MODULE 3 CET 202 ENGINEERING GEOLOGY

3. Stabilty, Earthquakes etc

81. Groundwater is found beneath the Earth's
surface in soil pore spaces and rock formation
fractures.
The flow of groundwater below the surface is a
fundamental property that controls the strength and
compressibility of soil impacting soil's ability hold up
on structural loads.
When soil is saturated, the soil media takes on very
specific physical characteristic due to the relative
incompressibility of water.
These characteristics come into effect below the
groundwater surface or table.

82. Groundwater tables can fluctuate with time.
Changes in groundwater surfaces can be slow
as they can change seasons, or they can be relatively
rapid such as in tidal basins or storm water detention
basins.
Groundwater pressure heads can exceed
elevation heads and, in those cases, result in water
flowing out on the ground surface as artesian flows or
springs or swampy wetlands.

83. Whenever construction must take place below the
water table or soil is used to retain water, groundwater
affects the project by impacting the function and
design of the facility, and the cost of its construction.
Groundwater is a frequent cause of disputes between
owners and contractors in construction projects.

84. Common ground water issues during construction:
Unstable subgrade
Unstable excavation and water seepage
Construction delays and cost overrun
Common ground water problems after construction:
Water leaks, wet basements, and mold growth
Cracked and uneven floors
Cracked and uneven walls
Unstable slopes and retaining walls
Delayed movements of foundations

85. How is groundwater identified and evaluated during
planning and construction?
Reconnaissance: Using imagery interpretation and
site visits to identify an overview of water table
conditions, but often requires subsurface
investigations.

86. Subsurface Investigation: Test borings and/or test pits
to depths below the anticipated excavation will be
required to define the ground water depth and
conditions including static, perched, and artesian
conditions.
Groundwater conditions can typically be visually
observed in cohesionless soils (sands, gravels, and
silty sands) because water can flow more readily
through these types of soils.
Groundwater conditions in cohesive soils (clay and
silty clay) cannot be visually observed for water flow
and often need to be tested in the lab. Flow velocities
in clays can be less than 1 foot/year.

87. Due to the slow rate of flow in cohesive soils
and wells, piezometers and other subsurface
instruments may take days to months to record
groundwater changes and pressure. When these
changes are paramount to a design process, ground
water reading may require a "zero" volume change
device such as a diaphragm transducer to read
changes in groundwater head in a real time
environment.

88. The engineering team responsible for all the phases
of a project, from initial planning and budgeting
through final construction, need to be aware of the
potential impact of groundwater during design,
construction and after construction so their decisions
will be effective.

140. THANK YOU