TM 5-818-1 / AFM 88-3, Chap. 7
foundation area is small. These preliminary borings
2-6). Detailed exploration should be carried to a depth
must be supplemented by intermediate borings as
that encompasses all soil strata likely to be significantly
required by the extent of the area, location of critical
affected by structure loading. If the structure is not
loaded areas, subsurface conditions, and local practice.
founded on piles, the significant depth is about 1 1/2 to 2
b. Depth of exploration. The required depth of
times the width of the loaded area. An estimate of the
exploration may be only 5 to 10 feet below grade for
required depth can be made using the stress influence
residential construction and lightly loaded warehouses
charts in chapter 5 to find the depth such that
∆q <= 0.1q0
and office buildings, provided highly compressible soils
(4-1)
where ∆q represents an increase in strata stress and q0
are known to not occur at greater depths. For important
or heavily loaded foundations, borings must extend into
is the foundation contact pressure. Note that in the case
strata of adequate bearing capacity and should penetrate
of a pile foundation, stresses are produced in the ground
all soft or loose deposits even if over- lain by strata of
to an appreciable depth below the tips of the piles.
stiff or dense soils. The borings should be of sufficient
Procedures to obtain ∆q apply as for other foundations.
depth to establish if groundwater will affect construction,
This depth criterion may not be adequate for complex
cause uplift, or decrease bearing capacity.
When
and variable subsurface conditions.
pumping quantities must be estimated, at least two
c. Plugging borings. All borings should be
borings should extend to a depth that will define the
carefully plugged with noncontaminating material if-
aquifer depth and thickness. Borings may generally be
(1) Artesian water is present or will be
stopped when rock is encountered or after a penetration
when the excavation is made.
of 5 to 20 feet into strata of exceptional stiffness. To
(2) Necessary to avoid pollution of the
assure that boulders are not mistaken for bedrock, rock
aquifer from surface infiltration, leaching, etc.
coring for 5 to 10 feet is required. When an important
(3) Necessary to preserve a perched
structure is to be founded on rock, core boring should
water table (avoid bottom drainage through borehole).
penetrate the rock sufficiently to determine its quality and
(4) Area is adjacent to stream or river
character and the depth and thickness of the weathered
where flood stage may create artesian pressure through
zone. Rock coring is expensive and slow, and the
the borehole.
minimum size standard core diameter should be used
d. Sample requirements. Table 4-2 may be
that will provide good cores. NX or larger core sizes may
used as a guide for required sizes of undisturbed
be required in some rock strata. Core barrels can
samples, and table 4-3 for general samples. The
remove cores in standard 5-, 10-, and 20-foot lengths
sampling program
(actual core may be much fractured, however; see para
Table 4-2. Recommended Undisturbed Sample Diameters
Test
Minimum Sample Diameter, in.
Unit weight
3.0
Permeability
3.0
5.0
Triaxial compressiona
5.0
Unconfined compression
3.0
Direct shear
5.0
a
Triaxial test specimens are prepared by cutting a short section of 5-in.-diam sample axially into four
quadrants and trimming each quadrant to the proper size. Three quadrants provide for three tests
representing the same depth; the fourth quadrant is preserved for a check test.
U. S. Army Corps of Engineers
4-3
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