SOIL LOADS AND HYDROSTATIC PRESSURE BASIC AND TUTORIALS

SOIL LOADS AND HYDROSTATIC PRESSURE BASIC
What Are Soil Loads And Hydrostatic Pressure?


The notation H is used for lateral soil loads, loads due to hydrostatic pressure, and the pressure of bulk materials. Soil lateral loads and hydrostatic pressure are introduced in ASCE 7 Sec. 3.2 and IBC Sec. 1610.

Soil lateral loading most commonly occurs at retaining walls. It is relatively unusual for wood to be directly loaded by retained soils. One notable exception to this is permanent wood foundations, used in some regions of the United States.

While soil lateral loading will most often come from a geotechnical investigation report, ASCE 7 Table 3-1 provides design lateral soil pressures (in psf, per foot of soil depth) for a range of soil classifications.

Where retaining walls are provided, it is possible to develop hydrostatic pressure in addition.

Hydrostatic pressure is most often avoided by providing drains behind retaining walls. In cases where it is not possible to provide drains, design for combined soil and hydrostatic lateral pressures is required.

In conditions where hydrostatic lateral pressures can develop, it is possible to also have upward hydrostatic pressures on adjacent floor slabs. These upward pressures would also use the notation H in load combinations.

The notation H is also defined in ASCE 7 and the IBC to include pressure of bulk materials. Although no discussion of this use is provided, it is thought to include pressure due to storage of grain, aggregates, or other bulk solids that can exert lateral pressures.

AGGREGATE MOISTURE CONTENT BASICS AND TUTORIALS

MOISTURE CONTENT OF AGGREGATES USED IN CONSTRUCTION BASIC INFORMATION
What Is The Value Of Moisture Content Of Aggregates?


Aggregates can hold water in two ways: absorbed within the aggregate porosity or held on the particle surface as a moisture film. Thus, depending on the relative humidity, recent weather conditions, and location within the aggregate stockpile, aggregate particles can have a variable moisture content.

For the purposes of mix proportioning, however, it is necessary to know how much water the aggregate will absorb from the mix water or how much extra water the aggregate might contribute. Figure 1.10 illustrates four different moisture states:

• Oven-dry (OD)—All moisture is removed by heating the aggregates in an oven at 105°C to constant weight.
• Air-dry (AD)—No surface moisture is present, but the pores may be partially full.
• Saturated surface dry (SSD)—All pores are full, but the surface is completely dry.
• Wet—All pores are full, and a water film is on the surface.


Of these four states, only two (OD and SSD) correspond to well-defined moisture conditions; either one can be used as a reference point for calculating the moisture contents. In the following discussion, the SSD state will be used. Now, to determine how much water the aggregate may add to or take from the mixing water, three further quantities must be defined:

• The absorption capacity (AC) represents the maximum amount of water the aggregates can absorb. From Figure 1.10, this is the difference between the SSD and OD states, expressed as a percentage of the OD weight:


AC = Wssd - Wod/ Wod x 100%


where W represents weight. It should be noted that, for most common aggregates, the absorption capacities are of the order of 0.5 to 2.0%. Absorption capacities greater than 2% are often an indication that the aggregates may have potential durability problems.

• The effective absorption (EA) refers to the amount of water required for the aggregate to go from
the AD to the SSD state:

EA = Wssd - Wad/ Wssd x 100%

To calculate the weight of the water absorbed (Wabs) by the aggregate in the concrete mix:

Wabs = (EA)Wagg

• The surface moisture (SM) represents water in excess of the SSD state, held on the aggregate surface:

SM = Wwet - Wssd/ Wssd x 100%

Thus, the extra water added to the concrete from the wet aggregates will be:

Wadd = (SM) Wagg