Geo tech project laboratory tests question - soil engineering

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· Grain-Size Analysis

· Sieve Analysis

This test provides a direct measurement of the particle size distribution of a soil by causing the sample to pass through a series of wire screens with progressively smaller openings of known size. The amount of material retained on each sieve is weighed. See ASTM C 136 (AASHTO T 27 or AASHTO T 311).

· Hydrometer Analysis

This test is based on Stokes Law. The diameter of a soil particle is defined as the diameter of a sphere which has the same unit mass, and which falls at the same velocity as the particle. Thus, a particle size distribution is obtained by using a hydrometer to measure the change in specific gravity of a soil-water suspension as soil particles settle out over time.

Results are reported on a combined grain size distribution plot as the percentage of sample smaller than, by weight, versus the log of the particle diameter. These data are necessary for a complete classification of the soil. The curve also provides other parameters, such as effective diameter (D10) and coefficient of uniformity (Cu). Tests shall be performed in accordance with ASTM D 422 (AASHTO T 88).

· Hydraulic Conductivity Analysis. - Permeability tests, two below are the tests method

· Constant Head Test

This test uses a permeameter into which the sample is placed and compacted to the desired relative density. Water (preferably de-aired) is introduced via an inlet valve until the sample is saturated. Water is then allowed to flow through the sample while a constant head is maintained. The permeability is measured by the quantity of flow of discharge over a specified time. This method is generally preferred for use with coarse-grained soils with k>10-3 cm/sec (Bowles 1984). Tests shall be performed in accordance with ASTM D 5856 or ASTM D 2434 (AASHTO T 215).

· Falling Head Test

This test uses an apparatus and procedure similar to the constant-head test (above), but the head is not kept constant. The permeability is measured by the decrease in head over a specified time. This method is often considered more economical for tests of long duration, such as tests on fine- grained soils with k between 5x10-5 and 10-3 cm/sec (Bowles 1984). Tests shall be performed in accordance with FM 5-513 or ASTM D 5856.

· Compaction Tests

· Standard Proctor Test

· Modified Proctor Test

Modified Proctor

This test method uses a 10-pound rammer dropped from a height of 18 inches. The sample is compacted in five layers. Tests shall be performed in accordance with ASTM D 1557 (AASHTO T 180).

· Atterberg Limits

The liquid limit, plastic limit and shrinkage limit are all Atterberg Limits. However, for classification purposes, the term Atterberg Limits generally refers to the liquid and plastic limits only. The tests for these two are described here; the shrinkage limit test is described in Section 5.1.8 of this chapter.

The liquid limit (LL) is the moisture content of a soil at the boundary between the liquid and plastic states. The plastic limit (PL) is the moisture content at the boundary between the plastic and semi-solid states. The plasticity index (PI) is the difference between the LL and PL. The results are generally reported as LL/PI values and can be plotted on the same graph as the moisture content above. These values are useful in soil classification and have been correlated with other parameters.

· Liquid Limit

The liquid limit is determined by ascertaining the moisture content at which two halves of a soil cake will flow together for a distance of 0.5 inch along the bottom of the groove separating the halves, when the bowl they are in is dropped 25 times for a distance of 0.4 inches at the rate of 2 drops/second. Tests shall be performed in accordance with ASTM D 4318 (AASHTO T 89).

· Plastic Limit

· Plastic Limit

· The plastic limit is determined by ascertaining the lowest moisture content at which the material can be rolled into threads 0.125 inches in diameter without crumbling. Tests shall be performed in accordance with ASTM D 4318 (AASHTO T 90).

· Strength Tests

Strength Tests

The shear strength of a soil is the maximum shearing stress the soil structure can resist before failure. Soils generally derive their strength from friction between particles (expressed as the angle of internal friction, φ), or cohesion between particles (expressed as the cohesion, c in units of force/unit area), or both. These parameters are expressed in the form of total stress (c, φ) or effective stress (c, φ). The total stress on any subsurface element is produced by the overburden pressure plus any applied loads. The effective stress equals the total stress minus the pore water pressure.

The common methods of ascertaining these parameters in the laboratory are discussed below. All of these tests should be performed only on undisturbed samples.

· Permeability Test

The laboratory determination of soil permeability can be performed by one of the following test methods. Permeability can also be determined either directly or indirectly from a consolidation test.

· Consolidation Test

Consolidation Test

When large loads such as embankments are applied to the surface, cohesive subsoils will consolidate, i.e., settle over time, through a combination of the rearrangement of the individual particles and the squeezing out of water. The amount and rate of settlement is of great importance in construction. For example, an embankment may settle until a gap exists between an approach and a bridge abutment. The calculation of settlement involves many factors, including the magnitude of the load, the effect of the load at the depth at which compressible soils exist, the water table, and characteristics of the soil itself. Consolidation testing is performed to ascertain the nature of these characteristics.