pile load testing procedure - archive
TRANSCRIPT
PILE LOAD TESTING
PROCEDURE
By
Zeyad T. Shukri
MSC. Geotechnicsal
Engineering
Introduction
The cost of ignorance has to be weighed against the cost of testing.
Piles can not readily be inspected once have been placed unlike almost all
other structural elements
The reason for testing piles are not always considered.
The information which may be gleaned from the results is sometimes not
clearly understood.
Piles are usually tested using direct measurement of the applied load and
the resulting settlement at the pile head.
In some instance, instrumentation of the pile shaft is added to provide
greater information (for research purpose or where soil condition is
unusual).
Questions to be answered
(for No. of piles needs to be tested)
How detailed and comprehensive should a program of testing be?
How many piles should be tested on a particular site?
The degree of validity of the ground condition?
Competence of piling contractor?
Difficulty of pile installation and supervision?
** Integrity testing can be used to guide the selection of piles for load testing.
Type of Test Program
1) Pre-contract test piles:
installed primarily to confirm design assumptions and therefore tested to
ultimate load whenever possible.
2) Pile test carried out within the main piling contract:
provide an indication of settlement of a contract piles and piles to be tested to ultimate load.
3) Contract piles:
- it is usual to load the piles to 1.5 x design load.
- a limiting settlement is normally specified.
- the test is to encourage the contractor to maintain a high standard of construction which comes from the knowledge that any pile may be subjected to test.
Selection of Piles for Test Depends on:
1 ) Theoretically:
Need more tests Need less tests
variable ground condition
bad piling contractor
poor supervision
Knowledge of difficulties during
installation of particular pile.
uniform ground condition
Good piling contractor
Good supervision
No difficulties during installation
of piles.
By federation of piling specialist
Selection of Piles for Test Depends on:
2 ) Quite often are selected for:
- provide an indication of settlement of a contract piles and piles to be tested to ultimate load.
- ease of access.
- location of neighboring piles.
- the effect on the overall program of testing one pile rather than another.
3) Good guide line:
- use integrity test to improve selection for load test.
Ultimate Load, Design Load and
applied Load
1) Ultimate Load (Pu):
a ) the load at which the resistance of the soil is fully mobilized (institute of civil engineering – u.k 1977)
b ) the load when the downward movement is 10% X Dia. Or 10% X least width of pile.
c ) as (a) but in most cases the value is taken as the force at which penetration is10% x pile Dia. (or least width) (BS – CP2004 – 1972).
2) Design Load:
- load applied on a pile without considering downdrag.
DL = Pu / FOS
3) Allowable Load:
- load applied on a pile safely after considering downdrag, pile spacing, overall bearing capacity of the ground below toe of pile, allowable settlement, etc...
AL = DL X reduction factor (Rf)
= Pu X Rf / FOS
4) Applied Load:
- theoretical calculated load to be imposed on a pile.
- applied load <= allowable load.
5) Proof Load:
- load applied on a selected working pile by load test.
- usually 1.5 X design load.
- for working piles normally not exceeding 150% of design load.
Test Pile Construction and Preparation
Additional Reinforcement:
- In some bored piles, additional reinf. May be required at the pile head, since
it is not tested normally to ultimate load, nominal reinf. Is usually adequate.
Low finished level:
- piles finished at low level may require extension and sleeving off to bring the pile head to a suitable level.
- test loads are not always perfectly axial, therefore, the unsupported extension length may suffer bending forces at the joint (should be carefully made).
- if water table is high or the distance to pile head is big, the pile must be pre-selected and brought to a higher level (allowance should be made for the resistance of the extension if it is not possible to sleeve the extension).
BM
Pile head preparation:
- remove blind concrete around the pile by breaking out around the pile
head.
- Excavate around the pile.
Pile head preparation:- reinf. Bars must be cut back or bent clear.
- a small pile cap may be constructed.
- a thick steel plate (25mm) should be bedded on top of the pile to spread
the load from the jack.
- reference plates for settlement measurement are usually clamped to the
top of the pile.
Concrete strength:
- Fcu concrete for pile and pile cap should at least
= 2 x applied stress
= 2 x 1.5 x allowable stress.
= 3 x allowable stress.
- the pile concrete is at least 7 days old.
Note: the beneficial effect of pile reinf. Is not usually taken into account.
Example:
Pile Dia = 1,000 mm
WL = 7,600 kN
= 7.6 x 106 N/mm2
Fcu = 50 N/mm2
Fcu(actual) =7.6x106
Area
= 9.67 N/mm2
Fcu(needed) = 3 x 9.67
= 29 N/mm2
Reaction Arrangement:
Depends on:
A ) soil condition (for anchor system).
B) Size of site (to decided type of reaction system).
C ) No of piles to be tested.
D ) cost.
Reaction
Dead load
(kentledge)Tension pile
Ground anchor Rock anchorConcrete blocks
(often 1 – 2m3)
iron blocks
(generally 2 ton wt.)
Rail sections
Small ingots
Water tanks
Total No. of Blocks = Pile Working Load x ( 1 + (10-20% )
(volume of cube x 2.5 ton)
Kentledge Reaction:
- the wt. is born on timber cribbage (or concrete blocks) with a sleeper over the ground surface to spread the load.
- bearing capacity of soil below should be checked.
Bearing capacity failure
Platform bearing failure
Failure of tension bar system
Kentledge Reaction:
- the cribbage pads should be spaced away from
the test pile to minimize interaction.
- distance between pads and face of
pile >= 1.0m.
- Increasing distance between pads and
the tested pile shall induce large B.M
- Insure toppling of any beam can not occur.
- Work should be carried out by skilled operator.
> = 1.0m
- Center the load on the pile.
- the load on grillage should be relived evenly.
- the max. load on pile = cribbage load x (10-20%) margin.
- If blocks are regular in size, lower margin is acceptable.
- Lifting condition is dangerous (changing from sagging prior to loading
to hogging after loading).
Ground Anchor Tension System:
- useful when rock exist at shallow depth below pile toe level.
- A more stable arrangement is provided by multiple anchors and
saddle blocks.
- the individual anchor should be proof loaded to at least 130% x their
maximum load prior to use.