Embankment, Abutment, and Foundation Seepage


“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

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 StructuresEarthquake Design and Evaluation of Concrete Hydraulic StructuresEarthquake Design and Evaluation for CW ProjectsSafety of Dams – Policies and Procedures.