5- Special Investigations

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Chapter 5

Site Investigation Manual - 2002 Special Investigations

Ethiopian Roads Authority Page 5-1

5. SPECIAL INVESTIGATIONS

Special investigations relate to such items as deep cuts, embankments over soft and

compressible soils, expansive soils and natural slopes.

5.1 Deep Cuts

This paragraph deals with additional considerations regarding cuts if common

investigations for the earthworks and subgrade have revealed particular problems (e.g.

significant rock excavation) and also with considerations regarding the side slopes to be

considered.

Regarding the investigations of deep cuts, it is desirable to conduct specific test pits,

borings, auger probes and/or geophysical investigations, since significant cost overruns

can result from erroneous estimates of rock excavation (as opposed to unclassified or

earth excavation).

The potential for water resurgence in deep cuts should also be investigated.

The stability of cut sections needs to be detailed, including provision of cut-off and

interface drainage, and treatment of other slope stability issues.

When new horizontal alignment is required, or when the vertical alignment is changed,

required new cuts should be designed and existing cuts deepened. It is very important to

avoid rocky cuts whenever possible. At the final design stage, visual inspection of these

areas is required.

If additional, possibly expensive, tests are deemed necessary, such as seismic tests or

drilling, a program should be proposed to ERA for approval, describing the scope, intent,

and price of the tests.

Specific drainage systems must also be designed to protect slopes prone to mud slides

from rainfall runoff.

The following factors should be considered with respect to deep cuts:

Type of material to be excavated, volume and position of different materials.

Level and flow of water table and springs

Stability of slopes

Drainage and protection against erosion

5.1.1 TYPE OF MATERIAL TO BE EXCAVATED, VOLUME AND POSITION OF DIFFERENT

MATERIALS

The type of material to be excavated influences its suitability as embankment material, as

developed in the section regarding subgrade strength class determination. As indicated

before, it is important to determine the volume of rock, rippable and normal material in

each deep cut. Dynamic penetration and seismic tests may usefully supplement boringsfor this purpose (See Chapter 7).


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Special Investigations Site Investigation Manual - 2002

Page 5-2 Ethiopian Roads Authority

The depth and degree of weathering may be very variable within short distances.

Laterization may also be present. The determination of the rock level is important not

only to estimate quantities and costs, but also because its interface with the overlying soil

may provide an opportunity for the presence of a perched water table. Springs may also

be a problem, depending on the type of rock and its structure.

5.1.2 WATER TABLE AND SPRINGS

The water table may be permanent, seasonal or perched. In any event, its presence and

characteristics (level, flow, etc.) should be determined, as they influence the stability and

any drainage system which may be required.

Also, the likelihood of springs occurring within the cut should be assessed.

5.1.3 STABILITY OFSLOPES

The complete analytical determination of the stability of a cut section is complex andbeyond the scope of this investigation manual. However, it is worth giving approximate

rules of thumb, or guidelines, given by experience, as listed below. It is essential to

remember that these guidelines do not take into account the presence of water or of

external loads. They are also limited to cases of homogeneous masses, and, in the case of

rock, do not account for strong patterns of jointing and their orientation.

Cohesionless sands: 1 (V): 2 (H)

Silty sands and silts: 1 (V): 1 (H)

Eluvial soils (e.g. red friable clays):

1.5 (V): 1 (H) for depths of cuts less than 4 meters

1 (V): 1 (H) for depths of cuts over 4 meters

Weathered rock: 2 (V): 1 (H) to 4 (V): 1 (H)

Sound rock: 5 (V): 1 (H) to 10 (V): 1 (H)

Note: V: Vertical, H: Horizontal

5.1.4 EROSIONCONTROL

Erosion control may occasionally conflict with stability requirements. One reason in

cohesive soils is that erosion is less pronounced for steep slopes than for 1 (V): 1 (H)

slopes, as exposure to rainfall decreases. The 1 (V): 1 (H) slopes may be considered for

cuts over 4 to 5 meters depth if planted with grass, otherwise 1.5 (V): 1 (H) slopes may

be more suitable in combination with berms at 4 meters vertical intervals.

