IRJET- Analysis of Cavities on Performance of Shallow Footing

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ANALYSIS OF CAVITIES ON PERFORMANCE OF SHALLOW FOOTING Thore Rehka Lahanu1, Amol B Saner2 1ME

Scholar, Matoshri College of Engineering Nashik 422213 Maharashtra, India Department of Civil Engineering, Matoshri Ciollege of Engineering, Nashik 422213 Maharashtra, India ---------------------------------------------------------------------------------***------------------------------------------------------------------------------ABSTRACT : The stability of any structure depends on bearing capacity of foundation soil, which plays major role in geotechnical engineering. The bearing capacity will change with presence of minerals in soil, with level of water table and with presence of cavities or voids in soil. The objective is to study the effect of number of voids, shape of void, size of void, spacing between voids and depth of voids and arrangement of voids on the stability of foundation. The parameters considered for the present study includes load inclination (0, 10, 20, and 30). This parameter effect is studied using PLAXIS 2D. 2Professor,

Keywords: Stability, cavities, Shallow footing, Bearing Capacity, PLAXIS 2D. Keeping in view the literature following objectives is planned.

1. INTRODUCTION: The existence of underground void affects stability of rigid surface structures such as foundations, rigid pavements over tunnels and underground pipe lines and also the integral stability of structure. Due to the population growth and in response to existing needs, the demand of tunnels for urban transportation has rapidly increased. These tunnels usually excavated close to the soil surface and their effects will develop to the ground level and can significantly affect the performance of shallow foundations located above these voids. Results of prior researches in this field indicated that the interaction between shallow foundation and underground voids has significant effects on the performance of shallow foundations.

i. ii. iii. iv.

2. METHODOLOGY

1.1 Necessity of Studying Effect of Cavities on the Performance of Footing: It is not uncommon that structures are founded above underground voids. A majority of the engineers contacted feel that cavities occur under structures at greater frequency in the areas having limestone and dolomite formations, and also in the areas which have been involved in active mining operations. The presence of underground void can cause serious engineering problem leading to instability of the foundation, incurring, severe damage to the super structure. If the void is located just below the footing at shallow depth, the consequence can be very costly and dangerous.

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Defining the geometry and finite element model layout.

2.

Specifying material parameters: appropriate selection of material strength and stiffness parameters from laboratory or in-situ tests.

3.

Generating stresses.

4.

Construction staging i.e. defining various stages of excavation using staged construction.

5.

Engineering judgment.

In many situations it necessitates that the placement of footings on the soil having cavities e.g. footings above the tunnels, conduits, sewer networks. The provision of footing on voids may results into failure and hence there is need to study the effect of void on interference of footing. Fig. 3.3.1 shows the strip footing of width B subjected to vertical load placed on soil model having circular void of area A and two footings subjected to vertical loading having circular void of diameter D beneath it. The soil model is rectangular in shape.

The literature review shows that studies on footing over void had been carry out considering many factor such as footing width, eccentricity of void, vertical distance of void from base of footing, size of void, shape of void, spacing of voids, depth of void by analytically and experimentally. However it is observed that the study of effect of void needs for effect of multiple voids (more than two), interference of footing in presence of void etc.

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1.

2.1 Problem Definition

1.2 Objectives:

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Effect of multiple voids on bearing capacity of footing Effect of void on interference of footing The factor influencing bearing capacity of footing in presence of void such as size, depth, shape, spacing and distance of void from footing Effect of type of loading in footing on voids.

B = Width of footing

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t = Thickness of footing

Figure 2.1: Problem Statement for Analysis

A= Area of void

3. Computational Analysis with PLAXIS 2D

Z = Depth of void from base of footing

The effect of cavities on the performance strip footing behavior subjected to vertical/ inclined load had been studied using PLAXIS 2D software. Initially the analysis was carried out only for determining ultimate bearing capacity of natural soil model (20B×15B) with footing placed at the top of the soil model at its center for vertical and inclined load. There after the analysis were carried out for ultimate bearing capacity of soil model having multiple voids of different shapes as shown in Fig. 3.1 in it parallel to axis of footing with different load inclination. The various cases studied using PLAXIS 2D is presented in Table 1.

S = Spacing between voids θ = Angle of load inclination with vertical axis D= Diameter of circular void Y= Spacing between two footing P = Applied load

PƟ t B

Z

Figure 3.1 Geometry for Shapes of Cavities

S P

P Y

B

B

Z

t

D Sr. No.

