The effect of water content on the strength of rock

| December 23, 2012

Introduction

In order to design all type of buildings, it is necessary to understand the ground condition i.e. soils and rocks which the building foundations are based on. The occurrence of buildings failure, therefore, can lead directly to the problem of underground conditions i.e. bedrock which support the whole building to standing. In addition, rock is one mainly of an aggregate in the cement production to build the building that means the strength of rock can affect directly to the building. The need to understand rocks type and their properties are significantly for the designers to design buildings and this enables designers to effectively design.

Rocks are comprised of many aggregate of mineral particles which is occurring in natural and bonded together with granular structure and texture. Rock in the geology can separate by the classification of grain size, genetic structure, hardness and quartz. but there are still have their individual characteristic and properties. The geologist separate rock in 3 types: igneous rocks, sedimentary rocks and metamorphic rocks.

Pore spaces or voids in rock are the most important that will effect of rock strength and rock properties, namely, the mechanical properties of rock derived from the physical characteristic of rock. Type and structure of rock are related to the amount of porosity and the method of formation of rock can relate to the degree of porosity. For instance, igneous rocks which magma is slowly cooling have no porosity or the least porosity of 3 types of rock, on the other hand, the rock with rapidly cooling then it will have more porosity than those slowly cooling lava but it not in the case of sedimentary rocks which the porosity depending on the sizing, grading and precipitation of materials. If the rock have many porosity or voids, the strength lead to reduction result of the loss of internal cohesion i.e. molecular and mechanical and that can lead water seep through inside, absorb and keep in the pore spaces these are the most effective for the strength of rock.

Many research are trying to understand the water content affect on rock strength but the results seem to be different on each other which depending on the type of rock and its source that means if the specimen come from difference areas then the results to show the strength of rocks will be different too, even if, the same type of rock.

In this research will comparing the strength between an unlikely of the two sedimentary rocks: limestone and chalk rock which both consist of the mineral calcite which is calcium carbonate or CaCO3 is most commonly marine organisms and used for Portland cements, mortars and concretes. Limestone is usually an organic sedimentary rock and made up mostly of calcium carbonate but some limestone have a high proportion of magnesium carbonate, whereas, chalk rock is a type of soft limestone, porous and made up almost 99.9% of pure calcium carbonate. The quantity of calcium carbonate should effect on the strength of rock that the result will show by using an unconfined compressive strength test in saturated conditions. In general, sedimentary rocks are weak in compressive strength which relates to amount of the pores space in rock, thereby, the sample of both specific rocks should be naturally lower in compressive strength and sensitively to the degree of moisture content. Namely, when the water content increases then the compressive strength is decreased.

Statement of the Problem

The research would like to know the effect of the water content on the two specific difference rock types: limestone and chalk rock. The following are the main questions that this study would like to answer by using the existing literature and the experimentation:

1. What are the differences of water content effect on the two specific types of rock?

2. How the water content can effect on the strength of the two different rock types?

3. Does the calcium carbonate which comprises of the both rocks have an effect of the

water content on the strength of rock?

4. If the calcium carbonate can have an effect on the strength of rock, how it can effect on

the strength of the two different rock types?

The scope of the study will be centered on concepts and topics related to the questions above

Objectives

This research focuses on the effect of water content of the differences of the two rock types. The study will provide an analysis and a comparison of the strength of the two difference types of rock. Thus, the following are the objectives of the study:

1. Provide an in-depth and comprehensive understanding of the strength of the differences

rock between the limestone and the chalk stone.

2. Compare the strength of the two specific rocks: limestone and chalk stone.

3. Integrating the knowledge which gained from the literature and relating to the result of

the experimentation and thoughts of researcher will give in conclusion.

These objectives will the guiding element of the study and the researcher hopes to stick to it.

Scope and Limitation

The research will be tackling the effect of the water content on the two differences rock types. Then, the comparisons of the strength will be made between the two specific rock types mentioned earlier which is comparing between dry states and saturation states. The data will be gathered from the existing literature and the experimentation and therefore, the comparisons that will be made will be based on it also. In this research project is concentrated on the strength of intact rock in saturated condition to understand the relationship between the water content and the rock strength. The study is limited to the two specific types of rock that were mentioned and the form of data will depend on the existing literature and the experimentation.