Inspection of existing cut slopes is necessary to benefit from local experience.

5.2 Embankments and Compressible Soils

This paragraph deals with additional considerations that may be required for

embankments which are founded on soft and compressible soils. The investigations will

include in-situ static and dynamic penetrometer tests, borings with undisturbed soil

sampling and occasionally field vane tests. In the laboratory, apart from identification


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(classification) tests among which moisture content and organic content determination

will be more extensive than usual, tests will include consolidation tests, unconfined

compression tests, and triaxial tests. The design parameters for stability analysis and

settlement analysis can be derived from these tests.

Failure in bearing also requires consideration in these embankments. Embankments,when built on weak foundation materials, may be subjected to sinking and spreading

failures, irrespective of the stability of the new overlying embankment material. A simple

rule of thumb based on bearing capacity theory can be used to make a preliminary

estimate of the factor of safety against circular arc failure for an embankment built over a

clay foundation. The rule is (Cheney and Chassie, 1982, Ref. 9):

F.S.6c

Hfill fill=

where F.S. = factor of safety

c = cohesion of foundation clay

fill = unit weight of embankment fillHfill = height of embankment fill

The factor of safety computed using this rule serves only as a rough preliminary estimate

of the stability of an embankment over a clay foundation and should not be used for final

design. If the factor of safety using this rule is less than 2.5, a more comprehensive

stability analysis is required.

Possible foundation treatment methods for constructing embankments on soft and

compressible soils include:

Partial excavation

Preloading with surcharge fills

Preloading with vertical drains

Pile-supported embankments

Detailed analysis of these methods is beyond the scope of this manual.

5.3 Natural Slopes

Many projects intersect ridges and valleys, and these landscape features can be prone to

slope stability problems. Natural slopes that have been stable for many years may

suddenly fail because of changes in topography, seismicity, groundwater flows, loss of

strength, stress changes and weathering. Careful and long-term study of a critical slope

along the proposed alignment is required to understand and predict the slopes behavior.Progressive failures of natural slopes are most likely due to materials such as clays and

shale. The presence of such materials and their properties should be investigated in slope

studies.

If slope stability problems are identified in mountainous portions of a road project,

protection against mudslides and falling rocks will be required in the area. Such reference

material as TRL Road Notes 14 and 16 (Ref. 10 and Ref. 11) may be consulted in this

respect. If new cuts are necessary, the slope angle needs to be determined by careful

analysis of existing data concerning the rock formation, and comparison to similar

existing cuts.

Specific drainage systems must also be designed to protect slopes prone to mudslides

from rainfall runoff.


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5.3.1 TYPES OFSLOPEMOVEMENTS

Figure 5-1 illustrates the general types of slope failures. Falling and toppling are features

frequently associated with rock slopes, whereas slide, spread and flow are related to soil

slopes.

5.3.2 FACTORSCONTRIBUTING TO SLOPEFAILURES

Slope failures are often caused by processes that increase shear stresses or decrease shear

strengths of the soil mass. Processes that most commonly cause an increase in the shear

stresses acting on slopes are listed in Table 5-1. Processes that most commonly cause a

decrease in the shear strength of slope materials are listed in Table 5-2.

Field explorations should be directed toward identifying these factors and to determine

the severity of each factor in predicting the slope stability.

5.3.3 GUIDELINES FORINVESTIGATIONS

Slope stability investigations are usually costly. A deep drill hole is difficult and time-

consuming, and still may not provide all the information needed. In this regard, it is

advisable to carry out a preliminary analysis to look for possible controlling features of

slope failures before starting a field investigation program. Once the features that control

the slide are identified, a suitable exploration program can be planned during the final

investigation phase.

The limits of slope failure surface can be approximated by observing the orientation of

trees and electric poles. Leaning trees are often a sign of surface movements. Telephone

poles or electric lines tend to tilt in a slide, which often causes tension or sagging of wires

between telephone or electric poles.

