3 a Review of Requirements on Ground Investigation and Load Test for Foundation Piles in Singapore by Er Dr Yet Nai Song

A review of requirements on ground investigation and pile load test for foundation piles in Singapore

Er. Dr Yet Nai Song Deputy Director  Building & Construction Authority BE seminar, 30 Sep 0!"

Joint BCA / IES I ES / ACES / GEOSS Circular 2016

Joint BCA / IES I ES / ACES / GEOSS Circular 2016

Outline of presentation

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Review of ground investigation requirement

 

Review of pile load test requirement

3

The Use of Rapid Load Test

# # $

Quality control test on bored piles Design and construction of bored piles

Hello How are Why a review of you? requirement is necessary?

BCA/IES/ACES advisory note 1/03 on site investigation and load test • Issued in Aug 2003 in consultation with IES / ACES • Used in conjunction with CP4:2003 • Cover buildings of 10 storeys and above • Most QP have also adopted it for buildings of less than 10 storeys • Has served Singapore well

BCA/IES/ACES Advisory note 1/03 on site investigation & load test

BCA/IES/ACES Advisory Note 1/03 Ground investigation requirement for foundation of buildings of 10-storeys and above

Number & Spacing Depth

BCA/IES/ACES Advisory Note 1/03 Pile Load test requirement for foundation of buildings of 10-storeys and above

Load test & integrity test

Why a review of requirement for foundation piles is necessary? •

Migration from BS code to Eurocode has significant implication on foundation pile design & execution.

•

Eurocode 7 assumes that structures are designed by appropriately qualified and experienced personnel. EN1997-1 Cl 1.3(2)

•

Eurocode 7 assumes that execution of geotechnical work is carried out according to standards & specification by personnel having appropriate skill and experience. EN1997-1 Cl 1.3(2)

•

These assumptions need to be considered by designer and client. EN1997-1 Cl 1.3(3)

Why a review of requirement for foundation piles is necessary? • Eurocode 7 is less prescriptive. • It provides greater room for pile designers to exercise engineering judgement in selecting partial factors for geotechnical design.

8 possible designs using alternative procedure under DA1-2 Design 1

Design 2

Design 3

Design 4

Model factor

1.2

1.2

1.4

1.4

Partial resistance factors (γs, γb)

1.4 1.7

1.6 2.0

1.4 1.7

1.6 2.0

2.05

2.38

2.39

2.77

Design 5

Design 6

Design 7

Design 8

Model factor

1.2

1.2

1.4

1.4

Partial resistance factors (γt)

1.7

2.0

1.7

2.0

2.24

2.64

2.62

3.08

Overall safety factor (approx.)

Overall safety factor (approx.)

• Overall geotechnical safety factor could be lower, as compared to CP4 ! 11

8 possible designs using alternative procedure under DA1-2 Design 1

Design 2

Design 3

Design 4

Model factor

1.2

1.2

1.4

1.4

Partial resistance factors (γs, γb)

1.3 1.5

1.5 1.7

1.3 1.5

1.5 1.7

1.85

2.11

2.16

2.46

Design 5

Design 6

Design 7

Design 8

Model factor

1.2

1.2

1.4

1.4

Partial resistance factors (γt)

1.5

1.7

1.5

1.7

1.98

2.24

2.31

2.62

Overall safety factor (approx.)

Overall safety factor (approx.)

• Overall geotechnical safety factor could be lower, as compared to CP4 ! 12

Illustration Fc;d •

1 m diameter bored pile is subject to axial compression load, with : Gk=4000 kN, Qk=1000 kN.

qs,1,k

•

(50 kPa)

L1 (14m)

The pile penetrates through 14m thick soil with characteristic unit skin friction of 50 kPa, and founded in firm stratum with : Characteristic unit skin friction = 200 kPa ; Characteristic unit end bearing = 7 MPa.

