Soil-Foundation-Superstructure

Interaction for Tall Buildings on

Combined Pile-Raft Foundations

Konstantinos Syngros, Ph.D., P.E.

Alan Poeppel, P.E. Langan Engineering & Environmental Services, Inc.

Langan International

OVERVIEW

• Definitions

• Process of Foundation Design for Tall Buildings

• Soil Modeling For Structural Design

• Case Study

• Final Remarks

DEFINITIONS

Foundation Differential Settlements

• Differential settlements due to gravity loads

• For tall towers even small differential settlements can offset the top by meters and overstress the structure

• A special study is needed to determine differential settlements

TOWER

Raft Foundation Differential

Settlements

Soil Modeling for Structural Design

Structural Approach

Winkler Springs Model

Geotechnical Approach

Continuum Model

Raft Foundations

• Raft foundations are considered when:

– Individual footings cover more than about half of the building area.

– Total and differential settlements are too high.

RAFT (a.k.a. MAT)

COLUMN LOCATIONS

• A raft consists of a reinforced-concrete slab that supports the columns and walls of a structure, and distributes the loads to the underlying soils.

Combined Pile-Raft Foundations

• Combined Pile-Raft foundations are considered when:

– Settlements and differential settlements are too high when using only raft foundations. PILES/

DRILLED SHAFTS

Piled Raft - Overview

• Combination of raft + piles – Raft to provide adequate

bearing

– Piles to reduce total and differential settlements

• Piled-raft consists of a limited number of piles strategically located below heavily loaded area (e.g.: shear walls)

Piled Raft - Overview Kuala Lumpur,

Malaysia Brooklyn, N.Y., US Brooklyn, N.Y., US

Kuala Lumpur,

Malaysia

Settlement Comparison between Raft only and Piled-Raft

• Project: Brooklyn, N.Y., US

A

B

C

D

E A

B

C

D

E

Raft Only

Max ∆/L = 1/335

Piled-Raft

Max ∆/L = 1/630

1 in

2.5 in 0.8 in

1 in

0.9 in

0.8 in

1.5 in 0.7 in

0.8 in

0.8 in

Process of Foundation Design for Towers

Project Team for Towers

• Developer

• Architect

• Structural Engineer

• Geotechnical Engineer

• Local Consultants

• Peer Reviewers

• Specialty Foundation Contractors

+

Developing Foundation System • Geotechnical Investigation

• Structural Engineer Provides Initial Tower Loads

• Initial Studies (Bearing Capacity, Settlement) by Geotechnical Engineer

• Refinement of Studies (Determine Constructability, Iterative Process between Structural + Geotechnical Engineer)

• Peer Review Process

• Contractor Selection Process

• Local Authorities Approval Process

Iterative Process General Steps - RAFTS COLUMN LOADS

WINKLER SPRINGS, FOUNDATION SETTLEMENTS+ STRESSES

REVISED FOUNDATION SETTLEMENTS+STRESSES

NEW WINKLER SPRINGS, NEW FOUNDATION SETTLEMENTS+STRESSES

Structural Engineer

Geotechnical Engineer

Iterative Process – Combined Pile-Rafts

In addition to Raft Settlements, require convergence on

• Pile Loads

• Pile/Subgrade Load Sharing

• Column/Wall Loads

Soil Modeling for Structural Design

Soil Modeling for Structural Design

Structural Approach

Winkler Springs Model

Geotechnical Approach

Continuum Model

Elastic

Space

Inelastic

Space

Determination of Soil Springs (K-value)

• The K-value (a.k.a. Winkler Spring or Modulus of Subgrade Reaction) is the ratio of pressure over displacement

• K = p/δ

Foundation Response

• Significant differences in response between Winkler and Continuum models.

– Contact pressure beneath “rigid” foundations

– Settlements beneath “flexible” foundations

Foundation Response – Rigid Raft

Settlements

Stress Distribution

Structural Approach

Winkler Springs Model

Geotechnical Approach

Continuum Model

Foundation Response – Flexible Raft

Settlements

Stress Distribution

Winkler Continuum

Foundation Response

• Solution:

– Stiffness of outermost springs should be increased to account for increase in soil rigidity.

– For “stiff” soil, outer springs may increase 30-40%

– For “soft” soil, they may need to increase 2-3 times!

