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.