Chapter 8 - POST FOUNDATION DESIGN
8.3 Soil Properties
8.3.1 General. The capability of a soil to handle loads transmitted to it by a post depend on such characteristics as: particle size and size distribu-tion (a.k.a. soil classificadistribu-tion), moisture content, density, and depth below grade. These soil characteristics control the allowable vertical and lateral soil pressures.
8.3.2 Soil Classification. Soil is classified by the size of individual particles and the distribu-tion of sizes within the sample. There are four major particle (grain) sizes: gravel, sand, silt, and clay. The most popular classification system in the U.S. (i.e., the Unified Soil Classification (USC) system) classifies gravels as grains be-tween 0.2 and 3.0 inches, sands as particles between 0.003 and 0.2 inches, silts as grains between 0.003 and 0.00008 inches, and clays as all particles finer than 0.00008 inches. The distribution of these particles within a given soil has a major impact of soil behavior. A soil with a wide distribution of particle sizes is referred to as a well-graded soil. A poorly graded soil is comprised of similar sized particles. The best soils for foundation design are gravels and sands, with well-graded gravels and sands, bet-ter than poorly graded gravels and sands. Or-ganic silt, peat and soft clay soils are not suit-able for post foundations, as they have neither the strength nor the stability to support structural loads.
8.3.3 Soil Moisture Content. The effective shear strength of a soil can be reduced signifi-cantly when soil is allowed to saturate with wa-ter. To avoid water saturation of soils around posts, install rain gutters, and slope the finish grade away from the building. A minimum 2%
slope for a distance of at least 6 ft (2 m) from the building walls is recommended.
8.3.4 Soil Density and Depth. Allowable verti-cal and lateral soil pressures increase with in-creases in soil density and depth. This is be-cause soil confinement pressures increase as both of these variables increase.
8.3.5 Tabulated Design Values. Table 8.1
contains soil properties as tabulated in ASAE are referred to as presumptive values and should only be used if there is no active building code in effect, and site-specific soil properties are unavailable.
The vertical soil pressures given in table 8.1 are for the first foot (300 mm) of footing width and first foot below grade. A twenty percent increase in allowable soil pressure is allowed for each additional foot (300 mm) of foundation width or depth, up to a maximum of three times the origi-nal value.
The lateral soil pressure values in table 8.1 are per unit depth. To obtain the allowable lateral pressure at a point below grade, SL, multiple the lateral soil pressure value, S, by the distance below grade of the point in question. For exam-ple, the lateral pressure per unit depth, S, for a firm sandy gravel is 300 lbm/ft2 per foot of depth.
This equates to an allowable pressure of 1200 lbf/ft2 (4 ft x 300 lbm/ft2 per ft x 1lbf/lbm) for points four feet below grade. [Note: use of variable SL to represent S when adjusted for depth, is unique to this design manual, and is done to avoid confusion between values that have and have not been adjusted for depth. It is important to realize that SL and S have different units.]
8.3.6 Soil Tests. Site-specific soil test results are often used to determine allowable soil pres-sures. Such calculations generally result in higher allowable design values than would be obtained using table 8.1. This is because pre-sumptive values are the lowest values associ-ated with a broad classification of soils, each at their minimum strength conditions.
8.3.7 Soil Sampling. Soil samples should be gathered from the applicable location in the soil profile: one-third the foundation depth for lateral soil pressure calculations for non-constrained posts; and at footing depth for lateral soil pres-sure calculations for constrained posts and for vertical soil pressure calculations. From each soil sample, the cohesion, c, angle of internal friction φ, and bulk density, w, must be deter-mined.
Table 8.1. Presumed Soil Properties for Post Foundation Design (ASAE, 1999). For use in ab-sence of codes or test.
