AN INTRODUCTION TO BUILDING FOUNDATIONS AND SOIL IMPROVEMENT METHODS

Transcription

1 AN INTRODUCTION TO BUILDING FOUNDATIONS AND SOIL IMPROVEMENT METHODS SEAONC 2008 Spring Seminar San Francisco, 16 April 2008 Hadi J. Yap, PhD, PE, GE 1

2 General Foundation Types Shallow Foundations Spread footings: isolated, continuous grid or waffle Post-Tensioned Slabs (PT Slabs) Mats Deep Foundations 2

3 Factors to be Considered in Selecting Foundation Type Subsurface conditions Column loads and spacing, basements Site constraints noise vibrations proximity to existing improvements, slope, channel Economics 3

4 Shallow Foundations Suitable where underlying material is strong Can be used in engineered fill if building load is light to moderate Mats can be used to span localized weak areas Mats can be used on weaker soil for structure with basements where net load (weight of structure minus weight of soil removed) is low 4

5 Sources of Settlement Immediate Settlement (sand and clay) Occurs as the load is applied Consolidation (saturated clay) A slow process of squeezing water out of the pores in soft clay when loaded Liquefaction (saturated sand) Temporary loss of shear strength in loose sand due to a rise in excess pore water pressure during cyclic loading such as seismic Seismic Densification (dry/moist sand) Densification of loose sand above the groundwater level due to ground shaking 5

6 Total and Differential Settlement Building can tolerate large total settlement if the differential settlement is within tolerable limits Where the total settlement is large, flexible connections should be provided to underground utilities where they enter the building 6

7 Allowable Differential Settlement for Buildings Angular Distortion = Differential settlement/distance Angular Distortion Limits (Bjerrum, 1963): 1/500 safe limit where cracking is not permissible 1/300 limit where first cracking in panel walls is to be expected 1/150 limit where structural damage to general buildings is to be feared 7

8 Design Parameters for Spread Footings Minimum width Minimum embedment depth Allowable bearing pressure Allowable passive pressure Allowable base friction coefficient 8

9 Spread Footing Excavations 9

10 When mat is to be considered When total footprint area of spread footings is more than, say, 50% of building footprint To reduce total and differential settlement To bridge areas of weak subgrade 10

11 Design Parameters for Mat Foundations Minimum embedment depth Allowable bearing pressures Allowable passive pressure Allowable base friction coefficient Subgrade modulus 11

12 Mat Subgrade and Mud Slab 12

13 Mat Rebars 13

14 Deep Foundation Types Drilled Piers/Cast-In-situ-Drilled-Hole [CIDH] Piles Driven Piles (Concrete, Steel H) Tubex Piles Auger Cast Piles Torque Down Piles Micropiles 14

15 Drilled Piers Can use one large diameter pier in lieu of several smaller, driven piles Lengths can be adjusted in the field reduce waste/build-up Derive axial capacity mainly from skin friction Need to use casing and/or drilling fluid if groundwater and/or loose soil is present 15

16 Drilled Pier Installation 16

17 Driven Precast, Prestressed, Concrete Piles Economical in San Francisco Bay Area Can be used where soft soil, non-engineered fill, or high groundwater level, is present Fabricated at yard good quality control Moderately high capacity up to 344 kips for 14 square piles using 6,000 psi concrete 17

18 18 Concrete and Steel Piles

19 Driven Steel H-Piles More expensive than driven concrete piles Suitable where depth to bearing soil layer varies; can conveniently be cut and spliced Design must consider corrosion Moderate to high capacity up to 456 kips for HP14X89 using 50 ksi steel Lateral resistance varies with load direction relative to pile axis 19

20 Soil Improvement If poor soil conditions are encountered: Bypass poor soil, use deep foundations Remove poor soil, replace with engineered fill Improve soil properties in place 20

