Introduction
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.
 
 
Part 1
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
Sample number C1 C5 C6 C7 C8 C9 C10
Borehole 1 5 6 7 8 9 10
Depth (m) 4.0 5.0 5.0 5.0 2.0 5.0 5.0
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
Specific gravity 76.37 2.51 2.55 2.53 2.53 2.57 2.56
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
Void ratio 72.68 2.23 2.01 1.80 1.43 1.44 2.00
Degree of saturation 2.49 94.49 99.24 95.88 74.53 85.93 98.61

 
 
Table 2

Projects : Broadlakes Development          
Location : Merrimac     Date : 15/2/93      
Test procedure : A.S. 1289            
Sample number C1 C5 C6 C7 C8 C9 C10
Sieve Size (mm) Percent Passing
2.360  100 100 100 100 100 100 100
1.180 100 98 95 95 98 99 99
0.600 99 97 93 92 96 98 99
0.425 98 96 91 90 91 91 99
0.300 98 96 88 87 77 77 96
0.150 95 93 81 77 42 35 95
0.075 93 90 75 62 23 19 93
0.020 87 85 68 54 21 14 87
0.006 81 79 61 49 18 11 81
0.002 73 70 56 41 14 9 75
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
Plasticity index 38 35 33 24 12 11 43
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  
Clayey
A-7
ClayeyA-7 ClayeyA-6 ClayeyA-6 ClayeyA-6 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
Table 3 – Data from laboratory tests on coarse-grained material.
 

Project: Broadlakes Development         Location: Merrimac         Date: 17/2/93
Sample number S1 S5 S6 S7 S8 S9 S10
Borehole number 1 5 6 7 8 9 10
Depth (m) 8.4 7.0 7.0 6.8 4.2 6.2 6.8
Density (g/cm3) 1.72 1.74 1.71 1.78 1.74 1.75 1.72
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
Specific gravity 2.52 2.55 2.53 2.57 2.54 2.6 2.57
Dry Density (g/cc) 1.46757679 1.46464646 1.43456976 1.545139 1.48717949 1.47554806 1.444164568
Void Ratio 0.71711628
 
0.74103448 0.76360234 0.663281 0.70793103 0.76205714 0.779575581
Sieve Size (mm) Passing
4.75 100 100 100 100 100 100 100
2.36 98 100 89 100 96 100 100
1.18 96 100 78 99 96 100 99
0.600 90 100 56 99 95 100 99
0.425 74 98 33 95 88 99 95
0.300 38 76 21 74 46 69 74
0.150 6 5 11 10 2 3 10
0.075 3 4 9 5 1 2 5
Interpretation and analysis (analyse only the samples from your boreholes)
D10 0.17 0.16 0.14 0.15 0.18 0.17 0.15
D30 0.26 0.20 0.39 0.20 0.25 0.21 0.20
D60 0.38 0.27 0.71 0.27 0.34 0.28 0.27
Cu = D60/D10 2.24 1.69 5.07 1.80 1.89 1.65 1.8
Cc = 1.06 0.93
 
1.53 0.99 1.02 0.93 0.99
Relative Density 53.06 58.00 45.80 79.23 68.51 51.18 51.85
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
 
S1
 
 
S5
 
 
S6
 
 
S7
 
 
S8
 
 
S9
 
 
S10
 
 
Table 4

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
 
Which gives:

Dry Density(Kg/m3) 16.5o 16.98 17.43 17.66 17.23 16.50

 
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.
Borehole 5
Borehole 1
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
 
 
Borehole 1
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
Borehole identification
 
Soil layers
Soil properties
Footing
Geogrid parameter
The anchors
Displacements
 
 
Conclusion
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