Slide 2: PRESENTED BY: Hafiz Muhammad Mukhtar (2007-MS-Geo-17) FRAMEWORK AND
SITE INVESTIGATIONS Slide 3: Frame Work of Site Investigation
Preliminary Site Investigations
Recent Advancements in Mapping
Planning of Site Investigation
Location of Boreholes
Depth of Boreholes
Spacing of Boreholes CONTENTS Slide 4: ** are to be done before construction and can be done during construction if any anomalies found. FRAMEWORK OF S.I Slide 5: A desk study is undertaken as the first stage of a site investigation in order to make an initial assessment of the ground conditions and to identify, if possible, any potential geological problems.
1-Available geological maps.
2-Air photographs, satellite imaginaries
4-Remote sensing (if available)
3-Historic ground water Data
6-Geotechnical Investigation Reports DESK STUDY a-Geological Maps : a-Geological Maps Geotechnical mapping involves the plotting on suitable scales, the locations of all data which assists in understanding the geotechnical conditions existing at the site.
generalized picture of geology of an area.
Information about the rock and soil exposed on the surface.
Shows extent of faults, and other geological features.
(but in real practice the boundaries and especially the faults are interpolated, so accuracy cannot always be taken as granted) Types of Geological Maps : Types of Geological Maps Regional (made on scale 1:1000 for immediate works area of site)
Geotechnical mapping essentially involves the identification of location of all surface features relevant to the establishment of geological models at the site. Geotechnical maps are usually produced at an intermediate scale (1:5000 or 1:25000) covering the general works sites, and at 1:1000 or 1:500 covering immediate area of the sites. Why Geological maps can also be called as superficial Maps? : Why Geological maps can also be called as superficial Maps? They give the information about the distribution of superficial deposits, landslide areas and potential sources of construction materials.
(Lessons from past Experience)
Representation of all type of data is impossible on one map. i.e. it is impossible to represent all environmental or engineering geological data on one map, more than one map on each topic can serve the purpose:
-Environmental geological maps
-Hazardous Maps Regional Geological Map : Regional Geological Map Sample Local Geological Map (Sukian Dyke Mangla Dam) : Sample Local Geological Map (Sukian Dyke Mangla Dam) SILTYSAND/SANDYSILT
(OVERBURDEN) LEGEND SANDY GRAVELS
(OVERBURDEN) SILTSTONE WITH DESIGNATED NUMBER CLAY WITH DESIGNATED NUMBER SPOIL TIPS HARD RIB IN SANDSTONE DIPS & STRIKE OF BEDS Reservoir Reservoir SANDSTONE WITH DESIGNATED NUMBER Hazardous Maps : Hazardous Maps Slide 12: Stress relief joints at base of left bank slope (Site C2) Recent Advances in Geological Mapping : Recent Advances in Geological Mapping Using GIS (geographical information systems)
It is digital acquisition of the geological data. the main advantage is that we can tag the properties of an individual mapped area digitally.
Using 3D models:
By inserting data from the cross sections developed and superficial details a 3D model can be obtained.
Looks very convincing to a non-geologist, but there are some times unpredicted deficiencies which only a geologist can solve during line work. 3-D Model of Basha Dam Site : 3-D Model of Basha Dam Site b-Remote Sensing : b-Remote Sensing Remote Sensing is a better tool now a days for geological investigations.
Used when we don’t have topographical maps.
(Remote sensing involves the identification and analysis of phenomena on the earth’s surface by using devices borne by aircraft or spacecraft.)
Soils and Rocks are recognized with respect to thermal inertia, Density, Thermal capacity. Remote Sensing of Basha Dam Site : Remote Sensing of Basha Dam Site c-Aerial Photographs : c-Aerial Photographs Aerial photographs are generally taken from aeroplane flying at 800m and 9000m.
Height is governed by the details required.
Aerial photographs are used to make topographical maps.?????
Relief present is exaggerated. So slopes seemed to be more steeper than actual on an aerial photograph. (Problem is commonly associated with mountainous areas) d-Satellite Images : d-Satellite Images Satellite images available on scales from 1:1000000 to 1:250000 provide a broad view of the project region indicating correlations between geological features or the position of geological boundaries
They are also taken for reviewing any hazards which occurs at the immediate and nearby sites. Satellite Imaginary of Basha Dam Site : Satellite Imaginary of Basha Dam Site Satellite Images : Satellite Images Satellite Images : Satellite Images e-Ground Water : e-Ground Water If you do not understand the groundwater
behavior then you cannot understand the
behavior of the ground” Wroth (1981) 2-Reconnaisance : 2-Reconnaisance The preliminary reconnaissance involves a walk over the site noting, where possible:
The distribution of soil and rock type
The relief of ground (Important in case of Dams and roads)
The surface drainage conditions (in case of dam it is surface runoff)
Past uses of sites (i.e. any building in past or present to on or nearby the site)
The previous cutting and filling occurred due to any development or road construction.
To identify the drainage conditions. (in case of dam one has to thoroughly see the catchment area adjacent to project site.) 2-Reconnaisance (Condt..) : 2-Reconnaisance (Condt..) In case of Building Construction
The best general indication of subsurface conditions is the performance of existing structures in the area. If it is generally good it follows that soil conditions are good, or at least that foundation designs are adequate. If performance is bad the opposite is true.??????????? 3-Preliminary Investigation : 3-Preliminary Investigation Includes limited no. of borings, test pits adits.
The importance of the preliminary investigation is that it should assess the suitability of the site for the proposed works.
If the site found to be satisfactory the data from desk study and reconnaissance forms the basis upon which the site exploration is planned.
