Diseño de la Cimentación y el Comportamiento de la Torre Latinoamericana.pdf

FOUNDATION DESIGN AND BEHAVIOUR OF TOWER LATIN0 AMERICANA IN MEXICO CITY by LEONARDO

ZEEVAERT,P~.D.

SYNOPSIS The foundation design for the forty-three storey building Tower Latino Americana in Mexico City introduced new and interesting problems in foundation engineering. The Paper describes the general philosophy adopted in the design of the foundation of this building. A detailed description of subsoil conditions and mechanical properties of the lacustrine deposits encountered at the site is given. The ground surface subsidence problem and investigations performed to discover the source of compression of the clay deposits are described, and the way this phenomenon was taken into account when consideration was given to the foundation design. Excavations into the lacustrine volcanic clay deposits in Mexico City produce large heave. The Author describes the procedure used to excavate to a 13-m depth for the foundation structure, and to avoid the heave of the bottom of the excavation and the excessive settlement of adjacent buildings and streets. Settlement observations are reported-of the building, of the ground surface, and other deepseated strata. Piezometric water-level observations during construction, and afterwards, are also dealt with. Finally, a comparison of observed and computed settlements is given in an attempt to predict the future behaviour of the foundation of the building.

Le plan de fondation du batiment de quarantetrois Ctages Tour Latino Americana a Mexico a pose de nouveaux et interessants problemes de travaux de fondations. L’article decrit la philoSophie generale suivie pour le plan de fondation de ce batiment. On y donne une description detaillee de l’etat du sous-sol et des proprietes mecaniques des depots lacustres rencontres sur le chantier. Le problitme d’affaissement de la surface du sol et les recherches faites pour decouvrir l’origine de compression des depots d’argile y sent decrits, ainsi que la man&e dont ce phenomene fut trait6 lorsque fut consider6 le plan de fondation. Les excavations dans les depots lacustres d’argile volcanique Q Mexico produisent de fort soulevement. L’auteur decrit la methode employee pour creuser a 13-m de profondeur afin de mettre en place la structure de fondation, en dvitant le soulevement du fond de l’excavation et le tassement excessif des batiments et rues avoisinants. Y sont rapportees des observations sur le tassement du bbtiment, de la surface du sol et d’autres couches profondes. On traite aussi des observations piezometrique de niveau d’eau pendant la construction ainsi qu’apres. Enfin, les tassements observes sont compares aux tassements estimes dans l’intention de predire le comportement futur de ce batiment.

INTRODUCTION

The forty-three storey building property, La Latin0 Americana Seguros de Vida, S.A., (Fig. 1, facing p. 118) was constructed in Mexico City at the corner of Madero and San Juan de Letran opposite the Palace of Fine Arts. The foundation surface occupied by the building is 1,114 sq. m. The weight, including the foundation structure and 20% live load, is 23,500 tons ; therefore the unit load at the foundation slab elevation is 21.1 tons/sq. m. The building is supported on a rigid reinforced-concrete mat foundation resting on 361 concrete piles driven to a depth of 33.5 m into a firm sand layer where they act as pointbearing piles. The foundation plan and the pile layout are shown in Fig. 2. The depth to the bottom of the foundation slab is 13 m below ground surface elevation. The total depth is occupied by two basements and the foundation structure. The foundation and retaining walls have been waterproofed to obtain effective use of the buoyant forces. In order to take care of the ground surface subsidence (typical of Mexico City) as the sidewalk settles away from buildings on pile foundations, the Author recommended a special design that would facilitate the lowering at any time of the ground floor of the building. The floor was divided into panels supported on wood blocks, permitting the panels to be lowered as required. This practice will avoid in the future the necessity to construct steps into the building as the sidewalk subsidence progresses. The piles were driven from a preliminary excavation 2.5 m deep made in advance to clean the site from old foundations. After the piles were inserted a I‘ Wakefield “-type of wood 115

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LEONARDO

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sheet-pile was driven in a single operation to a depth of 16 m. The wood sheet-pile served to create an impervious diaphragm to prevent water entering the excavation. Therefore the water-table in the upper pervious deposits was protected from a strong draw-down that might have initiated a large settlement of the neighbouring buildings. During excavation to the S-m depth the wood sheet-piles were shored from side to side Thereafter, the foundation beams in both the north-south and the east-west directions. were constructed in braced trenches excavated to the full depth required for the foundation structure. After the gridiron of beams was completed the panels between beams were excavated one after another, and the foundation slab resting on the piles was constructed. -4s substitutes for the excavated load, every panel was immediately filled with sand and gravel. After this the foundation was completed and loaded to obtain a reaction on the piles of 1‘2.5 tons/sq. m, equivalent to about half the weight of the building. The erection oi the steel structure then proceeded, and as more load was added the water-table was permitted to rise and exert under the foundation slab an equivalent reaction to the additional

43rd.srcrey

SAN

JUAN

DE

FOUNDATION

LETRAN

AVE.

