soil exploration

Soil Exploration

The knowledge of subsoil conditions at a site is a prerequisite for safe and economical design of substructure elements.  The  field  and  laboratory  studies  carried  out  for  obtaining  the necessary  information  about  the  surface  and  subsurface  features  of  the  proposed  area including  the  position  of  the  ground  water  table,  are  termed  as  soil  exploration  or  site investigation.

Objectives of soil exploration program

The  information  from  soil  investigations  will  enable  a  Civil  engineer  to  plan,  decide,  design and execute  a construction  project.  Soil investigations are  done  to  obtain  the information  that is useful for one or more of the following purposes.

1.  To know the geological condition of rock and soil formation.

2.  To establish the groundwater levels and determine the properties of water.

3.  To select the type and depth of foundation for proposed structure

4.  To determine the bearing capacity of the site.

5.  To estimate the probable maximum and differential settlements.

6.  To predict the lateral earth pressure against retaining walls and abutments.

7.  To select suitable construction techniques

8.  To predict and to solve potential foundation problems

9.  To ascertain the suitability of the soil as a construction material.

10. To determine soil properties required for design

11. Establish procedures for soil improvement to suit design purpose

12. To investigate the safety of existing structures and to suggest the remedial measures.

13. To observe the soil the soil performance after construction.

14. To locate suitable transportation routes.

The objectives of soil investigations from various requirements point of view is summarized in below table

Design requirements	·        define stratigraphy/geology ·        to determine soil properties required for design ·        aid material selection ·        to determine the type and depth of foundation
Construction requirements	·        to select suitable construction techniques ·        define equipment and techniques needed ·        to locate suitable transportation routes
Auditing	·        checking a site prior to sale/purchase ·        to establish procedures for soil improvement to suit design purpose
Monitoring	·        to observe the soil performance after construction ·        determine reasons for poor behavior ·        document performance for future reference

Identification of soil:

             Identification of soil:

                For the identification of soil perform we may perform the following      tests

1)      Visual examination:-  In the visual examination we observe with naked eye the following properties

- colour

- gradation

- angularity

                    And decide whether it is course grained or fine grained soil.

2)      Feel test:- In  feel test we take small amount of soil and add a few drops of water , rub it between our fingers and observe that

-          If large particle than it is sand.

-          If small particle than it is silt.

-          If small and soapy than clay.

3)      Rolling test:- In rolling test take the sample soil add few drops of water and make a ball out of it by keeping it on the palm and rolling it .Now convert it into a thread.

-          If immediately crumbles without assuming thread shape than it is sandy soil.

-          If it is converted to a ball and cracks appear than it is silty soil.

-          If it is converted to  thread of  3mm  than clayey soil.

4)      Strength test:- Take soil  and add few drops of water  make a cube out of it place it in sun or in oven at 100^oc when all the water is removed then try to break it by  hand.

-          In case of sandy soil it will break easily.

-          In case of silty soil some pressure is required.

-          In case of  clay it can’t break  easily.

5)      Dispersion test:- Take some dry soil and put it in a glass of water i.e.,  transparent glass

-          In case of sandy soil  the particles will settle down in a very short time i.e., few seconds.

-          In case of silty soil , it will take some time to settle down .

-          In case of clayey  soil,  it will take lot of time to settle down.

 All the above tests are simple and are performed to get the general idea about soil type.

Temporary Ground water level control

In excavation works, the presence of  groundwater in higher levels, makes the process very complicated. Generally, it is found more practical to lower the groundwater level far enough to make the excavation and other execution works easier. This is done in several ways depending on the original groundwater level, soil properties, importance of the structure and the length of the subsurface construction period.

Among the methods of groundwater control are the following:

·dewatering

·freezing

·electro-osmosis

·grouting 

·compressed air

CHOICE OF THE TYPE OF FOUNDATION

Choice of the Type of Foundation

The choice of the appropriate type of foundation is governed by some important factors such as

1.    The nature of the structure

2.    The loads exerted by the structure

3.    The subsoil characteristics

4.    The allotted cost of foundations

Therefore to decide about the type of foundation, subsoil exploration must be carried out. Then the soil characteristics within the affected zone below the building should be carefully evaluated. The allowable bearing capacity of the affected soil strata should then be estimated.

After this study, one could then decide whether shallow foundations or deep foundations should be used.

Shallow foundations, such as footings and rafts, cost less and easier to execute. They could be used if the following two conditions are fulfilled;

1.    The superimposed stress (Dp) caused by the building lies within the allowable bearing capacity diagram of different soil strata as shown in Fig.1.

This condition is fulfilled when , in Fig.1, is smaller than  and  smaller than  and  smaller than  and   smaller than and so on.

2.    The building could withstand the expected settlement estimated for that type of foundation

If one or both of these two conditions cannot be fulfilled the use of deep foundations should be considered.

Deep foundations are used when top layers of the soil are soft and there exists a good bearing stratum at a reasonable depth. Soil strata lying beneath the bearing stratum should be of ample strength to resist the superimposed stresses (Dp) due to the loads transmitted to the bearing stratum, as shown in Fig.2.

  

Deep foundations are usually piles or piers which transmits the load of the building to the good bearing stratum. They usually cost more and require well trained engineers to execute.

If the explored soil layers are soft for considerable depth and no bearing stratum is found at a reasonable depth, floating foundations could be used.

To build a floating foundation, a mass of soil, approximately equal to the weight of the proposed building, is to be removed and replaced by the building. In this case, the bearing stress under the building will be equal to the weight of the removed earth (γD) which is less than

(q_a = γD +2C)

and Dp will be equal to zero. This means that the bearing capacity under the building is less than ( q_a ) and the expected settlement equals theoretically to zero.

Finally, the engineer should prepare an estimate of the cost of the most promising type of foundation which represents the most acceptable compromise between performance and cost.