WHICH WAY IS IT MOVING?

Transcription

1 WHICH WAY IS IT MOVING? Guidelines for Diagnosing Heave, Subsidence and Settlement Authors: Presented by: Presented to : Ron Kelm, P.E., and Nicole Wylie, P.E. Forensic Engineers Inc. Ron Kelm, P.E. Presented on: 9 April 2008 Foundation Performance Association

2 Our Mission The mission of Forensic Engineers Inc. is to perform comprehensive forensic investigations of problematic civil, geotechnical, structural, and mechanical projects in order to determine the root cause of the reported problem. Ron Kelm, P.E. 2

3 Presentation Scope Clarify the movement types that commonly affect the performance of foundations for residential and other lowrise buildings in the Houston area. And feel free to ask questions during the presentation Ron Kelm, P.E. 3

4 OVERVIEW Introduction Definitions Causes Diagnoses Symptoms Remedies Other Movement Types Summary Ron Kelm, P.E. 4

5 Introduction Movement Types Three movement types commonly affect foundation performance in the Houston area: Heave ( ): upward; in clay soils Subsidence ( ): downward; in clay soils Settlement ( ): downward; in all soils Some still refer to all foundation movement as simply settlement Ron Kelm, P.E. 5

6 Introduction Evidence of Movement Ron Kelm, P.E. 6

7 Definitions OVERVIEW Introduction Definitions Causes Diagnoses Symptoms Remedies Other Movement Types Summary Ron Kelm, P.E. 7

8 Definitions Heave and Subsidence Heave is the vertical component of swell and subsidence is the vertical component of shrinkage Heave is due to moisture addition, while subsidence is due to moisture removal Both occur in clayey soil Both very common in Houston area See FPA-SC-13 for complete definitions Ron Kelm, P.E. 8

9 Definitions Heave and Subsidence Ron Kelm, P.E. 9

10 Definitions Heave Difficult to predict, diagnose and mitigate heave Foundation repair contractors often do not warrant against heave unless they can raise the entire foundation above a certain elevation Local geotechnical engineers often do not provide required testing or engineering needed to predict heave. (Causes some piers to be founded in active zone, instead of below it) Foundation design engineers may isolate foundation from soil if heave potential is large. Ron Kelm, P.E. 10

11 Definitions Isolated Structural System # Structural Slab with Void Space and Deep Foundations From: FPA-SC-01-0, Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils Ron Kelm, P.E. 11

12 Definitions Isolated Structural System # Structural Floor with Crawl Space and Deep Foundations From: FPA-SC-01-0, Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils Ron Kelm, P.E. 12

13 Definitions How deep should piers be for heave and subsidence? To prevent heave or subsidence, pier depth is: Computed based on suction testing Normally 1.5 to 2.0 times the depth of the active zone In Houston we have measured the active zone as deep as 20, thus piers/piles may need to be founded at 30 to 40 depth, depending on site characteristics. Ron Kelm, P.E. 13

14 Definitions What are heave forces on piers? Ron Kelm, P.E. 14

15 Definitions What are subsidence forces on piers? Ron Kelm, P.E. 15

16 Definitions Settlement Vertical loads from above are in excess of the bearing capacity of the soil strata directly below the foundation, causing the foundation and superstructure to move downward. Three common components of settlement immediate long-term slope instability See FPA-SC-13 for complete definition Ron Kelm, P.E. 16

17 Definitions Settlement - slope instability Ron Kelm, P.E. 17

18 Definitions Settlement Prevention + Ron Kelm, P.E. 18

19 Definitions How deep should piers be for settlement? To prevent settlement, pier depth: Is computed based on bearing capacity of soil Sometimes depends on location of water table and sand strata In Houston today, some designers and contractors still locate bell bottoms at Good Bearing Clays! Ron Kelm, P.E. 19

20 OVERVIEW Introduction Definitions Causes Diagnoses Symptoms Remedies Other Movement Types Summary Ron Kelm, P.E. 20

21 Causes Why does heave occur? Heave potential exists in clays that are at or below equilibrium moisture (= PL + 2 in Houston area). Clay particles exhibit a net negative charge and pack tightly when dry (below equilibrium moisture) Ron Kelm, P.E. 21

