Geotechnical Investigation For Foundation Designing

Haider Ali JafferiCivil Engineer

Hyderabad, Sindh

Introduction:Geotechnical investigations are

performed to obtain information on the physical properties of soil and rock underlying the site to design earthworks and foundations for proposed structures, and for repair of damages to earthworks and structures ,caused by subsurface conditions

Geotechnical Investigations

Site Details

Site Plan

In-situ Soil Density by Core Cutter MethodAtterberg LimitsSieve AnalysisSoil Classification by USCSPermeability of soil by Constant Head Method

Test Performed

Apparatus Used:Standard: ASTM D2937Significance:Field Work: Soil excavated up to a depth of 3 ft. The core cutter is driven inside the soil with the help of hammer. Soil is trimmed around the core cutter and core

cutter is removed.

Core Cutter Method

Lab Observations:Internal dia of core cutter =

4 inHeight of core cutter = 5 inCore cutter cross sectional area =πd2/4 = 12.56 in2

Mass of empty core cutter = 1383.8 g = 3.044 lbsMass of core cutter + soil = 3017.7 g = 6.639 lbsMass of soil =6.639 – 3.044 = 3.595 lbsVolume of core cutter = 62.8 in3

Bulk density = ρb= M2 –M1/V = 1588.1907 kg/m3

 Dry density = ρd= ρb/1 + m.c = 1489.5202 kg/m3

Core Cutter Cont……..

Apparatus Used:Standard Reference: ASTM D-422Significance:Field Work: Bore holes are drilled with the help of earth augers. Holes were drilled at 3 ft interval up to 12 ft and

disturbed samples of the soil were collected. Soil samples were collected in the bags and were

transported to the lab for laboratory tests and observations.

SIEVE ANALYSIS

LABORATORY OBSERVATIONS:Sieves were arranged in the order and were

tightly held on the sieve shaker.About 500 g of soil disturbed soil sample of

depth 3 ft was placed on the top sieve (sieve no 4).

Sieve shaker was then turned on and sieves were shaken for 5 minutes

SIEVE ANALYSIS Cont…

Gradation Test Results: Sample # 1 (3 ft.)Cu=4.8Cc=1.5So soil is Poorly Graded (SP) Sample # 2 (6 ft.)Cu=3.28Cc=1.2So soil is Poorly Graded (SP)

Soil Classification By USCS

Sample # 3 (9 ft.)Cu=2.9Cc=1.0So soil is Poorly Graded (SP) Sample # 4 (12 ft.)Cu=2.65Cc=1.1So soil is Poorly Graded (SP)

SIEVE ANALYSIS Cont…

Standard Reference: ASTM D431Significance: Liquid Limit:Apparatus Used:Lab Work:Soil sample was passed through sieve no. 40.Water was added to the taken soil sample gradually

in order to make a paste.The soil paste was filled in the Cassagrande

apparatus, and blows were applied.No of blows were counted and a graph was made to

determine the intermediate value of m.c at standard 25 blows.

ATTERBERG LIMITS:

LIQUID LIMITS Cont…Lab Observations:Sample # 1 (3 ft.)Trial A:No of blows = 31M.C = 29.5 %Trial B:No of blows = 23M.C = 33.5%

Sample # 2(6 ft.)Trial A:No of blows = 23M.C = 32.8 %Trial B:No of blows = 30M.C = 33.5%

LIQUID LIMITS Cont…Sample # 3(9 ft.)Trial A:No of blows = 27M.C = 26.7 %Trial B:No of blows = 21M.C = 29.6%

Sample # 4(12 ft.)Trial A:No of blows = 22M.C = 29.9 %Trial B:No of blows = 30M.C = 27.2%

PLASTIC LIMITSample # 1 (3 ft.)Trial A:M.C = 21.3 %Trial B:M.C = 23.1%Sample # 2 (6 ft.)Trial A:M.C = 12.5%Trial B:M.C =17.6 %

Sample # 3 (9 ft.)Trial A:M.C = 19.3 %Trial B:M.C = 16.7%Sample # 4 (12 ft.)Trial A:M.C = 17.7 %Trial B:M.C = 19.3%

Apparatus Used:Significance:Lab Work:Soil was passed through sieve # 200.Soil sample was filled in Permeameter in

three fills and were properly tampered.Time period was noted for the water to

discharge, in order to carry out important calculations.

PERMEABILITY OF SOIL

Laboratory Observations:h = 41.5 inL = 7.5 inQ = 20 ml = 0.02 L

t = 2:30 hr = 9000 secA = 75.42 cm2 = 11.69 in2

K = Q L/A h t K = 5.325 x 10-9 cm/sec

PERMEABILITY OF SOIL Cont…

In-situ Field Density:Bulk Density= 1588.19 kg/m3Dry Density = 1491.01 kg/m3

For well graded soil, Cu >6 1 < Cc <3

The results of experiment shown that the soil type was “poorly graded sandy soil” and its symbol is “SP”.

For permeability, if Vale of K is less than 0.0001, than the soil will be clayey other wise will be sandy.

Our result of experiment was 5.325 x 10^-9, and soil type came out to be clayey.

Summary Of Results

The foundation designing mainly depends upon the bearing capacity of soil.

During the test, no water table was intercepted during the bore holes.

The previous geotechnical studies shown that the bearing capacity of the soil in this vicinity is about 1 ton per square feet.

Conclusional Recommendations