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Rock Mechanics for Natural Resources and Infrastructure 
SBMR 2014 – ISRM Specialized Conference 09-13 September, Goiania, Brazil 
© CBMR/ABMS and ISRM, 2014 
 
SBMR 2014 
Soft Rocks: Relevant Aspects to the Resumption of the Studies in 
Brazilian Geotechnics 
 
João Cândido Valenga Parizotto 
São Paulo University, São Carlos, Brazil, jcvparizotto@gmail.com 
 
Rogério Pinto Ribeiro 
São Paulo University, São Carlos, Brazil, rogerioprx@sc.usp.br 
 
Antenor Braga Paraguassú 
São Paulo University, São Carlos, Brazil, nonus@sc.usp.br 
 
SUMMARY: Commitment to study soft rocks requires the understanding of two distinct areas from what 
we know as Science: engineering and geology. In this context, this report aims to expose to the scientific 
community the necessity of turning evaluations about soft rocks back to the scene, well treated in the 
eighties and nineties in Brazil. Nowadays, geologists and civil engineers from technical and scientific 
acquaintances, have not given substantial importance to this subject, as discussed in several occasions 
before. It is currently advisable to incorporate this theme in the agenda, since the substantial demand for 
infrastructure, such as the High Speed Train between the cities of Rio de Janeiro and Campinas, new 
airports expansions and constructions, highways and railways development, slopes stability in urban areas, 
etc. All these are indispensable to hold future (and actual) demands taking into account a regular sequence. 
Being in the interface between soil and rock masses, summing the relationship between two different points 
of view, qualitatively and quantitatively, the understanding of what a soft rock means ends up generating a 
diversity of classifications and interpretations, without actually bringing trustful parameters to support safe 
engineering projects. An overview about soft rocks will take, considering geological and geotechnical 
concepts, characterization and classifications, testing procedures, as analyzed in the available scientific 
literature. 
 
KEYWORDS: Soft rocks, characterization, testing procedures. 
 
 
1 INTRODUCTION 
 
Soft rocks have a great role and scope in Brazil, as 
they compound the foundations of current and 
upcoming dams, bridges and urban buildings; 
railways, highways, tunnel excavations, as well as 
hydroelectric projects. In these projects contexts, 
this type of rock is always avoided and/or removed 
by means of excavation. These facts were showed 
in the first report of the Sedimentary Rocks 
Geotechnical Committee from the Associação 
Brasileira de Geologia de Engenharia (ABGE, 
1988) and Dobereiner (1990), relating about one 
of the groups of soft rocks. 
The quoted author presented the distribution of 
sedimentary basins in Brazil, where 50% of the 
territory is formed by these younger rocks, and, by 
his understanding, with considerable presence of 
soft rocks. 
It is worth the emphatic return of this issue, due 
to the substantial demand for engineering works in 
Brazil, e.g. the High Speed Train (HST), among 
other projects of importance that will create great 
impact on society. In the mentioned project case, 
its design will intercept innumerous geological 
formations of soft rocks, including those originated 
 
 
 
 
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by weathering processes of crystalline rocks. 
Being in the interface between soils and rock 
masses, the understanding of a soft rock is ends up 
generating a diversity of classifications and 
interpretations, without actually bringing trustful 
parameters to support safe engineering projects. 
In this context, this paper presents geological 
and geotechnical concepts, characterizations and 
classifications, and relevant testing procedures 
available in the literature regarding to soil and rock 
mechanics, where both, given the right 
circumstances, are used in rocks of low resistance. 
 
 
2 NOMENCLATURE AND CONCEPTS 
 
In a generic and simplistic way, the soft rocks or 
rocks with low strength could be defined as 
geological materials with fragile mechanical 
characteristics, for example, with low deformability 
and strength materials, standing at the transition 
between soils and hard rocks (Figure 1). 
 
Figure 1. Soft rocks emplacement in geotechnics. (mod. 
from PINHO, 2003). 
 
The term weak rock is used because of a 
consensus among the institutions International 
Society for Rock Mechanics (ISRM), International 
Association for Engineering Geology and 
Environment (IAEG) and the International Society 
of Soil Mechanics and Geotechnical Engineering 
(ISSMFE). They considered the unconfined 
compressive strength as a limit between hard soils 
and soft rocks and hard rocks (lower and upper 
boundaries). 
There are several terminologies used to describe 
or name this material, placed between soils and 
hard rocks. Published articles and events in soil or 
rock mechanics used to take their classifications 
according to each author´s preference. Table 1 
presents a summary of the most common terms in 
both disciplines. 
 