21 Factors to be Considered in Selecting Soil Improvement Method Soil type; fines content (silt- and clay-size) Area and depth of treatment Soil properties strength, compressibility Proposed structure and settlement criteria Availability of skills, equipment, materials Adjacent improvements Economics 21

22 Primary objectives of soil improvement CLAY Increase bearing capacity or slope stability Reduce foundation settlement SAND - Reduce liquefaction potential - Increase bearing capacity - Reduce foundation settlement 22

23 Mechanisms of Soil Improvement for Clay Consolidation - Preloading Reinforcement - Soil-Cement Columns - Vibro-Replacement Stone Columns - Geopiers and Vibro Piers Mixing - Soil-cement columns 23

24 Mechanisms of Soil Improvement for Sand By vibration Impacts at surface: Dynamic compaction Depth vibrator: Vibro-compaction By vibration and displacement of backfill - Vibro-replacement stone columns - Vibro Piers By displacement of backfill material - Compaction grouting By binding particles - Permeation grouting (e.g. ultra-fine cement) By mixing Soil-cement columns 24

25 Soil Improvement Methods Method Preloading Dynamic Compaction Soil-Cement Columns Vibro-Compaction Stone Columns Geopiers and Vibro Piers Compaction Grouting Permeation Grouting Clay X X X X Sand X X X X X X X 25

26 Preloading Performed by placing fill over soft clay Improve foundation soil for buildings, embankments, runways, bridge abutments Type of preloads: earth fill, water, vacuum Use prefabricated vertical (wick) drains to reduce preloading time Wick drains: plastic core wrapped in geotextile; generally 4 wide and 1/8 to 3/8 thick 26

27 Wick Drain Installation 27

28 Preloading (cont d) Typical wick drain spacing is 3 to 6 feet, depending on soil permeability and time available Typical preloading period is 3 to 6 months, depending on soil permeability and degree of consolidation to be achieved Construction monitoring: settlement (settlement plates/probes), pore water pressure (piezometers), lateral movement (inclinometers) 28

29 Preloading with Wick Drains and Instrumentation Source: ASCE, Geotechnical Special Publication No. 69,

30 Dynamic Compaction Involves repeated dropping of heavy weights onto ground surface Effective for sand, waste, and rubble fills Pounders: concrete blocks, steel plates, or thick steel shells filled with concrete/sand Typical weight of pounders: 6 to 30 tons, depending on the depth of soil to be improved Typical drop heights: 40 to 100 feet 30

31 Dynamic Compaction Source: Hayward Baker 31

32 Dynamic Compaction (cont d) Most effective for soil with less than 25% fines (silt- and clay-size particles; material passing #200 sieve [0.075 mm opening]) Typical improvement depth is 10 to 30 feet D 0.5 (WH) where: D = improvement depth in m W = pounder weight in metric ton H = drop height in m 32

33 Dynamic Compaction (cont d) Ground Vibrations < 0.5 inch/sec to prevent cracks in walls < 2.0 inch/sec to prevent structural damage Construction monitoring Induced settlement Ground vibration Ground heave Pore water pressure Verification testing (SPT, CPT) 33

34 Scaled Energy Factor versus Particle Velocity Source: FHWA, Dynamic Compaction,

35 Soil-Cement Columns Mixing in-situ soil with cementitious materials using mixing shafts consisting of auger cutting heads, auger flights, or mixing paddles Produce soil-cement columns with higher strength, lower compressibility, and lower permeability than the native soil Used to improve bearing capacity and slope stability, and as shoring walls Typical compressive strength of cylinders ranges from 15 to 300 psi Typical permeability of mix ranges from 10-6 to 10-7 cm/sec 35

36 36 Soil-Cement Column Installation

37 Soil-Cement Shoring Wall 37

38 Soil-Cement Wall Installation 38

39 Steel Beam Installation 39

40 Vibro-Compaction Densifying granular soil by inserting a vibrating probe into the ground Probe spacing ranges from 6 to 14 feet Suitable for sand with less than 15% fines (silt- and clay-size particles) Vibrator is a torpedo shaped horizontally vibrating probe, 10 to 15 feet long, and weighs about 2 tons. The probe penetrates to the design depth under its own weight assisted by water jetting 40

