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Seepage

The slow movement of water through small openings and spaces in the surface of unsaturated soil into or out of a body of surface or subsurface water. Leakage through construction joints in concrete or sheet piles is also known as seepage and is particularly important it is monitored.

Seepage forces and erosion

When the seepage velocity is great enough, erosion can occur because of the frictional drag exerted on the soil particles. Vertically upwards seepage is a source of danger on the downstream side of sheet piling and beneath the toe of a dam or levee. Erosion of the soil, known as "soil piping", can lead to failure of the structure and to sinkhole formation.

Seepage pressures

Seepage in an upward direction reduces the effective stress within the soil. When the water pressure at a point in the soil is equal to the total vertical stress at that point, the effective stress is zero and the soil has no frictional resistance to deformation. For a surface layer, the vertical effective stress becomes zero within the layer when the upward hydraulic gradient is equal to the critical gradient.  At zero effective stress soil has very little strength and layers of relatively impermeable soil may heave up due to the underlying water pressures. The loss in strength due to upward seepage is a common contributor to levee failures. 

Sensitivity

An instrument's sensitivity is an indication of its ability to detect the smallest degree of change.

For example if a vibrating wire transducer has 2800 digits across its full range as opposed to 1400, then the sensitivity of the first one is twice that of the second one. 

Sliding

Sliding failure is a result of excessive lateral earth pressures with relation to retaining wall resistance thereby causing the retaining wall system to move away (slide) from the soil it retains.

Spider magnet

Named after its spring-steel legs, a spider magnet is used in a borehole to locate a magnetic field at various locations. These locations and any change (settlement or heave) can be measured using a Reed Switch Probe. It can have 3 or 6 legs. The 6 leg type has its legs compressed attached to the access pipe prior to installation and then the springs released. The 3 leg type allows it to be pushed into place after the pipe is installed and is easier to install than the 6 leg type. Grouting can be done either pre or post installation and the legs are only required to keep the magnetic target in place.

Slurry wall

A slurry wall (US term for diaphragm wall) is a technique used to build reinforced concrete walls in areas of soft earth close to open water or with a high ground water table. This technique is typically used to build diaphragm (water-blocking) walls surrounding tunnels and open cuts, and to lay foundations. See diaphragm wall.

Strain

Strain is expressed as the change in length ΔL per unit of the original length L of the material and is positive if the material is stretched and negative if compressed. Measures of strain in geotechnical instrumentation are usually expressed in micro strain.

Strain in concrete

  1. At stress below 30% of ultimate strength, the transition zone cracks remain stable. The stress-strain plot remains linear.
  2. At stress between 30% and 50% of ultimate strength, the transition zone micro cracks begin to increase in length, width and numbers. The stress-strain plot becomes non-linear.
  3. At 50 to 60% of the ultimate stress, cracks begin to form in the matrix. With further increase to about 75% of the ultimate stress, the cracks in the transition become unstable, and crack propagation in the matrix will increase. The stress-strain curve bends towards the horizontal.
  4. At 75 to 80% of the ultimate stress, the stress reaches a critical stress level for spontaneous crack growth under a sustained stress. Cracks propagate rapidly in both the matrix and the transition zone. Failure occurs when the cracks join together and become continuous.

Stress in rock

Unlike soils rock is rigid and permits the transfer and storage of stress. They consist of in-situ stress which is created by Geological processes and induced stress which is caused by excavation.

Stress in soil

Stresses at a point in a soil layer are caused by self-weight of the soil layers (Geostatic Stresses) and added load (Such as buildings, bridges, etc.)

Because of the presence of water in soil, there are three types of stresses. These are as follows:

  1. Total stresses (σ total)
  2. Effective stresses (σ effective)
  3. Pore Water Pressure (u)

Strut

Internal bracing transfers the lateral earth (and water pressures) between opposing walls through compressive struts.  Typically the struts are either pipe or I- beam sections and are usually preloaded to provide a very stiff system. Installation of the bracing struts is done by excavating soil locally around the strut and only continuing the excavation once preloading is complete.