22 Causes Why does heave occur? When water becomes available, it is attracted by the clay s negative charges and bonds tightly to the surface of the clay, pushing the microscopic clay particles apart, causing an expansion or swelling of the clay. Ron Kelm, P.E. 22

23 Causes In which direction will swelling occur? In any direction the laws of physics will allow Closer to grade, depending on the amount of overburden from the soil and structures, the soil movement will expand upward, (previously defined as heave ) and sometimes laterally, commonly known as walking when observed in flatwork or foundations. At deeper penetrations, the soil+water particles are restrained, reducing the potential for attraction of water particles and subsequent expansion. Ron Kelm, P.E. 23

24 Causes In which direction will swelling occur? Cracked and walking flatwork Ron Kelm, P.E. 24

25 Causes In which direction will swelling occur? Walking flatwork Ron Kelm, P.E. 25

26 Causes Why does heave occur? Most common reason: Tree is planted or naturally occurs in an active clay, matures, and is then removed. When tree is removed, the sudden lack of water uptake by the tree creates an imbalance, with the ongoing soil suction attracting available moisture from any direction. Moisture movement can happen over several months or several years depending on the quantities of cracks and root channels in the soil fabric. Rehydration continues until the soil reaches moisture equilibrium (= PL + 2 in Houston) or more. Ron Kelm, P.E. 26

27 Causes Heave due to a Tree Ron Kelm, P.E. 27

28 Causes Heave due to a Tree Ron Kelm, P.E. 28

29 Causes Heave due to a Tree Ron Kelm, P.E. 29

30 Causes Heave due to a Tree Ron Kelm, P.E. 30

31 Causes Can the opposite soil movement occur? Yes, it is a vicious cycle As water leaves an active soil, the gaps between soil particles close and shrinkage occurs, the vertical downward component of which is defined as subsidence. Subsidence is often the reason for needing to lift older foundations in the Houston area. Builder s piers are commonly added to new stiffened slabon-grade foundations in the Houston area to later prevent or minimize this effect from newly planted trees. Ron Kelm, P.E. 31

32 Causes Why does subsidence occur? Trees or other large vegetation desiccate active clays supporting the foundation, causing soil shrinkage. The shrinkage will be cyclical with low points usually in September and high points in March as the soil partially rehydrates and temporarily heaves. If poor drainage or an interior sewer leak is present, the tree will direct its roots towards/under the foundation, exacerbating the desiccation and subsidence. Ron Kelm, P.E. 32

33 Causes Subsidence due to a Tree Ron Kelm, P.E. 33

34 Causes Subsidence due to a Tree Ron Kelm, P.E. 34

35 Causes Cyclical heave and subsidence Ron Kelm, P.E. 35

36 Causes Cyclical subsidence (from Tree Root Damage to Buildings by P.G. Biddle, 1998) Ron Kelm, P.E. 36

37 Causes Why does settlement occur? Occurs because a soil stratum below is too weak to support the gravity loads from above Usually due to embankment instability Also due to perched water table and loss in bearing capacity of silty soils or slab-on-grade foundations founded on silts underlain by clay. Ron Kelm, P.E. 37

38 Causes How do I know if settlement has occurred? For embankment settlement, sloughing is apparent Measure the clay s shear strength at the bearing level: Use a pocket penetrometer or other test devices. Bearing capacity is directly related to clay shear strength. Ron Kelm, P.E. 38

39 Causes At what shear strength can settlement occur? Values for the ultimate shear strength for several structure types: Ron Kelm, P.E. 39

40 OVERVIEW Introduction Definitions Causes Diagnoses Symptoms Remedies Other Movement Types Summary Ron Kelm, P.E. 40

41 Diagnoses Who can properly diagnosis movement type? Forensic engineers that carry out Level C investigations have a reasonable chance to properly diagnose the movement type. Per Section 3.3 of the Foundation Performance Association s Document No. FPA-SC-13-0, Guidelines for the Evaluation of Foundation Movement for Residential and Other Low-Rise Buildings, published 15 Jul 07 at: the Level C investigation includes, but is not limited to the following steps: Ron Kelm, P.E. 41

