Load resistance in non-cohesive, granular soils

4.4.1 Prediction from soil tests - API method

Shaft friction

For tubular steel piles according to the offshore API RP2A code, the ultimate frictional resistance on a pile shaft R_s in cohesionless, granular soils for each soil layer can be estimated using the formula:

Rs = Ks poN tan (*). !s

where Ks = coefficient of lateral earth pressure against pile shaft wall poN = average effective overburden pressure over soil layer

!s = Exposed area of pile shaft in the soil layer

N^’ = average angle of internal friction within granular soil layer

* = effective interaction friction angle between the pile wall and the soil in the soil layer, which is equal to (NN_laboratory - 5E).

The method has been investigated in an SCI Technical Report⁽²²⁾ and found to be valid for H-piles and sheet piles provided that the whole steel surface area is used and that there are sufficient laboratory soil tests to determine the N^’ values for each layer.

Table 4.1 gives typical N^’ values for different types of granular soil.

Table 4.1 Typical N^’ values for granular soils (taken from API RP2A)

Density Soil

Sand-Silt 25 81.3 20 4.8

Dense

For tubular steel piles according to the API RP2A code, the ultimate pile end bearing Rb in cohesionless soils can be estimated using the formula:

Rb = poN Nq Ab

where poN = effective overburden pressure at pile tip

Nq = Dimensionless bearing capacity factor from Figure 4.5 Ab = as Section 4.4.

It should be noted that alternative offshore methods for the prediction of load resistance on steel tubular piles in both cohesive and granular soils are currently being developed by Imperial College, London. They have been reported in Jardine et al⁽³⁾ but their application to other steel pile sections has yet to be examined.

1000 800 600 400

200

100 80 60 40

20

10 8 6 4

2

1

20 30 40 50

10 0

φ

Soil friction angle, ' (deg.) Bearing capacity factors N , N and Nqcγ

N γ

N_c

N_q

2123.vcd

Figure 4.5 Bearing capacity factors (according to API RP2A⁽¹¹⁾)

4.4.2 Prediction from soil tests - SPT method

Use of the in situ driven SPT test for assessing granular soil properties is common practice within the UK and US piling industries and worldwide. Reference to both SPT and to CPT testing is made in Section 7.5.3. of BS 8004⁽¹⁵⁾.

A detailed explanation of the methods of SPT testing, reporting and calculation methods is available in CIRIA Report 143 The Standard Penetration Test (SPT):

Methods and Use⁽⁵⁰⁾.

The SPT test is conducted ‘down the hole’ as the borehole is excavated. The test is described in BS 1377: Part 9⁽²⁶⁾. An open-ended split barrel sample tube mounted on a string of screwed steel rods is driven into the bottom of the borehole by a 63.5 kg weight falling through a fixed height of 760 mm. The number of blows (N) required to drive the tube a distance of 300 mm is recorded, after its penetration under gravity and a seating drive of 150 mm. Usually the blow count is recorded as 6 values, one for every 75 mm.

Where driving is hard and a minimum penetration of 300 mm cannot be achieved without risking severe damage to the rods, an alternative method records the set for 50 blows of the hammer, after the initial seating drive of 25 blows. Since, in the full test, the number of blows required to drive the tube through a set of 300 mm is required, an SPT ‘N’ value has to be extrapolated from the smaller penetration in the alternative dataset.

The validation work for an SPT method⁽²²⁾ used the formula given in the earlier Steel Bearing Piles⁽¹⁰⁾ publication to predict capacity as follows:

Ultimate capacity Q_ult = f_s A_s + q_b A_b

where fs = 2N (kPa), and qb = 400N (kPa)

This formula has been in common use for 40 to 50 years and has proved to be appropriate for steel piling capacity predictions.

The phenomenon of ‘plugging’ did not occur on the 16 pile load test sites for which SPT data was available in the database, and it was therefore necessary to adjust the A_s and A_b values from those suggested by Cornfield in Steel Bearing Piles to be the total exposed pile surface area for As and the steel wall end area only for Ab (see Figure 4.1).

In accordance with Cornfield’s recommendations, where the soil was submerged below the ground water table, the values of N were factored by 0.67.

CIRIA Report 143 makes no reference to any factoring required to the N value measured in submerged soils, and given that these generally seem to account for the greater part of the pile length in the UK, this could be a very significant error.

However, the average value of the constants given in CIRIA Report 143 is very similar to the constants suggested by Cornfield when the 0.67 factor is applied to the constants rather than the N value. This would suggest that the two methods are essentially analogous in submerged soils.

4.4.3 Prediction from soil tests - CPT method

The Cone Penetration Test (CPT) formula for pile capacity prediction uses an in situ test measurement of soil resistance to penetration using a penetrometer with a 60E cone end and a friction mantle.

An instrumented device known as the Static Cone Penetrometer or Dutch Cone, as described in BS 1377: Part 9: In-situ tests⁽⁵¹⁾, is pushed into the soil at a constant rate of 2 cm/sec by a hydraulic jacking system. A load cell mounted just behind the cone continuously monitors the tip load q_c being applied to ‘fail’ the soil and

thereby to advance the cone. A sleeve section or ‘mantle’ just behind the cone is also strain-gauged and allows direct measurement of the cone ‘shaft’ friction f_s. A diagram of a typical static cone penetrometer tip is shown in Figure 4.6.

60°

Dirt seal

Seals

Friction sleeve

Watertight seal

Dirt seal

1000 mm

35.7 mm

Figure 4.6 A static cone penetrometer tip

Shaft friction

Several empirical formulae have been developed to relate the CPT qc value to foundation design parameters depending on soil type and loading regime (see Cone Penetration Testing: Methods and Interpretation⁽⁵²⁾).

It has been found that for the more consistent fine sands and silts encountered in Holland that the following formulae can be used in shaft capacity prediction for steel piles:

Compressive resistance: Unit shaft friction = 300^c

q

Tensile resistance: Unit shaft friction = 400

qc

The formulae are not valid in other types of granular soil.

These formulae use the cone resistance q_c to estimate the shaft friction; however, where cone friction mantle values fs are also available then the following formula has been derived (see reference 52):

Unit shaft friction = 0.35 fs

End-bearing

For plugged steel pile sections the ultimate unit endbearing is estimated from the cone values after applying Schmertmann’s averaging process for q_c values above and below the pile toe, and then adopting limits (see Reference 52). This will produce a conservative prediction of pile capacity that will limit pile settlement.

However where plugging is suspected, static pile load testing should always be carried out to confirm the design assumptions.

For unplugged steel pile sections, the qc value at any level can be used directly without modification as the ultimate unit endbearing resistance pressure under the steel wall area. This resistance also applies to the tip of the pile during driving.

4.4.4 Conclusions

For predicting the shaft friction in granular soils, the SCI validation work^(22)(29) indicates that either the SPT or CPT method can be used with confidence for fine granular soils and the SPT method for all granular type soils using the total exposed shaft surface area (see sections 4.4.2 and 4.4.3). To predict the ultimate base resistance on driven steel piles, only the wall cross-sectional area should be used together with a unit ultimate bearing pressure equal to the CPT value (as verified by Jardine et al⁽³⁾). If CPT values are not available then correlations can be used to deduce an equivalent value from correlations with the SPT.

The offshore pseudo-effective stress method for predicting shaft friction in granular soils (see Section 4.4.1) was generally not as reliable as the SPT method in the SCI database because of the absence of measurement of N0 in routine laboratory soil investigation testing.