STRUCTURE

7OUNDAliON ,-,F VI Wood Sheet-P,les

Fig.

2.

Foundation

plan

and layout

“LA

LATIN0

Forty

Three

of piles

LAYOUT

AMLRIGANA” -

5tcw.j

Bwldlng

FOUNDATION

DESIGN

AND

BEHAVIOUR

OF TOWER

LATIN0

AMERICANA

117

load. This procedure was followed until the total load of the building was applied and the water-table was restored to its original elevation. Settlement observations and piezometric water levels were carefully observed during the entire process of construction of the foundation, and thereafter. In order to design the foundation of Tower Latin0 Americana it was necessary to investigate the source of surface subsidence and the index and mechanical properties of the subsoil The results of these investigations are reported in materials at the site of the building. the Paper. SUBSOIL

CONDITIONS

The subsoil condition was investigated from continuous cores of undisturbed samples obtained from a 2.5-m depth to a depth of 70 m from the ground surface. The samples obtained were 5-in.-dia. undisturbed samples in the lacustrine clay deposits and 3-in.-dia. The results of the investigation are shown in soil profile, in the clayey sand and silt deposits. Fig. 3. The stratigraphic column was found as follows : Depth from : 0.0-5.55 m

5.555.70 m 5~70-6+30 m

6~80-6~85 6.85-7.45 7.45-7.55 7.55-9.15 9.15-l

m m m m

1-9 m

1 l-9-12.1 12.1-15.8

m m

15.8-15.85 m 15.85-16.5 m 16.5-21.4 m

21.4-21.50 21.50-22.50

m m

22.50-23.65 23.65-24.30

m m

24.30-27.20

m

27.20-29.10

m

Condition A fill was found of clayey silt and sand with humus. A large content of pottery remains of Aztec origin was encountered in these horizons. The average water content is about 4576. A layer of black volcanic ash with silt and little clay. Deposit of light grey plastic fissured silty clay with root-holes and high content of calcium carbonates, Caliche Barrilaco. The average water content is about lOOo/o. Pumice sand. Grey clayey silt with calcium carbonates. Pumice sand, and gravel. Greyish olive-green fissured clayey silt with little calcium carbonates. Average water content about 90%. Becerra sediments. Lacustrine volcanic clay, containing the mineral montmorillonite, diatoms, and ostracods. Tacubaya Clay I. Black volcanic ash. Brown and reddish brown lacustrine volcanic clay containing the mineral montmorillonite, diatoms, and ostracods. Tacubaya Clay I. Black volcanic ash. Grey clayey silty sand, with root-holes and calcium carbonates. Olive-green lacustrine volcanic clay, montmorillonite, diatoms, and ostracods with lenses of white volcanic glass at 19.75 and 20.80-m depth. Tacubaya Clay II. Brown pumice sand. Grey clayey silt and fine sand with root-holes and calcium carbonates “ caliche “. Brown and reddish brown volcanic clay. Tacubaya Clay III. Grey clayey silt and fine sand with root-holes and calcium carbonates “ caliche “. Olive-green lacustrine volcanic clay, contains montmorillonite, diatoms, and ostracods. Tacubaya Clay IV. Series of lacustrine deposits of volcanic montmorillonitic clay, pumice sand and ostracods sand. Ostracods and iiolites very abundant. Extremely pervious deposit in horizontal direction corresponding to Tacubaya Clay V.

