by Terzaghi (Equation (3.4)) was modified by Terzaghi and others for application to general foundations to incorporate the effects of shape, depth of foundation and inclination of the applied load. The general equation can be written as:

The following simplified equations are provided for calculation of bearing capacity:

For strip footing

For square footing

Allowable bearing capacity qa is calculated by dividing the ultimate bearing capacity by Factor of safety.

The FOS(Factor of Safety) is provided based on the designer’s experience, the type of soil, and the methods used to compute stresses. Usually for clay soils, FOS ranges from 2.5 to 4.

For analysis purposes, this design adopted a FOS of 3.0.

Design Output

DEVELOPMENT AREA 1The area consists of zone A and zone B soil types. For saturated clay the internal angle of friction ø=0

ZONE A

Two depths are considered of 1.5man average of 2.1m & 0.9m and 3.6m an average of 2.70m & 4.50m where cohesive stresses are determined as (cu) are 75kN/m2 and 90kN/m2 respectively.

The portal frames are subjected to two loading conditions 400KN and 800KN while the walls have a loading of 45kN/m and 60kN/m.

Table 1 : Shallow foundation sizing for zone A

qult (KN/m2)

y (dry)

y (sub)

depth (m)

q=y*d

FOS

Load (KN) Qall

Area (m2)

W (m)

ZONE A FOUNDATION DEPTH AT 1.5m SQUARE FOOTING

foot Load 1

582.75

1.5

5.7

400

1200

2.06

1.43

foot Load 2

582.75

1.5

5.7

800

2400

4.12

2.03

ZONE A FOUNDATION DEPTH AT 1.5m STRIP FOOTING

footing Load 1

454.5

1.5

5.7

135

0.30

footing Load 2

454.5

1.5

5.7

180

0.40

ZONE A FOUNDATION DEPTH AT 3.6m SQUARE FOOTING

foot Load 1

731.7

3.6

64.8

5.7

400

1200

1.64

1.28

foot Load 2

731.7

3.6

64.8

5.7

800

2400

3.28

1.81

ZONE A FOUNDATION DEPTH AT 3.6m STRIP FOOTING

footing Load 1

577.8

3.6

64.8

5.7

135

0.23

footing Load 2

577.8

3.6

64.8

5.7

180

0.31

ZONE C

Consists of Very Soft to Soft Clays. Two depths are considered of 4.4man average of 0.5m & 8.3m and 4.05m an average of 1.0m & 7.1m where cohesive stresses are determined as (cu) are 20kN/m2 and 35kN/m2 respectively.

Table 2 : Shallow foundation sizing for zone A

qult (KN/m2)

y (dry)

y (sub)

depth (m)

q=y*d

FOS

Load (KN) Qal

Area (m2)

W (m)

ZONE C FOUNDATION DEPTH AT 1.5m SQUARE FOOTING

foot Load 1

582.75

1.5

5.7

400

1200

2.06

1.43

foot Load 2

582.75

1.5

5.7

800

2400

4.12

2.03

ZONE C FOUNDATION DEPTH AT 1.5m STRIP FOOTING

footing Load 1

454.5

1.5

5.7

135

0.30

footing Load 2

454.5

1.5

5.7

180

0.40

ZONE C FOUNDATION DEPTH AT 4.4m SQUARE FOOTING

foot Load 1

227.4

4.4

79.2

5.7

400

1200

5.28

2.30

foot Load 2

227.4

4.4

79.2

5.7

800

2400

10.55

3.25

ZONE C FOUNDATION DEPTH AT 4.05m STRIP FOOTING

footing Load 1

272.4

4.05

72.9

5.7

135

0.50

footing Load 2

272.4

4.05

72.9

5.7

180

0.66

DEVELOPMENT AREA 2

Two sites are proposed for the location of the 6 story hotel. The bearing capacity for each site in Zone A and Zone B are computed. Columns are anticipated to carry up to 2200 KN.

ZONE A

Consists of Firm Clay from depth below ground level of 0.90m to 2.10m and from 2.70m to 4.50m in the two extreme situations identified on-site (cu = 75kN/m2 to 90kN/m2). In 80% of the assessed area Firm Clay is present at a depth of between 1.60m and 2.10m

Two depths are considered of 1.5man average of 2.1m & 0.9m and 3.6m an average of 2.70m & 4.50m where cohesive stresses are determined as (cu) are 75kN/m2 and 90kN/m2 respectively.

Table 3 : Shallow foundation sizing for zone A

qult (KN/m2)

y (dry)

y (sub)

depth (m)

q=y*d

FOS

Load (KN) Qall

Area (m2)

W (m)

ZONE A FOUNDATION DEPTH AT 1.5m SQUARE FOOTING

square foot

582.75

1.5

5.7

2200

6600

11.33

3.37

ZONE A FOUNDATION DEPTH AT 3.6m STRIP FOOTING

square foot

577.8

3.6

64.8

5.7

2200

6600

11.42

3.38

ZONE B – Consists of Firm Clay from depth below ground level of 2.70m to 3.30m and from 4.30m to 4.50m in the two extreme situations identified on-site (cu = 80kN/m2 to 95kN/m2).

