Engineering and Construction

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a. General.   In addition to the needed site investigations and data described in the manuals for nonfrost conditions, design of foundations in areas of significant frost penetration requires special studies and
data because of factors introduced by the special frost- related site conditions.  Detailed site investigation
procedures applicable for arctic and subarctic areas are described in TM 5-852-2/AFM 88-19, and TM 5-852-4/AFM 88-19, and may be adapted or reduced in scope, as appropriate, in areas of less severe winter freezing.  Methods of terrain evaluation in arctic and subarctic regions are given in TM 5-852-8.

b. Remote sensing and geophysical investigations.   These techniques are particularly valuable in selection of the specific site location, when a choice is possible.  They can give clues to subsurface frozen ground conditions because of effects of ground freezing upon such factors as vegetation, land wastage, and soil and rock electrical and accoustical properties.

c. Direct site investigations.  The number and extent of direct site explorations should be sufficient  to reveal in detail the occurrence and extent of frozen strata, permafrost and excess ice including ice wedges, moisture contents and groundwater, temperature conditions in the ground, and the characteristics and properties of frozen materials and unfrozen soil and rock.

(1) The need for investigation of bedrock requires special emphasis because of the possibilities of frost heave or ice inclusions as described in paragraph 18-2c(3).  Bedrock in permafrost areas should be drilled
to obtain undisturbed frozen cores whenever ice inclusions could affect the foundation design or performance.

(2) In areas of discontinuous permafrost, sites require especially careful exploration and many problems can be avoided by proper site selection.  As an example, the moving of a site 50 to 100 feet from its planned position may place a structure entirely on or entirely off permafrost, in either case simplifying foundation design. A location partly on and partly off permafrost might involve an exceptionally difficult or costly design.

(3) Because frozen soils may have compressive strengths as great as that of a lean
concrete and because ice in the ground may be melted by conventional drilling methods, special techniques are frequently required for subsurface exploration in frozen materials. Core drilling using refrigerated drilling fluid or air to prevent melting of ice in the cores provides specimens that are nearly completely undisturbed and can be subjected to the widest range of laboratory tests.

By this procedure, soils containing particles up to boulder size and bedrock can be sampled, and ice formations can be inspected and measured.  Drive sampling is feasible in frozen fine-grained soils above about 250F and is often considerably simpler, cheaper, and faster.

Samples obtained by this procedure are somewhat disturbed, but they still permit ice and moisture content determinations.  Test pits are very useful in many situations.  For frozen soils that do not contain very many cobbles and boulders, truck-mounted power augers using tungsten carbide cutting teeth will provide excellent service where classification, gradation, and rough ice- content information will be sufficient.  In both seasonal frost and permafrost areas, a saturated soil condition is common in the upper layers of soil during the thaw season, so long as there is frozen, impervious soil still underlying.  Explorations attempted during the thaw season are handicapped and normally require cased boring through the thawed layer.  In permafrost areas, it is frequently desirable to carry out explorations during the colder part of the year, when the annual frost zone is frozen, than during the summer.

(4) In subsurface explorations that encounter frozen soil, it is important that the boundaries of frozen and thawed zones and the amount and mode of ice occurrence be recorded.  Materials encountered should be identified in accordance with the Unified Soil Classification System (table 2-3), including the frozen soil classification system, as presented in TM 5-852-2/ AFM 88-19.

(5) In seasonal frost areas, the most essential site date beyond those needed for nonfrost foundation design are the design freezing index and the soil frost-susceptibility characteristics.  In permafrost
areas, as described in TM 5-852-4/AFM 88-19, the date requirements are considerably more complex; determination of the susceptibility of the foundation materials to settlement on thaw and of the subsurface temperatures and thermal regime will usually be the most critical special requirements.  Ground temperatures are measured most commonly with copper-constantan thermocouples or with thermistors. 

(6)Special  site investigations, such as installation and testing of test piles, or thaw-settlement tests may be required.

Assessment of the excavation characteristics of frozen materials may also be a key factor in planning and
design.

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Similar to the boring layout, the depth of subsurface exploration for a particular project must be based on judgment and experience. Borings should always be extended through unsuitable foundation bearing material, such as uncompacted fill, peat, soft clays and organic soil, and loose sands, and into dense soil or hard rock of adequate bearing capacity. In a general sense, the depth of subsurface exploration will depend on the size and loading of the proposed foundation, the sensitivity of the proposed structure to settlements, and the stiffness and coefficient of compressibility of the strata that will underlie the foundation.

In terms of additional general rules, Hvorslev (1949) states:

The borings should be extended to strata of adequate bearing capacity and should penetrate all
deposits which are unsuitable for foundation purposes, such as unconsolidated fill, peat, organic silt, and
very soft and compressible clay. The soft strata should be penetrated even when they are covered with a
surface layer of higher bearing capacity.

When structures are to be founded on clay and other materials with adequate strength to support the
structure but subject to considerable consolidation by an increase in the load, the borings should penetrate
the compressible strata or be extended to such a depth that the stress increase for still deeper strata is reduced to values so small that the corresponding consolidation of these strata will not materially influence the settlement of the proposed structure.

Except in cases of very heavy loads or when seepage or other considerations are governing, the borings may be stopped when rock is encountered or after a short penetration into strata of exceptional bearing capacity and stiffness, provided it is known from explorations in the vicinity or the general stratigraphy of the area that these strata have adequate thickness or are underlain by still stronger formations. When these conditions are not fulfilled, some of the borings must be extended until it has been established that the stiff strata have adequate thickness irrespective of the character of the underlying material.