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LEONARDO

29.10-3350 m 33.5~38.20

m

38.20-41.55 m

41.55-41.95 m 41.95-45.25 m 45.25-47.70 m 47.70-64.50 m 64.50-65.25 65.25-65.40 65.466660 66.60-68.75 6875-70.00

m m m m m

ZEEVAERT

Olive-green lacustrine volcanic clay, containing the mineral montmorillonite, ostracods, and some diatoms. Tacubaya Clay V. Series of alluvio-lacustrine deposits of andesitic sand, clayey silty sand with little andesitic gravel and pumice, root-holes and calcium carbonates in the upper part of the deposit. Tarango Sand I. Olive-green lacustrine volcanic montmorillonitic clay, with diatoms, ostracods, sponge spicules, with a black sand lens at 41.20. Tarango Clay I. Fine sand layer of white clean volcanic glass, wind deposited on the lake. Olive-green lacustrine volcanic montmorillonitic clay with white clean volcanic glass lens at 43.50-m depth. Tarango Clay I. Same lacustrine clay as above, interbedded with numerous thin lenses of volcanic sand. Tarango Clay I. Series of deposits of sand, clayey silt, or silty sand of andesitic origin. Little gravel and pumice grains. Tarango Sand II. Brown lacustrine volcanic clay. Lenses of volcanic sand. Olive-green lacustrine volcanic clay. Fine sand of white, clean volcanic glass. Olive-green iacustrine volcanic clay.

Fig. 3 shows the water content profile from which may be seen distinctly the lacustrine bentonitic clay deposits. The first lacustrine volcanic clay deposit corresponding to Tacubaya Clay I-V, assumes a high water content that remains practically constant with depth and reaches a height of 350%. Near the sand lenses the water content in the clay drops on account of higher content of coarser grains in the sediments. The large scattering of the water content appears to be because of the transgression and regression of the sediments as the water level in the lake assumed different elevations. This fact may be recognized also by the variation in the Atterberg limits. The liquid limit was encountered as high as 400% and as low as 260% regardless of depth, and the plasticity index between 264% and 110%. The unconfined compressive strength shows a large variation from O-7-1.4 kg/sq. cm. This variation may be associated with the different salinity of the water in the lake during the process of sedimentation. The minimum value of the unconfined compressive strength varies from 0.7 kg/ sq. cm in the upper part of the deposit to 0.85 kg/sq. cm at the bottom. From the permeability point of view it is important to notice the sandy and silty layers containing calcium carbonates at depths of 15.85, 21.50, 23.65, and 28 m. These horizons define shallow waters in the lake. Particularly important is the series of silt and sand layers with high content of microscopic shells, between 27.20-29.0 m deep. All these materials have a permeability from ten to one hundred times larger than the volcanic clay deposit. From geological considerations these layers may be considered continuous since they appear in the same stratigraphic position in the subsoil in many other places in the heart of the city. Therefore, from the hydraulic point of view, for consolidation purposes, they may be considered as drainage surfaces within the clay mass. Compressibility curves for the volcanic high compressible clay deposits are illustrated in Fig. 4(a). The first hard deposit Tarango Sand I has a variable compaction, its water content varies The upper part of the deposit, because of cementation with clay and calcium from 2570%. carbonates, has in the in situ state a higher strength ; but the strength may be variable in the horizontal direction because of the erratic development of calcium carbonates and clay content. The cohesion may be as large as O-4 kg/sq. cm and the angle of internal friction as high as 36”.

FOUNDATION

DES

GN AND

STANDARD PCN!lTRATlOh BLOWS PER Fool 50 I[x) 150200

BEHAVIOUR

OF TOWER

.ATINO

AMERICANA

FIATLRCONTLNTIN: UNCONFIN COMPRESS If DRYWM-ITOfSOLID:lRENGTUINI

sotoo IS0 200 250 3003so401

iI510 1.5

119

EFFECTIVE PRLSSURL IN KG/CM' 05 1.0I"520 25 30 35

.

m

m -

GRAY SILTYCLAY WITU CALCIUM

* WATER CONTENT

CARBONATES.ROOT-HOLES AND

X UNCONFINED COMPRESSIVE STRENGTH

SAND(CALlCUE)

0 INTERGRANULAR EFFECTIVEPRESSURE

iAND ATTERBERG LIMITS

l

X VOLCANIC CLAY @ SANDY J SILTYCLAY

INCLUDING SURFACE LOAD

4= BREAK INTHECOMPRESSIBILITY CURVE

ss SPECIFIC GRAVITY Fig.

3.

Subsoil

profile

.

120

LEONARDO

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PRCSSURC ‘P’ INK/C’

PRCSSLIAC :P’ IN K/C’ Fig.

4.