42 Diagnoses What is a Level C investigation? Interview the client, regarding history of property, performance of structure, Document visual observations made during a physical walk-through, Review pertinent documents including geotechnical reports, construction drawings, field reports, and repair documents, Observe factors influencing performance of foundation, Ron Kelm, P.E. 42

43 Diagnoses What is a Level C investigation? Continued.. Determine relative foundation elevations to assess levelness and establish baseline, Document locations of large trees/vegetation, Determine whether site drainage issues exist, Document the analysis process, data and observations, Site specific soil sampling and testing, if applicable, Hydrostatic leak test, with leak location and flow test, if applicable, Ron Kelm, P.E. 43

44 Diagnoses What is a Level C investigation? Continued.. Material testing, if applicable, Post-tensioning cable testing or steel reinforcing survey, if applicable, Aerial photographs to determine prior land usage or construction issues, and Observations of cut and fill. Ron Kelm, P.E. 44

45 Diagnoses What is a Level C report? If the client requests a report, it should contain, as a minimum: Scope of services, List of the reviewed documents, Description of factors that affect soil moisture, Observations, Scaled drawings, Site photographs, Survey elevation plan, Ron Kelm, P.E. 45

46 Diagnoses What is a Level C report? Continued... Detailed phenomena plan, Results from testing done as part of investigation, Discussion of factors identified as influencing the foundation performance and rationale in reaching opinions concerning foundation, Conclusions and recommendations for further investigation, remediation, or preventative measures. Ron Kelm, P.E. 46

47 Diagnoses What is foundation monitoring? Making periodic site visits. A single site visit provides only a snapshot of the foundation s condition. With more than one site visit the forensic engineer can see changes in distress and foundation movement. Monitoring should follow the procedures outlined in the Foundation Performance Association s Document No. FPA- SC-12-0, Guidelines for Evaluating Foundation Performance by Monitoring, published 9 Jan 06 at: When an external deep benchmark is available, it is easier to determine the direction of movement. Ron Kelm, P.E. 47

48 Diagnoses Why is monitoring used? To determine if the foundation is still moving To determine the type of movement To determine when movement has ended so that repairs to the foundation and superstructure can be made Ron Kelm, P.E. 48

49 Diagnoses Why is monitoring used? Monitoring the crack width (like this) Ron Kelm, P.E. 49

50 Diagnoses Why is monitoring used? Monitoring the crack width (not like this) Ron Kelm, P.E. 50

51 OVERVIEW Introduction Definitions Causes Diagnoses Symptoms Remedies Other Movement Types Summary Ron Kelm, P.E. 51

52 Symptoms Heave Symptoms Chicken-foot (or crossing) slab cracks. Damage began to occur during or soon after construction. Nearby trees removed within 1-2 years of first distress Flatwork adjacent to foundation translated upward and away. PI > 25. Soil moisture content (MC) increased in recent years. Gaps between grade beams and builder s piers. Loose foundation repair shims. Soil is tight around perimeter grade beams. Ron Kelm, P.E. 52

53 Symptoms Heave Symptoms Continued There have been leaks under or near slab. Much distress for small level distortion. Submerged water meters or irrigation valve pits. Nearby pool is out of level. Site drainage is flat or sloping to foundation. Soil supporting perimeter grade beam is easy to probe several feet down. Wet clayey soil in upper several feet below the perimeter grade beam. Ron Kelm, P.E. 53

54 Symptoms Heave Symptoms Chicken foot slab cracks Ron Kelm, P.E. 54

55 Symptoms Heave Symptoms Driveway concrete adjacent to foundation translated upward and away, relative to foundation Ron Kelm, P.E. 55

56 Symptoms Heave Symptoms Grade beam Gap between grade beam and builder s pier Gap Pier Rebar Ron Kelm, P.E. 56

57 Symptoms Heave Symptoms Water meter and irrigation valve pits are submerged Ron Kelm, P.E. 57

58 Symptoms Heave Symptoms Out of level pool Water flowing No water flowing Ron Kelm, P.E. 58

59 Symptoms Subsidence Symptoms Matured trees growing near the lower part of foundation. Superstructure cracks tend to open wider in drier months and close in wetter months. Foundation is at least ten years old and trees nearby are of similar age Large tree roots visible at grade, extending towards foundation. Owner is unable to maintain carpet grass below trees. PI > 25. Soil moisture content (MC) decreased in recent years. Pocket penetrometer readings of soil near grade is high, say 4 TSF. Single-line slab cracks, i.e., no intersections. Broken up concrete driveway and other flatwork. Ron Kelm, P.E. 59

