# Retaining Wall: Saturated Soil & Hydrostatic Pressure

I understand that Hydrostatic pressure and Soil pressure exist concurrently. I'm curious how these separate loads interact with each other.

Full disclaimer: I am not an engineer nor do I pretend to be one. I recognize that these are very broad & elementary questions; I am at the early stages of my understanding. Please be assured I am not, and will not, attempt to engineer actual structures.

1. Does the pore size limit, in any way, the surface area available for hydrostatic loading on the back of a retaining wall? I assume it does not. It would stand to reason that water under pressure would suspend the soil particles and push its way to the point where most of the surface area on the back of the wall was under a hydrostatic load?
2. Soil particules become suspended in water while a hydrostaic load is present. Does this (or any other water related factor) affect the overall soil load or does it remain the same?
3. Is the total load on the wall simply $\frac{H^2}{2}(\gamma_s K_a + \gamma_w)$ (Where $\gamma_s$ and $\gamma_w$ are the soil and water densities, respectively)?
• pore size and soil type will have large effects - a clay type soil can provide a water seal etc. Aug 20 '17 at 19:36
• If clay forms a water seal, with hydrostatic pressure behind it does the pressure get through to the wall by placing those clays under pressure? I understand clay certainly can affect the flow of water, and lead to hydrostatic pressure developing, but once present how does it interact with the soil load? Does the hydrostatic load increase linearly or as more water is added does some type of liquefaction occur that drastically changes the soil load? Aug 20 '17 at 19:52

Have a look on above photo (on your right hand side).

You will see how the loads are distributed if the water level is less the the retaining wall height. The area of the green surface will be the lateral loads on the wall. Hope this will give you a basic understanding about distribution of loads.

1. Does the pore size limit, in any way, the surface area available for hydrostatic loading on the back of a retaining wall? ...

It does not limit the surface contact area. The hydrostatic pressure and the soil load act on the wall concurrently. However, as you noted, the soil load is reduced because the effective weight of the soil is reduced due to buoyant forces. See below answer for more about that.

What the pore-size does do is potentially control other aspects of design- namely, whether the design should consider drained or undrained conditions. A small pore size should consider undrained conditions. Undrained conditions may not need to be considered for granular soils with proper draining, depending on the assumed location of the water table.

1. Soil particules become suspended in water while a hydrostaic load is present. Does this (or any other water related factor) affect the overall soil load or does it remain the same?

The presence of water impacts the soil load, as does movement of the wall away from the wall (active soil pressure) or toward the soil (passive soil pressure), or nonmovement of the wall (at-rest soil pressure).

1. Is the total load on the wall simply $\frac{H^2}{2}(\gamma_s K_a + \gamma_w)$ (Where $\gamma_s$ and $\gamma_w$ are the soil and water densities, respectively)?

If the effective unit soil weight is multiplied by the lateral pressure coefficient ($K_0$ which denotes at-rest, $K_a$, or $K_p$, depending on whether the wall is moving and what direction it is moving), it can be combined with the unit weight of water to determine an equivalent fluid pressure (EFP). The effective soil unit weight $\gamma'$ is the saturated weight $\gamma_{sat}$ minus the unit weight of water, $\gamma_W$.

Therefore:

$$\gamma_{EFP}=K\gamma'+\gamma_W$$

$$\frac{1}{2}H^2\gamma_{EFP}$$

The EFP has often been used in ASD design. However as things have moved towards LRFD design, it is less often used because the water and horizonal soil forces can have different load factors, making the EFP approach impractical for calculations. The loads are instead kept separate.

1. Does the pore size limit, in any way, the surface area available for hydrostatic loading on the back of a retaining wall (I assume it does not)?

Pore size has no bearing on hydrostatic surface area load on a retaining wall. What does change is the structural load-bearing strength of the soil in the water saturated soil. The more water the less strength in the soil. Imagine that where there is saturated soil it behaves exactly like a fluid (water causes mudslides because the water content exceeds the liquid limit of that soil under gravity and explosively collapses like a river down slopes.)

1. Soil particles become suspended in water while a hydrostatic load is present: does this (or any other water related factor) affect the overall soil load or does it remain the same?

Soil particles don't become suspended in an hydrostatic load but become inert structurally proportionally to the liquid limit state of that soil relative to the depth of soil. Think of doing a rock in a bucket of water. The surface of water rises volumetricaly. The problems arise when the rock particles become very small and are classed as clay. It is impossible to remove all the water from clay because it is bonded electronically to the soil itself. The capacity for clay to adsorb water is well understood and documented and causes immense damage to buildings worldwide but it also means that the mass of clay behind a retaining wall will be directly influenced by the water content. This water content will also determine the pressure against which a retaining wall must theoretically be designed to withstand in addition to the safety factor relevant to the local building regulations for such structures.