In simple words preliminary investigation also allows a check to be made on some of the conclusions reached in the desk study. 4-Detailed investigation : 4-Detailed investigation Includes probing of ground to identify shear zones.
After probing, the subsoil is examined by test pits or by soil borings. These may be spaced according to a rule-of-thumb of one pit or boring for every 10,000 square feet of area (C.B Crawford :1992), but they are more logically based on the geological evaluation of the site.
Sampling and testing is done and conclusions are made for setting goals for detail designing. Planning site investigations : Planning site investigations Location of Boreholes and test Pits
Depth Planning site investigations : Planning site investigations Usually these are based on the past experience of a geotechnical Engineer, but guidelines are there for a platform from where we can take start. Guidelines for Location of Boreholes : Guidelines for Location of Boreholes Boreholes may be located at every point of strategic importance where heavy concentrated loads occurred. For Dams : Basically the most important boreholes are along the center line of the Axis.
Also adjacent to structures, boreholes are made to access adjacent subsurface geology. For Dams For Dams : For Dams Boreholes During Construction : Boreholes During Construction In case of dams (even during construction supervision phase) boreholes are also done in line on both U/S and D/S of the Dam.
(Keeping in view the problematic Stara)????? For Buildings (Regular Areas) : For Buildings (Regular Areas) At the corners and at the center (points of concentrated loads). Rather be drilled at greater depth than other boreholes For Buildings (Irregular Areas) : For Buildings (Irregular Areas) If the area is irregular then we have to identity load sensitive points for boring which is based on experience.
Beneath heavy columns
Areas near to load bearing walls
One borehole beneath water tank if present (concentrated load)
(No hard and fast rule) Typical Borehole layout (from Simon Benzie) : Typical Borehole layout (from Simon Benzie) Depth of Borehole/Depth to Investigate : Depth of Borehole/Depth to Investigate Guidelines for Depth : Guidelines for Depth Based on No. of Storey (Empirical Approaches) Guidelines for Depth : Guidelines for Depth Revised Sower’s Approach (1979) : Revised Sower’s Approach (1979) The revised Sower's empirical approach depends on both No. of Storey and Foundation Depth D w.r.t soil conditions. Guidelines for Depth of Investigation based on Overburden Pressure : Guidelines for Depth of Investigation based on Overburden Pressure First Approach:
By Indian National Society of Soil Mechanics and Foundation Engineering Published (1963)
According to Prof. Burmister the critical boring depth is the depth where the 20% overburden pressure line intersects the foundation normal stress curve. Prof. Ede Beers contends that 10% overburden pressure line be taken. ∆σ / = qo Plot of Variation 0.2 * γh Guidelines for Depth of Investigation based on Overburden Pressure : Guidelines for Depth of Investigation based on Overburden Pressure Second Approach:
According to ASCE (1972)
D1= depth at which the effective stress increase (∆σ’) is equal to 10% of contact pressure.
D2= depth at which the ratio ∆σ’ /po’=0.05
i.e. ∆σ’=0.05 po’
Between D1 & D2 which
ever is greater is taken as depth to investigate. ∆σ / = qo Plot of Variation 0.05 * γh Qualitative Approaches:Guidelines for depth of Boring for foundations (Hvorselev (1949)) : Qualitative Approaches:Guidelines for depth of Boring for foundations (Hvorselev (1949)) Slide 47: For retaining walls:
1.5 times the width or height of retaining wall, whichever is more Depth to investigate for Large Structure on piles : Depth to investigate for Large Structure on piles Depth to investigate for Large Structure on piles (by Simon Menzie) : Depth to investigate for Large Structure on piles (by Simon Menzie) SPACING OF BOREHOLES : SPACING OF BOREHOLES Spacing basically decides no. of boreholes. Sower’s Aprroach(1979) Minimum No of Borings According to BG Clarke (Uni. Of New Castle Upon Tyne : Minimum No of Borings According to BG Clarke (Uni. Of New Castle Upon Tyne SPACING OF BOREHOLES : Minimum of 3 or 4 boreholes needed for small building site.
But if the site is large we have to plan by keeping in mind, some standards.
At Kala Shah Kaku campus of Uet, for 300 acres of area 30 no. of boreholes were proposed. SPACING OF BOREHOLES SPACING OF BOREHOLES : For earth dam usually the boreholes are done 25m- 50m distance depending upon economy and site conditions.
For highways and railways for every 250m-500m the boreholes are done.
For bridges, common practice is make one borehole for one pier. SPACING OF BOREHOLES Depth and spacing of Boreholes By John N.Surneka : Depth and spacing of Boreholes By John N.Surneka Test Pits : Test Pits According to M. J. Dumbleton and G. West, Transport And Road Research Laboratory Report LR403
Trial pits are shallow excavations going down to a depth no greater 6m.
An important safety point to note is that ALL pits below a depth of 1.2m must be supported.
0-2m By Hand 2-4m By Wheeled Back Hoe Test Pit (d/s of Main Dam(MDRP) : Test Pit (d/s of Main Dam(MDRP) Trench (D/s of Sukian Dyke) : Trench (D/s of Sukian Dyke) Conclusion Site Investigation-Less is More : Conclusion Site Investigation-Less is More Always create an optimum level of investigation just to save extra expenditures. References : References Mangla Dam Geotech Report
Basha Dam Geological Report
Book on Engineering geology and Construction by FG Bell.
Indian national society of soil mechanic and foundations
Paper on geotechnical investigation by CB Crawford
Satellite images from Nasa Earth observatory
A Short Course in Foundation Engineering By Simon Menzie