41 Vibrator and Water Jets 41

42 Vibro-Compaction (cont d) The action of vibrator and water jetting reduce intergranular forces between soil particles allowing them to become denser The vibrator starts at the bottom of the hole and raised to treat the next interval; the procedure is repeated as backfill sand is added If backfill is not added, craters with diameters of 10 to 15 feet can form around vibrator 42

43 Vibro-Replacement Stone Columns Extends the range of soil types that can be improved to silt and clay The probe is penetrated to design depth and gravel/crushed rock is placed in the hole as the probe is withdrawn in vertical increments of 2 to 5 feet A stone column is formed with the stone laterally compacted against the surrounding soil 43

44 Vibro-Replacement Stone Columns (Cont d) Three primary methods Wet, top feed method: hole is saturated with jetting water Dry, top feed method: hole formed by probe remains open without water Dry, bottom feed: stone backfill is fed through a hopper and tube to the bottom of the hole Construction Monitoring Settlement and ground heave Amount of stone backfill used Verification testing (SPT, CPT) 44

45 Wet Top Feed Method Source: Bauer 45

46 Dry Bottom Feed Method 46

47 Geopiers (Rammed Aggregate Piers) Locally installed by Farrell Design-Build Company Typically 24 to 36 inches in diameter and 6 to 30 feet deep, constructed by drilling and ramming crushed rock in 12-inch lifts The ramming equipment consists of excavators equipped with 2,000 to 4,000 lbs hydraulic hammers with beveled tampers The ultimate bearing capacity of a pier ranges from 100 to 300 kips in compression and kips in uplift (with steel anchor). Allowable bearing capacity range from 5 to 8 ksf 47

48 Completed Geopiers 48

49 Geopiers 49

50 50

51 Vibro Piers Installed using displacement and vibratory energy to depths of 10 to 30 feet by Hayward Baker Installation methods: Dry top feed method: stone is placed in preaugered hole, densified in 6 to 12-inch lifts with a vibrator Dry bottom feed method: for high groundwater level. The vibrator with tremie pipe attachment are penetrated to the design depth to install and densify the stone in place. Little or no waste results from this method. 51

52 Dry Top Feed Method Source: Hayward Baker Inc. Dry Bottom Feed 52

53 Compaction Grouting To compact loose soil or to produce control displacement to lift structure Involves injection of low-slump (less than 2 inches) grout (soil-cement mixture) which does not enter soil pores but remain in a homogeneous mass Grout material may consist of fine sand mixed with 12% cement and water to produce stiff, mortar-like mixture Grout pipe is installed to maximum treatment depth and grout is injected at high pump pressure as the pipe is withdrawn incrementally, forming a column of interconnected grout bulb 53

54 Compaction Grouting 54

55 Compaction Grouting (cont d) Can be performed stage down or, more commonly, in a stage up process, as follows: Advancing the grout pipe to the bottom of treatment depth Injecting the grout until refusal criteria is achieved, based on injected grout volume, injection pressure, or ground heave Extracting the grout pipe to the next depth interval and injecting the grout Repeat the process until reaching the upper limit of treatment zone 55

56 Compaction Grouting Source: Hayward Baker 56

57 Compaction Grouting (cont d) Construction monitoring: Injected grout volume Pressure loss/ground surface heave Verification testing: pre- and post-grouting SPT/CPT 57

58 Ultra-Fine Cement Grouting Uses micro-fine cement (particle size ranges from 1 to 10 microns); can penetrate fine sand Used to increase bearing capacity of sand under existing footings and/or reduce potential settlement Can be used to retain shallow excavation in loose sand Unconfined compressive strength can exceed 100 psi 58

59 Ultra-Cement Grouting 59

60 Elevator Pit Excavation 60