Simplified Method for Determining Ground Water Level in Soil Exploration

Bearing capacity of soil and settlement of foundation depends significantly on the location of ground water level (GWL). We will provide three cases of location of ground water table that influence foundation bearing capacity in our upcoming Post. During exploration of soil, we can determine level of ground water conveniently but may have some error.

In every exploration program, we can simultaneously determine the water level but in some especial cases we have to make exploratory boring only for determining water level. The purposes are like determination of artesian ground water.

Generally GWL can be determined by allowing water to reach equilibrium level in to the borehole. This will produce correct and simplified general procedure for determining GWL. Now we have to known how much time is required to reach this equilibrium.

Sandy soil:

We know that permeability of this soil is very high and equilibrium level is achieved very early. This is at most few hours after boring is done.

Clay soil:

The permeability of this fine grained soil is very low and it takes time to reach equilibrium. In most cases, it may take few weeks to be stabilized.

Depth of general water level is recorded by using a tape or any other measuring means like chain lowering into borehole.

Sometimes piezometer is used to measure depth of GWL. This method is used when accurate measurement is essential in geotechnical exploration. A piezometer used for this purposes, consists essentially of plastic standpipe which is attached to perforated pipe or any type of porous tip.

This tube is kept in position with packing of

-gravel

-coarse sand

It is usually sealed with impermeable clay (like bentonite)

What is Sample Disturbance in Soil Sampling?

During collection of soil samples from either shallow or deep location, there may have disturbance from small to large degree. When boring is of wash boring, a suspension of soil is collected; obviously this is a very disturbed sample. This method represents a soil that has almost lost all ambient properties (completely disturbed structure, altered natural moisture content etc); only provides information about changes in the layers.

With this complete disturbance, some methods provides less disturbed sample and there have some special method that provides least disturbed sample. There have some special method that provides nearly undisturbed sample. We have discussed about block sampling in our preceding post.

A soil sample may be disturbed in three consecutive stages either in any one stage or may be in combination of stages. These are:

a. Disturbance before sampling

b. Disturbance during sampling

c. Disturbance after sampling

Disturbance before sampling

During sampling generally we remove or cut out soils over the sample at desired depth. This method reduces vertical stress over the sample within a short duration reducing overburden pressure quickly. In some cases, soil layer beneath the sample may be deflected up for the same consequence of stress relief. In some sampling operation continuous collection of samples is applied depending on extent and requirement of soil investigation. In continuous sampling process, additional disturbance is observed due to shearing of previous sample by applying torsion to separate it from parent soil.

Disturbance During Sampling

During sampling the soil sample entered into the sampler by forcing them to take their position in sampler. We know there have some friction between sampler and soil and other mechanism that disturb the sample.

We can summarize this mechanism as below:

a. Disturbance by displacing soil by wall of sampler

b. Friction between sampler and sample

c. Adhesion between sampler and sample

Another factor that influences the disturbance is inside clearance between cutting shoe and sample.

Disturbance After Sampling

When a soil sample is collected carefully, least disturbing it which is practical during sampling and before sampling, may be disturbed by next steps of sampling process like

-transportation

-sealing

During transporting, in many cases, sample becomes disturbed. Sometimes in case of improper sealing of tube, moisture lose may be taken place.

In designing a soil sampler, the main criteria are that they don’t produce much disturbance. In determining disturbance of soil during sampling some parameters like ci, co, Ar, RQD and recovery ratio etc are used and specific limit of these parameters for different soil type are provided that will be discussed in upcoming posts.

What is Sampling of Soil?

Proper design of foundation of civil engineering structure, need sufficient knowledge about subsurface condition of proposed site upon which foundation is to be supported. Arrangement and orientation of underlying soil layers and physical properties of each layer, extent of each layer are the necessary information in deriving both bearing and settlement behavior of foundation soil. To obtain essential information like these, through laboratory and field testing, subsurface investigation is conducted. In this process soil samples are collected from shallow and deep strata. The method by which soil sample is collected is called sampling. Different types of sampler are used in this purpose depending on sample type and difficulties in collecting this.

The main methods of soil sampling are of two:

a. Block sampling

b. Borehole sampling

Block sampling:

In this technique, block samples are collected by hand cut. This is considered as the best techniques to collect samples in terms of sample quality i.e. this produce less disturbance to sample. In soil sampling, we all know the importance of undisturbed sample though it is a theoretical term.

In this technique surrounding soil of sample is excavated to collect sample and the sample is collected in a box. Being best quality sample, in many situations, however, it is not the best technique considering practicability and the most dominating factor economy.

Borehole sampling

This is widely adopted technique for collecting sample, especially form a considerable depth. A sampler is fitted to drill rod of boring apparatus is lowered through a dug hole in ground by removing soil using any of usual boring techniques.

Upon reaching to desired depth, the sampler is inserted in to soil either by

-pushing

-driving

After reaching sufficient soil sample into sampler, it is sheared at its base by rotating.