60 Symptoms Subsidence Symptoms Continued Exterior walls lean outward at the top. Foundation was raised around the perimeter several years prior and it now has a bowl shape with the low point near its center. Soil supporting perimeter grade beam is hard to probe several inches down. Large gaps between exterior foundation grade beam and adjacent soil at grade, particularly during drier months. Time-change elevations show level distortion of foundation to increase with time and seasonally, particularly in drier years. Ron Kelm, P.E. 60

61 Symptoms Subsidence Symptoms Single slab crack Ron Kelm, P.E. 61

62 Symptoms Subsidence Symptoms Tree roots growing near/towards foundation Ron Kelm, P.E. 62

63 Symptoms Subsidence Symptoms Surface tree roots growing towards foundation Ron Kelm, P.E. 63

64 Symptoms Subsidence Symptoms Tree roots beneath driveway Ron Kelm, P.E. 64

65 Symptoms Settlement Symptoms Pocket penetrometer readings for clay supporting a slab-ongrade at foundation grade beam bearing level < 0.5 TSF. More downward movement in areas of the foundation with high loads. Heavier structures have more damage than lighter ones. Nearby flatwork, pools or other lighter structures show no movement. Grade soil is loamy or silty material. Foundation is near an embankment without a proper retaining wall and there are signs of soil sloughing down the slope. Ron Kelm, P.E. 65

66 Symptoms Settlement Symptoms Continued Slab cracks are single lines, i.e., without intersections. PI < 15. No gaps between builder s piers and grade beams. Soil just below the perimeter beam is easy to probe. Ground surface near the foundation drains quickly with little slope, will vibrate when excited, and tends to become soft after heavy rains. Ron Kelm, P.E. 66

67 OVERVIEW Introduction Definitions Causes Diagnoses Symptoms Remedies Other Movement Types Summary Ron Kelm, P.E. 67

68 Remedies Heave Remedies There are three common remedies for heave: 1) remove the source of moisture (so that no more heave can occur), 2) wait for the heave to run its course (if due to tree felling) or, 3) lift the entire foundation (so that continued heaving soil will not contact the underside of the foundation). Ron Kelm, P.E. 68

69 Remedies Heave Remedy #1 - Comments 1) Removing the water source It may be costly to find the source and sometimes it is not feasible to stop it (may have to add moisture retarders) For a sewer leak, the obvious fix is to repair the leak. For poor site drainage, the fix is to repair the grade to provide proper drainage, perhaps including some underground piping for downspouts and area drains. For general wetting of soil due to rainfall or underlying soil phenomena, a moisture retarder may help. Ron Kelm, P.E. 69

70 Remedies Heave Remedy #2 - Comments 2) Letting heave run its course Often the best and most economical repair. Heave will continue until the soil reaches moisture equilibrium (can go beyond if poor drainage / leaks present). Periodic monitoring of the foundation s elevations and the superstructure s distress will allow the engineer to determine when the movement due to heave has ended. If the level distortion is not within acceptable limits after the foundation has stopped moving, the foundation can be underpinned much the same as for subsidence. Ron Kelm, P.E. 70

71 Remedies Heave Remedy #3 - Comments 3) Lifting entire foundation Considerably more expensive but allows contractors to offer a warranty, which they otherwise may not offer for heave conditions. Adds engineering challenges because the typical foundation that was originally engineered as a slab-ongrade must now be designed as a suspended structural slab foundation. There are plumbing and other interfaces to consider as well as new vertical steps at porches and at the attached garage apron/driveway junction. Ron Kelm, P.E. 71