Table 1. Normal nomenclatures for soft rocks. 
Rock Mechanics Soil Mechanics 
Weak rock Indurated soil 
Low strength rock Hard soil 
Soft rock Cemented soil 
 Structured soil 
 
Soft rocks are materials whose sampling and 
geotechnical characterization, as well as the 
prediction of their behavior are difficult to conduct. 
Sometimes they require complex construction 
techniques or special treatments, which ensure the 
stability of civil works during construction and 
lifework. Consequently, it is common to include 
these transitional materials in the problematic group 
materials (Rodrigues, 1990). 
According to Galván (1999), it is difficult to 
define the lower and upper limits of soft rocks, 
respectively between soils and hard rocks. While 
soil can be considered a discontinuous solid phase 
system, which consists of discrete particles, rock 
material is defined as a continuous system, where 
particles are closely connected/cemented. These 
connections were caused by melting processes 
generated from pressure and/or temperature 
variations, by molecular linkages or crystalline 
bonds. 
As previously mentioned, the unconfined 
compressive strength was the parameter used to 
provide the classification of materials and their 
limits. However, it is found that the soil/rock 
boundary is not well defined, since even in the 
ISRM (1978) classification, there is an overlap of 
strength classes proposed for soils and soft rocks, 
as shown in Figure 2. This also occurs when 
comparing with other resistance ratings of natural 
materials, as demonstrated by several authors 
(Dobereiner, 1984; Nóbrega, 1985, Hawkins, 
1998; Galván, 1999; Pinho, 2003; Kanji, 2012). 
 
 
 
 
 
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Figure 2. Strength limits for natural materials accordingly to ISRM (1978) classification. 
 
 
3 CLASSIFICATION 
 
The geological, geotechnical and geomechanical 
characterizations are the ways that involved 
professionals establish a common technical 
language, by one or several classification criteria. 
The first of these disciplines emphasizes the 
qualitative classification with prominence on the 
genesis and how they are in nature; the second one 
establish relationships and set quantitative criteria of 
physical and mechanical properties; the third uses 
the second and applies accordingly to their use or 
behavior in engineering works. 
Bieniawski (1989), Galván (1999) and Singh 
and Goel (1999) made a detailed review of 
geotechnical and geomechanical classifications.From these authors, the most relevant for the 
subject soft rock are presented. 
 
3.1 Geological 
 
In the geological point of view, the basic distinction 
made regarding rocks is the formation mode and 
mineral constitution. The few classifications 
involving soft materials have been created by the 
need to establish a geological differentiation 
between soils and rocks. 
Following these distinctions, Deere (1975) 
makes the following divisions of the soft rocks, 
being absent the fractures, “inherent to all rock 
masses": 
(a) plans or thin zones: long continuities with the 
presence of soft rock material; that is incidents 
of major fractures with spacing and large 
extensions associated with some kind of 
weathering, shear zones with foliation or absent, 
lodging plans and fault zones; 
(b) rock material of low strength: chemically 
altered rock (weathering or hydrothermal 
alteration), salt rock, clay shales, very friable 
and porous sandstones, volcanic tuffs, marls and 
basalt or tectonic breccias. 
Vardé (1989) assesses the rocks of low 
strength in three types according to the behavior 
aspect of the rock mass: 
- Type 1: sedimentary rocks, volcanic and 
metamorphic rocks intrinsically soft, weathered 
or hydrothermally altered rocks; 
- Type 2: structural weakness, including fault 
zones, zones of intense fracturing and 
foliated/laminated structures; 
- Type 3: potentially soluble rocks and/or with 
cavities (e.g. karst rocks). 
Dobereiner (1984) includes in a didactic way 
the genesis of soft rocks through a small diagram, 
inter-relating them with soil and rock mechanics. 
(Figure 3). 
 
 
 
Figure 3. Schematic representation of soft rocks evolution 
(mod. from Dobereiner, 1984). 
 
 
 
 
 
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Based on the classification and concepts of 
these and other authors (Rocha, 1975; Nóbrega, 
1985; Campos, 1989; Oliveira, 1993; Galván, 
1999; Pinho, 2003), from the geological 
perspective of evolution, these natural materials can 
be better divided into primary and secondary soft 
rocks, as concluded Giambastiani (2013), in the 
proposal presented in Table 2. 
However, as stated by Pinho (2003), 
classification systems with little quantitative 
parameters have low practical interest for a given 
geotechnical problem. While recognized that one 
granite is tougher than shale, the strength does not 
properly constitute a parameter for geological 
classifications. He concludes that alteration has a 
direct influence in the strength of a given rock; even 
so it does not change the geological rock 
classification. A granite will be always classified as 
a granite, regardless of its state of alteration. 
 