Man-made features such as catchpits, masonry walls and guard rails are usually built in

specific geometric shapes. Any deviations from these shapes would indicate that there

has been a differential movement from which the limits of a landslide can be determined.

5.3.4 REMEDIAL MEASURES

The information obtained from field investigations should be analyzed and compared

with the feasibility studies and laboratory test results. Design parameters should then be

derived to be used in the analysis of possible remedial measures. Remedial measures can

be:

Removing weight from the upper part of the slope

Removing all unstable materials

Flattening slopes

Benching slopes

Buttressing with counterberms

Mechanical stabilization (e.g. rock bolts)


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(i) Forms of falls in rock masses: (a) free fall and (b) toppling by overturning (ii) Slide forms in

soil formations. (a) Single block failed along slope as a result of high groundwater level, or

strength increase with depth in cohesive soils. (b) Single block in homogeneous cohesive soils

failed below toe of slope because of either a stronger or weaker soil boundary at base. (c) Failure

of multiple blocks along the contact with strong material. (d) Planar slide or slump in thin soil

layer over rock. Often called debris slides. Common in colluvium and develop readily into flows.

(e) Failure by lateral spreading.

Figure 5-1: Slope Failures (adapted from Ref. 12)


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Table 5-1: Factors that Cause Increases in Shear Stresses in Slopes

(1) Removal of Support

A. Erosion

1. By streams and rivers

2. By action of waves3. By successive wetting and drying (e.g., winds, rain)

C. Natural slope movements (e.g., falls, slides, settlements)

D. Human activity

1. Cuts and excavations

2. Removal of retaining walls or sheet piles

3. Drawdown of bodies of water (e.g., lakes)

(2) Overloading

A. By natural causes

1. Weight of precipitation (e.g., rains)

2. Accumulation of materials because of past landslides

B. By human activity1. Construction of fill

2. Buildings and other overloads at the crest

3. Water leakage in culverts, water pipes, and sewers

(3) Transitory effects (e.g., earthquakes)

(4) Removal of underlying materials that provided support

A. By rivers

B. By weathering

C. By underground erosion due to seepage (piping), solvent agents, etc.

D. By human activity (excavation or mining)

E. By loss of strength of the underlying material

(5) Increase in lateral pressure

A. By water in cracks and fissures

B. By expansion of clays

Source: Adapted from Highway Research Board, 1978. (Ref. 13)

Table 5-2: Factors that Cause Reduced Shear Strength in Slopes

(1) Factors inherent in the nature of the materials

A. Composition

B. Structure

C. Stratification

(2) Changes caused by weathering and physiochemical activity

A. Wetting and drying processes

B. Hydration

C. Removal of cementing agents

(3) Effect of pore pressures

(4) Changes in Structure

A. Stress release

B. Structural degradationSource: Adapted From Highway Research Board, 1978. (Ref. 13)

Detailed analysis of these methods is beyond the scope of this manual. Depending on thesite conditions, an appropriate method of remediation needs to be adopted.


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5.4 Expansive Soils

5.4.1 GENERAL

Expansive soils are those that exhibit particularly large volumetric changes (shrinkage

and swell) following variations in their in-service moisture contents.

Expansive soils, which usually contain the clay mineral montmorillonite, include

sedimentary and residual soils, claystones and shales. Relatively large areas are covered

with expansive soils such as black cotton soils and red silt clays. These clays have caused

persistent difficulties in road construction and are a relatively common problem in

Ethiopia. The following paragraphs give an outline of the problems associated with these

soils, in order for the personnel in charge of the investigations to be aware of their nature.

5.4.2 I DENTIFICATION

The investigations to identify and to classify expansive soils according to their

expansiveness are presented below:

Routine Investigations are those carried out during surveys of project

Extended Investigations include simple additional indicator testing in the laboratory

when expansive soils are suspected

In-Depth Studies include specialized laboratory testing and is used when extended

investigations show occurrence of expansive soils, and the required countermeasures

have high economic consequences.

Routine investigations are those analyses carried out during normal centerline soilssurveys and site observations, including simple geological and geomorphological

assessments, field reconnaissance, routine indicator tests of Atterburg limits and grading.

