Characterising a near river site in St · PDF fileCharacterising a near river site in ... Stiff Clay 30 200 kPa ... Carry out soil and rock testing - Triaxial tests

whitbybird

Paul Knight, Ramboll UK

Characterising a near river site in St Petersburg

How challenging soils impact on the foundation design for a high rise building

COMING UP…

• Project overview

• Geological context

• Ground conditions

• Challenging soils in context

• Site investigation

• Full scale load test

• Challenges faced due to ground conditions

• Impact on foundation design philosophy

PROJECT OVERVIEW

• Site is located in St Petersburg, Russia

PROJECT OVERVIEW

• Site is located in St Petersburg, Russia

• Situation on the banks of the Neva and Okhta Rivers

PROJECT OVERVIEW

• 396m high, 77 storey tower

• First structure of its kind to be built on these ground conditions

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509m 452m 442m 420m 396m 381m

PROJECT OVERVIEW



• Pentagonal pile cap

• Tower load 3,360MN

• 60% of load from core

• Remainder from columns

PROJECT OVERVIEW



• Pentagonal pile cap

• Tower load 3,360MN

• 60% of load from core, remainder from columns

• 13m deep basement

• 10 fin walls in basement to spread load from core

GEOLOGICAL CONTEXT

• Located on the edge of the Baltic and Eastern European Shields

• Vendian Clay ~570Ma old, extends from 43m to 200m

• Series of depositional environments during the Quaternary period

• Edge of the Baltic Sea

• Results in highly variable ground conditions, both vertically and laterally

0

28

43

200

Depth (m)

Glacier

Glacial lake

Glacial lake

Marine and lake

21

10

PROPERTIES OF GROUND

MaterialE

(MPa)cu or φ

Made Ground --- ---

Loose Sand 5 25 deg

Ground water 3m below ground level

0

5

10

Depth (m)


MaterialE

(MPa)cu or φ

Made Ground --- ---

Loose Sand 5 25 deg

Soft Clay and Loose Sand

1540 kPa 34 deg


0

5

28

10

Depth (m)


MaterialE

(MPa)cu or φ

Made Ground --- ---

Loose Sand 5 25 deg


1540 kPa 34 deg

Stiff Clay 30 200 kPa


0

5

28

43

10

Depth (m)


MaterialE

(MPa)cu or φ

Made Ground --- ---

Loose Sand 5 25 deg


1540 kPa 34 deg


Very Stiff Dislocated Clay

50 200 kPa


0

5

28

43

51

10

Depth (m)


MaterialE

(MPa)cu or φ

Made Ground --- ---

Loose Sand 5 25 deg


1540 kPa 34 deg


Very Stiff Dislocated Clay

50 200 kPa

Very Stiff Clay50 -150

400 –800 kPa


0

5

28

43

51

200

10

Depth (m)

CHALLENGING SOILS IN CONTEXT

Routine project work

Site investigation

Foundation design



Site investigation

Challenging soil identified

Foundation design



Site investigation

Understanding of problem

Foundation design

Additional site investigation

Research




Site investigation

Understanding of problem

Update foundation design

Foundation design

Additional site investigation

Research

Arrangement

Type

Construction method

Depth



CONSIDERATIONS

• Impact on foundation design

Does it affect the overall design philosophy?

Is there a change in load capacity, settlement, constructability?


CONSIDERATIONS


Does it affect the design philosophy?


• Research into problem

Cost of additional site investigation works

Delays with design and construction programme


CONSIDERATIONS


Does it affect the design philosophy?


• Research into problem

Cost of additional site investigation works

Delays with design and construction programme

• Options include

Investigation problem and optimise foundation design

Overcome problem with a conservative design

SITE INVESTIGATION

• Design work and research to scope site investigation

• Specific objectives

• Site investigation work carried out

• Interpretation of factual information

• Challenges faced during site investigation due to ground conditions

TO FOLLOW…

DESIGN WORK TO SCOPE SITE INVEST.