L2

qs,2,k (200 kPa)

Find required pile penetration depth using: a)

qb,k (7 MPa)

CP4

a) EC7 - Alternative procedure

EC7 vs CP4 – A comparison of results EC 7 : DA1-2 Alternative procedure Design 1

Design 2

Design 3

Design 4

(FOS=2.05)

(FOS=2.38)

(FOS=2.39)

(FOS=2.77)

1.2 1.4, 1.7

1.2 1.6, 2.0

1.4 1.4, 1.7

1.4 1.6, 2.0

Overdesign factor

1.002

1.081

1.07

1.032

Design pile depth

17.5m

21m

21m

23m

Partial factors γs, γb

MF R4

If 1 ULT and 1%(ULT+WLT) are carried out

If no pile load test are carried out

CP4 FOS=2.5

22m

ULT and WLT are usually carried out

In this example, if design 1 is adopted, there is 20% shorter penetration length 14 as compared to design using CP4.

Concern on migration to Eurocodes • There is a possibility of pile designed with smaller overall geotechnical safety factor, and yet without adequate provision of pile load test for verification. • And if this is unfortunately coupled with bad workmanship, constructed piles may not be able to satisfy design intent.

Considerations in review • Review taking into consideration:  – Old BCA/IES/ACES advisory note 1/03  – Recommendations in Eurocode 7  – GEOSS guideline “Guides on ground investigation and geotechnical characteristic values to Eurocode 7” issued on 24 Apr 2015

• In close collaboration with IES, ACES, GEOSS • HDB, LTA consulted

(1) Hello How are Review of you? Ground Investigation

For buildings of 10-storeys or more •

Minimum number of boreholes  – 1 per 300m2,  – 2 per block,  – 3 per site

•

Spacing of boreholes  – Between 10m to 30m apart

•

Minimum depth of borehole the deeper of:  – 5 m into hard stratum with SPT ≥ 100, and  – 3 pile diameters beyond intended pile depth

For buildings of 5 to 9-storey with footprint bigger than 100m 2 •

Minimum number of boreholes  – 1 per block  – 2 per site

•

Spacing of boreholes  – Between 15m to 40m apart

•

Minimum depth of borehole The deeper of :  – 5 m into hard stratum with SPT ≥ 100  – 3 pile diameters beyond intended pile depth.

Complex geology “The ground is full of surprises for the unwary.”

Karl Terzaghi 

Boulders

FCBB Cavity  3    5  m 20

Steeply inclined bedrock

Almost 45º bedrock slope

21

Hello (2) How are you? Review of Pile

Load Tests

BCA/IES/ACES Advisory note 1/03 on site investigation & load test Pile Load Tests for buildings of 10-storey or more

Joint BCA/IES/ACES/GEOSS BCA/IES/ACES/GEOSS Circular 2016 For buildings of 10-storeys or more •

Requ Re quir irem emen ents ts on pil pile e load load tes testt & int integ egri rity ty tes tests ts are are maintained maintained..

Table 1 Type of Pile Tests

Pile test schedule

(a)

Ultimate Load test (ULT)

1 number or 0.5% of piles, whichever is greater

(b)

Working Load test (WLT)

2 numbers numbers or 1% of piles piles,, whicheve whicheverr is greater

(c)

Non-destructive Integrity tests (High-strain dynamic test or sonic logging test)

2 number or 2% of piles, whichever is greater

Joint BCA/IES/ACES/GEOSS BCA/IES/ACES/GEOSS Circular 2016 For buildin buildings gs of 5 to 9 storeys storeys with footprin footprintt larger larger than 100 m2 Table 2 Type of Load Test

Pile Test Schedule

Working Load Test

1 number or 0.5% of the total number of working piles, whiche chever is greater.

QP to determine the minimum number of ultimate load tests and integrity test required for verification of pile design and capacity for their projects. .

Eurocode to take precedence • Clause 2.2 Eurocodes and Singapore National Annex are to take precedence if they impose more onerous requirement on pile testing than Table 1 or Table 2. • For example, BS EN 14199:2015 section 9.3.2.3 requires drilled micropile to be subject to a higher percentage of static load test.

Load test • Clause 2.3 Pile load test should be carried out by maintained loads. Pile load should be measured by both a calibrated load gauge and a calibrated pressure gauge.

Calibration of devices for pile load tests EN 1997-1:2004 Cl 7.5.2.1(2)

• Devices for determination of loads, stresses, strains and displacements should be calibrated prior to the test.