Cases

Shape

1

Case I

Square, Rectangular, Irregular, Circular

2

Case II

Square, Rectangular, Irregular

0, 0.5, 1

3

Case III

Square, Rectangular, Irregular

0, 0.5, 1

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Eccentricity of Combination (X/B) 0, 0.5, 1

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Combinations

Load Inclination

No of Footings

No. of Voids

All In One Row, All In One Column, Right Angle Placement, Skew Placement All In One Row, All In One Column, Right Angle Placement, Skew Placement All In One Row, All In One Column, Right Angle Placement, Skew Placement

0°

1

3

10°

1

3

20°

1

3

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4

Case IV

Square, Rectangular, Irregular

0, 0.5, 1

5

Case V

Circular

0

All In One Row, All In One Column, Right Angle Placement, Skew Placement ---

30°

1

3

0°

2

1

Table 1 Cases for Complete Analysis Program 3.1 Case I: Analysis of Single Footing above Three Voids for Different Combinations and Shapes Subjected to Vertical Loading

ALL IN ONE COLUMN

UBC (kN/m2)

3.1.1 Square Cavity: Figure 3.1.1.1 (a-d) illustrates the results of computational analysis for single footing placed on three square cavities with varying depth (Z/B) for all in One Row combination and varying area (A) for all in One Column, right angle placement and Skew Placement for different eccentricities (X/B). Here (Z/B) = 0.5, 0.75, 1, 1.5, 2, 3, A = 0.25, 1, 2.25, 4 m2 and (X/B) = 0, 0.5, 1

500 450 400 350 300 250 200 150 100 50 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Single Cavity Area (m²)

500 450 400 350 300 250 200 150 100 50 0

X/B=0

X/B=0.5

X/B=1

NO CAVITY

b RIGHT ANGLE PLACEMENT 500

0

0.5

1

1.5

2

2.5

UBC (kN/m2)

UBC (kN/m2)

ALL IN ONE ROW

3

Z/B X/B=0

X/B=0.5

X/B=1

NO CAVITY

400 300 200 100 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Single Cavity Area (m²)

a

X/B=0

X/B=0.5

X/B=1

NO CAVITY

c

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SKEW PLACEMENT

ALL IN ONE COLUMN

400 300

UBC (kN/m2)

UBC (kN/m2)

500

200 100 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Single Cavity Area (m²) X/B=0

X/B=0.5

X/B=1

NO CAVITY

500 450 400 350 300 250 200 150 100 50 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Single Cavity Area (m²)

d

X/B=0

X/B=1

X/B=2

NO CAVITY

Figure 3.1.1.1 Variation of UBC for Different Eccentricities of Combination for Vertical Loading

b

3.1.2 Rectangular Cavity:

RIGHT ANGLE PLACEMENT

Figure 3.1.2.1 (a-d) illustrates the results of computational analysis for single footing placed on three rectangular cavities (aspect ratio 2:1) with varying depth (Z/B) for All In One Row combination and varying area (A) for All in One Column, right angle placement and Skew Placement for different eccentricities (X/B). Here (Z/B) = 0.5, 0.75, 1, 1.5, 2, 3, A = 0.25, 1, 2.25, 4 m2 and (X/B) = 0, 0.5, 1.

500 UBC (kN/m2)

400 300 200 100

UBC (kN/m2)

ALL IN ONE ROW

0 0 0.5 1 1.5 2 2.5 3 3.5 4

500 450 400 350 300 250 200 150 100 50 0

Single Cavity Area (m²) X/B=0

X/B=0.5

X/B=1

NO CAVITY

c

0

0.5

1

1.5

2

2.5

3

Z/B X/B=0

X/B=0.5

X/B=1

NO CAVITY

a

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SKEW PLACEMENT

ALL IN ONE ROW

400 300

UBC (kN/m2)

UBC (kN/m2)

500

200 100 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Single Cavity Area (m²)

500 450 400 350 300 250 200 150 100 50 0 0 0.5 1 1.5 2 2.5 3 3.5 4

X/B=0

Single Cavity Area (m²)

X/B=0.5 X/B=1

X/B=1

NO CAVITY

X/B=0 X/B=0.5

d

NO CAVITY

Figure 3.1.2.1 Variation of UBC for Different Eccentricities of Combination for Vertical Loading

b

3.1.3 Irregular Cavity:

RIGHT ANGLE PLACEMENT

Figure 3.1.3.1 (a-d) illustrates the results of computational analysis for single footing placed on three irregular cavities with varying depth (Z/B) for all in One Row combination and varying area (A) for all in One Column, right angle placement and Skew Placement for different eccentricities (X/B). Here (Z/B) = 0.5, 0.75, 1, 1.5, 2, 3, A = 0.25, 1, 2.25, 4 m2 and (X/B) = 0, 0.5, 1.

