K Prakash

Consolidation behavior of soils

Based on Terzaghis consolidation theory, percent of consolidation, U, versus the time factor, T, relationship for constant/linear excess pore water pressure distribution, it is possible to generate theoretical log10(H2/t) versus U curves where H is the length of the drainage path of a consolidating layer, and t is the time for different known values of the coefficient of consolidation, cν. A method has been developed wherein both the theoretical and experimental behavior of soils during consolidation can be simultaneously compared and studied on the same plot. The experimental log10(H2/t) versus U curves have been compared with the theoretical curves. The deviations of the experimental behavior from the theory are explained in terms of initial compression and secondary compression. Analysis of results indicates that the secondary compression essentially starts from about 60% consolidation. A simple procedure is presented for calculating the value of cv from the δ-t data using log10(H2/t) versus U plot.

CONSOLIDATION BEHAVIOR OF CLAYEY SOILS UNDER RADIAL DRAINAGE

For Barrens degree of consolidation, U-r, versus the time factor, T-r, relationship for soils undergoing consolidation with radial drainage for the equal vertical strain condition, a simple method has been developed to determine the value of the coefficient of consolidation with radial drainage c(r). Theoretical log(10)(d(e)(2)/t) versus U-r curves where d(e) is the diameter of influence and r is the real time for the different known value of c(r) have been generated. A method has been developed wherein both the theoretical and experimental behaviors of soils undergoing consolidation with radial drainage can be simultaneously compared and studied on the same plot. The experimental log(10)(d(e)(2)/t) versus U-r curves have been compared with the theoretical curves. Effects of initial compression, secondary compression, and duration of load increment are studied. Simple procedures are presented for calculating the values of c(r) using the experimental log(10)(d(e)(2)/t) versus U-r curves. A comparative study of the coefficient of consolidation and the coefficient of permeability between the cases of vertical and radial drainage has been done.

Shrinkage Limit of Soil Mixtures

Shrinkage limit, one of the Atterberg limits, is widely linked with many plasticity-based soil behaviors. However, in a great majority of these cases, such correlations have been found to exhibit poor performance. Recently, it has been brought out that the shrinkage limit of a natural soil does not depend upon plasticity characteristics, and it is primarily governed by the relative grain size distribution of the soil. The present study confirms this mechanism with the results obtained using clay-clay, clay-non-cohesive soil, and non-cohesive soil mix systems. The present study gains importance from the point of view of criteria with respect to the design of back fill materials to be used in various applications, such as nuclear waste disposal projects.

Determination of Shrinkage Limit of Fine-Grained Soils by Wax Method

Shrinkage limit of fine-grained soils is one of the parameters that is used for predicting the volume stability of soils in the field. ASTM Standard D427-04 (2007) involves the use of mercury, a health hazardous substance. Many stringent precautionary safety and disposal measures have to be exercised before its use in the laboratory. The present technical note presents the wax method of determining the shrinkage limit of soil in the laboratory which does not require the use of mercury, and also to substantiate the method proposed by the ASTM standard. It involves the use of wax as a coating on the dry soil pat and the water displacement method for the determination of the dry volume of soil pat. The experimental results have shown that the values of shrinkage limit of soils determined by the wax method and by the mercury displacement method are within certain statistical bounds.

A note on the determination of plastic limit of fine-grained soils

Plastic limit of fine-grained soils is conventionally determined in the laboratory by the soil thread rolling method. Many adverse comments have been recorded in the geotechnical engineering literature on the method about its reproducibility and operator dependency. The present experimental study, which is based on a well-planned and meticulously executed experimental program, critically evaluates the effect of size of the rolled soil thread on the plastic limit of fine-grained soils and the operator dependency of the results. The results have shown that if the plastic limit tests are performed by a trained operator, then consistent results can be obtained and that the effect of size of the rolled soil thread on plastic limit is negligibly small.

A Simplified Approach of Determining the Specific Gravity of Soil Solids

Many computations in the field of geotechnical engineering require the use of specific gravity of soil solids. Presently, specific gravity of soil solids is determined in the laboratory by the sensitive pycnometer/density bottle method, which is characterized by many complexities and difficulties. The present technical note suggests the use of some of the measurements taken during the routine shrinkage limit test in the laboratory to compute the specific gravity of soil solids fairly accurately. It is shown through exhaustive experimental results that the values of specific gravity of soil solids obtained from the proposed method is in very close agreement with those determined from the conventional pycnometer/density bottle method.

Compaction Induced Yield Stress

AbstractIn order to realize more benefits and to have more effective output from the mechanical ground improvement projects, the field soils are subjected to over consolidation. This process induces a characteristic yield stress in the compacted soil mass similar to that exhibited by the soil masses subjected to natural processes of over consolidation during their geological life time. In view of the absence of any documented geotechnical engineering literature on such an important topic, this paper intends to throw some useful light on compaction induced yield stress in fine-grained soils. A detailed discussion of various factors contributing to the induced yield stress as a consequence of compaction is done. The results of the present experimental study indicates that the values of compaction induced yield stress increase from dry side of optimum to optimum compacted state. This stress may tend to achieve an equilibrium value or decrease beyond optimum compacted state depending upon the contribution from the coarser fraction composing the soil. It is also illustrated that the soils subjected to same compactive effort exhibit different values of yield stress as a consequence of clay mineralogical composition of the soils.

Limiting Compression Curves

Depending upon the initial water content of the soil sample, a family of compression curves is possible. Identification of characteristic limiting water contents for any fine grained soil system, along with the unique mechanisms controlling them, helps obtain the limiting compression curves. In the present note, it has been demonstrated that the compression curve obtained with its origin at the free swell limit of the soil represents the global upper bound compression curve, and that the compression curve obtained after the soil has reached equilibrium under cyclic consolidation represents the global lower bound compression curve. The compression curve obtained with its origin at the settling limit water content of the soil is the stress-free reference state curve as well as the upper bound compression curve for the homogeneous soil sample.

Permeability of Layered Soils: An Extended Study

The coefficient of permeability of stratified soil deposits, when the flow is normal to the orientation of the bedding planes, has been observed to deviate from the value calculated theoretically. This deviation has been successfully explained in the past for a two-layer soil system by considering the coefficient of permeability of the exit layer as the controlling factor. The present technical note deals with the results from the study of permeability behavior of three-layer soil system. This study reinforces the point that the coefficient of permeability of a layered soil system, when the flow is normal to the orientation of the bedding planes, depends upon the relative positioning of the layers with different values of coefficient of permeability in the system.

Discussion of "Atterberg Limits and Remolded Shear Strength—Water Content Relationships"

The present discussion tries to bring out the importance of clay mineralogical composition of fine-grained soils on their liquid limit behaviour. It reinforces the authors observation that the undrained shear strengths at liquid limit water content and at plastic limit water content are not unique.