72 Remedies Subsidence Remedies Removing or heavily pruning trees and other large vegetation. In foundation repairs, soil is often hard, desiccated clay with deep active zone, making it difficult to achieve sufficient penetration below active zone to stop future subsidence. Repair contractors normally offer lifetime warranties to adjust their pilings should downward movement continue. Ron Kelm, P.E. 72

73 Remedies Settlement Remedies Safest of the three movement types when a foundation repair lifetime warranty is offered. Knowing where active zone ends is not of as much concern as with subsidence and heave, only that sufficient capacity is achieved during driving. Goal should be to support foundation at a deeper stratum to achieve more bearing capacity than is available near grade. Ron Kelm, P.E. 73

74 OVERVIEW Introduction Definitions Causes Diagnoses Symptoms Remedies Other Movement Types Summary Ron Kelm, P.E. 74

75 Other Movement Types Other Movement Types There are other less common foundation movement types found in the Houston area: Active fault slippage Root heave And others Ron Kelm, P.E. 75

76 Other Movement Types Other Movement Types - Faults There are over 350 known active faults in the Houston area Only a third of Houston s faults are well documented Hundreds of structures have been built over faults, often inadvertently Fault slippage rates vary with the faster ones moving about 1 in./yr. Soil and foundations near the active fault line will translate at a rate of 3 vertical to 1 horizontal Ron Kelm, P.E. 76

77 Other Movement Types Other Movement Types - Faults Home and street built on active fault Ron Kelm, P.E. 77

78 Other Movement Types Other Movement Types - Root Heave Requires a large tree or trees near the foundation. Normally affects flatwork and unstiffened light foundations. For stiffened foundations, normally occurs in sandy soil with high bearing capacity and good oxygen transmission. Grade beams and walls are typically unaffected. The slab between stiffening grade beams exhibits heave-like cracks and the slab movement can occur over a short period of time. Ron Kelm, P.E. 78

79 Other Movement Types Other Movement Types - Root Heave Flat work is very susceptible to root heave Ron Kelm, P.E. 79

80 Other Movement Types Other Movement Types - Root Heave Tree too close! Heave like slab crack Slab crack wider at top Can core the root if lucky Ron Kelm, P.E. 80

81 Other Movement Types Other Movement Types - Root Heave Expose the roots Remove the roots Replace the slab Remove the tree Ron Kelm, P.E. 81

82 OVERVIEW Introduction Definitions Causes Diagnoses Symptoms Remedies Other Movement Types Summary Ron Kelm, P.E. 82

83 Summary Summary The three common foundation movement types requiring mitigation in the Houston area are heave (up), subsidence (down) and settlement (down). All three movement types are commonly misdiagnosed in the Houston area. Misdiagnosis is minimized when a forensic engineer performs a Level C investigation and when foundation monitoring is implemented. Heave is the most difficult movement type to mitigate Settlement is the simplest movement to diagnose and mitigate but is also the least common in Houston. Ron Kelm, P.E. 83

84 Summary Summary At least a dozen symptoms are commonly found for each type of foundation movement to help the forensic engineer identify the movement type. However: When heave is present there will be an available moisture source nearby When subsidence is present there will be mature trees or other large vegetation nearby and the movement will be cyclical When settlement is present the ultimate shear strength of the clay supporting a slab-on-grade at the grade beam bearing level will be below 0.5 TSF Ron Kelm, P.E. 84

85 Summary Summary Remedial foundation work varies depending on type of foundation movement present Heave repairs often do not work and are usually not warranted by repair contractors unless a) monitoring shows the movement has stopped or b) the entire foundation can be lifted above the soil s potential upward movement elevation Subsidence repairs are often difficult to maintain if offending trees are not removed or heavily pruned on a regular basis, or if active zone is too deep Settlement repairs have the best chance for no call-backs from a warranty standpoint Ron Kelm, P.E. 85

86 Summary Summary Other movement types exist in the Houston area, though are rarely found and are often mis-diagnosed. Some are active fault slippage and tree root heave. And, a word to the wise Ron Kelm, P.E. 86

87 Summary Summary Don t plant trees too close to your foundation! Ron Kelm, P.E. 87

88 Thank You! Download the publication WHICH WAY IS IT MOVING? Guidelines for Diagnosing Heave, Subsidence and Settlement at: Ron Kelm, P.E. 88