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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
• 396m high, 77 storey tower
• First structure of its kind to be built on these ground conditions
• Pentagonal pile cap
• Tower load 3,360MN
• 60% of load from core
• Remainder from columns
PROJECT OVERVIEW
• 396m high, 77 storey tower
• First structure of its kind to be built on these ground conditions
• 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)
PROPERTIES OF GROUND
MaterialE
(MPa)cu or φ
Made Ground --- ---
Loose Sand 5 25 deg
Soft Clay and Loose Sand
1540 kPa 34 deg
Ground water 3m below ground level
0
5
28
10
Depth (m)
PROPERTIES OF GROUND
MaterialE
(MPa)cu or φ
Made Ground --- ---
Loose Sand 5 25 deg
Soft Clay and Loose Sand
1540 kPa 34 deg
Stiff Clay 30 200 kPa
Ground water 3m below ground level
0
5
28
43
10
Depth (m)
PROPERTIES OF GROUND
MaterialE
(MPa)cu or φ
Made Ground --- ---
Loose Sand 5 25 deg
Soft Clay and Loose Sand
1540 kPa 34 deg
Stiff Clay 30 200 kPa
Very Stiff Dislocated Clay
50 200 kPa
Ground water 3m below ground level
0
5
28
43
51
10
Depth (m)
PROPERTIES OF GROUND
MaterialE
(MPa)cu or φ
Made Ground --- ---
Loose Sand 5 25 deg
Soft Clay and Loose Sand
1540 kPa 34 deg
Stiff Clay 30 200 kPa
Very Stiff Dislocated Clay
50 200 kPa
Very Stiff Clay50 -150
400 –800 kPa
Ground water 3m below ground level
0
5
28
43
51
200
10
Depth (m)
CHALLENGING SOILS IN CONTEXT
Routine project work
Site investigation
Foundation design
CHALLENGING SOILS IN CONTEXT
Routine project work
Site investigation
Challenging soil identified
Foundation design
CHALLENGING SOILS IN CONTEXT
Routine project work
Site investigation
Understanding of problem
Foundation design
Additional site investigation
Research
Challenging soil identified
CHALLENGING SOILS IN CONTEXT
Routine project work
Site investigation
Understanding of problem
Update foundation design
Foundation design
Additional site investigation
Research
Arrangement
Type
Construction method
Depth
Challenging soil identified
CHALLENGING SOILS IN CONTEXT
CONSIDERATIONS
• Impact on foundation design
Does it affect the overall design philosophy?
Is there a change in load capacity, settlement, constructability?
CHALLENGING SOILS IN CONTEXT
CONSIDERATIONS
• Impact on foundation design
Does it affect the design philosophy?
Is there a change in load capacity, settlement, constructability?
• Research into problem
Cost of additional site investigation works
Delays with design and construction programme
CHALLENGING SOILS IN CONTEXT
CONSIDERATIONS
• Impact on foundation design
Does it affect the design philosophy?
Is there a change in load capacity, settlement, constructability?
• 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
DESIGN WORK TO SCOPE SITE INVEST.
• Review of available information
Very little known
DESIGN WORK TO SCOPE SITE INVEST.
• Review of available information
Very little known
• Identification of options
Deep foundation required
DESIGN WORK TO SCOPE SITE INVEST.
• Review of available information
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
OBJECTIVES OF SITE INVESTIGATION
Item Methodology
Stratigraphy, geotechnical properties Standard field and laboratory work
Behaviour of Vendian Clay - soil or rock?
Carry out soil and rock testing- Triaxial tests- Unconfined compression tests
Petrographic examinations
Advanced small strain testing
OBJECTIVES OF SITE INVESTIGATION
Item Methodology
Stratigraphy, geotechnical properties Standard field and laboratory work
Behaviour of Vendian Clay - soil or rock?
Carry out soil and rock testing- Triaxial tests- Unconfined compression tests
Petrographic examinations
Advanced small strain testing
Lateral and vertical variation of quaternary soils
Very little known about upper materials
Cone penetration testing
Laboratory testing
OBJECTIVES OF SITE INVESTIGATION
Item Methodology
Stratigraphy, geotechnical properties Standard field and laboratory work
Behaviour of Vendian Clay - soil or rock?
