Article 4 Allowable Soil Bearing Pressures

For purposes of this subchapter, soil materials shall be classified and identified in accordance with table 11-1. In addition, the following supplementary definitions shall apply.

(a) Rock.

(1) HARD SOUND ROCK. Includes crystalline rocks such as Fordham geniss, Ravenswood geniss, Palisades diabase, Manhattan schist. Charateristics are: The rock rings when struck with pick or bar; does not disintegrate after exposure to air or water; breaks with sharp fresh fracture; cracks are unweathered and less than one-eighth inch wide, generally no closer than three feet apart; core recovery with a double tube, diamond core barrel is generally eighty-five percent or greater for each five foot run.

(2) MEDIUM HARD ROCK. Includes crystalline rocks of paragraph one of this subdivision, plus Inwood marble and serpentine. Characteristics are: all those listed in paragraph one of this subdivision, except that cracks may be one-quarter inch wide and slightly weathered, generally spaced no closer than two feet apart; core recovery with a double tube, diamond core barrel is generally fifty percent or greater for each five foot run.

(3) INTERMEDIATE ROCK. Includes rocks of paragraphs one and two of this subdivision, plus cemented shales and sandstone of the Newark formation. Characteristics are: the rock gives dull sound when struck with pick or bar; does not disintegrate after exposure to air or water; broken pieces may show weathered surfaces; may contain fracture and weathered zones up to one inch wide spaced as close as one foot; core recovery with a double tube, diamond core barrel is generally thirty-five per cent or greater for each five foot run.

(4) SOFT ROCK. Includes rocks of paragraphs one, two and three of this subdivision in partially weathered condition, plus uncemented shales and sandstones. Characteristics are: rock may soften on exposure to air or water; may contain throughly weathered zones up to three inches wide but filled with stiff soil; core recovery with a double tube, diamond core barrel is less than thirty-five percent for each five foot run, but standard penetration resistance in soil sampling is more than fifty blows per foot. Where core recoveries are less than twenty percent and the material is to be used for bearing, a minimum three inches diameter core shall be recovered and the material recovered shall be classified in accordance with table 11-1.

(b) Special soil types.

(1) FINE SAND. Soils of group SM, containing more than fifty percent (by weight) of particles passing a number sixty mesh sieve.

(2) HARDPAN. Soils of groups GM, GC, and SW, generally directly overlying rock, and which are sufficiently cemented to be difficult to remove by picking.

(3) CLAY SOILS. Soils of each group SC, CL, and CH shall be classified according to consistency as hard, medium, or soft in accordance with the following:

1. Hard clay. A clay requiring picking for removal, a fresh sample of which cannot be molded by pressure of the fingers.
2. Medium clay. A clay that can be removed by spading, a fresh sample of which can be molded by a substantial pressure of the fingers.
3. Soft clay. A clay, a fresh sample of which can be molded with slight pressure of the fingers.

(4) SILT SOILS. Soils of each group ML and MH shall be classified as dense, medium, or loose depending on relative difficulties of removal as described for hard, medium, and soft clays in paragraph three of this subdivision.

(5) VARVED SILT. A natural soil deposit consisting of alternating thin layers of silt, clay, and sand in which the silt or silt plus sand layers predominate.

The following materials, or any combination of them shall be considered as generally satisfactory bearing materials; hard sound, medium hard, intermediate, and soft rock; hardpan; granular soils of G and S groups; dense or medium silt soils of groups ML and MH; and hard or medium clay soils of groups CL and CH.

TABLE 11-1
UNIFIED SOIL CLASSIFICATION

FIELD IDENTIFICATION PROCEDURES FOR FINE-GRAINED SOILS OR FRACTIONS (Notes for Table 11-1)

These procedures are to be performed on the minus No. 40 sieve size particles, approximately 1/64 in. For field classification purposes, screening is not intended, simply remove by hand the coarse particles that interfere with the tests.

Dilatancy (reaction to shaking)

After removing particles larger than No. 40 sieve size, prepare a pat of moist soil with a volume of about one-half cubic inch. Add enough water if necessary to make the soil soft but not sticky.

Place the pat in the open palm of one hand and shake horizontally, striking vigorously against the other hand several times. A positive reaction consists of the appearance of water on the surface of the pat which changes to a livery consistency and becomes glossy. When the sample is squeezed between the fingers, the water and gloss disappear from the surface, the pat stiffens, and finally it cracks or crumbles. The rapidity of appearance of water during shaking and of its disappearance during squeezing assist in identifying the character of the fines in a soil.

Very fine clean sands give the quickest and most distinct reaction whereas a plastic clay has no reaction. Inorganic silts, such as a typical rock flour, show a moderately quick reaction.

Dry Strength (crushing characteristics)

After removing particles larger than No. 40 sieve size, mold a pat of soil to the consistency of putty, adding water if necessary. Allow the pat to dry completely by oven, sun or air-drying, and then test its strength by breaking and crumbling between the fingers. This strength is a measure of the character and quantity of the colloidal fraction contained in the soil. The dry strength increases with increasing plasticity.

High dry strength is characteristic for clays of the CH group. A typical inorganic silt possesses only very slight dry strength. Silty fine sands and silts have about the same slight dry strength, but can be distinguished by the feel when powdering the dried specimen. Fine sand feels gritty whereas a typical silt has the smooth feel of flour.

Toughness (consistency near plastic limit)

After particles larger than the No. 40 sieve size are removed, a specimen of soil about one-half inch cube in size, is molded to the consistency of putty. If too dry, water must be added and if sticky, the specimen should be spread out in a thin layer and allowed to lose some moisture by evaporation. Then the specimen is rolled out by hand on a smooth surface or between the palms into a thread about one-eighth inch in diameter. The thread is then folded and rerolled repeatedly. During this manipulation, the moisture content is gradually reduced and the specimen stiffens, finally loses its plasticity, and crumbles when the plastic limit is reached.

After the thread crumbles, the pieces should be lumped together and a slight kneading action continued until the lump crumbles. The tougher the thread near the plastic limit and the stiffer the lump when it finally crumbles, the more potent is the colloidal clay fraction in the soil. Weakness of the thread at the plastic limit and quick loss of coherence of the lump below the plastic limit indicate either inorganic clay of low plasticity, or materials such as kaolin-type clays and organic clays which occur below the A-line.

Highly organic clays have a very weak and spongy feel at the plastic limit.

Fill material, peat, (group Pt), organic silts and clays (grouping OL and OH), soft or loose soils of groups ML, CL, HM and CH, varved silt, or satisfactory bearing materials that contain lenses of, or are underlain by, these materials shall be considered as nominally unsatisfactory bearing materials.