• Design work required to identify:

– Potential ground challenges that require investigation

– Required depth of site investigation

– Relevant foundation design parameters required

– Specialised information required


• Review of available information

Very little known



Very little known

• Identification of options

Deep foundation required



Very little known

• Identification of options

Deep foundation required

• Preliminary foundation design

Load capacity using estimate of undrained shear strength

Block failure check

Preliminary modelling

• Foundations 80m deep required

OBJECTIVES OF SITE INVESTIGATION

Item Methodology

Stratigraphy, geotechnical properties Standard field and laboratory work


Item Methodology


Behaviour of Vendian Clay - soil or rock?

Carry out soil and rock testing- Triaxial tests- Unconfined compression tests

Petrographic examinations

Advanced small strain testing


Item Methodology






Lateral and vertical variation of quaternary soils

Very little known about upper materials

Cone penetration testing

Laboratory testing


Item Methodology






Lateral and vertical variation of quaternary soils

Very little known about upper materials

Cone penetration testing

Laboratory testing

In-situ properties of materials Pressuremeter testing

SITE INVEST. WORK CARRIED OUT

• 2500m rotary cored drilling down to 150m depth

• 17 CPTs to a depth of 40m

• 34 pressuremeter tests between 70 and 90m depth

• Geophysics

Seismic surface surveying

Cross-borehole seismic

Cross-borehole electrical resistivity

SITE INVEST. WORK CARRIED OUT

• 1290 laboratory tests

Triaxial testing

Unconsolidated undrained

Consolidated undrained

Consolidated drained

Shear box

Unconfined Compression

Oedometer

• Petrographic examinations

• Small strain testing

INTERPRETATION OF SI INFORMATION

• Identification of stratigraphy

• Physical properties

• Strength properties

Undrained and effective parameters

Peak and critical values

• Deformation properties

Deformation modulus over entire strain range including small strains

Laboratory and in-situ values

• Overconsolidation state of Vendian Clay

• Brittleness of Vendian Clay

CHALLENGES FACED DURING SITE INVEST. DUE TO GROUND CONDITIONS

• Drilling methods

Only single tube drilling could be carried out




• Thixiotropic upper soils

Soil samples could not be collected for upper materials for testing






• Pore pressure profile

Pore pressure could not be measured in the Vendian Clay






• Pore pressure profile

Pore pressure could not be measured in the Vendian Clay

• Limits of laboratory equipment

Problems saturating Vendian Clay

Difficulties applying and maintaining the very high loads

Long drainage and pore pressure equalisation times for tests

FULL SCALE LOAD TESTS

• Design work to specify full scale load test

• Testing carried out

• Analysis of tests

• Comparison of test results with design and site investigation results

• Additional soil challenges identified

TO FOLLOW…


• Objectives:

– Identify constructability issues

– Determine load settlement behaviour to calibrate numerical models

– Verify ultimate geotechnical capacity

– Obtain the ultimate skin friction of the soils using strain gauges

– Verify and calibrate the design methods

DESIGN WORK TO SPECIFY LOAD TESTS

• Need to determine:

– Depth of foundation

– Design working load of foundation elements


• Interpretation of site investigation information



• Foundation capacity

Total stress

Effective stress

Weak rock

• Settlement calculations



• Foundation capacity

Total stress

Effective stress

Weak rock

• Settlement calculations

• Numerical analysis

Robot

Plaxis 3D

Piglet


• 3 barrettes (3.3m long) and 1 pile (1.2m diameter) constructed to depths of 65m

• Loaded to three times the design working load


• 3 barrettes (3.3m long) and 1 pile (1.2m diameter) constructed to depths of 65m

• Loaded to three times the design working load

• Reaction piles and jacks used to apply loads to the pile

• Osterberg cells required to provide the high loads for the barrettes

ANALYSIS OF LOAD TESTS

• Load displacement curve


• Ultimate geotechnical capacity

Extrapolate load displacement curve


• Calibrate numerical models

Vary parameters to obtain load displacement and ultimate capacity


• Strain gauge analysis

1. Calculate axial stiffness by differentiating strain gauge results




2. Determine axial force in shaft





3. Calculate mobilisation in skin friction





3. Calculate mobilisation in skin friction

4. Determine ultimate skin friction





3. Then calculate mobilisation in skin friction

4. Determine ultimate skin friction

5. Carry out back analysis of geotechnical design parameters using design methods

COMPARISON WITH DESIGN AND SITE INVESTIGATION INFORMATION

Item Results of load test

Ultimate capacity of foundation elements

35% better than using site investigation and design methods

Settlement at design working load 30% better than using site investigation and design methods

