Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
King Saud University
College of Engineering
Civil Engineering Department
Improvement of Geotechnical Materials
(CE 486)
Term Paper:
Cavities in Limestone Formation
Submitted by
Ibrahim R. Al Mohanna (423 100 879)
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
INTRODUCTION
A cave is a natural opening or cavity within the earth, generally extending
from the earth's surface to beyond the zone of light. Three generic classes of caves
can be recognized according to the major sculpturing process: (1) caves formed by
pressure or flow, (2) caves carved by erosion, and (3) caves dissolved by solution.
The caves dissolved by solution include ice caves associated with glaciers
and the familiar limestone caverns or caves. Limestone caves are, by far, the most
common type of caves.
What Is A Cave?
A cave is usually considered to be any naturally formed underground cavity
big enough to enter. The term cave is usually used for horizontal passages and
where vertical passages are found, the term pothole is more common. A cavern is
another term for a cave, but is usually reserved for large chambers. The study of
caves is speleology.
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
Where do caves form?
Nearly all caves form by dissolving cavities in limestone (CaCO3) rocks
which underlie about half of Ireland's surface. Occasionally, some caves form in
marble, quartzite and rarely, other rocks. Sea caves are not formed by chemical
action away but are physically eroded by the sea, and are not usually extensive or
deep.
How do caves form?
Limestone caves are formed by chemical erosion by moving water.
Limestone is a permeable rock, which means that water can pass through it.
Limestone is also a soluble rock, which means that it will dissolve in rain-water
which is a natural dilute solution of carbonic acid. The continual flow of water
through the limestone, and the gradual solution of the stone by the water, causes the
enlargement of cracks. They eventually become caves over thousands of years. In
most limestone areas there are almost no rivers on the surface. Instead, these rivers
are found underground flowing through the caves. The river may enter a cave at a
swallow hole, often where the river first comes in contact with the limestone, after
flowing over a non soluble rock type.
A limestone cave or cavern is a natural cavity that is formed underneath the
Earth’s surface that can range from a few meters to many kilometers in length and
depth.
Most of the world’s caves are formed in porous limestone. Over millions of
years, acidic groundwater or underground rivers dissolve away the limestone,
leaving cavities which grow over time.
Early life forms appeared in the oceans about 3.8-billion years ago. These
were single-celled, blue-green algae, called cyanobacteria, which made their own
food through photosynthesis, releasing oxygen into the atmosphere in the process.
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
Dolomitic limestone, a sedimentary rock, was formed over millions of years
through chemical reactions generated by these early organisms.
With movements within the Earth’s crust, the sedimentary dolomitic
limestone eventually became exposed on dry land.
As time passed the limestone, which is permeable and soluble, was eroded
by water. Weak carbonic acid in rainwater, reacting with the chemicals in the rock,
dissolved and eroded away the limestone as the water filtered into the underlying
depths of sediments. Large hollow solution cavities were formed in the limestone
in this way.
Many cavities occur at various depths in a cave system due to the continual
seepage and flow of the mildly acidic water through the deposits, while
underground rivers may eventually carve their way through a mountainside,
creating openings and entrances to the outside.
What shape are caves?
Caves are found in all shapes and sizes. The shape is controlled by a
number of geological factors and many details of the cave's history can be
understood by studying the shape of the passage.
Low flat passage formed on horizontal cracks or 'bedding planes' between
the layers of limestone.
Tall narrow passage formed on vertical cracks or 'joints' in the limestone or
as a canyon passage cut by a single stream.
Circular passage formed when the water completely fills the passage like a
pipe so the water is able to dissolve the rock equally on all sides. These are called
phreatic passages.
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
Keyhole passage formed when a tall, narrow passage develops below
phreatic or bedding plane passage. Also known as T-shaped passage.
The location of swallow holes will also influence the type of cave. If the
swallow hole is high up on a hillside the water is able to cut quickly down into the
limestone and may form a single passage, several kilometers long, which may
contain vertical drops or 'pitches' where ropes and ladders are necessary. These are
often called 'pots' or 'potholes'. A swallow hole at a low altitude means the water
will flow slowly and will often produce a maze of passage within a small area.
How old are caves?