500 450

UBC (kN/m2)

400

UBC (kN/m2)

ALL IN ONE ROW

350 300 250 200 150 100

500 450 400 350 300 250 200 150 100 50 0

50 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Single CavityArea (m²)

0

0.5

1

1.5

2

2.5

X/B=0

X/B=0.5

X/B=1

NO CAVITY

c

3

Z/B X/B=0

X/B=0.5

X/B=1

NO CAVITY

a

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SKEW PLACEMENT

ALL IN ONE COLUMN

500

500

450

450 400

350

350

300 UBC (kN/m2)

UBC (kN/m2)

400

250 200 150 100

300 250 200 150

50

100

0 0 0.5 1 1.5 2 2.5 3 3.5 4

50

Single Cavity Area (m²)

0 0 0.5 1 1.5 2 2.5 3 3.5 4

X/B=0

X/B=0.5

X/B=1

NO CAVITY

Single Cavity Area (m²)

d

X/B=0

X/B=0.5

X/B=1

NO CAVITY

Figure 3.1.3.1 Variation of UBC for Different Eccentricities of Combination for Vertical Loading

b

3.1.4 Circular Cavity:

RIGHT ANGLE PLACEMENT

Figure 3.1.4.1 (a-d) illustrates the results of computational analysis for single footing placed on three circular cavities with varying depth (Z/B) for all in One Row combination and varying area (A) for all in One Column, right angle placement and Skew Placement for different eccentricities (X/B). Here (Z/B) = 0.5, 0.75, 1, 1.5, 2, 3, A= 0.25, 1. 2.25, 4 m2 and (X/B) = 0, 0.5, 1.

500 450 400 UBC (kN/m2)

350

ALL IN ONE ROW

300 250 200

UBC (kN/m2)

150 500 450 400 350 300 250 200 150 100 50 0

100 50 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Single Cavity Area (m²)

0

0.5

1

1.5

2

2.5

X/B=0

X/B=0.5

X/B=1

NO CAVITY

c

3

Z/B X/B=0

X/B=0.5

X/B=1

NO CAVITY

a

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SKEW PLACEMENT

ALL IN ONE ROW

500

400

450

350

400 300 UBC (kN/m2)

UBC (kN/m2)

350 300 250 200 150

250 200 150

100

100

50

50

0 0

0.5

1

1.5

2

2.5

3

3.5

0

4

0

Single Cavity Area (m²)

0.5

1

1.5

2

2.5

3

Z/B X/B=0

X/B=0.5

X/B=1

NO CAVTY

X/B=0

X/B=0.5

X/B=1

NO CAVITY

d

a

Figure 3.1.4.1 Variation of UBC for Different Eccentricities of Combination for Vertical Loading

ALL IN ONE COLUMN

4.1 Case II: Analysis of Single Footing above Three Voids for Different Combinations and Shapes Subjected to 10° Inclined Loading

400 350 300 (kN/m2)

4.1.1 Square Cavity:

UBC

Figure 4.1.1.1 (a-d) illustrates the results of computational analysis for single footing placed on three square cavities with varying depth (Z/B) for all in One Row combination and varying area (A) for all in One Column, right angle placement and Skew Placement for different eccentricities (X/B). Here (Z/B) = 0.5, 0.75, 1, 1.5, 2, 3, A = 0.25, 1, 2.25, 4 m2 and (X/B) = 0, 0.5, 1

250 200 150 100 50 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Single Cavity Area (m²) X/B=0

X/B=0.5

X/B=1

NO CAVITY

b

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RIGHT ANGLE PLACEMENT

ALL IN ONE ROW 400

400

350 300

300 250

UBC (kN/m2)

UBC (kN/m2)

350

200 150 100

250 200 150

50 100

0 0 0.5 1 1.5 2 2.5 3 3.5 4

50

Single Cavity Area (m²)

0 X/B=0

0

X/B=0.5

X/B=1

0.5

1

1.5

2

2.5

3

Z /B

NO CAVITY

c

X/B=0

X/B=0.5

X/B=1

NO CAVITY

SKEW PLACEMENT

a

400 350

ALL IN ONE COLUMN 400

250

350

200 300 150

UBC (kN/m2)

UBC (kN/m2)