Table 2. Proposed classification of soft rocks in terms of 
origin and geological evolution (mod. from Giambastiani, 
2013). 
 
Division Subdivision 
P
ri
m
a
ry
 
I) Pyroclastic and clastic sedimentary rocks 
of low to moderate compaction and lithification 
(sandstones, siltstones, mudstones, tuffs, 
conglomerates, marls, etc.); 
II) Chemical sedimentary rocks formed by 
primary minerals of Mohs hardness <3.5 
(gypsum, sylvite, halite, carnallite, limestone, 
etc.); 
III) Metamorphic rocks formed by mineral 
hardness <3.5, such as shales, schists, slates, 
comprising chlorite, mica, sericite, graphite, talc, 
among others, related to low metamorphic 
levels. 
S
e
c
o
n
d
a
ry
 
I) Including all other types of rocks that 
have undergone processes of physical and/or 
chemical change due to weathering and/or 
hydrothermal alterations. 
 
3.1 Geotechnical and Geomechanical 
 
Several authors have addressed geotechnical 
and geomechanical classifications: Williamsom 
(1959, in Williamsom and Kuhn, 1988), John 
(1962), Deere and Miller (1966), Obert and 
Duvall(1967), Stapledon (1968), Guidicini et al 
(1972), Bieniawski (1974, in Bieniawski, 1989), 
Rocha (1975), ISRM (1978), IAEG (1981), 
Dobereiner (1984), Barton et al (1990), British 
Standard BS-5930(1999). Among these, the 
following are mentioned. 
The classification of Williamson (Williamson and 
Kuhn, 1988), called Unified Rock Classification 
System (URCS) was developed in 1959 and 1960, 
and has been extensively used in civil projects of 
the U.S. Army Corps of Engineers. The system 
proposes the in situ rock identification, predicting a 
subsequent adjustment of the collected information 
in order to refine the classification according to 
project requirements. State of weathering, 
estimated strength, linear and planar elements and 
specific gravity were the four parameters chosen. 
Among the four parameters, the second one is 
related with the soft rock subject. The strength is 
achieved through the blow of a spherical peak 
hammer, as shown in Table 3. 
 
Table 3. Estimated strength criteria of the Unified Rock 
Classification System (URCS), Williamson (1959) (mod. 
from Williamson and Kuhn, 1988), based on the hammer 
blow effect. 
 
Cat. UCS 
Strike 
Reaction 
Quality 
Distinctive 
characteristic 
A 
103 
(15,000) 
Rebound 
 
B 
103 - 55 
(15,000 - 8,000) 
Pit 
 
C 
55-21 
(8,000 – 3,000) 
Dent 
 
D 
21-7 
(3,000 – 1,000) 
Crater 
 
E 
<7 
(<1,000) 
Moldable 
 
*MPa and (psi). 
 
Stapledon (1968), commissioned to write an 
overall study about field investigations in Australia, 
proposed to include a rock classification based on 
the unconfined compressive strength, as seen in 
Figure 4. This classification considers a soft rock 
when its strength is between 7 to 20 MPa (1,000 
to 3,000 psi), and very soft when lower than 7 
 
 
 
 
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MPa (1,000 psi). The author explains that 
overlapping occurs within very cohesive soils and 
very soft rocks. 
 
 
 
Figure 4. Classification of rock materials based on 
unconfined compressive strength. (mod. from Stapledon, 
1968). 
 
Rocha (1975) defined the upper and lower 
boundaries of low resistance materials taking into 
account not only the compressive strength and 
friction angle as well as the deformability module of 
rock mass and its relation to the module of the 
intact rock. In his opinion, soft rocks comprehend 
strengths between 2 to 20 MPa. It also presents 
some considerations about the influence of 
discontinuities in the rock mass and the relationship 
between the strength properties of intact rock and 
rock mass. 
Dobereiner (1984), based on a deformability 
study of sandstones, provided a range of 
compressive strength for this type of soft rock. The 
author defined the lower limit at 0.5 MPa, based on 
the saturation of sandstones in vacuum, in a limit 
where the structure was not loosened. The upper 
limit, 20 MPa, was based on microscopic 
inspection of sandstone samples tested in triaxial 
compression tests, showing disruption 
accompanied by grain breakages where strength 
exceeded the established limit. 
 
