Granular Soil Uses

THAT'S GOOD DIRT

 Alrighty Then;  We were discussing granular graded soils, and we ended with a brief look at classification charts (schemes); ASTM, USCS, and the AAHTO and ( a lot of other acronyms we haven't looked at ), said these standards are used by our engineers in determining soil types.

  We also mentioned, that soils are in combinations of each other with diverse compositions and varying consistencies. The granular soils are for the most part non-plastic, meaning; no bonding agent or attraction of particles, and no shear strength if unconfined; the particles don't sticktogether that well but, are preferred in construction use, over the cohesive (clay) types, and that's because of their distinctive characteristic like; stability, excellent drainage capability, low frost heave, best back-fill potential, dense, coarse-grained, and resistance to foundation settlement, and typical high bearing capacity. Building codes often list maximum bearing capacities for soils of general classes that may be used without soil testing in light frame construction. But these are generic and may lead to erroneous conclusions regarding the true load-carrying capability of the soil at a specific site, and so these should be used with caution.

Presumptive Bearing Values of Foundation Materials

Class of materials                                                     Bearing Pressure lb./sq.ft.

1. Crystalline bedrock                                                                             12,000
2. Sedimentary rock                                                                                    6,000
3. Sandy gravel or gravel                                                                          5,000
4. Sand, silty sand, clayey sand, silty gravel, clayey gravel       3,000
5. Clay, sandy clay, silty clay, clayey silt                                           2,000

  No Problem Construction Soil

  Soils that are dense, coarse-grained, and properly drained. Cause very few problems in light buildings for slab-on-grade foundations.

  The soils here have excellent bearing capacity, and they are not subject to significant volumetric change (we will see this later). Gravels, occur in a dense state and not likely to settle, or prone to pose that problem. The settlement that does occur is very sudden and happens immediately when the load is applied.

  During trenching and grading operations, other precautions need to be heeded, ie; the natural confinement of the soil should not be disturbed, (except when compacting);, because further disturbance of the soil bed would necessitate measures that half to be taken to increase the necessary lateral support, ie; (earth retention systems; soil stabilization systems; water and Portland Cement for bonding). 

  Lateral displacement, is horizontal movement of soil, displaced by compression of heavy loads or the foundation. Generally this is not a problem in light frame construction. Lateral displacement is more likely to occur under heavy building loads and this problem is addressed in the foundation design. At any rate, the bearing soil must be undisturbed and stable.

  Soils acceptable for use in the construction of fills, back-fills, and embankments for commercial, institutional, and industrial buildings and for large multifamily housing projects are generally limited to those in the GP, GM, GC, SP, and SC categories.

  General back-filling outside of the structures is generally limited to those soil classified as GP, GM, GC, SP, and SM. However, in certain situations, some other classifications may be permitted, but that decision should be left to a competent soil engineer, and in all circumstances additional liquid limit and plastic limit requirements will apply. 

  Gravel and sands; The shear strength of these soils depend solely upon the internal friction between grains. Generally speaking, bearing strength is high and foundation failure is relatively infrequent. The sudden settlement that does occur takes place immediately upon application of the load and does not materially effect the foundation. The shear strength increases as the grain size increases and a well graded soil is best.

  They do not hold water at all, with the exception of some sands and silts, which do exhibit a weak tendency to bond together with a negative water charge, but will easily collapse. You'll notice this while excavating in a sandy soil type; the sides will hold for some time and then start dropping off as the soil becomes unsaturated and then falls apart or collapses. It is very permeable and excellent material to use for drainage.

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