Shallow foundations of tall buildings designed on the ... · Morumbi 31 4.0m (13.1 ft) 3,000-10,000 400 KPa Figures 1 to 4 show the footings of the four build-ings and one of the

Shallow foundations of tall buildings, designed on the basis of DMT results

Prof. Dr. Antônio Sérgio Damasco Penna

Damasco Penna Engenheiros Associados S/C Ltda

Universidade Mackenzie, São Paulo, Brazil

Keywords: Shallow foundations, tall buildings

ABSTRACT: The objective of this paper is to describe the use of DMT in the design of shallow founda-tions for tall buildings in Sao Paulo / SP, Brazil. This city in Brazil has a well known geologic formation, witha sedimentary basin in its central area, surrounded by residual soils. Shallow foundations are often economi-cal, both for sedimentary over-consolidated clays and sands, and residual silty soils. The design of those shal-low foundations, for typical 20–25 floor buildings, is controlled by settlements. Dilatometer tests DMT, per-formed with SPT and CPT, were used in those settlements evaluations and provided the necessary support fordesign decisions.

1 GEOLOGICAL CONDITIONS OF SAO PAULO

Sao Paulo is a 1.516 Km2 (585 mi2) city, and when including suburbs its area is about 3.000 Km2 (1,158 mi2).

The elevations above sea level generally vary be-tween 730 m (2,395 ft) and 830 m (2,723 ft), with a maximum of 1.126 m (3,694 ft) at Jaragua Peak.

During the tertiary geological age, a sedimentary basin was formed, with many layers of clays and sands, reaching elevations of 830 m (2,723 ft) above sea level.

Gradually, the two principal rivers, Tiete and Pinheiros, partially eroded valleys to about elevation 730 m (2,395 ft).

Many mountains of gneissic or granitic residual soils surround this basin.

In this area a large number of tall buildings have been constructed both in the central area (sedimen-tary basin) and in the contour area (mountains of re-sidual soils).

2 SITE CHARACTERIZATION PRACTICE

Generally, the practice of foundation engineering in Brazil is based on Standard Penetration Test (SPT) results.

The use of additional site characterization, based on field (CPT, DMT, PMT) or laboratory tests is rare.

Since 1997 we have been encouraging the use of DMT as an additional field test.

This practice has been growing slowly in some construction companies. We have demonstrated that a better site characterization can be obtained with DMT. With the more accurate DMT data a better foundation design can occur resulting, in some cases in lower foundation construction costs.

3 BUILDING FOUNDATIONS IN SAO PAULO

The development of building constructions in Sao Paulo started around 1930 - 1940.

Five to fifteen floors were common at that time. Shallow foundations on spread footings, drilled

piers with enlarged base, obtained by manual under reaming and with or without the aid of compressed air, “Franki” piles, precast concrete piles and steel piles, were all used then.

Since 1970 - 1980 slurry method of drilled pier construction and concrete flight auger piles have been used.

Nowadays, most tall buildings in Sao Paulo are constructed on piled foundations.

The number of floors is growing as well as the to-tal weight of the building.

PROCEEDINGS FROM THE SECOND INTERNATIONAL FLAT DILATOMETER CONFERENCE

162

STANDARD PENETRATION TEST (SPT) MARCHETTI DILATOMETER TEST (DMT)

Figure 1. Pompéia building

CONE PENETRA-TION TEST (CPT)

SHADED AREA CORRESPONDSTO EQUIVALENT FOOT

MEASURED SETTLEMENT=35 mm


MEASURED SETTLEMENT=32 mm MEASURED SETTLEMENT=28 mm


PREDICTED SETTLEMENT=59 mm



PREDICTED SETTLEMENT=0,629 mm

D

C

B

B B

5830 NK

5920 KN

3570 KN3570 KN

7690 KN 7690 KN

4500 KN4500 KN

5960 KN

5280 KN 5280 KN

7600 KN

3290 KN3290 KN4240 KN

4570 KN3080 KN3080 KN

7980 KN

7350 KN

3060 KN

5210KN

4660 KN

C B

DA

650

425

220

710

220

645

455 540

480

555

295

350

295

350

350

440

220

500 220

500

450

565

700

570265

675

265

675

485 1060

590

1350

775.50

778.34

781.22

784.82

ELE

VATI

ON

(m)

FLOOR

1º SS

2º SS

2.10

YELLOW SANDY CLAY

SANDY CLAY

6.50

8.50

9.40PURPLE SILTY CLAY

WITH GRAVEL

15.70

16.50

20.15

20.45

MEDIUM CLAYEY SAND

(TERTIARY SEDIMENT)

SILTY CLAY (T.S.)