Aims

The purpose of this research project is to determine the effect of water content on the strength of intact rock which is usually in compressive strength for intact of limestone and chalk rock which both intact rocks are formed of calcite minerals and consist of many pores insides that should lead directly effect on the strength on rocks. To understanding the strength, properties and characteristic of the two specific intact rocks. To the end of this research project will understand how the strength will be performance after the both intact rocks contain water within pore spaces in saturated conditions compare to the dry conditions.

Literature Review

Properties of Limestone

Limestone is one of the sedimentary rock groups and called as calcite mineral or CaCO3 in geology. It is a carbonate rock that made by the deposition of carbonate sediments in the sea with inorganic chemicals processes that do not involve all of life such as the aggregation of certain types of carbonate mud and the fossils which deposit in the sea e.g. carapace of sea creatures, coral animals, algae, shellfish and aquatic plant. They are deposited within the pressure and then to be crystallizing of calcite mineral which can be reaction with an acid substance. Limestone has a white, grey, soft-pink or black colour with a solid texture details and its perhaps having a fossils in the rock e.g. shellfish or coral animals. The sources of limestone are usually as the mountain with a raised sharp cliffs and solubility. Limestone are the rock which having around a 50% of the calcite mineral. The limestone elements consist of calcium carbonate which most of the calcite and can be easily test by drops of dilute hydrochloric acid, using more kinds then the effervescent bubbles of carbon dioxide out. The areas where the accumulations of plaque are a great shallow sea water in tropical.

The limestone used for industry of roads, railways, mortar, cement, calcium refractory, to make fertilizer and paint et cetera. The limestones are normally porous rock, have high permeability and have holes in the rock which those mean limestone characteristic are usually suitable for water storage. The pores will absorb water better but it can evaporate quickly too. Limestone is lighter because it has a high porosity. The limestone will be built to CaO, the phase mainly in cement by the grain cement is the main phase in 4 phases (often referred to as a potential phase), which phase they will react with water (hydration) to cement features we need such as strength, setting time, spitting out the heat during a reaction and resistance to chemicals. Since the production of cement, limestone is used so many important things to remember are that the consistency of a component or a little of the variance components that can make adjustments to control the quality of ingredients with ease.

Many research for limestone strength by using the unconfined compressive test report that the strength at dry condition was higher than at the saturated condition as the result of higher porosity in dry condition than those in saturated condition, however, the saturated condition should provide in the full saturated but they did not mentioned that how long it should take for making the saturated condition.

Limestone

Figure 1: Limestone

 

Properties of Chalk Stone

Chalk rock is a one kind of limestone which is a pure soft limestone. The rock texture is like stone with fine soil which can break and corrode easily as the result of the calcium carbonate crystals with non-bonding or loose since the arrangement of molecular of calcium carbonate and born from the sea as well as limestone. Sometimes, chalk rocks are related as clay but its different by withstand to weathering and less slumping which compare to clay. The chalk rock colours are usually white, soft-grey or soft-yellow and consist of 90-99% of calcite mineral and mixed with a little of silica which naturally occurring by combination of limestone from fossils, dead plants and shells in the sea. The chalk rock was made from the deposit of different bacteria and with calcareous algae and mixed with the sediment of calcite in the shallow water. Often crumbly texture, porosity chalk rock and often contain with fossils of living by the sea. The process of chalk rock is the biochemical or the inorganic chemistry. Chalk rocks have a small pore bulk everywhere that makes porosity of chalk rock up to 50% which called as high porosity rocks, high permeability and low density, hence, it absorb plenty of water within the rock better. Because of it consists of the mainly calcite mineral, it therefore react with acid. Chalk rock stores plenty of water by many pores of the stone. Chalk rocks with varying strength depend on mineral type interface but often have low retention in saturated conditions.

