Measuring and testing – Hardness – By penetrator or indentor
Reexamination Certificate
2000-05-26
2002-02-26
Noori, Max (Department: 2855)
Measuring and testing
Hardness
By penetrator or indentor
Reexamination Certificate
active
06349590
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method for estimating the load-bearing capacity of a pile. It includes a method and apparatus for predicting the capacity or load bearing of a proposed or virtual pile, and a method and apparatus for measuring the estimated load bearing of an actual driven pile, including means for acquiring data from a pile being driven or an installed pile for the purposes of applying the present method to estimate its load bearing.
BACKGROUND ART
Of the few categories of foundation pile installation methods (such as driven, bored, injected, cast-in-place, caisson or floating, etc.) the method of this invention is based on the impact-driven or hammer-driven testing on an installed pile or pile under installation.
As the load bearing capacity of a pile is of the utmost importance in considering the design and installation of a building structure's foundation, various methods have been employed to estimate the pile's load bearing capacity. As the ultimate load bearing of a foundation is limited by either the structural strength of the pile or the capacity of the supporting soil (i.e. the “pile-soil” system), geotechnical engineers may, in most instances, choose or design the appropriate piles. The soil factor, on the other hand, are but predetermined by the proposed structure's site and, hence, the soil profile can only be analysed and studied through soil tests.
The manner in which the adequacy of a pile chosen or designed in meeting the allowable loads is generally governed by building by-laws or standards such as the BS 8004:1986. The pile's load bearing capacity may then be verified by one or more of the conventional tests such as static tests, dynamic tests or wave equation-based tests.
Static Test
Traditionally, the most relied upon pile testing has been the static load test which involves waiting for up to 30 days after pile driving to allow soil remoulding and settlement before stacking incrementally static weight loads onto the pile top and then measure the corresponding pile movements. In practice, static tests are performed to either (i) prove that it can safely hold the design load (proof test), or (ii) to determine a design load based on the ultimate load bearing capacity of the pile (i.e. load until failure).
Proof testing is more prevalent as the ultimate pile strength may often never be known. This results in higher capacity of the piles being laid and thus incurring greater foundation costs than are actually required. Moreover, due to the physical difficulty of stacking large weight blocks onto the pile top, the length of time and costs involved, only a small percentage of the piles on site are actually tested.
Dynamic Test
From the number of hammer blows in advancing a pile per unit of penetration, the energy from the hammer blows has been equated to the work done in advancing the pile against soil resistance, Based on Newton's Second Law of Motion, i.e. the impulse-momentum principles, theoretical and semi-empirical formulae have been derived to express this relationship between energy and work by assuming the hammer and pile are rigid bodies and soil resistance as a constant static force. These formulas are generally known as Dynamic Formulae or Energy Formulae.
The model common to all the simple dynamic formulae may be illustrated in
FIG. 1
where if potential energy stored in the ram is equated to the work done by advancing the pile against a constant soil resistance, then
Wh=Rs.
or
R
=
W
h
s
where
W
is
the
ram
weight
;
R
is
the
soil
resistance
constant
;
h
is
the
drop
height
of
the
ram
;
s
is
the
pile
set
.
This basic dynamic pile capacity formula, which is also termed “rational pile formula”, and most of the other dynamic pile formulae currently in use are derived from this equation. [For examples, see Joseph E. Bowles,
Foundation Analysis and Design,
4th edition, McGraw-Hill, 1988, p. 791].
The more common formulas derived from the above rational pile formula have incorporated various assumptions on certain parameters in order to improve on the reliability of predictions on the pile capacity; however, each formula gives a different value of the capacity of the same pile. Some are these formulas are given in the following Table 1 [from Bowles, p. 794].
TABLE 1
VARIOUS DYNAMIC PILE FORMULAS
Danish formula [Olson and Flaate (1967)](use SF = 3 to 6)
P
u
=
e
h
E
h
s
+
C
1
;
wherein
&AutoRightMatch;
C
1
=
e
h
E
h
L
2
AE
Eytelwein formula (use SF = 6) [Chellis (1961)]
P
u
=
e
h
E
h
s
+
0.1
(
W
p
W
r
)
Modified ENR [Engineering News-Record (1965)] (use SF = 6)
P
u
=
1.25
(
e
h
E
h
)
s
+
0.1
·
W
r
+
n
2
W
p
W
r
+
W
p
Hiley Formula (1930)
P
u
=
e
h
W
r
h
s
+
1
2
(
k
1
+
k
2
+
k
3
)
·
W
r
+
n
2
W
p
W
r
+
W
p
wherein
P
u
= ultimate pile capacity, F.
A = pile cross-section area, L
2
.
E = modulus of elasticity, FL
−2
.
e
h
= hammer efficiency.
E
h
= manufacturers' hammer-energy rating, FL.
h = height of fall of ram, L.
k
1
= elastic compression of cap block and pile cap and is a form of P
u
L/AE, L.
k
2
= elastic compression of pile and is of a form of P
u
L/AE, L.
k
3
= elastic compression of soil, also termed quake for wave-equation analysis, L.
L = pile length, L.
n = coefficient of restitution.
s = amount of point penetration per blow, L.
W
p
= weight of pile including weight of pile cap, driving shoe, and cap block (also includes anvil for double-acting steam hammers), F.
W
r
= weight of ram (for double-acting hammers include weight of casing), F.
Each of the formulas has its own advantages under different circumstances. For example, the Engineering News-Record (ENR) formula (1965) is thought to be reasonably valid over the entire range of load tests and has been cited as the most widespread dynamic formula in the U.S.A. It defines the soil resistance, R, in terms of the ultimate pile capacity, P
u
, as the safe load in kips (1 kip=1000 lbs=453.6 kg) with “loss” terms e
h
and E
h
introduced and safety factor (SF) of 6 assigned. [Hussein, Likens & Rausche,
Testing Methods of Driven Piles,
Pile Buck, Inc., 1988].
The Hiley formula (1930) is found to have the least statistical deviation or the highest statistical correlation. In fact, the British Standard BS 8004:1986 cites the Hiley formula as one of the more reliable dynamic formulas and is probably the most commonly used in Britain.
Although dynamic formulas have been widely used to predict pile capacity, more accurate means is needed to determine when a pile has reached a satisfactory load-bearing value other than by simply driving it to some depth predetermined by the formulas. The pile driven to a predetermined depth may or may not obtain the required bearing value due to the complex pile and soil characteristics. Today, dynamic formulas are generally acknowledged as inaccurate due to their oversimplification of the modelling of the hammer, driving system, pile, and soil. In fact, they are not applicable to most hammer types used on today's sites, long elastic piles, or cohesive, impermeable, compressible soils.
Wave Equation Analysis
Modern foundation engineering recognises that pile driving may be better estimated by wave propagation theories which may include complicated partial differential equations With the advent of digital computers in the 1950s it became possible to arrive at a discrete solution of wave propagation equations by algorithm using comp
Merchant & Gould P.C.
Noori Max
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