PURPLE AND YELLOW

SILTY CLAY (T.S.)

30 4010 20

5

4

SPT784.450

SP-01

4

5

5

5

5

5

16

18

10

12

(TERTIARY SEDIMENT)MEDIUM BROWN AND

MEDIUM RED POROUS

(TERTIARY SEDIMENT)

(TERTIARY SEDIMENT)

MEDIUM RED SANDY CLAY

VERY STIFF YELLOW AND(TERTIARY SEDIMENT)

10

11

14

14

7

5

9

10

MEDIUM RED AND YELLOW

STIFF PURPLE AND YELLOW

STIFF PURPLE AND YELLOW

(TERIARY SEDIMENT)

FINE SANDLOOSE TO MEDIUM

785

780

775

770

765

760

CPT-1

12108642

2.00

1.00

0.00

7.00

6.00

5.00

4.00

10.00

9.00

8.00

12.00

11.00

3.00

17.00

16.00

15.00

14.00

13.00

021.00

20.00

19.00

18.00

DEPT

H (

m)

DEPT

H (

m)

Point Resistance


163

TO EQUIVALENT FOOTSHADED AREA CORRESPONDS

B

B B

D

C

B

DA

C

3090 KN

4180 KN

9190 KN

10760 KN

4180 KN

4520 KN

11500 KN

9140 KN

4520 KN

3090 KN

2740 KN

2740 KN

4180 KN4520 KN

3090 KN

7720 KN

7720 KN

3960 KN 3960 KN

3960 KN3960 KN

4520 KN

4180 KN

2740 KN2740 KN 3090 KN

445

730

445

730

200

450

200

450

230

615

230

615

230

615

615

230

315

1265

335

270

270 335

200

485

190

510

275

355

275

355

430

305

430

305

430

305

305

430

490

590

590

490

310 1160

480

260

480

260

480

260

260 480

W.T.

SEDIMENT)

779.00

781.79

784.67

787.65

ELE

VATI

ON

(m)

40

FLOOR

1º SS

2º SSGREY AND YELLOW SAND

16.00

FINE CLAYEY SAND

17.70

YELLOW AND GREY CLAYEY

20.40

FINE CLAYEY SAND

22.1022.50 CLAYEY SAND (T.S.)

24.95

25.45MEDIUM SAND (T.S.)

CLAYEY SAND (TERTIARY

790

SP-01

5

787.13SPT 10 20

4

4

785

7

5MEDIUM TO VERY STIFFCLAY (TERTIARY SEDIMENT)

3

9780

13

20

12

18

21775

17

18

15

15770 STIFF TO VERY STIFF

16

24

(TERTIARY SEDIMENT)

STIFF TO VERY STIFF

15

15STIFF GREY AND PURPLE

(TERTIARY SEDIMENT)

19

39

VERY STIFF GREY FINE

765

30SAND (TERTIARY SEDIMENT)

36

22STIFF GREY AND YELLOW

760

HARD PURPLE AND YELLOW

30

DEPT

H (

m)

12

CPT-10.00

3.00

4.00

1.00

2.00

7.00

8.00

9.00

5.00

6.00

12.00

13.00

10.00

11.00

1020 4 6 8

14.00

15.00

DEPT

H (

m)

Figure 2.Moóca building

STANDARD PENETRATION TEST (SPT) MARCHETTI DILATOMETER TEST (DMT) CONE PENETRATION

TEST (CPT)

Point Resistance

(MPa)


164

These conditions combined with a large number

of available equipment for different types of piles, and a poor site characterization techniques, lead to the use of pile foundations for most buildings.

Better information about soil properties can change this practice.

The use of DMT, as illustrated in this paper, can give the necessary information for settlement evaluation, allowing in some cases, the use of shal-low foundations, with a substantial reduction in costs, when compared with pile foundations.

4 FOUR CASE HISTORIES IN SAO PAULO

This paper presents four case studies of buildings constructed on spread footings.

On Table 1 a summary of those four building cases is presented.

