Week 9 Determination of Bearing Capacity from Field Tests

Bearing Capacity from Field Tests

Standard Penetration Test (N-values) Cone Penetration Test (qc ) Plate Load Test

Major Advantages of field tests over laboratory tests: There is no need to extract soil sample (sampling not

required) The conditions during testing are identical to the actual

situation (soil disturbance is minimum)

[Tests performed in the field to obtain the required soil properties]

Standard Penetration Test

Bore Hole

Split Spoon Sampler

Tripod

65 kg Hammer

750mm

Split Spoon Sampler

It consists of a split spoon sampler 50.8 mm OD, 35 mm ID, min 450-600 mm long and 63.5 kg hammer freely dropped from a height of 750 mm.

The test is performed on a pre-prepared clean hole 50 mm to 150 mm in diameter.

Split spoon sampler is placed vertically in the hole, allowed to freely settle under its own weight or with blows for first 150 mm which is called seating drive.

The number of blows required for the next 300 mm penetration into the ground is the standard penetration number N

Apply the desired corrections (such as corrections for overburden pressure, saturated fine silt and energy)

[N is correlated with most soil properties; such as friction angle ϕ, undrained cohesion, density etc..]

Standard Penetration Test

Bearing Capacity correlations with SPT-value

Bearing Capacity of footings on Sand

Advantages & Disadvantages of Standard Penetration Test

Advantages:• Relatively quick & simple to perform• Equipment & expertise for test is widely available • Provides useful index for relative strength & compressibility of soil• Able to penetrate dense & stiff layers• Results reflect soil density, fabric, stress strain behavior

 Disadvantages:• Requires the preparation of bore holes.• Dynamic effort is related to mostly static performance. • If hard stone is encountered, difficult to obtain reliable results. • Not possible to obtain properties continuously with depth.

Cone Penetration Test

• Cone Penetration Test can either be Static or Dynamic. • Continuous record of penetration resistance with depth is achieved.• Consists of a cone 36 mm diameter (1000 mm2) and 60o vertex angle.• Cone is carried at the lower end of steel rod that passes through steel tube of

36 mm dia.• Either the cone, or the tube or both can be forced in to the soil by jacks.• Cone is pushed 80 mm in to the ground and resistance is recorded, steel

tube is pushed to the cone and resistance is recorded. Further, both cone and tube are penetrated 200 mm and resistance is recorded. Total resistance (qc) gives the CPT value expressed in kPa.

• Cone resistance represents bearing resistance at the base and tube resistance gives the skin frictional resistance. Total resistance can be correlated with strength properties, density and deformation characteristics of soil.

• Correction for overburden pressure is applied.• Approximately, N = 10qc

Cone Penetration Test

CPT-SPT values for different soil types

A correlation between the cone penetration resistance qc and values of Ø′ was proposed by Meyerhof, and is shown below:

The Cone Penetrometer Test (CPT)

Advantages:• Continuous resistance with depth is recorded.• Static resistance is more appropriate to determine static

properties of soil.• Can be correlated with most properties of soils. Disadvantages: • If a small rock piece is encountered, resistance shown is

erratic and incorrect.• Involves handling heavy equipment.

Advantages & Disadvantages

Plate Load Test

Foundation Soil

Sand Bags

Platform for loading

Foundation LevelTesting Plate

Dial Gauge

To determine bearing capacity & settlement characteristics of the ground through preparing a test pit down to the desired foundation level.

A rigid steel plate (round or square in shape) 300 mm to 750 mm in size, 25 mm thick acts as model footing.

Dial gauges, at least 2, of required accuracy (0.002 mm) are placed on the plate at corners to measure the vertical deflection.

Loading is provided either as gravity loading or as reaction loading. (For smaller loads gravity loading is acceptable where sand bags apply the load). In reaction loading, a reaction truss or beam is anchored to the ground. A hydraulic jack applies the reaction load.

At every applied load, the plate settles gradually. The dial gauge readings are recorded after the settlement reduces to least count of gauge (0.002 mm) and average settlement of 2 or more gauges is recorded.

Load vs. Settlement graph is plotted; load is plotted on the horizontal scale and settlement is plotted on the vertical scale, (Refer to BS5930)

Plate Load Test

Advantages & Disadvantages of Plate load test

Advantages:1- It provides the allowable bearing pressure at the location considering both

shear failure and settlement.2- The loading techniques and other arrangements for field testing are

identical to the actual conditions in the field.3- It is a fast method of estimating the average bearing pressure and load-

settlement behavior of the ground. Disadvantages:1- Test results reflect the behavior of soil below the plate not that of actual

footing which is generally larger.2- It is essentially a short duration test. Hence, it does not reflect the long term

consolidation settlement of clayey soils.3- Size effect is pronounced in granular soils. Correction for size effect is

essential in such soils.

Bearing Capacity from

Field Tests

A study of the load–settlement relationships for footings of different widths on granular soil shows that it is only when the width B is less than 1m that bearing capacity considerations are likely to control the design.

For foundations on granular soil, the conventional approach is to design from the standpoint of settlement and then check that there is an adequate factor of safety.

In the case of footings, we generally specify that the settlement should not exceed 25mm and that the factor of safety should not be less than 2.

DESIGN PROCEDURE FOR SHALLOW FOUNDATIONS ON GRANULAR SOIL (Standard Penetration Test)

• The bearing pressure that produces any given settlement in a loose soil will clearly be less than the bearing pressure that produces the same settlement in a dense soil. It should then be possible to produce some rough correlation between the bearing pressure that produces a given settlement and the N-values obtained from the Standard Penetration Test.