Lateral Pressure Per Unit Depth, S
↑
Vertical
Pressure, Sv ↓ Density, w ± Class of Material
Density or Con-sistency
← lbf/ft2 per ft
kPa per m
Lateral Sliding
Coeffi-cient
→ lbf/ft2 kPa Friction
Angle, degrees
° lbm/ft2 kg/m3
1. Massive crystalline bedrock - 1200 180 0.79 4000 200 - - -
2. Sedimentary and foliated rock - 400 60 0.35 2000 100 - - -
3. firm Sandy gravel and/or gravel (GW 300 45 - - - 38 120 2000
and GP) loose 200 30 0.35 2000 100 32 90 1500
4. firm Sand, silty sand, clayey sand, 200 30 - - - 30 105 1750
silty gravel and clayey gravel
(SW, SP, SM, SC, GM, and GC) loose 150 22.5 0.25 1500 75 26 85 1400
5. Clay, sandy clay, silty clay and medium 130 20 ″ - - 15 120 2000 clayey silt (CL, ML, MH and CH) soft 100 15 - 1000 50 10 90 1500
← Firm consistency of class 4 and the medium consistency of class 5 can be molded by strong finger pressure, and the firm con-sistency of class 3 is too compact to be excavated with a shovel.
↑ The hydrostatic increase in lateral pressure per unit depth has been included in the equations of this chapter. Source: Table 29-B U29-BC modified with the addition of firm and medium values from Hough (1969).
→ Sliding resistance source: Table 29-B UBC.
↓ Allowable foundation pressures are for footings at least 1 ft (300 mm) wide and 1 ft (300 mm) deep into natural grade. Pressure may be increased 20% for each additional 1 ft (300 mm) of width and/or depth to a maximum of three times the tabulated value.
Source: Table 29-B UBC.
° Soil friction angle varies from soft to medium density for clay materials, and from loose to firm for sand and gravel materials.
Source: Merritt (1976).
± Soil density varies from soft to medium density for clay materials, and from loose to firm for sand and gravel materials. Source:
Hough(1969).
″ Multiply an assumed lateral sliding resistance of 130 lbf/ft2 (6 kPa) by the contact area. Use the lesser of the lateral sliding resis-tance and one-half the dead load.
8.3.8 Allowable Vertical Soil Pressure From Soil Test Data. The allowable vertical soil pressure for round or square footings, Sv, can be estimated from site-specific soil test as:
Sv = SBC / FS (8-1)
where:
Sv = allowable vertical soil pressure, lbf/ft2 (kPa)
FS = factor of safety (2.3 to 3.0)
SBC = ultimate soil bearing capacity, lbf/ft2 (kPa)
SBC = 0.6 g w b (Nq + 1) tan φ +
(Nq - 1+ Nq tan φ)(g w y + c/tanφ) (8-2) Nq = eπ tanφ tan2(φ/2 + 45)
c = soil cohesion, lbf/ft2 (Pa)
φ = soil angle of internal friction, de-grees
w = soil bulk density, lbm/ft3 (kg/m3) g = gravitational constant, 1 lbf/lbm
(0.00981 kPa m2/kg)
y = depth where soil allowable pressure is calculated, ft (m)
b = footing diameter or length of one side, ft (m)
For shallow foundations, a factor of safety be-tween 2.3 and 3.0 is typically applied to vertical soil pressure (Whitlow, 1995). Equation 8.2 is a modified Terzaghi-Meyerhoff equation taken from Whitlow (1995). Values compiled in table 8.2 can be used to facilitate calculation of the ultimate soil bearing capacity, SBC.
Table 8.2. Ultimate Bearing Capacity*
* See Equation 8.2 for variable descriptions.
8.3.9 Allowable Lateral Soil Pressure From Soil Test Data. The allowable lateral pressure per foot of depth, S, can be estimated from site-specific soil test data as:
S = SRP / FS (8-3)
where:
S = allowable lateral soil pressure, lbf/ft2 per ft, (kPa per m)
FS = factor of safety (1.5 to 2.0)
SRP = Rankine passive pressure for drained, cohesiveless soils, lbf/ft2 per ft, (kPa per m).
For lateral earth pressures in drained soils, a factor of safety between 1.5 and 2.0 is typical (Whitlow, 1995). Equation 8-2 assumes drained soils (i.e., the water table is located below the top of the footing). Equation 8-2 does not ac-count for soil cohesion, therefore the equation is conservative for clays. Values for the Rankine passive pressure are given in table 8.3.
