Site investigation is an important aspect in any structural design. It establishes the suitability of the soil to support a specified load, either from a building, bridge, canal or even a road among others. However, clay soils have unsuitable geological properties that makes them not good for some types of foundation. This is because they are highly plastic and a major component of expansive soils.
In line with this, there are some serious considerations about the location of the canal that has been proposed at the Merrimac Site of Broad lakes Development. As per the geotechnical investigation that has been done by Coffee partners international Pty ltd, there are deposits of clay on the site. In essence, the report will be in two categories: the site analysis and the foundation design.
As with the site analysis, the main task is to analyze the data that has been collected from the field. Generally, the analysis is based on the soil constituents of the different soil samples collected from the different boreholes. Moreover, the other analysis will be focused on soil classifications.
On the other hand, the second part will involve the use of the Plaxis 2D software to determine the size of footings proposed for the development of a building covering an area of 100m x 200m. Moreover, the building will exert a load of 20000 tons to the foundation. Nevertheless,the location and sizing of each footing will also bedetermined.
Soil analysis and classification
Table 1 – Data from laboratory tests on fine-grained soil.
|Project: Broadlakes Development
Location: Merrimac Date: 12/2/93
|Test procedure: A.S. 1289|
|Wet Density (t/m3)||1.47||1.43||1.51||1.52||1.48||1.56||1.51|
|Moisture Content (%)||78.3||83.8||78.2||68.2||42.1||48.2||77.2|
|Soil particle density (t/m3)||2.53||2.51||2.55||2.53||2.53||2.57||2.56|
|Interpretation and analysis (analyse only samples from your boreholes)|
|Dry density (g/cm3)||0.82||0.78||0.85||0.90||1.04||1.05||0.85|
|Volume of water (m3)||2.50||127.95||130.04||120.95||86.05||99.57||129.10|
|Weight of solids (N)||0.27||1.50||1.63||1.74||2.01||2.03||1.64|
|Degree of saturation||2.49||94.49||99.24||95.88||74.53||85.93||98.61|
|Projects : Broadlakes Development|
|Location : Merrimac||Date : 15/2/93|
|Test procedure : A.S. 1289|
|Sieve Size (mm)||Percent Passing|
|Liquid Limit (%)||67||59||51||40||32||33||73|
|Plastic Limit (%)||29||24||18||13||20||22||30|
|Linear Shrinkage (%)||17||15.5||14||12||2.5||4||19|
|Interpretation and analysis|
|Water content (%)||1.30||1.71||1.81||2.18||1.84||2.38||1.10|
|Liquidity index (classification based on Liquidity index)||0.03||1.34||1.90||3.9||5.3||10.6||-0.46|
|Activity (classification based on Activity)||0.52- Inactive||0.50- Inactive||0.59- Inactive||0.59- Inactive||0.86- Normal||1.22- Normal||0.57- Incative|
|Classification : (AASHTO symbols)||ClayeyA-7||
Soils which are less than 0.75 are inactive while those greater than 0.75 but less than 1.25 are normal. Soils greater than 1.25 are active
Table 3 – Data from laboratory tests on coarse-grained material.
|Project: Broadlakes Development Location: Merrimac Date: 17/2/93|
|Moisture content (%)||17.2||18.8||19.2||15.2||17||18.6||19.1|
|Maximum void ratio||0.932||0.951||0.901||0.859||0.921||0.933||0.96|
|Minimum void ratio||0.527||0.589||0.601||0.612||0.61||0.599||0.612|
|Dry Density (g/cc)||1.46757679||1.46464646||1.43456976||1.545139||1.48717949||1.47554806||1.444164568|
|Sieve Size (mm)||Passing|
|Interpretation and analysis (analyse only the samples from your boreholes)|
|Cu = D60/D10||2.24||1.69||5.07||1.80||1.89||1.65||1.8|
|Classification (AASHTO symbols)||A-3||A-3||A-1-b||A-3||A-3||A-3||A-3|
The values of D 30, D60 and D10 are obtained from the following sieve analysis logarithmic curves
|Mass of compacted soil (g)||1867||1956||2044||2106||2090||2036|
|Moisture Content (%)||11||13||15||17||19||21|
Mold is 1000 cubic centimeters.
In essence the bulk unit weight is calculated by multiplying the mass of the soil by the gravitational pull 9.81
|Bulk Unit Weight (KN/m3)||18.315||19.19||20.05||20.66||20.50||19.97|
Finally, the dry density is calculated from the following formula
The optimum moisture content is obtained by plotting the graph of moisture against dry density
From the graph the optimum moisture content is about 17.5%
Part 2: Design of the foundation
We may take a foundation of 600mm by 600mm. considering that the total area to be occupied is 100m by 200m, we may take rectangular 15 rectangular shaped footings with the area sun divided into 10 blocks. Therefore, the total number of footings is: 150.
Therefefire, each loasinfg is about 2000/150=133KN
Considering that there is presence of a water table .the following formula may be used. This is dependent on the location of the borehole:
Where q is the surcharge load, and are the saturated unit weight of water and dry unit weight of water respectively.
Nevertheless, we use the data from borehole 5 and borehole 1 for the analysis.
Based on the above data and formula:
For the foundation to be located in sois stipulated by borehole 1:
We may assume that the maximum depth of the foundation to be 2 meters, therefore D1=0.45 m while D2= 1.55
Nevertheless, the saturated unit weights of gravel, sand, silt and clay are in the ranges of 20-22,18-20,18-20 and 16-22 respectively On the other hand, the dry units weights are between 15-17,13-16,14-18 and 14-21 respectively.
Based on this and the fact that silty clay and sand silty clay is the predominant sand,we may take the saturated weight of 19kN/m3 and 16kN/m3 respectively
Therefore, q at the first borehole=0.45x.16+ (19-9.81)= 19.60 KN/ m3
The width of the footing can be taken as
20000/19600 as 1.0 meters
We may furthermore assume that it is a rectangular footing
As with borehole 5.the predominant soil is clay and as such, the foundation must be deep
The dry unit weight and saturated unit weight of clay are 20 and 18 respectively. Nevertheless, the foundation may be located just below the first soil layer. The depth may be approximated as 11m
Therefore D1= 9.95m while D2 may be taken as 1.05 m
Therefore q= 20X 9.95 + 1.05(18-9.81)= 207.60KN/M3
Therefore B=20000/207600 =0.3 square meters
These results are dependent on the actual conditions and variables taken from the actual; geotechnical analysis.
The Plaxis 2D modelling
Based on the analysis above and the plaxis modelling, it can be clearly be seen that there is need for proper geotechnical investigation prior to actual developments. In this case, the, number of footings are 150 and from the displacement diagram, they form a relatively good base for the structure. The displacement is between o.10 and 0.20