Settlements

Predicting settlements of shallow foundations

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Predicting settlements of shallow foundations is the No. 1 application of the DMT, especially in sands, where undisturbed samples cannot be retrieved. Many world experts consider DMT one of the best presently available tool for predicting settlements, notoriously not well predicted by conical probes.

Settlements are generally calculated, both in sands and in clays, by means of the one-dimensional formula.

Where Ds_v is calculated according to Boussinesq and M_DMT is the constrained modulus by DMT. In clays the predicted settlement is the primary settlement (net of immediate and secondary).

The validity of the method has been confirmed by a large number of observed agreement between measured and DMT-predicted settlements. (see e.g. summaries of comparisons by Schmertmann 1986, Monaco et al. 2006, Failmezger 2015).

Settlements estimates are useful, for instance, when the designer has to choose between a shallow foundation and a piled foundation.

Settlements Calculation using “DMT Settlements”

A computer program for calculating settlements according to Eq. 1 is freely available. The DMT Settlements Software is freely downloadable in the section SOFTWARE.

The validity of the method has been confirmed by a large number of observed agreement between measured and DMT-predicted settlements. (see e.g. summaries of comparisons by).

Settlements estimates are useful, for instance, when the designer has to choose between a shallow foundation and a piled foundation.

(a) Blades cause penetration distortions lower than axy-cylindrical probes.
(b) Modulus by a mini load test relates better to a modulus than a penetration resistance.
(c) Availability of the parameter K_D, providing stress history information (note that DMT is a 2-parameter test. It is fundamental to have both: E_D and K_D). The availability of at least two measurements is indispensable, if deformation properties have to be investigated.
(d) The soil is loaded at a lower, more appropriate, strain level.

The dilatometer modulus E_D should not to be used, as such, for predicting settlements, because E_D is only slightly sensitive to Stress History, while the operative modulus increases considerably with Stress History. E_D must first be corrected for Stress History:

M_DMT = E_D × R_M (K_D, I_D) (2)

where E_D is the dilatometer modulus and R_M is a correction factor applied to E_D depending primarily on the stress history index K_D (Marchetti 1980). As to the operative Young’s modulus E, if required, it can be estimated from Mdmt using the theory of elasticity (E ≈ 0.8-0.9 M_DMT).

Incorporating stress history information in the settlement calculation, using K_D, is not a refinement, but a necessity. The availability of the parameter K_D is important. There are not many alternatives to K_D for obtaining in situ information on stress history. On the other hand if the investigation is carried out with probes of modest sensitivity to stress history, the benefits of stress history are ignored, leading to a settlement overprediction and a more expensive design.

Not rarely M estimated in situ are higher than M estimated in the laboratory, due to sample disturbance. E.g. Schmertmann (1988) compared M obtained by different methods at Sunshine Skyway Bridge, Tampa Bay in Florida. On the average, M estimated by DMT was 200 MPa, by laboratory oedometers 50 MPa, back calculated from observed settlements 240 MPa. The laboratory oedometers were in this case 4 times too soft, possibly due to sample disturbance and stress relief. On the other hand already Terzaghi and Peck, in their 1967 book, had warned that even a good oedometer of OC clay may 2 to 5 times softer than in situ. In sands in situ estimates of M are even more useful, due to the difficulty of recovering undisturbed samples in sand.
Eq. (1), based on linear elasticity, provides a settlement proportional to the load, and is unable to provide a non linear prediction. The predicted settlements is meant to be the settlement in working conditions
Immediately after a DMT is completed, the predicted settlements may give an idea of the proper type of foundation. E.g. in case of buildings, very roughly, if the predicted settlement is < 3 cm (or possibly 4 or 5 cm), then a shallow foundation can be adopted, otherwise a deep foundation has to be adopted. In general the entity of settlements has a significant economical consequence. Accurate estimates may permit a more economical design.
M can also be predicted as M = a Q_c. The problem is that a depends on stress history or OCR, a missing information if only Q_c is available. The range of variation of a is quite wide (a = 2 to 20).

