Prediction of compaction characteristics of soils

Abstract

PREDICTION OF COMPACTION CHARACTERISTICS OF SOILS Fine-grained soils with appreciable clay fractions are being placed in earth dams and highway embankments by geotechnical engineers more often than in the past, sometimes by choice and sometimes by necessity. Every day thousands of cubic metres of fine-grained soils are compacted throughout the world, since compaction is the only treatment that can produce marked changes in soil properties economically. Properly placed and compacted soil mass has strength and stability that are as good or better than many natural formations.

Characterisation of compacted soil behaviour involves time-consuming laboratory testing. It is always desirable in geotechnical engineering practice to evolve simple testing methods, both in the field and laboratory, such that with minimum input parameters, the compacted soil behaviour is predicted within the limits of accuracy at the engineering level. Prediction would help to form an independent check on the laboratory investigations to increase the level of confidence in handling test data, and to minimise, if not eliminate, the time-consuming laboratory investigations.

(i) There are two approaches for property characterisation and prediction of compacted soil behaviour pursued independently by practicing engineers and researchers. One is an engineering approach, in which parameters needed for solving field problems are derived by empirical methods of correlations based on experimental test data. Another is a scientific approach, in which complexities are unraveled for a meaningful interpretation and generalisation with simple models to predict compacted soil behaviour. So far, it has not been possible to provide a satisfactory link between the two, so as to render them applicable directly to practice, using only normally measured and observed input parameters.

It is the aim of this investigation to examine the possibility of providing simple links between the two, both behavioural and parametric. This is intended to culminate in evolving a unified engineering approach to generalise and predict the compacted soil behaviour, within the framework of a well-established scientific principle. In the present investigation, truncated diffuse double layer theory has been used for this purpose. Chapter one introduces compaction as a construction process and gives, briefly, the development of compaction techniques.

(ii) In Chapter two, soil has been viewed as a construction material. Soil has been regarded as a particulate material, and the influence of size, shape, and surface area of the constituent solid particles of soil and their relative interaction with the other phases of the soil system have been examined. Based on behavioural aspects, soils have been broadly grouped as non-interacting and interacting soils. Non-interacting soils are soils made up of coarse-grained material, and whose behaviour is governed by gravitational and mechanical forces. Interacting soils are soils either partly or fully composed of fine-grained clay-sized fractions, whose behaviour is influenced by the physico-chemical interaction forces.

With regard to natural soils, which contain both coarse and fine-grained constituents, the floating matrix concept has been introduced. According to this, after a minimum percent of clay fraction, the coarser solid constituents float in the clay matrix, resulting only in the dilution of the physico-chemical interaction. Tentatively, the possibility, or otherwise, to determine the liquid limit of the soil is regarded to define the interacting or non-interacting soils. In this investigation, generalisation and prediction of compaction characteristics of only interacting soils, i.e., fine-grained soils, is attempted.

(iii) Chapter three details the scope of this investigation. Compaction curves of fine-grained soils follow a specific pattern. Hence, it is very likely that the basic mechanism controlling the compaction behaviour of fine-grained soils should be a singular one. Several investigators have advanced their theories in an attempt to explain the observed responses of fine-grained soils to compactive effort. Yet these theories have not been completely substantiated by diagnostic laboratory experiments. In Chapter four, an attempt has been made to model the compaction behaviour of fine-grained soils, and a phenomenological theory of compaction has been evolved based on physico-chemical interaction theory.

When water is added to a fine-grained soil and mixed prior to the compaction process, truncated double layers are formed and interact with each other to result in stable clusters. Stability of clusters to a compactive effort depends on the physico-chemical interaction forces mobilized, which in turn depend on the availability of water and the liquid limit water content of the soil. For a given soil, the density achieved by compaction is a function of water content, degree of saturation, and compactive effort. It has been shown phenomenologically that for any soil compacted with a given compactive effort and at the same degree of saturation, the dry density ( / d) is a function of (w/wL).

(iv) In Chapter five, based on the phenomenological theory of compaction evolved in Chapter four, generalized compaction curves for two standard compaction tests (Standard and Modified) have been generated. These generated curves have been used to predict compaction characteristics of a number of soils. The level of prediction obtained is within the limits of accuracy needed in engineering practice.

Due to the lack of uniformity of soil from place to place, it is important to exercise utmost control over earthwork operations during construction of earth structures. In the field, soil is spread in layers and compacted to a density specified in the earthwork specification. Proper accomplishment of these requirements involves consideration of the soil type, its water content, and the type of equipment used for the compacting operation. In Chapter six, generalizing the compaction characteristics with respect to compactive effort, a simple method for field compaction control has been evolved. Also, using the generalized compaction curves developed in Chapter five, a simple and quick method of determining moisture content in the field for compaction control has been developed.

In Chapter seven, a summary of the investigation done and conclusions arrived at have been presented.