Table 8.3. Rankine Passive Soil Pressures for Drained, Cohesiveless Soils
SRP, lbf/ft2 per ft
8.3.10 Adjustment to Allowable Vertical Pressure. Most codes allow for a 33% increase in the allowable vertical pressure values, Sv, when post loads result from wind and seismic forces acting alone or in combination with verti-cal forces (see Section 3.9.4). This adjustment would apply directly to the Sv value from equa-tion 8-1, and is cumulative with the adjustments described in Section 8.3.5 for the presumptive Sv values listed in table 8.1. In this manual, a prime (‘) will be used to denote an allowable Sv
value that has been adjusted (i.e., Sv Î Sv’).
8.3.11 Adjustment to Allowable Lateral Pressure. In addition to the 33% increase gen-erally allowed when post loads result from wind
and seismic forces (acting alone or in combina-tion with vertical forces), the allowable lateral pressure, S, can be doubled when posts have a spacing at least six times their width. This in-crease is due to the multi-dimensional nature of pressure distribution in the soil around isolated posts as depicted in figure 8.1, and described in Section 8.1.3. In this manual, a prime (‘) will be used to denote an allowable S value that has been adjusted (i.e., SÎ S’).
8.4 Footings
8.4.1 General. Typically, the soil is not able to resist applied vertical loads when those loads are transferred through the post alone. There-fore, the post is set on some type of footing, which is installed in the hole prior to post place-ment. Footings in post-frame construction are usually poured concrete. This type of footing is depicted in Figure 8.4. Generally there is no mechanical attachment of the footing to the post.
8.4.2 Friction. A footing is assumed to only re-sist vertical loads; the friction between the foot-ing and the post is assumed to be negligible
when assessing the post lateral load resistance capabilities. Also, the friction between the post (and/or collar) and the surrounding soil are as-sumed to be negligible when assessing the ver-tical load-carrying capability of a given post foundation design.
8.5 Collars
8.5.1 General. When lateral soil pressures ex-ceed allowable values, additional lateral surface area can be obtained by increasing post depth, or by adding a structural element called a collar.
A collar is typically either concrete cast around the base of the post (and considered to be at-tached to the post) or built-up wood atat-tached to the post. These structural elements are repre-sented in figure 8.4.
8.5.2 Location. The collar increases the lateral load resistance capability of the post foundation by increasing the bearing area in the region of the post where lateral soil capability is relatively high. Collars are typically not placed at the top of the post foundation (at the surface of the ground) due to the possibility of frost heave.
Figure 8.4. Examples of common post foundation elements with (a) a poured concrete collar, and (b) a built-up wood collar.
Ground level
Post
Original excavated post hole and backfill region
Poured concrete collar Built-up wood collar
Footing
(a) (b)
8.5.3 Attachment. Whether poured concrete or wood, the collar must be attached to the post in a manner sufficient to carry the structural loads involved. As with any wood structural element exposed directly to the soil, appropriate pre-servatives and fastener systems must be em-ployed to maintain structural integrity over the design life of the building.
8.6 Backfilling
8.6.1 General. The details of backfilling are of-ten overlooked by the designer, and with poof-ten- poten-tially dire consequences. After the footing and post are installed (and the collar, if required), the hole that was dug or drilled is backfilled. Essen-tially, the material used for backfill is the medium through which some, if not all, transverse loads are passed from the post to the virgin soil. Back-fill material is subjected to higher pressures than the surrounding virgin soil due to its proximity to the post. Therefore, material used for backfill and its installation are critically important for the successful performance of a post foundation design.
8.6.2 Materials. Typical materials for backfill include concrete, well-graded granular aggre-gate, gravel, sand, or soil initially excavated from the post hole. These alternatives are listed in the order of decreasing stiffness.
8.6.3 Concrete. While concrete is the stiffest backfill material, it is also the most expensive.
Concrete backfill essentially increases post width, b. It must be installed with attention to the possibility of frost heave (discussed later).
8.6.4 Excavated Soil. The most common back-fill material is the excavated soil. If used as backfill, it should be free of topsoil and organic matter. Silt- or clay-based soils should be moist (not wet) and well packed.
8.6.5 Compaction. Backfill materials should be tamped or vibrated upon backfill in maximum layers (a.k.a. lifts) of 8 inch (400 mm).