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Because of the variance in underpinning types, a standard load test, ASTM D 1143-81 (Reapproved 1994) was chosen as the method for field testing. This method is referred to as the “Standard Test Method for Piles Under Static Axial Compressive Load.” As described in the introduction, “This standard has been prepared to cover routine methods of testing to determine if a pile has adequate bearing capacity”. This method is a static load test where in the Quick test as defined in ASTM D1143, the determined load will be applied for 2 minutes while watching the deflection gauge to determine failure.

Although cyclic loading has been used, it does not appear to contribute to the interpretation of static load bearing capacity and even makes it harder to interpret (England and Fleming 1994). Another factor in the selection of a two (2) minute static test is that some studies have shown an increase risk of influence of the time-dependent movements if left loaded for over 15 minutes and this may impair the test results (Butler and Hoy 1977).

Other sources of testing criteria were reviewed to confirm parameters for failure load. Ultimate failure is a peak load above which the foundation does not take more load and will plunge downward if the load is increased further. In other cases, the peak may not appear in the load-deforming plots; instead, a plateau type loading curve is established. In such cases, the deformation criterion should be used to establish ultimate

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Stabilzation

of Oklahoma,

failure load of a foundation. For augercast piles, failure load is recorded at 3 in. of downward movement (Neely 1990) whereas ultimate loads for helical piers will produce a plunging failure (Smith 2004). According to FHWA-HI-96-033, “the failure load of a pile tested under axial compressive load is that load which produces a settlement at failure of the pile head equal to”:

Sf = ∆ + (4.0 + 0.008b) (2.24)

where: Sf = Settlement at failure in mm (in).

b = Pile diameter or width in mm (in).

∆ = Elastic deformation of total pile length in mm (in).

If we discount pile deformation, allowable settlement for the steel and concrete piles would be computed as:

Steel piles = 4.0 mm + 0.008(73.03) = 4.584 mm or 0.178 inches Concrete piles= 4.0 mm + 0.008(152.4) = 5.219 mm or 0.204 inches

Therefore, it will be important to record incremental deflection to load for the entire deformation so that all failure modes will be easily measured.

Elastic deformation in a pile is computed as follows:

∆= Qa L/(A E) (2.25)

Where: ∆ = Elastic compression of pile material (in), (mm) Qa= Design axial load in pile (lb), (kN)

L = Length of Pile (in), (mm)

A = Pile cross sectional area (in2), (m2)

E = Modulus of elasticity of pile material, (psi), (kPa)

Soil

Stabilzation

of Oklahoma,

For the steel piles used in this research, we will assume a modulus of elasticity of 207,000 MPa (30,022,813 psi), Length of 13,000 mm (44 ft), Design load of 11.24 kN (50 kips), Pipe cross sectional area of 0.00418 sq.m. (0.0451 sq.ft.).

Therefore: ∆ = ) 000 , 000 , 207 )( 0041881 . 0 ( ) 000 , 13 )( 24 . 11 ( = 1.685 mm = 0.066 inch

For the concrete piles of this experiment, we will assume a modulus of elasticity of 27,800 MPa, length of 8231.7 mm (27 ft), design load of 11.24 kN (50 kips), concrete cross sectional area of 0.0182 sq.m. (0.1965 sq.ft.)

Therefore: ∆ = ) 000 , 800 , 27 )( 01824 . 0 ( ) 7 . 231 , 8 )( 24 . 11 ( = 0.18 mm = 0.007 inch

Criteria established for settlements at failure loads were followed in the present research.

While the purpose of this testing is to determine ultimate axial capacity, there is a theory that evaluates the rebound curve (curve of deflection vs. load when the load is released), which will provide a proportion of tip bearing to skin friction resistances. This method of analysis by England (England 2000) establishes that the shaft friction quickly reaches its ultimate level while base resistance increases until failure. It is also proposed that when the load is decreased to no load that the skin friction reverses itself to counteract base resistance rebound (Davies 1987; England 2000). Elastic Shortening

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must obviously be accounted for so that actual pile/soil interaction is known (Fleming 1993). This theory has relevance but should only be used as a supplementary data tool in conjunction with proven empirical methods based upon the large and diverse data pool available for a particular test. It is also important that the foundations are set in a homogeneous soil condition that certainly does not exist at this site. Another difficulty in application to this research is the usage of the ASTM 1143 Quick Load Test that can enhance the skin friction approximation because of the rapid static loading (England 1992 and 1993).

It has been mentioned that knowing the distribution of forces along the shafts would be beneficial to the total understanding of pier and pile functional performance and also that there are residual loads along the shaft that could alter an understanding of these forces (Fellenius 2002). This testing, however, only focused with the ultimate resistance of piers and piles in vertical compression. Results presented in this research show load versus deflection plots depicting the failure loads for each underpinning element (ASCE 1985).

2.4 Summary

This chapter provides a comprehensive literature review on the six most common remedial underpinning methods used in practice. Each of the six foundation types (drilled straight shafts, drilled and belled piers, augercast piles, helical anchors, pressed steel and pressed concrete piles) are presented with background information applicable to testing for determining the axial compression capacity. Available literature, including: papers, books, website reviews and conversations with field and academia experts were reviewed to gather this information.

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CHAPTER 3

RESEARCH METHODOLOGY