Compressibility

curves

The second lacustrine volcanic clay deposit corresponding to Tarango Clay I, has an Compared with the almost constant water content of about 190% in its entire thickness. The silt and very fine sand upper volcanic clay deposit the Atterberg limits are smaller. The variations in content is larger and has less content of ostrocod shells and diatoms. The unconfined compressive strength assumes minimum liquid limit are from 260-108%. values of about 0.9 k&q. cm in the upper part of the deposit, Fig, 3. Compressibility curves of this volcanic clay are shown in Fig. 4(b). The second hard deposit, Tarango Sand II, consists of a series of alluvio-lacustrine strata of sand, silt, and clayey silts with gravel, and may be considered in a semi-compact state. The compressibility is low. The cohesion is zero for sand and silt stratifications and as large The angle of internal friction may reach values as 0.67 kg/sq. cm in the clayey sediments. up to 45”. The second lacustrine clay deposit, Tarango Clay II, encountered at 65-m depth has a water content of 150%, liquid limit of 153% and plasticity index of 105%. Compressive strengths are as low as 1.65 kg/sq. cm. HYDRAULIC

CONDITIONS

The investigation of the hydraulic conditions in the subsoil is extremely important in relation to the ground surface subsidence of the area in question and the value of the effective To perform this investigation piezometers were installed overburden pressures in the subsoil.

FOUNDATION

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at different depths. The horizons selected to install the porous point of the piezometers were The curve marked B the most pervious strata at 48, 34, 28, 21, 16, 12, 8, and 2-m depth. in Fig. 3 shows the effective overburden pressure computed with the piezometric pressures encountered, and the curve marked A shows the effective pressures with static hydraulic conditions (as if all piezometric water level elevations would reach the water-table found at 1.15 m from the ground surface). The curve marked B shows that the effective pressures increased by the drop in piezometeric pressures, because of downward water flow. The investigation demonstrates that there is a small drop in the piezometric water levels for piezometers installed at 28-m depth or less, but the strong change in the piezometric levels starts at 34-m depth. The semi-pervious layers at 28 m appear to provide sufficient water to maintain, at present, the hydrostatic pressure practically unchanged at this elevation. Therefore, an The seepage important downward hydraulic gradient is established only after 28-m depth. forces have increased the effective pressures in the fifth layer of the upper clay deposit Tacubaya, and in deposit Tarango Clay I, as shown in Fig. 3. From this investigation it was concluded that the source of ground surface subsidence was mainly the compression of Tacubaya Clay V of the upper volcanic clay deposit, and that of the second volcanic clay deposit, Tarango Clay I. Benchmarks ST48 and 9T34 (see Fig. 9) installed at the site at 48-m and 34-m depth, respectively, show the quantitative values of the compression of these two clay deposits and of the total ground surface subsidence with respect to benchmark ABN49 installed at 49-m depth in the Alameda Park, 280 m away from the site, Fig. 7. The location of the benchmarks and reference points used in this investigation are shown in Fig. 8. From observations in the Alameda Park, illustrated in Fig. 9, it will be seen that starting in 1950 the rate of drop in piezometric water pressures has diminished and also the velocity of ground surface subsidence to about half of its value during the period 1949-1950. This phenomenon may be due to the suppression of part of the deep water supply wells in the central part of Mexico City. FOUNDATION

DESIGN

The foundation was designed with piles, covering an area of 1,004 sq. m on the first hard deposit, Tarango Sand I, Fig. 3. This layer was selected to avoid excessively large negative friction on the piles, and the emerging effect of the building from the surface of the ground ; in contrast to a design using piles bearing on Tarango Sand II, which would cause the effects referred to above to be of an unacceptable magnitude. Furthermore, the piles were more economical with a length to reach the first hard stratum. A safe average load of 1.2 kg/sq. cm was assigned to the upper part of Tarango Sand I, taking into account the reduction of pressure because of excavation, the rigidity of the foundation structure, and the distributing effect of the supporting sand layer itself. The weight of the building is 2.10 kg/sq. cm : therefore, to obtain an increment of pressure in Tarango Clay I that could be taken safely, it was necessary to support with uplift water Thus it was decided to place the pressure the balance foundation pressure of 1-O kg/sq. cm. foundation slab at a depth of 13.0 m from the ground surface. The probable settlement caused by the increment of load in the second clay deposit, Tarango Clay I, may be estimated using the following settlement equations, taking into consideration the secondary consolidation :

.

St = s, + -52 Primary

consolidation

S, = L’ mqll. H . p” . t . F,(T,) S, = L’ m,z . H . ;[F(Tm)(t 4*

.

.

.

.

.

(1)

.

.

.

.

.

(2)

t, < t < tc .

.

(32

: -

ta) +

o