Soil Types. The easiest way to group soils is into two categories: cohesive and non-cohesive. Fine- grained soils such as clay are considered cohesive. Sand and other coarse-grained soils are non-cohesive. The classification of cohesive and non-cohesive soils may be further sub-divided based on origin, method of deposition and structure. Soil structure may be classified as deposited or residual. Deposited soils have been transported from their place of formation to an anchor location. Residual soils are formed by physical and/or chemical forces breaking down parent rocks or soils to a more finely divided structure. Residual soils are often referred to as weathered. Soil structure properties can be divided into loose, dense, honeycombed, flocculated, dispersed or composite. Often these soils are in layers of different thickness of unlike soils, These soils do not necessarily retain consistency of materials at various depths. Trouble free anchoring demands the careful evaluation of local soil conditions and anchor types. Without proper soil/anchor planning, maximum anchor performance an never be assured. Almost all of the soil types classified below can be found in the Denver area.
Shallow and deep foundation anchors A couple of soil failure may occur depending on the depth of the helix; shallow and deep. Foundations expected or proven to exhibit a specific mode are usually referenced as shallow or deep foundations. Which basically points out the location of the surface and a sudden drop in load resistance to almost zero. With deep foundations, the soil fails sequentially, maintaining significant post-ultimate load resistance , and exhibits little or no surface deformation. The dividing line between shallow and deep foundations has been reported by a number of investigators to be three to eight times the foundation diameter. The five-diameter depth is the vertical distance from the surface to the top of the helix. The five-diameter rule is often simplified to 5 feet minimum. Anytime a foundation anchor is considered, a deep foundation should be exercised. Deep foundations have two key advantages over shallow foundations. (1) it provides an increased ultimate capacity, (2) failure will be incremental with no sudden decreases in load resistance after the ultimate capacity has been obtained.
Soil Groupings Throughout the Denver area there are a wide variety of soil conditions present. Some areas consist of very sandy conditions that will not support even load designs of 1000 pounds per square foot. While no more than 2 miles away the soils are so expansive that additional design requirements are needed.
Below depth (ft) probe (ft) description
13 0 0-100 Top soil
10 5 150-175 Gray clay
13 10 150-175 Gray clay some red clay
13-15 175-225 Gray clay, traces of sand
20 20 225-300 Gray clay with sand, light rock-wet
Soil test probe
Foundations have long been dependent upon excavation, penetration and lab tests of core samples. When construction is concentrated in an area, this is still desirable, but for an overhead line or underground pipeline, which may extend for hundreds of miles, economic feasibility requires a less costly yet dependable determination of soil properties. The portable soil test probe provides a new dimension. This instrument, portable and operable by one man, will provide reproducible numerical data related to resistance of the soil to flow under load . It may be used in soils up the uniformity of hardpan , to any depth below the surface and without the need to make an excavation or otherwise disturb the soil. A probe consists of a head on a square shaft with a number of extensions, all of which may be coupled together. A ratchet wrench with a torque-measuring handle is used to install, remove or take readings. Corner marks at 1-foot intervals provide means to determines the depth below the surface when a reading is taken. The hub of the probe head is forced into the ground by application of torque acting on the blade of the probe. Thus the torque required to turn the probe is proportional to the resistance of the soil to penetration of the hub. When determining end bearing foundation work , the bearing strength of the soil can be calculated directly from the probe reading. Most heavy-duty probes can withstand torque to 1800 foot pounds. That basically means that the probe will not penetrate packed gravel, shale or rock.
Bearing capacity theory
This theory suggests that the capacity of a foundation anchor is equal to the sum of the capacities of individual helixes. The helix capacity is determined by calculating the unit bearing capacity of the soil and applying it to the individual helix areas. Friction along the central shaft is not used in determining ultimate capacity. Friction or adhesion on extension shafts may be included if the shaft is rounded and at least 2.5” in diameter.
Water, Frost and Soil
The typical minimum foundation depth required by code is three feet. This measurement is taken from the top of the soil to the bottom of the footing. The depth is important so that freezing of the ground will not lift the footing and foundation. If an anchor helix is in a zone of deep frost penetration (cold winter seasons), frozen soil will behave as a stiffer soil and will basically yield greater holding capacity, by hard soil with brittle cracking instead of softer depressible soil. However, when spring begins to thaw out the soil, the overlying zone will be water-saturated while the layer housing the helix remains frozen. The condition is comparable to a hard layer under a soft layer, and may result in sudden anchor failure. Occasionally anchor jacking or movement out of the ground occurs during these conditions. When dealing with permafrost, the helix should be at least three to five feet below the permafrost line; provisions made to prevent solar energy from being conducted down the anchor. Anchor holding capacity decreases as moisture content increases. If a helix is installed at the water table level, anchor capacity can reduce helix capacity by as much as 50 percent in granular soil. Water, draining from fine grain soil under load, will permit creep, slow movement of the ground under a steady load. This is similar to the consolidation marvel under a foundation. Rapidly applied loads due to wind or ground tremors have little effect on creep so long as they do not exceed soils shear water slowly drain off. Under such circumstances, creep could become troublesome even though the anchor/ soil system has not structurally failed.