COMPARISON WITH DESIGN AND SITE INVESTIGATION INFORMATION

Item Results of load test

Ultimate capacity of foundation elements

35% better than using site investigation and design methods

Settlement at design working load 30% better than using site investigation and design methods

Geotechnical parameters from strain gauge analysis

Slightly lower strength parameters than from site investigation

Geotechnical parameters from calibration of numerical models Better than from site investigation

Geotechnical parameters from calibration of design methods Better than from site investigation

ADDITIONAL SOIL CHALLENGES IDENTIFIED DURING LOAD TESTS

• Variable ground conditions

Best construction method difficult to determine




• Thixiotropic material

Significant concrete over consumption (80%), mostly in upper section






• Strength of Vendian Clay

Slow rate of construction leading to a long exposure to bentonite







Slow rate of construction leading to a long exposure to bentonite

• Strain gauges indicated that the mobilised skin friction was lower than expected in some locations

Effect of construction method?

Unreliable strain gauges?


• Strain gauges showed soil properties may be less than measured from the site investigation

However, ultimate capacity and load-settlement behaviour were better than expected

Back analysis of design methods and numerical models showed that soil properties were better than expected


• Two views

Soil properties are WORSE than from site investigation

Soil properties are BETTER than from site investigation


• Two views



Strain gauges are reliable

Axial stiffness can be calculated from theoretical values

Design methods are conservative so underestimate load capacity

Construction methods resulted in lower skin friction than expected


• Two views



Strain gauges are reliable Strain gauges are faulty/unrealiable


Actual axial stiffness (EA) is difficult to determine

Design methods are conservative so underestimate load capacity Design methods are appropriate


Conservative assessment of SI or sampling disturbance


• Two views



Strain gauges are reliable Strain gauges are faulty/unrealiable


Actual axial stiffness (EA) is difficult to determine

Design methods are conservative so underestimate load capacity Design methods are appropriate


Conservative assessment of SI or sampling disturbance

Need to consider the impact on the foundation design

REVIEW OF PRIMARY SOIL CHALLENGES

• Vendian Clay is an intermediate soil

Does it behave like a soil or rock at the design stress?

Does effective stress or rock strength govern design?

Does pore pressure affect the performance?


• Vendian Clay is an intermediate soil

Does it behave like a soil or rock at the design stress?

Does effective stress or rock strength govern design?

Does pore pressure affect the performance?

• Limits of standard lab equipment

Samples couldn’t be saturated

High loads difficult to apply



Correct construction method difficult to choose

Significant concrete over consumption is upper materials






Construction Delays leading to long exposure to bentonite






Construction Delays leading to long exposure to bentonite

• Strain gauges in load test indicated that the skin friction was lower than expected in some locations

Effect of construction method?

Unreliable strain gauges?

IMPACT ON FOUNDATION DESIGN PHILOSOPHY

• Load capacity determined using three methods considering Vendian Clay as an intermediate soil

• Overall factor of safety of three adopted

• Load capacity and settlement checked for post-peak strength





• Design methods verified by full scale load tests

Load tests showed overall performance of foundation is better than predicted by the design

Settlement is acceptable at design working load





• Design methods verified by full scale load tests

Load tests showed overall performance of foundation is better than predicted by the design

Settlement is acceptable at design working load

• Detailed numerical analysis carried out

Structure-foundation and foundation-soil interaction

Calibrated to full scale load tests

THANK YOU

Documents

Characterising a near river site in St · PDF fileCharacterising a near river site in ... Stiff Clay 30 200 kPa ... Carry out soil and rock testing - Triaxial tests