Caves vary in age from region to region and it is often difficult to put an
exact age on any cave. Caves are a part of the landscape and as such are slowly
developing and evolving. Rivers continually cut downwards through the rock, so
often the higher parts of the cave are older. Dry or 'Fossil' passages are sometimes
found, originally formed by the river, but now inactive. They often contain
preserved mud and sediments. These may contain important information about the
environment outside the cave, when the sediment was deposited, many years ago.
Sediments can sometimes be dated to find a minimum age for the cave. Fossil
passages often contain beautiful displays of 'stalactites and stalagmites.' These
'formations' can be dated by measuring the decay of tiny amounts of radioactive
Uranium in them. Some Irish examples were found to be 70,000 years old. As a
general rule, the larger a cave is, the older it is. Over long periods of time the rock
will fall from the cave walls making it larger and forming caverns or chambers that
may reach 20m high and 50m in diameter.
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
Cavities in Saudi Arabia:
Caves are airfilled underground voids developed by the former action of
water on rock that over a long period of time was dissolved, opening up holes and
tunnels in the ground. The holes and tunnels in cave systems are normally
interconnected, depending on how the water seeped through the rock along joints
and cracks, working its way down to the water table below the surface of the
ground.
The caves illustrated here are in limestone - the most common type of rock
to have caves - which in this part of Saudi Arabia consists of calcium carbonate and
small amounts of magnesium carbonate.
The rocks found 50 million years ago from the calcareous shells and
skeletons of countless organisms that flourished in shallow warm seas that covered
the Arabian Peninsula. Over time, the shelly deposits were cemented by additional
calcium carbonate, became hard and turned into limestone, forming a geologic unit
referred to as the Umm er Radhuma and Rus Formations. Starting 25 million years
ago, these formations were raised above sea level by earth movements affecting the
whole of the Middle East, and were exposed to wind and rain at the surface.
The action of water percolating down through soluble rock is critical for the
formation of most caves. Some caves develop because sulphuric acid rises from
deep below the surface, but this is rare and it is not known if any of the caves in the
Ma'aqala area formed in this fashion. Most likely they all formed by water action either by falling rain or by streams that sink into the ground through joints and
holes.
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
The process of forming caves in soluble rock is very slow. As rain falls
through the air, it absorbs a small amount of carbon dioxide and picks up additional
carbon dioxide from the soil. The result is a weak solution of carbonic acid that
seeps downward, dissolving the limestone bedrock and opening up cavities and
inter-connected channels. If streams flow on the surface - and in the recent geologic
past, Saudi Arabia was much wetter and probably had permanent drainage additional weakly acid water would enter the limestone where streams sink down
holes and cavities. The underground water will move along the bedding and down
the joints, forming long tunnels by combined solution and normal erosion.
Sometimes the water moves very slowly and dissolves the rock instead of wearing
it away, creating an intricate labyrinth of passageways. In other places, joints
become enlarged, resulting in vertical shafts and chimneys.
Some of the well known caves in Saudi Arabia:
UPM Cave: It has passages on three different levels and is impressive
because of a huge hall (45 by 80 meters and 17 meters high) which once housed an
underground lake.
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
Dharb al Najem: Located in the desert east of Majma'ah this cave consists
of a single chamber roughly measuring 100 meters in depth by 100 meters in
diameter.
In the Dhahran region, large-sized solution cavities were discovered during
construction at three sites: KFUPM, ARAMCO, and RSAF. In 1981, during
excavation activities at the CIM and adjacent building sites at KFUI'M, large
solution cavities were found within the limestone. The cavities were formed
through solution of limestone by groundwater charged with atmospheric carbon
dioxide, Jado and Johnson (1983). They appear to be controlled by fractures
formed when the limestone slumped after dissolution of underlying anhydrite and
gypsum.
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
Problems of cavities with respect to engineering aspects:
In general, cavities in limestone cause problems during construction
because they are not easily found in advance. Excavation may break into large
cavities, which have to be filled, adding to the cost of the job. There may be
cavities just below the foundations, undetected until failure occurs later.
There is an increasing awareness among developers and owners, of the need
to conduct subsurface exploration studies before planning a project. The use of
geophysical methods in subsurface exploration has been established to be valuable
in locating sinkhole activity and suggesting pro-active measures. However, the
value of geophysical exploration to supplement geotechnical investigation has not
yet been stressed, primarily out of cost concerns. Ground penetrating radar and
electrical resistivity tests, which were used to determine void spaces below
proposed buildings are gaining wider acceptance. As a result of information
obtained from geophysical tests, foundation designs were modified, to use posttensioned mats or to include grouting.