Table 5-3 shows information usually collected during a field reconnaissance, with typical

features of expansive soils given in the second column:

Table 5-3: Features of Expansive Soils- Soil Descriptions

Soil Description Typical Features of Expansive Soils

Soil Type More clayey soils are likely to be expansive

Consistency when slightly moist to dry Stiff to very stiffConsistency when wet Soft to firm and sticky

Structure Typical cracked surface, slick-sided fissures

Color Only a reliable indicator when combined

with local knowledge

Extended investigations are advisable if:

The results of the field reconnaissance indicate expansive soils, and

PIw >20%Where PIw = Plasticity Index tested on fraction


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Extended investigations include simple additional laboratory tests to estimate

expansiveness and shall be routinely employed where special measures against damage

from expansive soils are proposed in the design. Extended investigations include:

Shrinkage Limit (ASTM D4943-89)

Calculation of Expansiveness from Formula

Classification of low/medium/high expansiveness (see Table 5-4)

Expansiveness is calculated from the following formula:

ex = 2.4 wp 3.9 ws +32.5

Where

wp = PI x (% passing 425mm)/100

ws = Shrinkage Limit x (% passing 425m)/100

In-depth studies are carried out where extended investigations have revealed the

occurrence of expansive soils and the required countermeasures are costly. Treatment of

expansive soils has far-reaching economic consequences on major road projects

traversing long sections of expansive soils. Such projects may warrant in-depth studies of

the expansiveness of the soil, including determination of clay minerology. In-depth

studies to quantify swell potential include:

Oedometer compression test with unloading and consolidation stages

Determination of swell index from unloading stages

Calculation of expansiveness from formula relating to swell index below

Instrumental anaylsis to identify characteristic clay minerology, e.g. X-ray

diffraction, differential thermal analysis, and electron microscopy

For in-depth studies, the formula for expansiveness is:

ex = 644 Cs 18.4

Where

Cs = Swell Index determined in accordance with TRL Report No.

PR/OSC/012/93

5.4.3 CLASSIFICATION OFEXPANSIVESOILS

The system for relative classification of expansive soils is given in terms of ex in Table

5-4:

Table 5-4: Expansive Soils- Classification

Expansivenessex Classification

50 High


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5.4.4 DESCRIPTION OFPROBLEMS

Volume changes: expansive soils shrink and crack when they dry out and swell when

they get wet. The cracks allow water to penetrate deep into the soil, hence causing

considerable expansion. This results in deformation of the road surface, since the

expansion and the subsequent heave are never uniform. Furthermore, these volumechanges may produce lateral displacements (creep) of the expansive clay, if the

side slopes are not gentle enough. Seasonal wetting causes the road edges to wet and

dry at a different rate than those under the surfacing. This mechanism in turn causes

differential movements over the cross section of the road and associated crack

developments, first occurring in the shoulder area, and subsequently developing in

the carriageway, as indicated in Figure 5-2.

Bearing capacity: when the moisture content increases, expansion occurs and thebearing strength of the expansive soil decreases dramatically. The CBR may be

reduced to less than 2 if the soil becomes completely saturated.

Susceptibility to erosion: when they are or become dry, expansive soils may present a

sand like texture. In this state, they are prone to erosion to a much greater extent than

that normally anticipated from their plasticity and clay content.

Figure 5-2: Moisture Content in Expansive Soils

5.4.5 DESIGN AND CONSTRUCTIONCONSIDERATIONS

Some design and construction considerations specific to expansive soils are mentioned in

the following paragraphs, to the extent that they may influence the scope of the

investigations undertaken in the field and the laboratory.


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The measures chosen to minimize or eliminate the effect of expansive soils shall be

economically realistic and proportionate to the risks of potential pavement damage and

increased maintenance costs.

The design engineer may consider the following four main approaches to mitigate or

overcome the problem of expansive clays:

Avoid expansive clays areas by realignment

Excavate the expansive clays and replace them with suitable material (backfilling)

Treat the expansive clays (with lime)

Minimize moisture changes and potential swelling in the expansive clays.