Citations by world experts on settlements

On Predicting Settlements by CPT
On Predicting Settlements by DMT

Lambrecht and Leonards (1978)

Prestressing increased the modulus by one order of magnitude, while Qc had only a slight increase.

Robertson (1986)

CPT predictions of soil modulus from the cone resistance can be rather poor, especially for overconsolidated soils, with a large potential error.

Jamiolkowski (Isopt-1 1988)

Without Stress History it is impossible to select a reliable E (or M) from Qc (similar statement by Terzaghi, Leonards, Schmertmann…)

Leonards & Frost (1988)

Correlations between penetration resistance and soil modulus will seriously overestimate settlements if the deposit has been prestressed.

Schnaid (2009)

Correlations between cone tip resistance and soil stiffness are unreliable.

Been SOA at CPT (2010)

If purpose is parameters, CPT cannot be used in isolation – must be supplemented by laboratory or other methods. CPT can easily mislead in terms of soil type, strength and particularly modulus.

Powell (BRE) (2012)

The scarce ability to predict modulus is a well known weakness of CPT.

Schmertmann (1986)

A DMT sounding can usually provide the data needed for the calculation of expected settlements with an accuracy adequate for most practical purposes.

Leonards (1988)

It has been argued that, at the present time, the Marchetti dilatometer is the most generally applicable practical tool for sensing soil compressibility.

Geopac Quebec (1992)

Settlements predicted by PMT and DMT were very similar, but cost and time for DMT were a fraction of PMT.

Woodward & McIntosh (1993)

Use of modulus from DMT permitted considerable savings. Using DMT, the structure was able to be constructed successfully on a shallow foundation system without utilizing more expensive and time consuming soil improvement techniques.

Steiner (1994)

An earthfill on a loose sandy-silt produced settlements substantially higher than anticipated based on conventional soil borings. DMT were then performed. “The DMT-predicted settlements agreed well with observed settlements”.

Tice & Knott (2000)

Good agreement was observed between DMT-predicted and measured settlements at the sandy site under Cape Hatteras Light Station.

KCI Technologies Usa (2000)

By DMT a more cost effective design can result, producing savings in construction cost.

Crapps (2001)

Users prefer DMT data over any other soils data to estimate settlements. A user with Law Engineering in Atlanta told me yesterday that the settlements are “always right on (meaning close to) the predictions from DMT”.

Mayne (2004)

Over two decades of calibration between the DMT and measured foundation performance records have shown its value & reliability in settlements computation.

Failmezger & Bullock (2011)

DMT is a static deformation test that strains the soil to intermediate strains. Tests are generally performed at depth intervals of 0.20 m. Tests typically take about 1 minute to perform. DMT is therefore the best choice of in-situ tests for settlement prediction of shallow foundations.

Penna (2013)

In my practical experience the DMT is the best approach for settlements prediction.

Case History

Cost savings by using DMT to redesign the foundation system

Roger A. Failmezger, P.E., F. ASCE, D GE – In-Situ Soil Testing, L.C., Lancaster, Virginia, U.S.A.