The useful of chalk rocks are variety used in many manufactured but commonly used for a manufactured substance such as filler, production of ceramics, cosmetics, plastics, rubber, paints, crayons, plasters, variety of materials for pigment, for engineering used in the process of making mortar, mixed in cement and making a quicklime or lime mortar rather than pure chalk.

Many researches of the strength of chalk rock by using the unconfined compressive strength test mentioned that it is generally low compressive strength in dry conditions as it has many pores insides and outsides since its strength was decreased in saturated conditions. As they indicated that chalk rock is a type of very weak rocks as the result of a weak bonding strength and very porosity that water can get in to the rock and stay insides of the rock. Nevertheless, its strength will be various by depending on the source and the weathering that processes the chalk rocks.

Chalk stone

Figure 2: Chalk stone

Porosity and Permeability

Porosity is all of the space volume which is not replaced by solids and consists of liquid inside as indicated as the characteristic of the rocks by the ratio of the volume of pore space within the rock mass to the volume of the rock mass by expressed in percentage of pore space in the rock. The degree of porosity is more or less depending on the selected size, shape, matrix, classification of rock type, rock texture, arrangement of sediment, the interface of various sediments together and fractures that occur in the intact rock. High porosity means more than 15 of the percentage of porosity, on the other side, the percentage of porosity of lower than 5 is called as low porosity while the percentage of porosity between 5 and 15 is called as medium porosity. The value of porosity that measurement on rock outside is the same porosity value as inside.

In general, pore spaces within sedimentary rock contains of the volume of liquid which is water, sometimes gases, air or oils by depending on the porous of intact rock which called the porosity. The pore space in intact rock is interconnected. The amount of water is usually proportional to porosity of the rocks. Pore spaces can be seen by eyes sometimes but it looks uncertain made by crack, break, water erosion, transmutation or fossils inside before. In the dry conditions, the pore spaces are contained with gases and liquids are absented, on the contrary, gases are absented on the pore space in saturated conditions. The pore space is naturally consisting of the groundwater.

High porous rock can store more water inside as the result of the sediments with a good grading in similar size sediment or with a rounded shapes since when sediments fall to deposits will provide more space between the sediment, on the other hand, sediments with the classification of the size of the sediment is not well with both small and large mixed together or with a square shapes so when sediments fall to deposits will lead to less space between the sediment which low porous rocks are made and less storage of water insides. Moreover, if the sediment interface with object coordination occurs, porosity of the intact rock will go down but if the intact rock is eroded or crack in this way it will have a higher porosity.

Permeability is stated as a condition to allow absorbing the liquid permeability of the intact rock under differences pressure. The degree of permeability is generally depending on porosity, matrix, size of rock, characteristic of rock, texture of rock, the bonding inside of rock and continuity between the pore spaces such as low porous rock with a large pore spaces and less attractive force between inside of intact rock and water then the movement of water possible through without any trouble. Even if the intact rock is a high porosity but if the pore space of intact rock is small so the water movement possible through very difficult because the attractive force between the inside of intact rock and water are high.

Pore spaces within sedimentary rocks occurred by the washing away of matrix that is when groundwater flow through the crack of the rock or the horizontal layers of rock, thereby, the matrix were washed away so that the void of rock will larger by times and store the water inside of pore spaces. Sedimentary rocks with mainly a carbonate minerals are generally having a crystalline texture and a high percentage of porosity due to the both specimen intact rocks which will use in this research project are stated in sedimentary type which mean there both have a high percentage of porosity too i.e. the both intact rocks have higher the percentage of 15 with absorb and store water inside voids of the intact rock better. Porosity of both limestone and chalk rock are usually around 30% and up to 50% respectively.

Since the both are high porosity then their permeability is high as well because of the pore insides and the cohesion of the mineral crystal is loose which water can through and store in the intact rock. As can be seen that porosity depend on the texture of rock, matrix, structure of rock and the process of rock made. In additional, the depth can decrease the value of porosity as the deep-depth are lower porosity than the shallow-depth as a result of the pore space is closed by the pressure on the deep-depth.