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Sonic Logging test • Also known as Ultrasonic Crosshole Testing, Crosshole sonic logging • To detect pile defects such as soil intrusion, necking, sand lenses, voids etc, and to detect the extent, nature and depth of these defects

Sonic Logging test

• Clause 2.4 Selected test piles shall be constructed in a manner representative of all working piles. • ASTM D6760-08 - one duct per 0.25m to 0.3m pile diameter, with minimum of 3 access ducts per pile

Supervision of Load test • Clause 2.5 QP shall submit Certificate of Supervision on Pile Load test for every pile load test conducted (Piling Annex B).  – To be submitted within 28 days upon completion of load test

Certificate of supervision of pile load test (Piling Annex B) • QP supervising pile load test shall submit Piling Annex B  – Declaring that they have inspected test equipment to be functional and properly calibrated

Allowable settlement for pile load test • Clause 2.6 Allowable settlement should be the same as SS CP4:2003. • Working load test should be carried out to a minimum 1.5 times or 2 times characteristic load acting on the pile, with allowable maximum settlement at pile top generally not exceeding 15 mm or 25 mm respectively.

Hello How are (3) you? The use of rapid load test

What is Rapid Load ? •

ISO 22477-10 : 2016 Clause 3.1.5 Rapid load - force applied to the pile in a continuously increasing and then decreasing manner (typically less than 1s) which causes the pile to compress over full length and translate as a unit during full loading period

•

Test duration is typically 200 ms.

•

ASTM 7383-08 Cl 3.2.4 Force pulse shall exceed preload for a duration of at least 12 times the pile length (L) divided by stress wave speed ( c).

Static load test using maintained loads SS CP4 Cl 7.5.5.4 Limiting rate of movement = 0.25 mm/hr

Typically 4 - 6 days

A maintained load test may take 4-6 days, with each load increment maintained for a duration till a limiting rate of movement is reached.

36

Use of Rapid Load Test • Clause 3.1 QP can propose to use rapid load test to verify capacities of working piles, provided that:  – QP have full understanding of limitation of rapid load test  – QP have adequately addressed these limitations

• QP should ensure that competent personnel are involved in conducting the test and interpreting the test results. • Due engineering allowance should be made to account for time-dependent creep effect and loading rate effect.

For buildings of 10-storeys or more • Clause 3.2 Conditions for partial replacement of static load test with Rapid load test  – Rapid load test should be calibrated against a static maintained load test on the same site  – Replacement with rapid load test should be limited to 50% of pile working load tests.

Why calibration is needed of rapid load test? 1. Rapid load test is not static load test and do not have the same degree of reliability and precision as static load test using maintained load. 2. Loading duration on pile is too short for settlement equilibrium to be reached for any load increment. 3. Results of the load test are not direct and require interpretation by competent and experienced personnel.

39

Reduction factor ASTM D7383-08 •

.. a reduction factor to account for additional load resistance due to faster rate of loading.

•

Reduction factor can be 2 for clayey soil, in European experience

ISO/FDIS 22477-10 Cl 4.1 •

Force applied to pile head during a rapid load test for measuring ultimate bearing capacity may exceed pile design compressive static resistance by a factor of 2 to 3 due to soil specific rate effect.

40

Use of Rapid Load Test BS 8004:2015

Example of calibration test Statrapid (3/3/2014)

Static load test (24/3/2014)

Settlement at 1xWL

3.7mm

3.9mm

Settlement at 2xWL

9.5mm

7.9mm

Pile diameter 800mm Working load 3770kN Depth = 33m

Statrapid

Static

42

Hello (4) How are you? Quality control test on bored piles

Why Quality Control Tests ? • Execution conditions and workmanship of bored piles affects design. • Quality control test will assist bored pile designer to assess and verify the appropriateness of design parameters chosen.

Problems with bored piles • Soft toe – due to pile bore stability problem, due to improper base cleaning, contaminated stabilizing fluid • Concrete quality – due to improper concreting method, collapse of unstable pile bore due to inadequate bore support

Changing level of stabilizing fluid

Temp Steel Casing Stabilizing fluid

Soft soil Potential Soil collapse

Zone of concern

Competent Soil layer Soil debris

Defects of bored pile • Defects of bored piles can affect pile structural integrity and their load carrying capacity. Loss of continuity along pile length Defective concrete, segregation Cavity within piles, incomplete pile section Inclusion of foreign particles Soft toe

Execution will affect design

Design

Execution

BS EN 1536:2010 Clause 7.1.2

Bored pile design shall take into account the construction conditions.