Two depths are considered of 3.0man average of 2.70m & 3.30m and 4.4m an average of 4.30m & 4.50m where cohesive stresses are determined as (cu) are 80kN/m2 and 95kN/m2 respectively.

Table 4 : Shallow foundation sizing for zone A

qult (KN/m2)

y (dry)

y (sub)

depth (m)

q=y*d

FOS

Load (KN) Qall

Area (m2)

W (m)

ZONE B FOUNDATION DEPTH AT 3.0m SQUARE FOOTING

square foot

646.8

3.0

5.7

2200

6600

10.20

3.19

ZONE B FOUNDATION DEPTH AT 4.4m STRIP FOOTING

square foot

620.7

4.4

79.2

5.7

2200

6600

10.63

3.26

DEVELOPMENT AREA 3

Low rise residential areas residential areas. Wall loading between 30KN/m and 50KN/m. The entire area lies within soil profile zone C.

For zone C – Consists of Very Soft to Soft Clays. Two depths are considered of 4.4man average of 0.5m & 8.3m and 4.05m an average of 1.0m & 7.1m where cohesive stresses are determined as (cu) are 20kN/m2 and 35kN/m2 respectively.

qult (KN/m2)

y (dry)

y (sub)

depth (m)

q=y*d

FOS

Load (KN) Qall

Area (m2)

W (m)

ZONE A FOUNDATION DEPTH AT 4.4m STRIP FOOTING

Load 1

227.4

4.4

79.2

5.7

0.40

Load 2

227.4

4.4

79.2

5.7

150

0.66

ZONE A FOUNDATION DEPTH AT 4.05m STRIP FOOTING

Load 1

272.4

4.05

72.9

5.7

0.33

Load 2

272.4

4.05

72.9

5.7

150

0.55

PILED FOUNDATIONS
Pile Load Capacity

The load-carrying capacity of piles is governed both by its structural strength and the supporting soil properties. Usually, the pile capacity based on soil properties governs the design except probably in timber piles.

The capacity of piles based on structural strength can be obtained by multiplying the area of pile cross-section with the allowable compressive strength of the material of the pile as given for timber pile

The pile capacity determination based on soil properties is done using: (a) dynamic pile driving formula, (b) static methods, (c) correlation with field penetration tests such as SPTand CPTand (d) prototype pile load tests at site.

Static Method

These methods are developed for piles and deep foundations using the soil properties in which they are founded. They assume equilibrium of the pile under the applied loads and resistance offered by the soil in terms of point bearing capacity and the friction and adhesion of the shaft. A single pile subjected to a vertical load and the mechanism of load transfer to the soil is shown in Figure 9.4. Thus the load is transferred to the soil partly as point bearing pressure at its base and partly as friction and adhesion along the surface of the shaft. Hence the ultimate load-carrying capacity of the pile can be obtained from its static equilibrium equation as

Qaf can be estimated using the principles of mechanics and lateral earth pressure distribution along the embedded length of the shaft. Accordingly

DEVELOPMENT AREA 1

ZONE A

Firm Clay – from depth below ground level of 0.90m to 2.10m and from 2.70m to 4.50m in the two extreme situations identified on-site (cu = 75kN/m2 to 90kN/m2). Note in 80% of the area it is assessed that the Firm Clay is present at a depth of between 1.60m and 2.10m

Stiff to Very Stiff Clay at all locations beneath the firm clay extending down to depths varying between 6.50m and 10.00m (cu = 125kN/m2 to 200kN/m2).

Length

Qult (KN/m2)

sat

y (dry)

y (sub)

depth (m)

q=y*d

FOS

Load (KN)

Area (m2)

D (m)

ZONE A 400KN LOAD PILE SIZE

0.9 – 2.1

652.35

577.35

1.2

21.6

5.7

2.1-4.5

673.95

598.95

2.4

43.2

5.7

4.5-6.5

1087.25

962.25

125

5.7

6.5-10

1745

1545

200

3.5

5.7

400

1200

0.29

0.61

ZONE A 800KN LOAD PILE SIZE

0.9 – 2.1

652.35

577.35

1.2

21.6

5.7

2.1-4.5

673.95

598.95

2.4

43.2

5.7

4.5-6.5

1087.25

962.25

125

5.7

6.5-10

1745

1545

200

3.5

5.7

800

2400

0.58

0.86

ZONE B

Firm Clay – from depth below ground level of 2.70m to 3.30m and from 4.30m to 4.50m in the two extreme situations identified on-site (cu = 80kN/m2 to 95kN/m2).

Stiff to Very Stiff Clay – at all locations beneath the firm clay extending down to depths varying between 13.50m and 14.50m (cu = 150kN/m2 to 225kN/m2).

ZONE C

Very Soft to Soft Clay – from depth below ground level of 0.50m to 8.30m and from 1.00m to 7.10m in the two extreme situations identified on-site (cu = 20kN/m2 to 35kN/m2).

Firm Clay – Not present under this Zone.