300

100 50 0 0 0.5 1 1.5 2 2.5 3 3.5 4

250 200 150 100

Single Cavity Area (m²)

50 X/B=0

X/B=0.5

X/B=1

NO CAVITY

0 0 0.5 1 1.5 2 2.5 3 3.5 4 Single Cavity Area (m²)

d Figure 4.1.1.1 Variation of UBC for Different Eccentricities of Combination for Load Inclined at 10°

X/B=0

X/B=0.5

X/B=1

NO CAVITY

b

4.1.2 Rectangular Cavity: Figure 4.1.2.1 (a-d) illustrates the results of computational analysis for single footing placed on three rectangular cavities (aspect ratio 2:1) with varying depth (Z/B) for All In One Row combination and varying area (A) for All In One Column, right angle placement and Skew Placement for different eccentricities (X/B) subjected to 10° inclined loading. Here (Z/B) = 0.5, 0.75, 1, 1.5, 2, 3, A = 0.25, 1, 2.25, 4 m2 and (X/B) = 0, 0.5, 1

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eccentricities (X/B) subjected to 10° inclined loading. Here (Z/B) = 0.5, 0.75, 1, 1.5, 2, 3, A = 0.25, 1, 2.25, 4 m2 and (X/B) = 0, 0.5, 1.

RIGHT ANGLE PLACEMENT 400

ALL IN ONE ROW

300

400

250

350

200

300

UBC (kN/m2)

UBC (kN/m2)

350

150 100 50

250 200 150 100

0

50

0 0.5 1 1.5 2 2.5 3 3.5 4

0 0

0.5

1

1.5

Single Cavity Area (m²) X/B=0.5

X/B=0 X/B=1

2

2.5

3

Z /B

NO CAVITY

X/B=0

X/B=0.5

X/B=1

NO CAVITY

c a SKEW PLACEMENT ALL IN ONE COLUMN

350

400

300

350

250

300

200

UBC (kN/m2)

UBC (kN/m2)

400

150 100

250 200 150

50 100 0 0

0.5

1

1.5

2

2.5

3

3.5

4

50

Single Cavity Area (m²) 0 X/B=0

X/B=0.5

X/B=1

NO CAVITY

0 0.5 1 1.5 2 2.5 3 3.5 4 Single Cavity Area (m²)

d Figure 4.1.2.1 Variation of UBC for Different Eccentricities of Combination for Load Inclined at 10°

X/B=0

X/B=0.5

X/B=1

NO CAVITY

b

4.1.3 Irregular Shape: Figure 4.1.3.1 (a-d) illustrates the results of computational analysis for single footing placed on three irregular shape cavities with varying depth (Z/B) for All In One Row combination and varying area (A) for All In One Column, right angle placement and Skew Placement for different

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RIGHT ANGLE PLACEMENT

Y/B=0

400 UBC (kN/m2)

350 UBC (kN/m2)

300 250 200 150 100 50

500 450 400 350 300 250 200 150 100 50 0

0

0

0.5

1

0 0.5 1 1.5 2 2.5 3 3.5 4

1.5

2

D/B

Single Cavity Area (m²) X/B=0

X/B=0.5

X/B=1

NO CAVITY

Z/B=0.5

Z/B=1

Z/B=2

NO CAVITY

a

c

Y/B=0.5

SKEW PLACEMENT 500 450

350

400

300

350 UBC (kN/m2)

UBC (kN/m2)

400

250 200 150

300 250 200 150

100

100

50

50

0

0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

0

0.5

1

Single Cavity Area (m²)

1.5

2

D/B

X/B=0

X/B=0.5

Z/B=0.5

Z/B=1

X/B=1

NO CAVITY

Z/B=2

NO CAVITY

d

b

Figure 4.1.3.1 Variation of UBC for Different Eccentricities of Combination for Load Inclined at 10° 5.1 Case-III Analysis of Two Footings above Single Circular Cavity Subjected to Vertical Loading Figure 5.1.1 (a-d) illustrates the results of computational analysis for two footing placed on single circular cavity with varying depth (Z/B), varying diameter (D/B) and varying spacing between footings (Y/B). Here (Z/B) = 0.5, 1, 2, D/B = 0.5, 1, 2, and (Y/B) = 0, 0.5, 1, 2

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6. CONCLUSIONS

Y/B=1 500

i.

Bearing Capacity Ratio (BCR) of shallow foundations in presence of underground cavities is less than in case of without cavities, depending on the geometry and configuration of cavities.

ii.

Bearing Capacity Ratio (BCR) increases with increase in crest depth of cavities.

iii.