Quality Control Tests on bored piles •

For bored piles supporting buildings of 10-storeys or more, recommended quality control tests are as in Table 3 below. Quality Control Test

(a)

Interface coring test at pile base

Recommended test schedule 5% of total number of piles, but may be reduced to 2% if base grouting is carried out. Applicable for piles using characteristic unit bearing resistance qb;k > 10 MPa in design

(b)

Concrete core test

5 % of total number of piles,

below pile cut-off-level Applicable for piles designed using : (i) characteristic concrete cube strength fcu > 40 MPa and (ii) concrete compressive stress under working condition > 7.5 MPa.

Quality Control Tests on bored piles •

For bored piles supporting buildings of 10-storeys or more, recommended quality control tests are as in Table 3 below. Quality Control Test

(c) Pile Profile Test (to assess verticality and shapes of pile bore prior to concreting)

Recommended test schedule 100% of working piles with

Φ

>1.8m, which either

(i) have no steel casing going through soft or loose soil layer during construction or (ii) have no redundancy*.

e.g. Koden, SoniCaliper For piles ≤1.8m, QP shall determine the number of pile profile tests needed. *Redundancy can be provided by having more than one pile to support a single column or having tie beams in 2 directions joining individual pilecaps together or having integrated pilecap with base slab.

Interface coring test • Core drilling at concrete / rock interface inside reservation tube (normally from 1m above interface to 1 m below the interface) • To ascertain soundness of interface at pile base

50

Why concrete core test on bored pile ? •

If adopting fcd,pile = 0.3636 fck in structural design for lightly reinforced bored pile using EC2, minimum characteristic concrete cube strength f cu of 40 MPa is required to achieve allowable stress of 7.5 MPa. Concrete grade, fcu

C25

C30

C37

CP4

6.2

7.5

7.5

EC2 0.3636fck

4.7

5.6

6.9

C40

7.5

C45

C50

C55

7.5

7.5

7.5

8.4

9.3

10.3

Use of higher concrete grade in design using EC2 can result in larger compressive stress acting on bored pile, as compared to CP4.

Concrete core test • With no compaction during bored pile concreting, soil acting as formwork, possible soil inclusion in pile, the concern is “Can the high design strength of concrete used in design be achieved at site ?” • Concrete core test can help to address this concern when larger compressive stress is used.

Why Pile Profile test ? To assess verticality and shapes of pile bore prior to concreting of bore pile.

Example of pile profile test : Koden test •

KODEN test utilizes principle of ultrasonic wave travel from sensor to pile shaft and reflect to the sensor through the bentonite slurry.

•

Emission/reception of signal is carried out in 2 directions (X-X, Y-Y).

Pile Profile test Example of pile profile test : SoniCaliper  – can provide 3-dimensional model of excavated pile shaft

(5) Hello How are Design and you? construction of bored pile

Ultimate test pile should be representative of working piles •

Clause 5.1 Supervising QP, site supervisors and builders are to ensure that working piles are installed using :  – same method of construction as ultimate test pile;  – workmanship no less inferior than ultimate test pile.

•

Ultimate test pile shall NOT be constructed under more favourable condition than working piles.

EC7-1 Clause 7.6.2.2

Avoid over-reliance on pile base resistance • Clause 5.2 Characteristic shaft resistance of bored piles should not be less than 1.3 times characteristic load acting on them.  – unless method of enhancing the pile base resistance is employed at site (like base grouting technique), verified by testing (like interface coring).

Rs,k ≥ 1.3 (Gk + Qk)

Lateral force acting on pile • Clause 5.3 Bored piles shall not be constructed without any steel reinforcement. • Bored pile should be designed for lateral force acting on them during construction and long term conditions. • Reinforcement of bored piles should go beyond any soft or loose soil layer.

What is Soft soil ? EN ISO 14688-2:2004

What is Loose soil ? EN ISO 14688-2:2004

Execution of bored pile EN 1536:2010

7.1.2 Bored piles design shall take into account construction tolerances given in 8.1 and the execution conditions as set in Clause 8. 7.1.7 A bored pile should be reinforced over any length of soft or loose soil.