Stiff to Very Stiff Clay – at all locations beneath the soft clay extending down to depths varying between 16.50m and 18.00m (cu = 150kN/m2 to 225kN/m2).

Length

Qult (KN/m2)

sat

y (dry)

y (sub)

depth (m)

q=y*d

FOS

Load (KN)

Area (m2)

D (m)

ZONE C 400KN LOAD PILE SIZE

0.5-8.3

434.75

399.75

7.8

140.4

5.7

8.3-16.5

1409.1

1259.1

150

8.2

147.6

5.7

16.5-18.0

1919.25

1694.25

225

1.5

5.7

400

1200

0.32

0.64

ZONE C 800KN LOAD PILE SIZE

0.5-8.3

434.75

399.75

7.8

140.4

5.7

8.3-16.5

1409.1

1259.1

150

8.2

147.6

5.7

16.5-18.0

1919.25

1694.25

225

1.5

5.7

400

1200

0.32

0.64

DEVELOPMENT AREA 2

ZONE A

Stiff to Very Stiff Clay at all locations beneath the firm clay extending down to depths varying between 6.50m and 10.00m (cu = 125kN/m2 to 200kN/m2).

Length

Qult (KN/m2)

sat

y (dry)

y (sub)

depth (m)

q=y*d

FOS

Load (KN)

Area (m2)

D (m)

ZONE A 400KN LOAD PILE SIZE

0.9 – 2.1

652.35

577.35

1.2

21.6

5.7

2.1-4.5

673.95

598.95

2.4

43.2

5.7

4.5-6.5

1087.25

962.25

125

5.7

6.5-10

1745

1545

200

3.5

5.7

1000

3000

0.72

0.96

ZONE A 800KN LOAD PILE SIZE

0.9 – 2.1

652.35

577.35

1.2

21.6

5.7

2.1-4.5

673.95

598.95

2.4

43.2

5.7

4.5-6.5

1087.25

962.25

125

5.7

6.5-10

1745

1545

200

3.5

5.7

2200

6600

1.59

1.42

ZONE B

Firm Clay – from depth below ground level of 2.70m to 3.30m and from 4.30m to 4.50m in the two extreme situations identified on-site (cu = 80kN/m2 to 95kN/m2).

Stiff to Very Stiff Clay – at all locations beneath the firm clay extending down to depths varying between 13.50m and 14.50m (cu = 150kN/m2 to 225kN/m2).

Length

Qult (KN/m2)

sat

y (dry)

y (sub)

depth (m)

q=y*d

FOS

Load (KN)

Area (m2)

D (m)

ZONE B 400KN LOAD PILE SIZE

2.7 – 3.3

683.6

603.6

0.6

10.8

5.7

3.3-4.5

820.55

725.55

1.2

21.6

5.7

4.5-13.0

1414.5

1264.5

150

8.5

153

5.7

13-14

1910.25

1685.25

225

5.7

1000

3000

0.62

0.89

ZONE B 800KN LOAD PILE SIZE

2.7 – 3.3

683.6

603.6

0.6

10.8

5.7

3.3-4.5

820.55

725.55

1.2

21.6

5.7

4.5-13.0

1414.5

1264.5

150

8.5

153

5.7

13-14

1910.25

1685.25

225

5.7

2200

6600

1.37

1.32

GROUND IMPROVEMENTS TECHNIQUES

Foundations on Clays or Silts

Silts and clays may vary from very soft, normally consolidated to very stiff and overconsolidated soils. Problems mainly occur in the case of soft deposits. Extra care should be taken to estimate the design soil pressures in terms of bearing capacity and allowable soil pressures (from allowable settlement criterion) in such cases

Such soils undergo volume changes due to wetting and drying. These are found mostly in arid and semi-arid regions and contain large clay minerals. Following remedial measures may be useful in such cases.

Alter the expansive nature of the soil by stabilizing the soil with lime, cement, or other admixtures.
Control the direction of expansion by allowing it to expand into cavities. A common practice is to build waffle slabs (Bowles, 1996).
Control the water by excavating the soil to such a depth that the weight of the soil will be sufficient to control the heave, lay a geotextile plastic fabric which is impermeable and then backfill.
Increase the depth of the footing such that the heave does not cause any detrimental effect. One can use belled piers with the bulged bell of sufficient depth (Bowles, 1996) or use under reamed piles.
Increase the surcharge load on the soil surface to counter the swell pressure caused by heaving

Uneven settlement creates cracks in connecting structural components such as beams and slabs. Even connecting walls and slabs develop cracks due to these uneven settlements. The cracks usually develop in the diagonal direction through vertical cracks are also possible. Remedies include;

Use a raft foundation with or without stiffening beams in one or more directions.
Reduce the net pressure transmitted to the soil by providing deep basements
Use piles, piers or basement slab foundations, pile–raft systems to transfer large loads from the superstructure to strong deeper soils with low compressibility
Provide jacking pockets or brackets in columns to re level the alignment of the superstructure when necessary.
Provide additional loads on lightly loaded parts of the structure if feasible.