During the siting, construction, and maintenance of facilities in the karst
terrain, the stability of the residual soils that overlie the cavitose limestone is often
a concern. The development of arching in the residual soils and the associated
distribution of stress is important to the stability. The economics of the landfill
construction are usually improved by excavating a portion of the residual soil to
increase the capacity of the landfill. However, it is generally accepted that for a
given diameter of potential limestone cavity, the residual clay soil becomes less
stable as the thickness of the soil cover decreases. Geotechnical engineers are often
called upon to evaluate the stability of the residual soils as a part of the permitting
process or prior to the construction of critical structures such as municipal landfills;
yet, simple, rational methods are not available. Simplified methods are needed to
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
estimate the thickness of the soil required to provide stability against soil collapse
and the subsequent formation of sinkholes or distress to the surface structures.
Upper Surface of Limestone Rock:
Generally foundations are terminated on top of bedrock or at shallow depths
immediately below. Since the surface of limestone rock is located at relatively
shallow depth and its topography is highly variable, any tunnel, shaft, pile
foundation or deep excavation is likely to encounter the upper layers of the rock.
Therefore knowledge and understanding of the characteristics of the upper part of
the limestone rock is essential. With this in mind an area in and around zone 1 was
studied in detail making use of information available from 37 boreholes over a
distance of approximately 1km.
There are several cavities situated very near the upper surface of the
limestone rock (typically <3m depth). This leads to the inference that the upper
surface could be in the form of a limestone pavement.
Since the top of the limestone rock has withstood mechanical and chemical
weathering processes, there is a possibility that the rock at this level may possess
relatively high strength. However, out of a total of 6 boreholes considered, only one
core from the first 1m depth was found suitable for testing. This recorded an
unconfined compressive strength of 20MPa which is very low when compared with
the average strength of 80MPa estimated for limestone rocks in Singapore. One of
the boreholes met an isolated core of limestone below which stiff silty clay was
found for considerable depth. This block of limestone is believed to be a floater.
The averages of total core recovery (TCR), solid core recovery (SCR) and RQD are
77%, 39% and 32%, respectively, with a standard deviation of 34%. This confirms
that the upper layer of limestone rock is not very strong and is closely jointed or
fractured.
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
Locating the Upper Surface of Limestone Rock
The upper level of the limestone rock is highly variable even over short
distances. Borehole investigations give most accurate results to locate the upper
surface. However these are expensive and the results cannot be used to interpolate
with sufficient confidence regardless of the proximity of the holes. Geophysical
methods such as electrical resistivity and seismic surveys give a reasonably
accurate indication of the top level of bedrock but were found to be not successful
in detecting local features such as cavities or sink holes. A comparison of rock head
levels obtained by geophysical surveys and boreholes show variations in the range
of 0 to 5 m. The results from the geophysical methods predicted generally lower
levels of limestone rock head. There was no significant difference noted in the
bedrock levels indicated by resistivity survey and seismic survey.
Cone Penetration Testing (CPT) is an inexpensive as well as expedient
method of sampling the top of limestone rock at closely spaced intervals. This
method is particularly suitable for site investigations underwater. It is effective in
most of the soil types and has proved to be practicable and reasonably accurate to
depths sampled 15m below sea bed level. Information obtained from boreholes
near the CPT sites show that hard residual soils (sand and stiff clays) and limestone
fragments could over estimate the top of limestone by up to 4m.
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
1st Sem. 1429-1430
CE 486
Solving the cavity problem:
If the project could be relocated that would be better, but usually the cavity
will be filled or grouted using sand or cement depending on the project.
Here is a cavity found under Southern Circular Street "Daeri Aljonobi" and
how it was treated by figures.
Student Name
Student ID
Ibrahim R. Al Mohanna
423 100 879
Term Paper
Improvement of Geotechnical Materials
Student Name
Student ID
1st Sem. 1429-1430
Ibrahim R. Al Mohanna
423 100 879
CE 486
Term Paper
Improvement of Geotechnical Materials
Student Name
Student ID
1st Sem. 1429-1430
Ibrahim R. Al Mohanna
423 100 879
CE 486
Term Paper