Project Name	Cost Savings with DMT Redesign of Foundation System
Westminister Village	$100,000
Ocean Landing Shopping Center	$750,000
Old Town Crescent	$150,000
Fox Run Village	$100,000
Monarch Landing	$150,000
MD Live!	$2,000,000
Towson Circle	$200,000
Retirement Community, Glen Mills, PA	$150,000
Xfinity Live!	$500,000
Obery Court	$200,000
Residences at Rivermarsh	$100,000
Residences at River Place	$80,000
Ocean Pines	$200,000
Four Seasons	$100,000
912 King Street-116 S Henry Street Mixed Use	$500,000
Dumfries Town Square	$200,000
Seacobeck Hall—Mary Wash- ington University	$500,000
Motown	$150,000
Richmond Area Collegiate Sci- ence Building	$85,000
Richmond Area Collegiate Re- search Building	$150,000
Food Processing Addition and Tank Farm	$100,000
13th and U Street	$100,000
55 M Street	$150,000
Alexan Dunn Loring Develop- ment	$250,000
Association of Manufacturing Technology Building	$150,000
Excelsior Parc Development	$100,000
Glenmont WMATA	$150,000
Halley Rise	$150,000
Howard Hughes HHMI Expan- sion	$100,000
I-64 Widening	$500,000
JHU-NIH-NCI	$150,000
Mark Center Plaza Building 5	$150,000
Mosaic Parcel CE	$100,000
National Gateway Land Bay “E” West	$250,000
Potomac Yard Bay D	$150,000
Ripley Street Development	$100,000
Rock Spring Centre	$250,000
Route 7 over Dulles Toll Road	$150,000
Route 7 Widening	$350,000
Tysons Archstone	$150,000
Tysons Central	$250,000
Upper Rock Blocks G & H	$100,000
West Falls Church WMATA	$150,000
McWane Hall—Lynchburg Col- lege	$100,000
Brooktrout	$50,000
Mecklenburg Schools	$100,000
Abingdon Elementary School	$225,000
Abingdon Heights	$400,000
Fauquier High School	$250,000
Prince William Commons	$400,000
PWCPS Administration Bldg	$225,000
Warrenton Aquatic & Recrea- tion Facility	$250,000
Washington Center	$350,000
WMATA White Flint Parking Garage	$625,000
3800 Glenwood	$350,000
Homewood Suites	$150,000
Johnson County WWTP	$1,000,000
1011 M Street	$200,000
14th and W Street	$200,000
1600 7th Street	$50,000
300 8th Street	$125,000
A-1 Glass	$100,000
B-CC High School	$350,000
Carlisle	$200,000
Fairfax Blvd Center	$100,000
Forest Oak Middle School	$100,000
Grimke	$1500,000
Kilmer Place	$50,000
Liberty Tank	$100,000
Sumner Suites	$125,000
Windsor	$50,000
Wood Middle School	$75,000
Wootton	$75,000
14th and P Street	$150,000
Culpepper Farmers’ Coop	$50,000
Indian Head Water Tanks	$75,000
Portals Phase 3	$175,000
Thomas Jefferson Library	$75,000
Avalon Mosaic Parcel H	$800,000
Cabin John Middle School	$200,000
Navy Federal Credit Union	$100,000
Cambridge Village	$150,000
Ben Oaks Water Tower	$75,000
Apple Greene Water Tower	$50,000
Oyster Bay Condos	$100,000
North Beach Various Parcels	$50,000
Fort Meade-DINFOS	$200,000
Fort Meade-Building #8605	$50,000
Fort McNair-Building #48	$250,000
535 Broadwater Road	$50,000
15 Judith Sound Circle	$8,000
318 Ironside Circle	$50,000
Courthouse Professional Build- ing	$60,000
Dahlgren Hotel	$75,000
Doc Stone MOB	$40,000
Hamptons at Hunton Park	$60,000
Kaeser Compressors Warehouse Expansion	$80,000
New Post Site	$50,000
Oakwood Estates	$85,000
Sophia and Hanover Streets	$40,000
1112 First Street Hotel	$250,000
Courthouse Village Bridge	$250,000
Arbor House	$100,000
William Square Hotel	$250,000
James Madison University— Phillips Hall	$150,000
1336 H Street	$80,000
Reston Crescent	$90,000

Total Cost Savings

$21,258,000

Settlements

Predicting settlements of shallow foundations

Settlements Calculation using “DMT Settlements”

Possible reasons of the DMT’s ability to predict settlements

Importance of the stress history parameter KD to predict settlements