When the structure is to be founded on rock, it must be verified that bedrock and not boulders have
been encountered, and it is advisable to extend one or more borings from 10 to 20 ft (3 to 6 m) into sound
rock in order to determine the extent and character of the weathered zone of the rock.

For localized structures, such as commercial or industrial buildings, it is common practice to carry explorations to a depth beneath the loaded area of 1.5 to 2.0 times the least dimension of the building (Lowe and Zaccheo, 1975). Table 2.8 presents additional guidelines for different types of geotechnical and foundation projects.

Another commonly used rule of thumb is that for isolated square footings, the depth of subsurface exploration should be two times the width of the footing. For isolated strip footings, the depth of subsurface exploration should be four times the width of the footing. These recommendations are based on the knowledge that the pressure of surface loads dissipates with depth. Thus, at a certain depth, the
effect of the surface load is very low. For example, a common guideline is to perform subsurface

exploration to a depth where the increase in vertical pressure from the foundation is less than 10 percent of the applied pressure from the foundation. There could be problems with this approach because as will be discussed in Chap. 7, there could be settlement of the structure that is independent of its weight or depth of influence. Settlement due to secondary influences, such as collapsible soil, is often unrelated to the weight of the structure. Especially when geologic conditions are not well established, it is always desirable to extend at least one boring into bedrock to guard against the possibility of a deeply buried soil strata having poor support characteristics.

For some projects, there may be specifications on the required depth of borings. For example, the
Standard Specifications for Highway Bridges (AASHTO, 1996) states:

When substructure units will be supported on deep foundations, the depth of subsurface exploration shall extend a minimum of 20 ft (6 m) below the anticipated pile or shaft tip elevation. Where pile or shaft groups will be used, the subsurface exploration shall extend at least two times the maximum pile group dimension below the anticipated tip elevation, unless the foundation will be end bearing on or in rock. For piles bearing on rock, a minimum of 10 ft (3 m) of rock core shall be obtained at each exploration loca- tion to insure the exploration has not been terminated on a boulder.

At the completion of each boring, it should immediately be backfilled with on-site soil and com- pacted by using the drill rig equipment. In certain cases, the holes may need to be filled with a cement slurry or grout. For example, if the borehole is to be converted to an inclinometer (slope monitoring device), then it should be filled with weak cement slurry. Likewise, if the hole is to be converted to a piezometer (pore water pressure monitoring device), then special backfill materials, such

as a bentonite seal, will be required. It may also be necessary to seal the hole with grout or bentonite if there is the possibility of water movement from one stratum to another. For example, holes may need to be filled with cement or grout if they are excavated at the proposed locations of dams, levees, or reservoirs.

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The general purpose of subsurface exploration is to determine the following (AASHTO, 1996):

1. Soil strata
   a Depth, thickness, and variability
   b Identification and classification
   c Relevant engineering properties, such as shear strength, compressibility, stiffness, perme-
ability, expansion or collapse potential, and frost susceptibility.

2. Rock strata
   a Depth to rock
   b Identification and classification
   c Quality, such as soundness, hardness, jointing and presence of joint filling, resistance to weathering (if exposed), and soluble nature of the rock.

3. Groundwater elevation

4. Local conditions requiring special consideration

In terms of the general procedures and requirements for subsurface exploration, Hvorslev (1949) states:

Investigation of the distribution, type, and physical properties of subsurface materials are, in some
form or other, required for the final design of most civil engineering structures. These investigations are
performed to obtain solutions to the following groups of problems:

Foundation problems or determination of the stability and deformations of undisturbed subsurface materials under superimposed loads, in slope and cuts, or around foundation pits and tunnels; and determination of the pressure of subsurface materials against supporting structures when such are needed.

Construction problems or determination of the extent and character of materials to be excavated or location and investigation of soil and rock deposits for use as construction materials in earth dams and fills, for road and airfield bases and surfacing, and for concrete aggregates.

Groundwater problems or determination of the depth, hydrostatic pressure, flow, and composition of the ground water, and thereby the danger of seepage, underground erosion, and frost action; the influence of the water on the stability and settlement of structures; its action on various construction materials; and its suitability as a water supply.

There are many different types of subsurface exploration, such as borings, test pits, or trenches.

Table 2.3 presents general information on foundation investigations, samples and samplers, and subsurface exploration. Table 2.4 (from Sowers and Royster, 1978, based on the work by ASTM; Lambe, 1951; Sanglerat, 1972; Sowers and Sowers, 1970) summarizes the boring, core drilling, sampling and other exploratory techniques that can be used by the geotechnical engineer.

As mentioned earlier, the borings, test pits, or trenches are used to determine the thickness of soil and rock strata, estimate the depth to groundwater, obtain soil or rock specimens, and perform field tests such as Standard Penetration Tests (SPT) or Cone Penetration Tests (CPT). The Unified Soil Classification System (USCS) can be used to classify the soil exposed in the borings or test pits (Casagrande, 1948).

The subsurface exploration and field sampling should be performed in accordance with standard procedures, such as those specified by the American Society for Testing and Materials (ASTM, 1970, 1971, and D 420-03, 2004) or other recognized sources (e.g., Hvorslev, 1949; ASCE, 1972, 1976, 1978).

App. A (Glossary 1) presents a list of terms and definitions for subsurface exploration.