Determination of Modulus of Subgrade Reaction (K-value)

• The K-value can be obtained through:

• Method A: Plate load tests

• Method B: Tables of typical values /

correlations (based on plate load tests)

• Method C: Modeling of the loaded foundation

Determination of Modulus of Subgrade Reaction (K-value) – Method C

• Estimate settlements and bearing pressures at many points within the raft

• Determine the K-value from:

– K = p/δ (pressure/settlement)

• Draw K-value contours

• If piles are present, pile springs can be determined as pile force/pile settlement at the pile locations

CASE STUDY

Tower in Kuala Lumpur, Malaysia

Geotechnical Investigation

• Geotechnical Borings

• In-situ Testing (i.e., SPT, Pressuremeter Tests)

• Laboratory Tests (Index properties, Triaxial, 1-D Consolidation, etc)

• Geophysical Methods

Model Parameter Selection

Determine critical parameters such as

• Raft thickness

• Locations, geometry of piles

• Soil Modulus of deformation

• Method of construction

• Pile/Soil interface strength

Hand Calculations

Finite Element Modeling Input

• Geometry (raft, piles, stratigraphy)

• Material Properties (raft, piles, stratigraphy)

• Element Selection (Volume, area, line elements)

• Boundary Conditions

• Loading (Construction Staging, Self Weight, Tower Loads)

17

0m

RAFT: ~ 40m x 60m x 4m PILES D=2m, L=75m

Typical Soil/Rock Constitutive Models Soil

• Elastic (E, , unit weight)

• Mohr-Coulomb (friction angle, cohesion)

• Modified M-C/Soil Hardening

• Soft Soil/ Modified Cam-Clay

Rock

• Elastic (E, , unit weight)

• Mohr-Coulomb (friction angle, cohesion)

• Hoek-Brown

Typical Structure Models Raft

• Elastic (E, , unit weight), Volume Elements

• Plate Elements (thickness)

Piles

• Volume Elements (w or w/o interface)

• Beam Elements (w or w/o interface)

• Embedded Beam Elements w interface

This is the best but model may become too big for the computer to handle

Sufficiently accurate and can handle large number of piles.

Intermediate situation

Typical Loading Conditions • Soil/Foundation Self Weight

• Column/Wall Loads as

–Point Loads

– Line Loads

–Pressure Loads

• Foundation Construction Stages

Settlement Results

~100mm

~35mm

Settlement Results

40 m

70 m

Soil Subgrade Stresses

Vertical Strains (Block Behavior)

Pile Settlements

Pile Axial Loads

Pile-Soil Relative (Slip) Displacement

Soil/Pile Springs Determination

• Determine Pile Springs (Reaction/Settlement)

• Determine Area Springs (Pressure/Settlement)

• Determine Pile/Raft Load Share

• Provide the above to Structural Engineer

Pile Loads + Settlements

25

27

26

25

30

27

27

24

39

24

28

27

24

27

25

24

26

27

38

24

25

26

29

41

25

30

28

24

29

23

23

25

24

25

27

37

25

31

28

27

30

2631

32

25

25

40

36

32

28

2740

2736

25

24

31

32

23

26

30

27

29

29

25

24

2630

2526

26

27

2625

33

32

30

Pile Head Axial Load (MN)

Pile Loads

Raft Settlements

Soil/Pile Springs PILE SPRINGS

AREA SPRINGS

283

337

366

304

351

324

361

261

497

268

334

437

346

312

310

306

365

298

454

338

322

285

351

475

278

461

343

265

499

261

253

280

289

282

297

465

304

516

448

296

336

297344

499

367

264

486

502

456366

425466

379476

306

296

397

488

457

279

372

301

460

332

298

316

279370

292308

335

405

423288

436

406

385

Pile Springs (MN/m)

Winkler Springs Adjustment

Based on Structural Engineer’s

• Settlements

• Subgrade Stresses

• Pile Loads

• Pile/Raft Load Sharing

• Wall Loads

Re-run Foundation Model + adjust Area and Pile Springs appropriately

Comparisons: Iteration 1

Pile Loads > 20% Settlements >20%

Pile Loads > 20% Settlements >10% Wall Loads>10%

Comparisons: Iteration 2

Pile Loads > 5% Settlements >5% Wall Loads >10%

Comparisons: Iteration 3

Pile Loads > 5% Settlements >5% Wall Loads <10%

Comparisons: Iteration 4

Pile Loads ~5% Settlements ~5% Wall Loads <10%

Comparisons: Iteration 5

Shear-Wall Load Changes

RAFT EDGES RAFT CENTER

FINAL REMARKS

• The iterative process gives insight on the redistribution of column/wall loads.

• The redistribution is more pronounced when the superstructure is “stiff” compared to the foundation.

• The redistribution may necessitate “stiffening” the foundation.

Thank you

Questions?

KSyngros@Langan.com

APoeppel@Langan.com