Artigo - Prova de carga sobre areia

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be performed. If, instead, one relies on single pile load

tests, which indicate that, at working loads, a pile will settle

only a few millimeters, the impression is that piles do not

settle. However, these pile load tests generally last a few hours,

1Spencer J. Buchanan Prof., Dept. of Civ. Engrg., Texas A&M Univ.,

College Station, TX 77843-3136. E-mail: briaud@tamu.edu

2Geotech. Engr., Kleinfelder Inc., 9555 Chesapeake Dr., Ste. 101, San

Diego, CA 92123-6300.

Note. Discussion open until February 1, 2000. To extend the closing

date one month, a written request must be filed with the ASCE Manager

of Journals. The manuscript for this paper was submitted for review and

possible publication on July 10, 1996. This paper is part of the Journal

of Geotechnical and Geoenvironmental Engineering, Vol. 125, No. 9,

September, 1999. qASCE, ISSN 1090-0241/99/0009-0787–0796/$8.00

1 $.50 per page. Paper No. 13680.

and the load is often increased when the settlement under the

previous load step has become less than 0.25 mm/h (ASTM

D1143). If such a rate is maintained for the design life of the

bridge, say 50 years, the settlement at 50 years would be 110

m. While it is unreasonable to assume that the settlement rate

will remain constant, the point is made that observations on

short-term tests and on single piles can be misleading for the

long-term behavior of pile groups.

The best foundation is the one that meets the design requirements

while minimizing cost and optimizing safety. Except

for obvious cases, spread footings should always be considered

as a foundation alternative and eliminated only on the

basis of calculations.

SOIL

The spread footing tests were performed at the National

Geotechnical Experimentation Site on the Texas A&M University

Riverside Campus near College Station, Tex. The soil

at the site is a medium dense, fairly uniform, silty fine silica

sand with the following average properties near the footings

and within the top 5 meters: mean grain size D50 = 0.2 mm,

SPT (standard penetration test) blow count 18 blows per 0.3

m, CPT (cone penetrometer test) point resistance 6 MPa, PMT

(pressuremeter test) limit pressure 800 kPa, PMT modulus 8.5

MPa, DMT (dilatometer test) modulus 30 MPa, borehole shear

test friction angle 327, estimated total unit weight 15.5 kN/m3,

and crosshole shear wave velocity 240 m/s. The water table is

4.9 m deep. Summary profiles of soil tests performed near the

footings are shown in Figs. 1–3. The exact location of the

soundings is shown in Fig. 4. Additional data can be found in

Briaud and Gibbens (1994).

Geologically, the top layer of sand is a flood plain deposit

of Pleistocene age (Jennings et al. 1996) about 3 m thick with

a high fine content. The next layer of sand is a river channel

deposit of Pleistocene age about 3 m thick, clean and uniform.

The third layer is a mixed unit with an increasing amount of

clay seams and gravel layers; it is also of Pleistocene age and

was deposited by a stream of fluctuating energy. Below these

200,000-year-old sand layers and about 10 m below the

ground surface is the 45-million-year-old Eocene bedrock; this

bedrock is a dark gray clay shale that was deposited in a series

of marine transgressions and regressions. Erosion of the Eocene

marine clay took place before the Pleistocene river sediments

were deposited.

TEST SETUP AND LOAD SETTLEMENT CURVES

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