Carry out soil and rock testing- Triaxial tests- Unconfined compression tests
Petrographic examinations
Advanced small strain testing
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
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
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
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
• 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…
FULL SCALE LOAD TESTS
• 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
DESIGN WORK TO SPECIFY LOAD TESTS
• Interpretation of site investigation information
DESIGN WORK TO SPECIFY LOAD TESTS
• Interpretation of site investigation information
• Foundation capacity
Total stress
Effective stress
Weak rock
• Settlement calculations
DESIGN WORK TO SPECIFY LOAD TESTS
• Interpretation of site investigation information
• Foundation capacity
Total stress
Effective stress
Weak rock
• Settlement calculations
• Numerical analysis
Robot
Plaxis 3D
Piglet
FULL SCALE LOAD TESTS
• 3 barrettes (3.3m long) and 1 pile (1.2m diameter) constructed to depths of 65m
• Loaded to three times the design working load
FULL SCALE LOAD TESTS
• 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
ANALYSIS OF LOAD TESTS
• Ultimate geotechnical capacity
Extrapolate load displacement curve
ANALYSIS OF LOAD TESTS
• Calibrate numerical models
Vary parameters to obtain load displacement and ultimate capacity
ANALYSIS OF LOAD TESTS
• Strain gauge analysis
1. Calculate axial stiffness by differentiating strain gauge results
ANALYSIS OF LOAD TESTS
• Strain gauge analysis
1. Calculate axial stiffness by differentiating strain gauge results
2. Determine axial force in shaft
ANALYSIS OF LOAD TESTS
• Strain gauge analysis
1. Calculate axial stiffness by differentiating strain gauge results
2. Determine axial force in shaft
3. Calculate mobilisation in skin friction
ANALYSIS OF LOAD TESTS
• Strain gauge analysis
1. Calculate axial stiffness by differentiating strain gauge results
2. Determine axial force in shaft
3. Calculate mobilisation in skin friction
4. Determine ultimate skin friction
ANALYSIS OF LOAD TESTS
• Strain gauge analysis
1. Calculate axial stiffness by differentiating strain gauge results
2. Determine axial force in shaft
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
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
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
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
• Strain gauges indicated that the mobilised skin friction was lower than expected in some locations
Effect of construction method?
Unreliable strain gauges?
ADDITIONAL SOIL CHALLENGES IDENTIFIED DURING LOAD TESTS
• 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
ADDITIONAL SOIL CHALLENGES IDENTIFIED DURING LOAD TESTS
• Two views
Soil properties are WORSE than from site investigation
Soil properties are BETTER than from site investigation
ADDITIONAL SOIL CHALLENGES IDENTIFIED DURING LOAD TESTS
• Two views
Soil properties are WORSE than from site investigation
Soil properties are BETTER than from site investigation
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
ADDITIONAL SOIL CHALLENGES IDENTIFIED DURING LOAD TESTS
• Two views
Soil properties are WORSE than from site investigation
Soil properties are BETTER than from site investigation
Strain gauges are reliable Strain gauges are faulty/unrealiable
Axial stiffness can be calculated from theoretical values
Actual axial stiffness (EA) is difficult to determine
Design methods are conservative so underestimate load capacity Design methods are appropriate
Construction methods resulted in lower skin friction than expected
Conservative assessment of SI or sampling disturbance
ADDITIONAL SOIL CHALLENGES IDENTIFIED DURING LOAD TESTS
• Two views
Soil properties are WORSE than from site investigation
Soil properties are BETTER than from site investigation
Strain gauges are reliable Strain gauges are faulty/unrealiable
Axial stiffness can be calculated from theoretical values
Actual axial stiffness (EA) is difficult to determine
Design methods are conservative so underestimate load capacity Design methods are appropriate
Construction methods resulted in lower skin friction than expected
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?
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?
• Limits of standard lab equipment
Samples couldn’t be saturated
High loads difficult to apply
REVIEW OF PRIMARY SOIL CHALLENGES
• Variable ground conditions
Correct construction method difficult to choose
Significant concrete over consumption is upper materials
REVIEW OF PRIMARY SOIL CHALLENGES
• Variable ground conditions
Correct construction method difficult to choose
Significant concrete over consumption is upper materials
• Strength of Vendian Clay
Construction Delays leading to long exposure to bentonite
REVIEW OF PRIMARY SOIL CHALLENGES
• Variable ground conditions
Correct construction method difficult to choose
Significant concrete over consumption is upper materials
• Strength of Vendian Clay
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
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
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
• Detailed numerical analysis carried out
Structure-foundation and foundation-soil interaction
Calibrated to full scale load tests
THANK YOU