Soft clay

Competent soil layer or bedrock

61

Lesson learnt BS EN 1536:2010 Clause 7.1.7

When bore piles penetrate through soft or loose soil to embed in firm soil stratum, reinforcement should be reinforced over this soil layer. 62

Proposed changes in forms relating to specialist builder

NEW CHANGES TO PERMIT, NOTIFICATION OF BUILDER FORM AND BUILDER CERTIFICATE (BC) FORMS

1. Revised BCA-BE-Pe!i" #$$%i&#"i' '! SECTION F . DECLARATION B* BUILDER

i.

General Builder to declare types of specialist building works undertaken by him (if he also holds a Specialist Builder license for the works) • • • • • •

Ground support and stabilization works Piling works Pre-cast concrete works Site n!estigation works n-situ post-tensioning works Structural steelworks

ii. For the specialist building works which the General Builder is not the Specialist Builder, he will need to declare that Specialist Builder(s) will be appointed to carry out the specialist building works.

NEW CHANGES TO PERMIT, NOTIFICATION OF BUILDER FORM AND BUILDER CERTIFICATE (BC) FORMS

Revised BCA-BE- Bi%de-A$$'i"!e" '!

SECTION C . APPOINTMENT OF BUILDER B* BUILDER

ii. SECTION B . APPOINTMENT OF BUILDER B* DE+ELOPER

i.

"e!eloper  to notify #B# on the Specialist Builders appointed by him

Builder  to notify #B# on the Specialist Builders appointed by him

NEW CHANGES TO PERMIT, NOTIFICATION OF BUILDER FORM AND BUILDER CERTIFICATE (BC) FORMS

. Revised BCA-BE- Bi%de-A$$'i"!e" '! SECTION D . DECLARATION B* SPECIALIST BUILDER

i.

Specialist Builder to declare that his works will be carried out in accordance with B# $ct notwithstanding that he ha!e sub-contracted all or part of the specialist building works

NEW CHANGES TO PERMIT, NOTIFICATION OF BUILDER FORM AND BUILDER CERTIFICATE (BC) FORMS

/. Revised BCA-BE- Bi%de-Te!i#"i' '!

SECTION B. TERMINATION OF GENERAL BUILDERSPECIALIST BUILDER B* DE+ELOPERBUILDER

i.

"e!eloper%Builder to notify #B# on the termination of his General Builder and Specialist Builder 

SECTION C. NOTIFICATION ON TERMINATION OF APPOINTMENT BUILDER

i.

General builder%Specialist Builder to notify #B# on the ceasing of duties

NEW CHANGES TO PERMIT, NOTIFICATION OF BUILDER FORM AND BUILDER CERTIFICATE (BC) FORMS

0. Addi"i' ' Bi%de Ce"ii&#"e (BC) ' S$e&i#%is" Bi%de  i.

Specialist Builder to declare that he has e&ecuted the following specialist building works in accordance with the plans as supplied by the 'ualified Person(s) and in accordance with all the pro!isions in the $ct and egulations

A comparison of the OLD with the NEW Items 1. Site investigation (10-storeys or more) 1. Site investigation (5 to 9 storeys) 2. Pile load test & integrity test (10-storeys or more) 2. Pile load test & integrity test (5 to 9 storeys) 3. The use of rapid load test 4. Quality control tests on bored piles (10-storey or more) 5. Design and construction of bored piles

New Circular 2016

Old advisory note 2003

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√

√ √ √ √

√

Summarising remark •

Since July 2015, BCA has engaged IES, ACES and GEOSS to work on a joint circular on requirements of piled foundation, covering:     

•

Ground investigation Load test & integrity test Use of Rapid load test Quality Control tests for bored piles Design and construction of bored piles

The working group on the joint circular has taken into consideration the following:  –  –  –  –  –

Eurocodes CP4 BCA/IES/ACES advisory note 1/03 GEOSS guidelines Current industry practice

Summarising remark The joint circular will take effect on projects where ST01 is submitted on and after 1 April 2017, and shall supersede Advisory Note 1/03. The requirements shall be complied when making submissions of structural plans to CBC for approval.