• Such a correlation has been proposed by Peck, Hanson & Thornburn (1974) for the case of a foundation of width B situated on granular soil in which the water table is at a depth B or more below foundation level. This suggests that for a total settlement of 25mm, the net allowable bearing pressure, denoted by qnall, can be expressed

as: qnall = 11N (kN/m2)

DESIGN PROCEDURE FOR SHALLOW FOUNDATIONS ON GRANULAR SOIL (Standard Penetration Test)

However, if the water table lies at ground level, the effective stresses within the soil are reduced by about 50%, and correspondingly the stiffness of the soil is reduced. Hence, if the water table is at ground level, the bearing pressure that will produce a settlement of 25mm is about half of that which would produce 25mm of settlement if the water table is at a depth of B or more below foundation level. Thus, for the water table at ground level:

qnall = 0.5 x 11N (kN/m2) = 5.5N (kN/m2)

For intermediate positions of the water table, we can perform a linear interpolation between the values 11N and 5.5N using the mathematical expression:

Where Dw denotes the depth of the water table below ground level 0 ≤ Dw ≤ (Df + B)

DESIGN PROCEDURE FOR SHALLOW FOUNDATIONS ON GRANULAR SOIL (Standard Penetration Test)

f

Shallow foundations on granular soils

Fsd = tanФ /′ tanФ′mFactor of safety against drained shear failure Ф′m = mobilised shear friction

Figure 7.4 Relationship between Ng and Ø′ for shallow foundations in granular soil

Figure 7.6 Correlation between relative density, standard penetration value N, bearing capacity factor Ng and effective angle of friction Ø′

A vertical concrete column is to carry a total load at ground level of 2200kN, and is to be supported on a square concrete footing founded at a depth of 2m below ground level in a thick deposit of sand as shown in Figure 7.11. A ground investigation revealed that groundwater level was static at a depth of 2m, and Standard Penetration Tests carried out in the sand gave N values in the range 30 – 34. The sand has a bulk density above groundwater level of 1.60Mg/m3 and a saturated density of 1.90Mg/m3.

(a) If the total settlement of the footing is not to exceed 25mm and the factor of safety based on drained bearing failure is not to be less than 2, determine a suitable size for the square footing.

(b) For this size of footing, calculate the factor of safety based on drained shear failure. Neglect the difference in density between the concrete and the sand.

Classwork (1):

Figure (7.4)

Subs. FS=2.0 to find Nᵧ mobilised then m;

Classwork (2):

(a) A square footing is to be located at a depth of 2m below ground level in a thick deposit of sand. The footing will support a column load at ground level of 2000kN. A site investigation revealed that the water table was static at a considerable depth below foundation level, and Standard Penetration Tests in the sand gave an N value of 30. The sand was found to have a bulk density above the water table of 1.60Mg/m3and a saturated density of 1.90Mg/m3. If the total settlement is to be limited to 25mm, determine a suitable size for the square footing and estimate values of the factor of safety based on drained bearing failure and the factor of safety based on drained shear failure of the foundation. (b) Repeat the above calculations for conditions where the water table lies: (i) at the base of the footing, and (ii) at ground surface.

[Ans: (a) B = 2.46m; Fbd = 5.70; Fsd = 1.80 (b) (i) B = 2.94m; Fbd = 7.09; Fsd = 2.09 (ii) B = 3.48m; Fbd = 7.40; Fsd = 2.11]

Figure 7.12 shows the details of a bridge pier that is founded in the bed of a river in a deep deposit of sandy gravel having a saturated density of 2.05Mg/m3 and an effective angle of friction of 35°. The depth of water in the river is 4m, and the base of the pier is located at a depth of 2.5m below the riverbed level. The pier is 3m wide and the total load at foundation level including the weight of the structure and the foundation is 1500kN per metre run. Determine values for the factor of safety based on drained bearing failure and the factor of safety based on drained shear failure.

Classwork (3):

In conjunction with the conventional design approach explained above, there are methods available for predicting the likely settlements of foundations on granular soil. The Buisman–De Beer method (1965), for example, relates the cone penetration resistance qc to a constant of compressibility C for the soil and calculates the settlement of the foundation S as the summation of the settlements of a number of incremental sub-layers of thickness Δz using the expression:

However this method is strictly applicable only to normally consolidated granular soils, and in the case of over-consolidated soils it will therefore overestimate the likely settlements.

DESIGN PROCEDURE FOR SHALLOW FOUNDATIONS ON GRANULAR SOIL (Cone Penetration Test)

Plate load test is often used to assess the likely settlements of foundations on granular soils. The results of the test are extrapolated to predict the settlement of the full-scale foundation, the extrapolation is generally based on an empirical relationship proposed by Terzaghi & Peck (1967) which is in dimensionless form and approximated by the expression:

(7.26)

Sp=settlement of a plate of width Bp and Sf =settlement of a foundation of width Bf when the bearing pressure is the same for both

the plate and the foundation.

However, there is considerable evidence that the correlation between foundation settlement and foundation size is not quite as simple as that suggested by equation (7.26). This evidence taken together with the doubts expressed earlier about the truly representative nature of plate load tests suggests that predictions of foundation settlement based exclusively on equation (7.26) should be treated with considerable caution.

DESIGN PROCEDURE FOR SHALLOW FOUNDATIONS ON GRANULAR SOIL (Plate Load Test)

Factor of Safety

It is the factor of ignorance about the soil under consideration. It depends on many parameters such as,1- Type of soil2- Method of exploration3- Level of uncertainty in soil strength4- Importance of structure and consequences of failure5- Likelihood of design load occurrence, etc.

Important:Assume a factor of safety F=3, unless otherwise specified for bearing capacity problems. Table 7.5 provides the details of factors of safety to be used under different circumstances:

Table 7.5 Typical factors of safety for bearing capacity calculation in different situations

End of Shallow

Foundations