Pore spaces of rock

Figure 3: Pore spaces of rock

Permeability

Figure 4: Permeability

Strength and Failure in Intact Rocks

The strength of rock is ability of rock to resist to maximum stress which the structure of rock is not deformed permanently. In naturally, the intact rock strength depends on the structure of crystalline formations in intact rock which the component of minerals are bonded together.

Types of rock strength are compressive strength, tensile strength and shear strength. Compressive strength is commonly representing the strength of rock in order to resist to compressive stress while tensile strength is rarely measured and shear strength is usually used in unconfined condition which it is ability to resist directly to shear stress. If intact rock cannot resist the maximum stresses then it will permanent deformation by this will performed the change of the geological structure that is fault, joint, shear fracture, shear zone, fold, cleavage, foliation and lineation by depending on types of rocks, temperature, water pressure, confining pressure and strain rate. In the other way, if the intact rocks can be stored after the stresses were gone, it called as elastic deformation and cannot change the geological structure and therefore, we cannot analyze the intact rock strain. If the acting force exceeds the capacity of the intact rock on the fragile environmental that fracturing will occurs in intact rock and it lost the intact rock strength. Namely, the fractures are the most effective on the strength of rocks.

 

Shear failure

Figure 5: Shear failure

Effect of Water on Rock Strength

Nowadays, the effect of water content on the 3 types of strength of rock i.e. compressive strength, tensile strength and shear strength has no certainly theory explanation exactly. However, those theories are used for help us to understand the direction how the water content affect to the intact rock strength which is normally occur in saturated conditions.

Many researches before have noted that when the moisture content in intact rocks that it cause to alter the properties of intact rocks and their behavior. Water content is the most influence factor that effect on the strength of intact rocks. In general, the mechanical properties of intact rocks decrease when the moisture content increases. The strength of intact rock will decrease after only 1% of saturated of water. The sensitively of water content on intact rocks depend on the effective porosity that is the intact rocks with high porosity contain more water in pore spaces than the intact rocks with low porosity. However, the strength is not depending on only porosity of intact rock but also various factors such as physical characteristic, saturated condition and nature of formation of intact rock et cetera.

The strength of intact rocks is commonly changed in saturated conditions that are when the water contain in the pore spaces of intact rocks and stored inside. Normally, the intact rocks strength in the dry conditions is higher than in saturated conditions as it relates to the water content in the voids. Namely, the strength of intact rock is reduced larger in the saturated conditions usually compressive strength which is the mainly strength of intact rocks. Even if the strength in saturated states is less than in dry states but there are no relationship between them. However, it is not only the compressive strength is changed in the saturated conditions but also the effective normal stress where as the shear stress is unchanged.

The pore spaces seem to be significant factor to make the strength changing in saturated conditions by contain water inside by doing this the level of pore-water pressure is raised up too. Particularly, weakened rocks which are rocks with a low pore-water pressure will absorb more water because of high porosity, permeability and saturated that the strength of rock is weaker than those rocks with a high pore-water pressure which is low porosity, permeability and saturated so it will absorb little water. In general, the saturation is the most effective to the amount of pore-water pressure, nevertheless, it still depends on other necessary factors such as porosity, permeability and pressure of acting forces i.e. pressure in deep-depth, lateral pressure or earth pressure.

Pore-water pressure usually reduces the effective normal stress and the capacity of shear resistance where as the saturation decreases the cohesion of grain and internal friction of intact rock, in additional, water is effective to greatly reduce the strength of intact rock with high porosity which is commonly a sedimentary rock.

When moisture content comprise in intact rock and its increasing will cause not only the pore-pressure is raised up but also the percentage of the porosity of intact rock is raised up higher as well as the value of the permeability, on the other hand, the intact rock lost the cohesion between matrix or grains and cementation, loose the intact rock structures then intact rock is deformed, the friction is reduced, the destruction of chemical of intact rock, the intact rock is eroded and as well as the reduction of the overall strength capacity and the shear resistance.

Hence, the overall strength is significant decreased when the intact rock with moisture content. Furthermore, the mechanical properties of intact rocks are generally altering as well as the characteristic of intact rocks by this the intact rock start to crack that reach the rocks failure later on.