In addition, some considerations are mentioned further below relative to pavement

selection, embankment side slopes and drainage structures.

5.4.6 MITIGATIONMEASURES

(a) Realignment: this solution is possible only if the areas covered with expansive clays

are of limited extent. It is still possible to consider at this stage of design.

(b) Excavation and replacement: this simple procedure effectively eliminates the

problems and is therefore recommended as much as possible. However, backfill materials

are to be obtained from borrow pits, thereby increasing the need for such investigations.

The investigations should focus on minimizing haulage of the materials, and this method

will be economically viable only if suitable backfill material is available in the vicinity of

the road.

It is usually considered sufficient to excavate the expansive soil to a depth of about 1 m

(even if some expansive soil remains under the backfill material, it will be confined and

protected from moisture changes). This may consequently be used for preliminary

estimates of the required quantity of backfill material. Such backfill material should

exhibit strength (CBR) characteristics similar to those of the overlying embankment

materials (preferably at least CBR on the order of 5, i.e. subgrade strength class S3) and

should not be too pervious in order not to act as a drain.

(c) Treatment with lime: Treatment of expansive soils with hydrated lime can give good

results. The addition of 4 to 6% of lime is usually required and provides the following

improvements:

- Reduction of the plasticity index to less than 20- Considerable increase of the shrinkage limit

- Reduction of the swell to negligible values

- Increase of the CBR to a minimum of 10 (after 7 days cure) and 15 (after 28 days

cure), with corresponding improvement of the subgrade strength class.

- Modification of the particle size distribution (by agglomeration of the clay particles),

the final grading being similar to that of a silt.

This treatment is, however, costly, in particular because it is necessary to treat a

substantial thickness of soil (minimum 30 cm compacted thickness). Lime treatment

would therefore be considered advantageous only where investigations failed to locate

suitable backfill or improved subgrade material, and when pavement savings can bemade by taking advantage of the enhanced strength of the treated clay.

(d) Minimizing Moisture Changes and Consequent Movements


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If the above methods cannot be utilized, because of excessive costs or the absence of

suitable fill or replacement material, expansive clays may be used for fill and subgrade.

Special practices are then necessary to avoid detrimental moisture and volume changes in

the swelling soils, as follows:

Confining expansive clays under improved subgrade and protective blankets

Placing selected materials over weak subgrades can reduce subbase thicknesses and

hence pavement costs. In the case of expansive soils, this practice also protects them

from moisture changes. It is then recommended that the selected material be at least 30

cm thick and relatively impervious.

Expansive soils may also be used to form shallow embankments (up to about 3 m),

provided that a protective blanket (at least 30 cm thick) is placed on the slopes. The

blanketing material should be at least conducive to a subgrade strength class of S3 quality

and be impermeable and resistant to erosion.

Surcharging expansive clays

Placing a substantial thickness of non-swelling material over expansive clay reduces

heave. The minimum thickness required depends on the expansion pressure of the

swelling soil, but will usually be 1-3 m to produce a useful reduction of swell. It is

therefore possible to use expansive soil to form the lower part of an embankment. It is

recommended that the total thickness of pavement plus improved subgrade be at least 60

cm, irrespective of the other protective measures taken.

Limiting the compaction of expansive clays

Expansion pressure and potential volume change increase significantly with the dry

density of swelling soils. High degrees of compaction may therefore be detrimental and

should be avoided. It is recommended that the dry density of expansive soils in no case

exceeds 95% MDD (AASHTO T 180).

Placing expansive clays at equilibrium moisture content

This should prevent moisture changes. If possible, the equilibrium moisture content

should be measured under existing roads in the region concerned. Otherwise, it can be

assumed that the equilibrium moisture content is near the plastic limit. This applies in

areas where the mean annual rainfall exceeds 500 mm and the water table is non-existent

or deep (more than 5-6m). In arid areas or in the case of a water table close to groundlevel, a special study will be required to determine the equilibrium moisture content.

