GEOTECH: CH 11 - CH 13

Elastic settlement (or immediate settlement)

caused by the elastic deformation of dry soil and of moist and saturated soils without any change in the moisture content

Primary consolidation settlement

the result of a volume change in saturated cohesive soils because of expulsion of the water that occupies the void spaces.

Secondary consolidation settlement

observed in saturated cohesive soils and organic soil and is the result of the plastic adjustment of soil fabrics. It is an additional form of compression that occurs at constant effective stress.

Elastic settlement

occurs directly after the application of a load without a change in the moisture content of the soil.

consolidation

This gradual process of drainage under an additional load application and the associated transfer of excess pore water pressure to effective stress cause the time-dependent settlement in the clay soil layer.

consolidometer

The one-dimensional consolidation testing procedure was first suggested by Terzaghi. This test is performed in a

Normally consolidated

present effective overburden pressure is the maximum pressure that the soil was subjected to in the past

Overconsolidated

present effective overburden pressure is less than that which the soil experienced in the past.

preconsolidation pressure

The maximum effective past pressure is called the

logarithm of time method

square root of time method

For a given load increment on a specimen, two graphical methods commonly are used for determining cv from laboratory one-dimensional consolidation tests.

preloading

This technique of achieving the probable settlement of soil before construction is referred to as

sand drains and prefabricated vertical drains

are used in the field to accelerate consolidation settlement in soft, normally consolidated clay layers and to achieve precompression before the construction of a desired foundation.

Sand drains

are constructed by drilling holes through the clay layer(s) in the field at regular intervals. The holes then are backfilled with sand.

Prefabricated vertical drains

originally were developed as a substitute for the commonly used sand drain. With the advent of materials science, these drains are manufactured from synthetic polymers such as polypropylene and high-density polyethylene

Prefabricated vertical drains

have been used extensively in the past for expedient consolidation of low permeability soils under surface surcharge

Total settlement

is the sum of elastic settlement and consolidation settlement

primary and secondary

Consolidation settlement has two components

Elastic settlement

is primarily a function of the size (length and width) and rigidity of the foundation, the modulus of elasticity and Poisson’s ratio of the soil supporting the foundation, and the intensity of the load applied.

Consolidation

is a time-dependent process of settlement of the saturated clay layer(s) located below the groundwater table

Sand drains and prefabricated vertical drains

may be used to accelerate the consolidation process in the field

Precompression

is a technique to reduce postconstruction settlement by application of a temporary surcharge on the ground surface for a limited period of time. The surcharge is removed when construction begins.

shear strength

is the internal resistance per unit area that the soil mass can offer to resist failure and sliding along any plane inside it.

Mohr

presented a theory for rupture in materials that contended that a material fails because of a critical combination of normal stress and shearing stress and not from either maximum normal or shear stress alone.

direct shear test and the triaxial test

are the two commonly used techniques for determining the shear strength parameters.

direct shear test

is the oldest and simplest form of shear test arrangement

stress-controlled tests

the shear force is applied in equal increments until the specimen fails.

strain-controlled tests

a constant rate of shear displacement is applied to one-half of the box by a motor that acts through gears.

ultimate shear strength

After failure stress is attained, the resisting shear stress gradually decreases as shear displacement increases until it finally reaches a constant value called the

critical void ratio

At large shear displacement, the void ratios of loose and dense sands become practically the same, and this is termed the

drained test (consolidated drained)

is made on a saturated soil specimen by keeping the rate of loading slow enough so that the excess pore water pressure generated in the soil is dissipated completely by drainage.

triaxial shear test

is one of the most reliable methods available for determining shear strength parameters. It is used widely for research and conventional testing

consolidated-undrained test

is the most common type of triaxial test. In this test, the saturated soil specimen is first consolidated by an all-around chamber fluid pressure

unconsolidated-undrained tests

drainage from the soil specimen is not permitted during the application of chamber pressure

unconfined compression test

a special type of unconsolidated-undrained test that is commonly used for clay specimens. In this test, the confining pressure is 0

sensitivity

may be defined as the ratio of the unconfined compression strength in an undisturbed state to that in a remolded state

quick clays

Some clays turn to viscous fluids upon remolding. These clays are found mostly in the previously glaciated areas of North America and Scandinavia. Such clays are referred to as

Thixotropy

is a time-dependent, reversible process in which materials under constant composition and volume soften when remolded.

Anisotropy

is caused primarily by the nature of the deposition of the cohesive soils, and subsequent consolidation makes the clay particles orient perpendicular to the direction of the major principal stress.

shear vane

usually consists of four thin, equal-sized steel plates welded to a steel torque rod

Triangular

Shear strength mobilization is cu at the periphery of the soil cylinder and decreases linearly to zero at the center. This assumption is referred to as

Uniform

Shear strength mobilization is constant (that is, cu) from the periphery to the center of the soil cylinder. This assumption is known as

Parabolic

Shear strength mobilization is cu at the periphery of the soil cylinder and decreases parabolically to zero at the center. This assumption is known as

Torvane

is a handheld device with a calibrated spring. This instrument can be used for determining cu for tube specimens collected from the field during soil exploration, and it can be used in the field.

stress paths

Results of triaxial tests can be represented by diagrams called

stress paths

is a line that connects a series of points, each of which represents a successive stress state experienced by a soil specimen during the progress of a test.

strength anisotropy of clay

Due to the nature of disposition of clay soils, the shear strength may vary depending on the direction of load application . This is referred to as

vane shear test

is another method to determine the undrained shear strength of clay soils in the laboratory and field

• Consolidated-drained

• Consolidated-undrained

• Unconsolidated-undrained

Triaxial tests can be conducted under three different drainage conditions:

Consolidated drained shear strength

can be used to determine the longterm stability of structures such as earth embankments and cut slopes.

Consolidated undrained shear strength

can be used to study stability problems relating to cases where the soil initially is fully consolidated and then there is rapid loading.

unconsolidated-undrained shear strength

can be used to evaluate the end-of-construction stability of saturated cohesive soils with the assumption that the load caused by construction has been applied rapidly and there has been little time for drainage to take place.

plastic equilibrium in soil

refers to the condition where every point in a soil mass is on the verge of failure

Rankine

investigated the stress conditions in soil at a state of plastic equilibrium

rigid retaining walls and mechanically stabilized earth walls

the common types of retaining walls constructed can be divided into two major categories:

Gravity retaining walls

are constructed with plain concrete or stone masonry. They depend on their own weight and any soil resting on the masonry for stability. This type of construction is not economical for high walls.

semigravity walls

a small amount of steel may be used for the construction of gravity walls, thereby minimizing the size of wall sections. Such walls generally are referred to as

Cantilever retaining walls

are made of reinforced concrete that consists of a thin stem and a base slab. This type of wall is economical to a height of about 8 m

Counterfort retaining walls

are similar to cantilever walls. At regular intervals, however, they have thin, vertical concrete slabs known as counterforts that tie the wall and the base slab together

at-rest earth pressure

When the wall does not yield toward the backfill or away from the backfill, the lateral earth pressure is referred to as