Field Testing of Soils
|Publication Date:||1 January 1962|
In the development of modern techniques for evaluating the engineering properties of soils, field test procedures have received only a fraction of the attention accorded to laboratory methods. The profession has put forth much effort to refine laboratory methods and apply the results to conditions assumed to exist in the field. Yet, the inherently variable nature of soils and soil-water systems frequently makes representative sampling and laboratory testing impossible. The structural or hydraulic action of a mass of soil may be influenced greatly by undetected and untested strata or discontinuities. Field tests, on the other hand, if properly conducted and interpreted, can reflect this variable nature and indicate the true action of the soil mass.
Field tests and measurements can also be used profitably to monitor changes in soil or soil-water systems during and after construction. Examples of such applications include the use of piezometers to measure the dissipation of pore pressure in weak foundations underlying fills and to monitor water levels during excavation dewatering. Also, mass movements in slopes and in soils adjacent to excavations can be detected by inclinometers placed in plastic casings. For stability problems of this sort, nature has provided us with an effective warning system: before rupture, small but accelerated deformations will appear. A few field measurements at selected locations in a construction area can be positive indicators of impending trouble.
This symposium focuses attention on the value of field procedures in soils and foundation engineering. It is hoped that it will encourage wider use of those tests currently available to engineers and stimulate the development of new methods and devices. Many tests are in use today, but without ASTM guidance in the form of standards, tentatives, or even suggested methods. The need for refinement of test procedures is evident. Some field tests are rather elementary in performance, but their interpretation is not simple. Examples of such are the plate-loading test, the pile-loading test, and the percolation test. Papers on these three have been included, and it is hoped that the symposium will provide some necessary guidance to their interpretation.
This symposium does not suggest that field tests should be substituted for laboratory tests on soil samples. Field and laboratory tests should be complementary, each being used at the optimum time in the design-construction sequence. An excellent illustration of combined use is provided by the practice in pile foundation design and construction. Although laboratory test data on undisturbed soil samples can predict the load capacity of a single pile driven to given embedment, it has become good practice on major construction to require the confirming evidence of field-loading tests on test piles. These tests are usually performed in the early stages of construction. It is also good practice to observe and record the pile penetration resistance throughout the entire construction period for comparison with test pile data and with the early soil data. Thus, the three steps-laboratory soil tests, field-loading tests and field measurement of resistance-combine to ensure satisfactory foundation performance. Through engineering imagination and development, similar thoroughness is possible and necessary in other soils engineering applications.
[EDITOR'S NOTE.-At the Fourth Pacific Area National Meeting a number of papers were presented having a direct bearing on some aspects covered in the present symposium. These dealt for the most part with landslides and subsidence, including papers on foundation problems in areas as far apart as San Francisco and Mexico City. They have accordingly been included in this volume, as have also two papers covering the more esoteric aspects of soils engineering-the present and expected difficulties in sampling and testing soils of the ocean bottom and of the moon.]