 
4 TECHNOLOGICAL TESTS 
 
Soft rocks are problematic materials, being difficult 
to apply test techniques developed for soils or hard 
rocks. The increasing knowledge about these 
materials and the technological advances made it 
possible to adapt some of the tests to their 
characterization. In general, the used procedures as 
well as the major types of laboratory tests for soft 
rocks are the ones shown in Table 5, described by 
Nóbrega (1985) and by several authors in Akai 
(1997). 
 
Table 5. Main techniques used in the characterization of 
hard rocks and low strength rock masses.Tests 
In
-s
it
u
 t
e
st
s 
Permeability 
Geophysics 
Direct shear 
Plate test 
Dilatometer 
Pressure meter 
P
h
y
si
c
a
l 
a
n
d
 C
h
e
m
ic
a
l 
te
st
s Moisture content 
Porosity/Permeability/specific gravity 
Grain size 
Swelling 
Durability 
Ultrasonic 
Petrographic analysis 
Scanning electron microscopy 
Methylene blue adsorption 
Sohxlet extraction 
X-ray diffraction 
M
e
c
h
a
n
ic
a
l 
te
st
s 
Point load test 
Rebound Hammer 
Slake durability 
Los Angeles Abrasion 
Uniaxial compression test 
Triaxial compression test 
Creeping 
Deformability 
Direct shear 
Consolidation 
Scratch test 
Indentation test 
 
 
 
 
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The difficulties on testing begin from the 
sampling process, collection and specimen 
preparation. In occasions which the rock mass is 
strong, uniform and slightly anisotropic, samples, as 
a rule of thumb, present good quality and high 
recovery rate. In the context of soft rocks, there is 
a great variability in the quality of the rock material, 
such as the presence of discontinuities (lamination, 
stratification, fractures), composition (cement type, 
matrix, framework, and hardness), grain size, 
density, compaction degree, consistency, 
permeability and especially weathering grade. 
These elements make sampling a hard task, 
many times resulting on specimens of low recovery 
ratio and quality, as can be seen in an antagonistic 
situation in Figure 5. Both samples represent 
massive (fine laminated) (A) and laminated (B) 
mudstones from Itararé Group, collected in the 
vicinities of the HST alignment near Campinas city, 
Brazil. In B, the high weathering degree hindered 
the sampling process due to the high expansion of 
clay minerals by natural cycling, causing partial rock 
mass disintegration. This high disintegration effect, 
also hinder sampling, preventing the extraction of 
cylindrical or prismatic standardized samples 
(ASTM D4543) for UCS. Test applied as index 
for rock strength classification. 
 
 
Figure 5. Pelitic samples from Itararé Group, taken from the 
HST alignment in Campinas. Note that the same rock mass 
(fine laminated and laminated mudstones) has adverse 
conditions. In B, the disintegration achieved a level that 
turns sampling impossible. 
 
Faced with this problem, Galván (1999) 
assembled and ordered parameters of soft rocks 
from several studies aiming to establish patterns of 
behavior in response to requests by loading. 
Through simulations of artificially manufactured 
materials, set up great correlations between 
parameters, concluding that by making a database 
and/or using the professional expertise of 
geotechnical materials of the same nature, it can be 
inferred, without many pretensions, rock 
parameters through simple tests: specific gravity, 
porosity, etc. 
The results obtained by Galván (op.cit.) opens 
new perspectives, additionally with the studies 
developed by Pinho (2003) and Campos (1981). 
The first author studied silty-clayey rocks in 
Portugal (shales and greywackes), to contribute to 
their uses in areas of exposure of the Baixo 
Alentejo Flysch. In Brazil, Campos (1981) studied 
the use of the sandstones from the Caiuá Formation 
as construction material, due to the shortage of 
competent rocks (e.g. basalts), in the region of a 
hydroelectric plant construction. Both studies 
included intense geological and geotechnical 
characterizations, resulting conveniently suitable for 
certain purposes, even with less noble utility. 
 
 
5 CONCLUSIONS 
 
The soft rocks study is wide, as they are still great 
uncertainties of this subject mainly about the intense 
variability of each property given the transition zone 
each is found. The most appropriate method to 
understand their behavior involves the establishment 
of correlations between the various physico-
chemical and mechanical properties, possible only 
with the advent of specific and detailed studies. 
Researches of this nature would provide greater 
reliability in applications of soft rocks, both as 
foundation, rockfill or even aggregates. These 
would decrease risks and costs of future Brazilian 
infrastructural projects, located in zones where 
“problematic” rock masses predominate. 
 
 
ACKNOWLEDGEMENTS 
 
The authors thanks the Coordenação de 
 
 
 
 
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Aperfeiçoamento de Pessoal de Nível Superior 
(CAPES) for supporting this research. 
 
 
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