Unconfined Compressive Strength Test

A common strength of intact rock is taken from grains cohesion which is formed by chemical bonds and grains friction which is formed by particle interlocking and surface friction. An unconfined compressive strength used for determine the approximation value of cohesion of grains which this test is used to be widespread to find out the intact rock strength which is the shear resistance without the lateral pressure i.e. in unconfined states.

The unconfined compressive strength test has another name called as the uniaxial compressive strength test. In general, this testing is done under the standard of ASTM D2938-86 or ISRM (1972) specification for unconfined compressive strength test. The testing is generally the most commonness and easiest to define the strength of intact rock because of the result gives in conservative values. The testing determine the rock strength which is the maximum stress that crushing the intact rock i.e. an unconfined intact rock fails in shear which is the primary failure of intact rock and the failure of intact rock subject to stress in one direction.

Unconfined compressive strength equipment

Figure 6: Unconfined compressive strength equipment

The testing is commonly in two methods which is stress control and strain control but the general method which used in widely is strain control. The strain control method is easier than the stress control because of the unconfined compressive strength test is usually control the strain around the rate of 0.5-2.0 percent per minute. For instance, if the specimen length is 50 millimeter and the strain is controlled at 1% which means the specimen in the compressive strength test is subsided at the rate of 0.5 millimeter per minute and repeatedly until at the desired point or the points of failure in shear.

Failure of specimen

Figure 7: Failure of specimen

 

The unconfined compressive strength should be done quickly approximately 10 minutes in one time because the specimen can alter the moisture content inside, hence, the result of the compressive strength test will be higher than it could be.

Unconfined compressive strength test

Figure 8: Unconfined compressive strength test

The specimen is commonly chosen as circular cylinder shape with smoothness rather than rectangular prism shape because prevent the edge effects on the specimen. However, the circular cylinder shape is still having an end effects which it is not possible to avoid in the unconfined testing.

To preparing the specimen, the standard of the diameter (D) of the circular cylinder shape is not less than 54 millimeter or NX core size and the length (L) are depended on the length/diameter (L/D) ratio of 2-2.5. After getting the specimen then make the specimen with smoothness, flat blade to insult and free from irregularly abruption. The end of specimen must be lapped.

Nevertheless, the size of specimen should be equal all the bar lengths with the difference is less than 0.127 millimeter to prevent the irregularities, the vertical axis tilt up not exceed 0.25o by the both ends of a flat parallel difference does not exceed 0.025 millimeter to prevent the both ends lapping. To storing the specimen should be within 30 days to preserve the natural moisture content.

Arrangements test and stress distribution

Before starting to compression the specimen, the both ends of the specimen lay on the base curve range to force the press to spread across the specimen in the vertical throughout the test. After that the machine will force the press to specimen by compressive strength with the rate of stress should be within the limit of 0.5-1.0 MPa/s until the failure point which is generally take place about 5-15 minutes and the specimen will fracture in shear directions or sometimes, cracking in the vertical axis together.

If the ratio of length/diameter (L/D) is less than 2, the results need to be adjusting the values before calculating the compressive strength. The testing must be doing at least 10 of specimen and the time of saturation depends on the diameter of the core rock.

Methodology

This research project method is relying on the experiment of specimen of the intact rocks which are limestone and chalk rock by using the unconfined compressive strength test in both of dry conditions and saturated conditions to determine the maximum load at failure point which is the result from the measuring instrument on the compressive strength machine after that calculate the effective compressive strength which is derived from the maximum load at failure point divides by the cross-sectional area of the specimen. The amount of specimen will use in this research project on the total of 40 rock cores from 20 of intact limestone and 20 of intact chalk rocks which come from a part of England. The saturated conditions in this research project need to be done under the vacuum at least 24 hours. The value of the effective compressive strength indicates the capacity of the shear resistance at the point of failure. The researcher then tried to compare the compressive strength of both intact rocks which mentioned before and compare the compress strength on intact rock itself between the dry conditions and the saturated conditions.

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