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JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING / SEPTEMBER 1999 / 787

BEHAVIOR OF FIVE LARGE SPREAD FOOTINGS IN SAND

By Jean-Louis Briaud,1 Fellow, ASCE, and Robert Gibbens,2 Member, ASCE

ABSTRACT: Five square spread footings ranging in size from 1 to 3 m were load tested up to 150 mm of

settlement. They were all embedded 0.75 m into a medium dense, fairly uniform, silty silica sand. Loadsettlement

curves are presented, as well as creep curves relating settlement and time under a constant load. Since

the soil mass was instrumented with telltales and inclinometers, vertical and horizontal movements in the soil

mass were obtained as a function of depth and lateral extent. Conclusions are reached regarding how best to

measure footing settlement, how to present load test results, new correlations for use in design, creep settlement,

effect of cyclic loading and preloading on creep rate, zone of influence under the footing, mode of deformation

of the soil mass, and volume change observations. Twelve settlement methods, six bearing capacity methods,

and the WAK (wave activated stiffness) test are evaluated by comparing the predictions with the measurements.

Many results of these large-scale instrumented tests confirm findings at small scale of previous researchers.

INTRODUCTION

This article describes a series of load tests performed on

five square spread footings ranging in size from 1 3 1 m to

3 3 3 m. The load-settlement curves have already been presented

by Briaud and Jeanjean (1994) as an aid to the development

of a new load-settlement curve method for spread footings

and by Briaud and Gibbens (1994) as the backbone of an

international prediction symposium. This article focuses on the

presentation and analysis of three new items related to these

tests: creep deformation as a function of time, vertical displacement

of the soil mass as a function of depth below the

center of the footings, and horizontal displacement of the soil

mass as a function of depth and lateral extent near the edge

of the footings. The soil data, footing test setup, and load settlement

curves are summarized to place the new results in

perspective.

SPREAD FOOTINGS OR DEEP FOUNDATIONS?

There are approximately 600,000 bridges in the United

States. If these bridges had to be replaced today, it would cost

about $300 billion. Each year some 6,000 new bridges are

built, and the Federal Highway Administration is conducting

research to minimize the cost of this infrastructure while optimizing

safety and reliability. One such effort consists of helping

engineers place more bridges on spread footings by improving

the confidence in the design predictions. Indeed,

spread footings are generally less expensive than deep foundations,

with savings up to 20% of the cost of the bridge

(Briaud 1993).

Thinking that spread footings are more prone to settlement

than deep foundations would be a misconception, as shown in

two separate studies by Moulton et al. (1985) and Hearn

(1995); both engineers showed that, on the average, the settlement

of bridges on spread footings is very similar to that

of bridges on deep foundations.

Proper settlement analysis for bridges on deep foundations

should 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: [e-mail]

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).