Bearing Capacity Ratio (BCR) increases with increase in distance of the cavity from the center of footing.

iv.

Bearing Capacity Ratio (BCR) decreases with increase in size of cavity.

v.

Bearing Capacity Ratio (BCR) for circular cavity is more than that of other shapes such as rectangular cavity, irregular shaped cavity and square cavity.

vi.

The inclination of load in presence of cavity on footing has negligible effect of on bearing capacity.

vii.

Bearing Capacity Ratio (BCR) increases with increase in inclination of load on footing with vertical.

450 400

UBC (kN/m2)

350 300 250 200 150 100 50 0 0

0.5

1

1.5

2

D/B Z/B=0.5

Z/B=1

Z/B=2

NO CAVITY

c Y/B=2

6.1 Two Footings above Single Circular Cavity:

500 450

i.

Bearing Capacity Ratio (BCR) increases with increase in crest depth of circular cavity from bottom of footings.

ii.

Bearing Capacity Ratio (BCR) increases with increase in spacing between two footings.

iii.

Bearing Capacity Ratio (BCR) decreases with increase in diameter of circular cavity for constant crest depth and spacing between footings.

400

UBC (kN/m2)

350 300 250 200 150 100

6.2 Future Scope:

50

The present knowledge of effect of cavities on performance of footing is limited. The present study may be extended further considering following aspects:

0 0

0.5

1

1.5

2

D/B Z/B=0.5

Z/B=1

Z/B=2

NO CAVITY

i.

The effect of cavities on the performance of footing can be analyzed for multilayered soil.

ii.

The effect of cavities on performance of footing can be analyzed for eccentric vertical and inclined loading.

iii.

The effect of cavities on performance of footing can be analyzed for sand.

d Figure 5.1.1 Variation of UBC for Different Diameter and Crest Depth of Circular Void for Vertical Loading

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iv.

The combinations studied for single footing can be studied for two footing.

v.

Effect of dynamic forces can be also studies on footings above the cavities.

4.

Wang M. C. and Badie A., (1985), “Effect of Underground Void on Foundation Stability”, ASCE, Journal of Geotechnical Engineering, Vol.111, No. 8, pp.

1008-1019.

5. Azam, G., Hsieh C. W. and Wang M. C., (1991), “Performance of Strip Footing on Stratified Soil Deposit with Void”, ASCE, Journal of Geotechnical and Geoenvironmental. Engineering, Vol. 117, No. 5, pp.

753-772.

6. Peng F. L., Kiyosumi M., Ohuchi M. and Kusakabi O., (2006), “Cavity Effect on the Bearing Capacity of Footing Foundation and the Calculation Method”, ASCE, Journal of Underground Construction and Ground Movement, Vol. 199, No. 4, pp. 50-57.

7. Kiyosumi M., Osamu O., Ohuchi M. and Peng F., (2007 ), “Yielding Pressure of Spread Footing above Multiple Voids”, ASCE, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 133, No. 12, pp.

1522-1531.

8. Lavasan A., Talsaz A., Gazavi M. and Schanz T., (2016), “Behavior of Shallow Strip Footing on Twin Voids”, Springer International Publishing Switzerland, Journal of Geotechnical and Geological Engineering, pp.1-15.

9. Baus R. L. and Wang M. C., (1983), “Bearing Capacity of Strip Footing above Void”, ASCE, Journal Geotechnical Engineering, Vol. 109, No. 1, pp. 1-14.

10. Badie A. and Wang M. C., (1984), “Stability of Spread Footing above Void in Clay”, ASCE, Journal of Geotechnical Engineering, Vol. 110, No. 11, pp. 15911605.

11. Kiyosumi M., Kusakabe O., and Ohuchi M., (2011), “Model Tests and Analyses of Bearing Capacity of Strip Footing on Stiff Ground with Voids”, ASCE, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 137, No. 4, pp. 363-375.

6.3 Applications: Due to the population growth and in response to existing needs, the demand of tunnels for urban transportation has rapidly increased. These tunnels usually excavated close to the soil surface and their effects will develop to the ground level and can significantly affect the performance of shallow foundations located above these cavities. Results of prior researches in this field indicated that the interaction between shallow foundation and underground cavities has significant effects on the performance of shallow foundations. The knowledge acquired can help to select suitable position of underground structure such as tunnels, aqueducts, conduits etc. to minimize its effect on stability of existing foundations. Also the knowledge will be useful for design of foundation of soil bed with existing multiple cavities.

REFERENCES: 1.

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