Preventing moisture changes under the pavement

It is pointed out that expansive clays are the product of weathering of basic igneous

rocks, by leaching out of salts, to leave a clay with an open chain structure and a sugary

granular appearance. This is not confined to black cotton soils alone: these are the

products of weathering in poor drainage conditions. However, the red silty clays present

in great quantities in the southern part of Ethiopia are also the products of weathering of

basic igneous rocks, but under good drainage conditions. They are also expansive clay

soils, but to a less startling degree. Because of these less obviously detrimentalcharacteristics and their good handling and compaction properties, these are often used as


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embankment materials. Unfortunately, possibly due to the methods by which we measure

moisture content, which shows the extra water ion present in the meta-halloysite mineral

that makes up these red clays, as free water, which it is not; the specified OMC for these

materials is not the equilibrium moisture content. Subsequently large vertical cracks

develop in these embankments as seasonal drying and wetting of embankments takes

place abetted by root growth. These lead to depressions in the wheel paths, in granularbased pavements (often erroneously ascribed to base failure) and longitudinal cracks in

bound base pavements.

These unwanted effects can be avoided by placing a slippage layer, consisting of a thick

polythene sheet, and a thin layer of sand, at the interface between subgrade and

pavement. They inhibit the upward travel of the cracks. Alternatively, not using or

modifying these materials can reduce the problems.

It is essential that the swelling soils under the pavement are protected from moisture

changes, whether caused by external water or by internal variations. To this end, the

following practices should be adopted:

1. The pavement shall be as impermeable as possible. In particular, the use of an

impervious bituminous surfacing is required (multiple surface treatment or asphalt

concrete).

2. The shoulders must be sealed. They should be extended to a width at least equal to the

depth of the zone affected by seasonal moisture changes. Their width should in no case

be less than 2 m.

3. The side ditches should be dispensed with, or if this is impractical, they should belocated as far away as feasible from the pavement. They should have sufficient section

and grade to ensure that no water ponding can occur.

5.4.7 ADDITIONAL CONSIDERATIONS

Pavement selection

Since small differential movements of the subgrade are almost inevitable, even if

mitigation measures are implemented, flexible pavements are required. Double surface

treatment is generally adequate if the pavement layers are impervious. Asphalt concrete

is preferred over pervious roadbase and subbase layers.

Embankments slopes

If the side slopes of the embankments are protected by non-swelling materials, usual

slopes (e.g. 1(V):2(H)) may be used. However, if left unprotected, expansive soils are

prone to erosion and also to creep due to lateral expansion movements. Safe slopes in

expansive soils are therefore very gentle. If expansive soils are to be left exposed, as may

be the case in shallow cuts or side ditches, it is recommended that the slope does not

exceed 1 (Vertical) : 4 (Horizontal).

Drainage structures


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Culverts must not be cast and drainage pipes not be laid directly against expansive clays.

Surrounds or haunches, made from non-swelling material, must be placed. The use of

impermeable and, if possible, gravelly material is essential.

Current Consequences

The site investigation capacity available at present in Ethiopia is limited, and the

recommendations to drill and sample deep cuts, rock quarries, and two holes at each

bridge abutment and pier, have a number of consequences:

If this is included as part of a Design Contract, it will be expensive and time

consuming, and much of it cannot be started until the Final design is well under way.

The Design Contract times will be extended by many months.

Design Consultants will be put at greater risk due to having to let sub-contracts tovariable performers.

In the present system, the Design Consultant obtains surface samples from rock quarries

and investigates bridge sites by a combination of trail pits, probe holes and geosysmic

and resistivity survey, and then provides a full site investigation rig for the Supervision

Engineers use in the Construction Contract. It is suggested that this system is more

economic and better suited to Ethiopia at present, where constraints of access inhibit site

investigation contractors working in isolation.

If it is considered mandatory to obtain this information during the Design Contract, it issuggested that a separate contract for the SI should be let, during the Final Design Stage,

supervised by the Design Consultant, but directly paid for by ERA.

One drill hole per abutment and pier, three to four holes per rock quarry, and one to two

holes per major deep cut should be sufficient and this program will take approximately

three months to carry out per 100km of average road.

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