Foundation Specifications 0111

Sub Sections: 1 2 3 4 5 6 7 8 9 10 11 12
Pages: 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124

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Economics often dictate hammer size selection by contractors. Large hammers with

increased energy will reduce driving time. They will also help achieve penetration where

hard driving is encountered, thus enabling completion of the work without the need of

supplemental measures (jetting or predrilling). On the other hand, heavy hammers require

heavy leads and heavy cranes; the result being decreased mobility and increased equipment

costs. Ideally the Contractor will come up with a “mid-range” selection. Another consider-

ation is that a larger hammer will deliver more usable energy to the pile. Hence, the

probability of pile damage (heavy spalling or other) is increased as hammer size is in-

creased. Ram impact velocity is another important factor. In general, a large ram weight

with a short stroke and low velocity at impact will not produce the magnitude of pile stress

that a light ram with a long stroke and high velocity will induce. At constant driving energy,

the driving stress on the pile will decrease as the ram weight increases.

In lieu of a static load test, the typical method for determining load bearing capacity of a

pile depends on knowledge of the energy used to drive the pile. This was stated earlier in the

chapter by saying, “not only is the pile hammer the production tool for the Contractor, it is

also a measuring device for the Engineer”. Various methods and procedures are available

when using the known driving energy to determine the bearing capacity of the pile. These

procedures can be categorized into three areas: (1) Pile driving formulas, (2) wave equa-

tion analysis of pile driving, and (3) dynamic pile driving analysis.

Pile driving formulas all utilize the energy delivered per blow, the resistance to the move-

ment of the pile per blow, pile penetration, and some acknowledgement of the unknown

produced by all the components which act to drive the pile. All of the driving formulas

make use of the conservation of energy theory:

(HAMMER ENERGY) – (ENERGY LOSSES) = (WORK PERFORMED)

Soil resistance multiplied by pile penetration represents work performed, hammer stroke

multiplied by ram weight represents hammer energy, and various factors and/or constants

in driving formulas represent energy losses in the piling system. The desired objective is to

account for the most significant energy losses so that soil resistance can be estimated.

Some of the energy losses associated with pile driving are hammer combustion and

mechanical inefficiency, hammer and pile cushion restitution, dynamic soil resistance and