Site Investigation William J. Likos, Ph.D. Department of Civil and Environmental Engineering University of Wisconsin-Madison GLE/CEE 330 Lecture Notes Soil Mechanics

A. Purposes for Site Investigation: 1.Site Characterization (geometry) –Define type and extent of soil and rock –Define location of groundwater –Define stress history of site –Identify potential problem conditions (e.g. karst, contaminants)

Purposes for Site Investigation con’t 2.Obtain samples for testing –Index and characterization tests u moisture content u Atterberg limits u grain size distribution –Strength tests for stability problems u unconfined compression u direct shear u triaxial –Deformation tests for settlement u consolidation tests –Other tests u hydraulic conductivity u compaction u contamination

B. General Considerations: l Structure –more critical and complex => more extensive investigation l Site –More complex site => more extensive investigation l Familiarity with site geology –More familiarity => less extensive investigation l Economics –How much benefit will one more boring provide? –Drilling/sampling ~ $2k-$5k per day (depending) l Client –Can influence extent of investigation, but not too much! l Codes

C. Preliminary Investigation: l Plans for site development –Site plan with finished grades and structure location(s) –Approximate magnitudes of loads –Sensitivity of structure to settlements l General site conditions –Company reports/files –Geologic maps –Topographic maps –USCS soil surveys (surficial soils) –Air/Satellite Images (Google Earth) –Newspaper articles

Geologic Maps

Topographic Maps

Aerial Photographs See

D. Site Visit – Items to take: l Info from preliminary investigation (e.g. reports, topo’s, geo’s, aerial photos, etc.) l Note pad, notebook, or site inspection form (take notes on everything!! – time, weather…) l Camera l Small shovel or hand auger l Compass l Tape l Level

Site Visit – items to look for: 1.Site address/location 2.Date and time 3.Current and recent weather conditions 4.Site accessibility (e.g. terrain, right of way, gates, etc.) 5.Power lines 6.Availability of water and electricity 7.Ground slopes and any evidence of movement 8.Locations of trees, brush, streams, marshes 9.Rock outcrops (note type and condition) 10.Evidence of prior filling 11.Existing or prior structures 12.Evidence of contamination 13.Manholes, utilities, etc. 14.Others…

E. Methods of Subsurface Exploration l Test Pits l Hand Auger l Truck-mounted Power Auger l Wash Boring l Rotary Wash Boring l Rock Coring l Cone-penetration Test l Other in-situ tests

E.1 Test Pits l Excavated by hand or backhoe l Generally < 20-ft deep l Uses –Locate active faults –Locate thin seams –Investigate potential borrow areas

Excavations and Road Cuts

E.1 Test Pits l Advantages –Can view soil directly (helps locate small but critical seams) –Can obtain large quantities of disturbed soil for testing –Can hand carve undisturbed samples –Can perform field tests (e.g. plate load tests) l Disadvantages –Economically limited to 10-ft to 20-ft depth –Dewatering required below water table –Numerous test pits disrupt site –Cost of excavation in rock may be prohibitive

E.2 Hand Augers l 2-in to 6-in diameter l Can drill to perhaps 25-ft l Used where access is restricted or mobilization costs for motorized rigs are prohibitive

E.2 Hand Augers l Advantages –Low cost –Portability –Requires minimal headroom l Disadvantages –Maximum depth restricted –Cannot obtain high quality “undisturbed” samples –Cannot bore through rock, hard clay, or sand below water table –Slow and labor intensive

E.3 Truck-mounted Power Auger l 4-in to 12-in continuous flight augers l Can drill to depths of 150-ft.

Hollow and Solid-stem Augers

E.4 Wash Boring l Tripod Rig with “cathead” l Hole advanced by chopping/cutting head and fluid flow to return cuttings

E.4 Wash Boring l Advantages –Low equipment costs –Adaptable to any terrain –Can drive sampler at bottom of hole l Disadvantages –Depth of drilling limited by weight of drilling rods (generally <50-ft. –Slow drilling through hard clays and gravel –Labor intensive –Difficult to get location of GWT –Cannot push samplers –Set up time is several hours

E.5 Rotary Wash Boring l Same principle as wash boring but with added motorized rotary action l Often combined with power auger

E.5 Rotary Wash Boring l Advantages –Relatively rapid drilling through almost any soil –Maximum depth from several hundred to several thousand feet –“Undisturbed” samples can be obtained by pushing or driving samples –Can drill in soft clay or sand is drilling mud (slurry) is used l Disadvantages –Drilling rigs are relatively expensive –Difficult to accurately measure GWT is slurry used –Set-up/clean-up time may be several hours per hole –Rig accessibility is primary concern –Drilling mud must be disposed – environmental concern –Difficult to retain drilling mud in highly permeable strata

Rig Types

Special Rigs

Safety First

Sampling

l Important Engineering Properties –Shear Strength –Compressibility –Permeability l Causes of disturbance 1.Remolding of soil - Friction/smearing along side walls - “Squeezing” soil into sampler 2.Unloading of soil - May cause change in soil structure

Sampler Characteristics - As A R increases, sample quality decreases - For high quality samples, A R <=15% Area Ratio Inside Clearance Ratio - Typical values - Need some friction to hold sample

Sample Recovery Ratio Ideally, R R = 1 but may be 1

Standard Sampler (a.k.a. split spoon) l Primarily used for Standard Penetration Test (SPT) l Can be used to sample in difficult soils l Samples obtained are badly disturbed l A R is 110%

Standard Sampler

Shelby Tube Sampler l Commonly 2-inch to 3-inch O.D. (up to 5-inch) l Wall thickness of 0.06-inch to 0.15-inch l AR is 10 to 15 % for 3-inch tubes l Lengths vary from 24 to 36 inches

Lined Samplers l Generally rugged, thick- walled samplers with large A R l Intent is to minimize disturbance due to unloading l Liner can be made convenient size for: –Consol. Tests –Permeability tests –Direct shear tests –Triaxial tests

Core Barrels l Use for drilling and sampling in rock l Come in several common sizes (e.g. RX) l Use hollow diamond bit to drill through rock l Can use stationary inner barrel within outer rotating barrel to protect rock core l Use recovery ratio and other rock core characteristics to establish suitability of rock (don’t want to break due to coring!) l Can test intact pieces of rock in lab

Distribution of Disturbance

Effects of Disturbance l Sands: –Loose sands generally become more dense (unsafe!) –Dense sands generally become more loose (conservative) l Clays: Specimen Source Avg. Undrained Strength 1.9” Shelby1333 psf 3.4” Shelby1990 psf 3.4” Shelby – trimmed to 1.5” 2120 psf Strength Compressibility

In-situ Testing l Standard Penetration Test (SPT) l Cone Penetration Test (CPT) l Others: –Vane Shear Test –Pressuremeter (PMT) –Flat Plate Dilatometer (DMT) –Geophysical Tests

Standard Penetration Test l Most common in-situ test l SPT “N-value” is number of blows of special hammer required to penetrate standard sampler 12 inches l Procedure: –140-lb hammer –30-inch drop –Penetrate total distance of 18-inches, measure the number of blows required for each 6-inch increment –Compute “N-value” by summing number of blows for last 12-inches of penetration

Common SPT Hammers

Cone Penetration Test

Cone Penetration Rig

Cone Penetration Log

Other In-situ Tests

Geophysical Tests