Table 1 – Building characteristics LOCATION

IN SAO PAULO

NUMBER OF

FLOORS

DEPTH OF EXCAVATION FOR SUBSOIL

FLOORS

LOAD IN EACH COL-UMN (KN)

APPLIED STRESS IN

SPREAD FOOTING

Pompéia 25 5.5m (18ft) 3,000-8,000 300 KPa

Moóca 25 5.5m (18ft) 3,000-8,000 350 KPa Água Rasa 20 8.0m (26.2 ft) 1,000-5,000 275 KPa Morumbi 31 4.0m (13.1 ft) 3,000-10,000 400 KPa

Figures 1 to 4 show the footings of the four build-

ings and one of the tests results combining SPT, DMT and CPT.

5 FOUR CASE HISTORIES IN SAO PAULO

It is well known that settlement governs founda-tion design for tall building over spread footings.

That is the reason why when predicted settle-ments are high pile are preferred, instead of footings, eventhough a pile foundation is usually more expen-sive than a footing foundation (about 1.3 to 1.6 times).

Using DMT, the design engineer can accurately predict settlement, than with only SPT results.

The DMT method used to compute settlement for those buildings is very simple.

The reduction in stress imposed by the excavation is considered as acting in the whole area and this induces reductions in the layers below the footings. No heaving is considered. The subsoil below the footing is divided in to numerous 20 cm thick layers, each one having a M value determined from the DMT test.

All the footing are considered together, as a large stressed fictitious rectangle, having an area repre-senting the sum of the individual areas of the foot-ings, receiving the total load of the building.

Four points are considered in this fictitious rec-tangle, the center (A), the corner (B), the middle of the length (C) and the middle of the width (D), as showed in figures 1 to 4.

The stresses induced in the subsoil by the rectan-gular loaded area are calculated in the centre of each 20 cm thick layer, using Newmark formula.

Thickness reduction in each 20 cm layer is calcu-lated by the expression

Settlement evaluated using this method does not

consider the effect of the building structure, which will reduce the differences at points on the fictitious rectangle.

Results obtained in the predictions based on this method, for those four buildings are shown in Fig-ures 1 to 4.

6 SETTLEMENT MEASUREMENTS

Unfortunately in Brazil, it has been difficult to persuade managers of construction companies to use better quality tests for site characterization to com-plement the SPT.

With only SPT, settlement predictions have been almost impossible to obtain.

We have been working hard to show the advan-tages of special field tests, as DMT. For “Moóca” site there were no settlement measurements, and the building is now finished. For the “Pompéia” site the settlement was measured at five columns, with a simple approach, and the building is also finished. For “Água Rasa” and “Morumbi” sites, both are un-der construction, and a specialized company is measuring settlements monthly.

For “Pompéia Building”, Figure 1 shows the pre-dicted and measured settlements. The mean meas-ured value is 31,4 mm and the mean predicted value is 24,1 mm. This prediction is good enough for de-sign decisions.

For “Morumbi Building”, until now (February, 2006), only 40% of the total loads have been ap-plied. Measured settlements are compared to pre-dicted settlements in Figure 5 (for 40% of the total load).

The mean predicted settlement (13.1 mm) for 40% of the total loads, are compared with the mean measured settlement (8.9 mm) also for 40% of the total loads.

i

ii M

v′Δ=Δ

σε (1)


165

W.T.5.67

30/0

3/05

(TERTIARY SEDIMENT)

788.00

3º SS789.43

792.23

795.03

797.83FLOOR

1º SS

2º SS

ELE

VA

TIO

N (m

)

40302010SPT796.55

SP-01

1/17

2/27

1/20

4

8

13

15

16

17

23

15

15

17

29

32

39

16

18

14

17

0.100.35

1.70

5.00

13.80

17.40

20.45

800

795

790

785

780

775

CONCRETE SLABDIVERSE SOIL FILL

VERY SOFT BROWN

STIFF TO VERY STIFF

STIFF TO VERY STIFF

POROUS CLAY(TERTIARY SEDIMENT)

SOFT POROUS RED CLAY

YELLOW AND GREY SILTY CLAY (TERTIATY

SEDIMENT)

SEDIMENT)SILTY CLAY (TERTIARY

PURPLE YELLOW AND RED VERY STIFF TO HARD

BROWN AND GREY SILTYCLAY (TERTIARY SEDIMENT)


