Category Archives: Design & Policy

Soil Compaction for Geotechnically Engineered Embankments or Structural Fills.

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Compaction of soils is engineeringly controlled by the use of compaction curves as determination by the Modified or Standard Proctor laboratory tests. These are part of a system called end-product methods.

Other called procedural methods are also frequently used to control compaction by the number of passes of a compactor per specified lift heights. Tests fills can be used to determine the minimum number of passes need to achieve the specified minimum dry density with varying moisture contents for the same soils intended to be used during production.

Proctor tests:

The compaction curves of soils are based on a specific input energy to the soil layers in the laboratory. They are not plainly called the maximum density of the soil. The are the maximum density of the soil for a certain input energy. Input energies greater than the modified proctor are most likely used in the field during heavy civil construction. Some construction specifications have even used 100 percent maximum dry density as a minimum threshold.

Soil compaction curves as determined from laboratory Proctor tests are plotted with dry density versus moisture content. Fine-grained soils such as clays are more influenced by changes in moisture than coarse-grained soils and thus develop more classically “defined” compaction curves.

If it is stated in the specifications that the soil is to be compacted to 95% of maximum dry density and within plus or minus 2 % of the optimum moisture content, such requirements are shown. This represents a minimum standard. For less critical jobs, a compliance window can be developed based on the specifications. But for larger jobs or where there’s more soil variability, for example from a natural borrow source, then a family of curves (FOC) will need to be developed. The FOC will follow the slope of the ZAV line for larger or smaller maximum dry densities. Therefore, a family of compaction curves will need to be developed for each soil type at the project. Each curve should also have companion gradation (including hydrometer) and plasticity tests completed so they can be correlated to the properties of the soils (or soil classification).

Embankment, Abutment, and Foundation Seepage

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“All Dams Leak” is a common and true phrase. But what makes seepage (or leakage) a potential to cause damage. Basically velocity and pressure. There has to be sufficient pressure (head) that generates a velocity in the voids of the soils that will erode particles. The basic equation for velocity (k) is: v = ki

where; k = permeability (ft/day), i = gradient = delta h / L, h = head (ft), L = seepage path length (ft).

Erodibility of soils is mainly based on particle or grain size (coarse vs fine sands) and cohesion (non-plastic silts vs clays).

Earth Embankment with Left Side Spillway

Seepage and Flownets

Seepage pressures and gradients must be analyzed for all types of dams and foundations using flownets. In order to fully understand the seepage pressures and velocities at a dam, graphical flownets must be developed and verified with field observations when appropriate. Foundations that are analyzed based on seepage through broad geologc units, will most of the time miss important details within the units that may present seepage problems at the dam project.

Seepage Control

Seepage berms are extremely beneficial when added to a seepage collection system, some of those are:

-increased drainage

-reduction in uplift pressures

-increased resistance to sliding

-increases in effective stresses and available shear strength

Drainage capacity of a gravel section can be determined by the procedure outlined in SD&FN (Cedergren 1989).

for the filter aggregate kh = 10 ft / day [1], for the coarse gravel aggregate kh = 100,000 ft / day. The area is approx. = 3 sq ft. The slope of the pipe and the hydraulic gradient in the gravel section are equal to 0.01. Using Darcy’s law . Q = 22,000 gallons per day (gpd) or 15 gallons per minute (gpm).

with a 6″-dia drainage pipe, approx. Q = 260,000 gpd or 180 gpm. Using 0.1 as the gradient, the Q or flow volume values of course are 10 times larger.

Dam Cores

What’s the purpose of the “core of a dam?” Basically it’s economics. By reducing the reservoir hydraulic gradient in a shorter distance than more pervious materials, adding a core allows the use of steeper slopes for zones of materials downstream of the core, thus reducing the volume required to build the dam. Technically speaking, adding a core increases the effective stresses of the embankment and foundation materials downstream of the core. Higher effective stresses allow for greater available shear strength. Dams that are built with cores of higher permeability require flatter downstream slopes.

Design Earthquakes – Definitions

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For Concrete Structures: OBE (operational basis earthquake) = 144-yr Return Period (50 percent chance of being exceeded in 100 years).

Max. Design EQ: for Structures Defined As Critical use MCE, In general for other structures, MDE = 950-yr return period (10 percent chance of being exceeded in 100 years).

MCE is defined as the greatest earthquake that can reasonable be expected to be generated by a specific source (which comes from a deterministic site hazard analysis).

USACE references: Stability Analysis of Concrete Structures, Earthquake Design and Evaluation of Concrete Hydraulic Structures, Earthquake Design and Evaluation for CW Projects. Safety of Dams – Policies and Procedures.