Variability of Shear Strength of Foundation Soil

Shear strength

Soil is a particulate or granular material. When a soil mass beneath foundation is subjected to excessive deformation, these particles allow rolling, sliding or rearrangement to some extent. In simple word shearing is happened. This capability of sliding to support a structure, its own weight (overburden pressure) and sustaining a slope is known as shear strength. This definition is for understanding the term shear strength easily; in engineering expression, shear strength is ultimate or maximum shear stress which a soil mass can sustain and this strength is always related to surface or plane of rupture.

In this blog our attempt is to simplify all civil engineering terms to understand easily. The shear strength that we are discussing can be measured both in laboratory or filed. In determining shear strength several factor have to be considered and strength value for different factor will be definitely different. We will discuss each test method according to respective standard.

 Now come to the point ‘variability’. The variability in shear strength due to flowing facts:

a. Soil is naturally a heterogeneous material and it is very rare to deal with a foundation that is supported on homogeneous soil.

b. Pore water pressure in relation to drainage facility

c. The nature and type of construction

d. Stress history greatly alter shear strength and its parameters c and φ especially in clay

e. Disturbance of soil structure

f. Various chemical and environmental action

What is Consolidation of Soil? Realizing 1D, 2D and 3D Consolidation below Foundation

Consolidation term is generally used in case of cohesive soil. The settlement that occur due to consolidation beneath foundation may continue several years depending on loading intensity, permeability of respective soil and drainage path available for this process.

Definition of consolidation:

when a soil layer having low permeability having certain degree of saturation is loaded to increase stress in soil mass to generate excess pore water pressure within clay layer, the soil being less permeable takes time to dissipate this pressure. Obviously this dissipation will be associated with certain volume change which leads to certain degree of settlement of soil mass. This phenomenon is termed as consolidation and respective settlement is consolidation settlement.

We will discuss about different types of settlement in our upcoming posts.

Depending on direction, nature of drainage and deformation, consolidation can be classified as:

a. One dimensional consolidation

b. Two dimensional consolidation

c. Three dimensional consolidation

a. One dimensional consolidation:

In this condition, only vertical drainage and subsequent strain occurred. This consolidation is generally happened below a load of wide extent i.e. there is no way but to drainage in vertical direction as drainage in other two horizontal directions is impossible due to

-long travel distance

-impermeability of clay deposit

Consolidation phenomenon can be happened in other than wide loading area where thickness of layer is small relative to its width.

General examples are:

• Under large fill

• Beneath center of circular or square footing

• Alteration of ground water table i.e. lowering

• Beneath center of mat/raft foundation

Two dimensional consolidations:

Terzaghi provided solution for one dimensional consolidation which is extended next for two dimensional and three dimensional consolidations. Normally two dimensional consideration taken where drainage can be happened in one of the two horizontal direction with as usual in vertical. In natural condition, without providing any external consolidation drainage path, this type of consolidation is observed when strip load i.e. any narrow loading beneath which with vertical drainage, in short direction of loading some horizontal drainage is occurred.

This drainage generally happen when drainage and strain are both in vertical and any of horizontal direction. The strain may be vertical but drainage occur in both directions.

Examples are:

• Strip footing

• Embankment

• dam

Three dimensional consolidation

In this condition of consolidation drainage and consolidation are happened in three directions i.e. vertical and two horizontal directions. The strain may be in vertical but resulting drainage in three directions.

When vertical sand drains or PVD drains are used, radial drainage can be obtained. Simultaneously there will also vertical drainage. Radial drainage associated with vertical drainage reduces consolidation time abruptly. Design of sand drain will be discussed in our upcoming posts.

Effects of two dimensional and three dimensional consolidation:

The effects of 2D and 3 D consolidations are decrease in time which will help engineers to implement construction stage in short time. Now we will learn how time is reduced:

• Footing area is deceased as new drainage path introduced

• Thickness of soil layers is decreased for same cause of reducing drainage path

• Horizontal permeability of soil increased

Soil Mineralogy: Characteristics of Clay Mineral

Clay mineralogy is an essential portion of soil mineralogy. Clay are constituted from some essential group of very small particles in crystalline form of which one/more member fall in these group are termed as clay minerals. These minerals are of hydrous-aluminum silicates with some exceptions where aluminum from some minerals can be substituted by magnesium and iron. Sometimes alkaline earth present in them as an essential constitutes. The clay mineral groups are:

-kaolin

-semecites

- palygorskite

-illite etc

We will discuss each of these groups and other groups not listed here also.

The character tics of clay minerals are governed by some properties they are also follows:

a. They have a layered structure of which one of the dimensions is in nanometer range:

1. Thickness of 1:1 layer=0.7 nm

2. Thickness of 2:1 layer= 1nm

b. Anisotropy remains in particles or layers

c. Surfaces of several types are exist in their structure

d. Plasticity is the one of the main properties of clay mineral we will learn how water is absorbed in clay interstices in the next posts.

e. Under firing drying they become harden. Although most clay minerals show this property, some exceptions are also available.