D

C

B

B B

4600 KN(2380+3960) KN

4780 KN

4700 KN

10560 KN

4700 KN2180 KN

2180 KN

(3960+2380) KN 4600 KN

4500 KN

(2380+3960) KN

4500 KN

(3960+2380) KN

6100 KN 6100 KN

14780 KN

4780 KN4780 KN

4780 KN

A

C B

D

220

790

220

790

220

790

220

790

74552

0

745

725

500

445

500

445

35066

0305

540

350

660

540

305

305

550

305

550

350

660

350

660

410

420 305

260

305

260

41042

0

CPT-1

8640 2 10

0.00

15.00

14.00

11.00

10.00

13.00

12.00

6.00

5.00

9.00

8.00

7.00

2.00

1.00

4.00

3.00

Figure3. Água Rasa building

(MPa)

Point Resistance

DEPT

H (

m)

DEPT

H (

m)

CONE PENETRATION TEST (CPT)

MARCHETTI DILATOMETER TEST (DMT) STANDARD PENETRATION TEST (SPT)


166


D

CBB

B

34870 KN 34690 KN

44110 KN

14830 KN 15090 KN

4280 KN

3300 KN

3850 KN

4420 KN

3410 KN

2970 KN

19400 KN

5230 KN

19590 KN

5160 KN

B

DA

C620

1780

800

515

795

540

380

400

255

330

360

400

385

400

260

330

300

395

540

940

235

600600

1675

565

910

225

615 600

1675

CPT-6

201816141218.00

17.00

16.00

3.00

4.00

1.00

2.00

7.00

8.00

9.00

5.00

6.00

12.00

13.00

10.00

11.00

14.00

15.00

0.00

1020 4 6 8

ELE

VATI

ON

(m)

DENSE TO VERY DENSESAND SILTY (RESIDUAL SOIL)

SILTY FILL

798.00

2º SS

1º SS

FLOOR 806.75

803.60

800.45MEDIUM TO VERY DENSE

DENSE TO VERY DENSE

RED AND GREY SANDSILTY (RESIDUAL SOIL)

YELLOW AND GREY SAND SILTY (RESIDUAL SOIL)

12

15

16

22

23

46

41

42

49

50

48

44

44

34

36

40

0.30

10.60

15.00

16.00

810

805

800

795

790

785

SP-11805.798SPT 10 20 30 40

(* Não foi atingido o lençol freático)

Figure4. Morumbi building

STANDARD PENETRATION TEST (SPT)

DEPT

H (

m)

DEPT

H (

m)

Point Resistance

(MPa)

MARCHETTI DILATOMETER TEST (DMT)


167

Figure 5.” Morumbi building” in constrution, with 40% of the total load

PREDICTED 40%SETTLEMENT=7,7 mm

SETTLEMENT=20,0 mmPREDICTED 40%








D

BC

D

B

B C

Distance (m)

1675

600615 225

910

565

1675

600

600

235

940

540

395300

330260

400

385

400360

330

255

400

380

540

795

515

800

1780

620

C

A D

B

Dis

tanc

e (m

)

Settlement (mm)

Set

tlem

ent (

mm

)

PRO

CEED

ING

S FRO

M TH

E SECO

ND

INTER

NA

TION

AL FLA

T DILA

TOM

ETER C

ON

FEREN

CE

168

7 CONCLUSIONS

The Marchetti Dilatometer “DMT” is a powerful tool to predict settlements for buildings on spread footings, where no primary or secondary consolida-tion is involved.

The mean values predicted with dilatometer re-sults, area accurate for design decisions.

The influence of building structure in settlement distribution is somewhat complex. In profile view the predicted settlements give a more curved “dish” shape than what is measured, because of the rigidity of the building frames.

REFERENCES

ASTM D6635-01. “Standard Test Method for

Performing the Flat Plate Dilatometer”. American Society for Testing and Materials ASTM, 2001.

EUROCODE 7 (1997). Geotechinical Design -

Part 3: Design assisted by field testing, Section 9: “Flat dilatometer Test (DMT) Final Draft”, ENV 1997-3, Apr. 66-73, CEN-European Committee for Stardardization.

MARCHETTI, S. (1975). “A New In Situ Test for

the measurement of horizontal soil deformability”. Proc. Conf. On In Situ Measurement of Soil Proper-ties, ASCE, Special Conf., Raleigh, Vol. 2, 255-259, June.


169

Documents

Shallow foundations of tall buildings designed on the ... · Morumbi 31 4.0m (13.1 ft) 3,000-10,000 400 KPa Figures 1 to 4 show the footings of the four build-ings and one of the