Section 27-678. Allowable soil bearing pressures

The allowable bearing
  pressures on satisfactory bearing materials shall be  those  established
  in   table   11-2.   The   allowable   bearing  pressures  on  nominally
  unsatisfactory  bearing  materials  shall  be   those   established   in
  accordance  with  section  27-679  of  this  article.  Allowable bearing
  pressure shall be considered to be the allowable pressure at a point  in
  the  soil  mass in excess of the stabilized overburden pressure existing
  at the same point  prior  to  construction  operations.  The  stabilized
  overburden pressure existing at a point shall be defined as that portion
  of  the  weight  of  the  overlying  soil  material that is supported by
  granular  interaction  rather  than  pore  pressure.  In  general,   the
  magnitude  of  the stabilized overburden pressure may be approximated as
  follows:
    (a) The overlying soil material  shall  have  been  in  place  for  an
  adequate  length  of time to produce a stable condition of pore pressure
  in, or below the foundation level. Where the bearing stratum consists of
  soils of classes  5-65  through  8-65,  the  bearing  stratum  shall  be
  considered  to be fully consolidated except with regard to the weight of
  that portion of the overlying soil material that consists of added  fill
  material.
    (b)  Where  all  or  a  portion of the overlying soil consists of fill
  material, the weight of the fill material shall not be included  in  the
  stabilized  overburden  pressure  unless  the  magnitude  of  stabilized
  pressure is verified by  an  architect  or  engineer  on  the  basis  of
  laboratory or field tests on undisturbed material.
    (c)  Where  the  bearing  stratum  consists  of  soils of classes 9-65
  through 11-65, the stabilized overburden pressure shall be considered as
  zero unless the magnitude of the stabilized pressure is  established  by
  an  architect  or  engineer on the basis of laboratory or field tests on
  undisturbed material.
    (d) The stabilized overburden pressure shall not include the weight of
  any soil removed by excavation and not replaced. For footings, the total
  stabilized overburden pressure shall not exceed  the  weight  of  a  one
  square  foot column of soil (considering submerged weight where the soil
  column is partly submerged) measured from the bottom of the  footing  to
  the  lowest  level  of  the  final  grade  above  the footing. For a box
  foundation, where the strength of the slab is adequate to stabilize  the
  underlying soil, the stabilized overburden pressure shall not exceed the
  weight  of  a one square foot column of soil measured from the bottom of
  the box to the lowest level of the adjacent grade.
    (e) Where the bearing  stratum  consists  of  soils  of  classes  9-65
  through  11-65, the allowable bearing pressure shall be adjusted for the
  effects of rebound due to excavation as  determined  from  consolidation
  test data.
    (f) Where the bearing stratum consists of rock of classes 1-65 through
  3-65, the stabilized overburden pressure shall be neglected.
 
                 Table 11-2 Allowable Soil Bearing Pressures
  ========================================================================
                                           Basic Allowable Bearing Values
  Class of         Description              (Tons per sq. ft.)--See Notes
  Material        See Notes (1)                  (10), (11) and (12)
  ------------------------------------------------------------------------
  1-65      Hard Sound Rock                60   See Notes (2) and (8).
  2-65      Medium Hard Rock               40   See Notes (2) and (8).
  3-65      Intermediate Rock              20   See Notes (2) and (8).
  4-65      Soft Rock                       8

  5-65      Hardpan                        --   See Notes (3) and (8).
  6-65      Gravel and Gravel Soils (Soil
              Groups, GW, GP, GM&GC and
              soils of Soil Groups SW, SP,
              and SM containing more than
              10% of material retained on
              a No. 4 sieve)               --   See Notes (4) and (8)
              and (9).
  7-65      Sands (other than Fine Sands)
              (Soil Groups SW, SP&SM but
              containing not more than 10%
              of material retained on a No.
              4 sieve)                     --   See Notes (5), (8) and (9).
  8-65      Fine sand                      --   See Notes (6), (8) and (9).
  9-65      Clays and Clay Soils (Soil
              Groups SC, CL&CH)
                Hard                        5   See Note (7).
                Medium                      2   See Note (7).
                Soft                        See Sec. 27-679.
  10-65     Silts and Silt Soils (Soil
              Groups ML&MH)
                Dense                       3
                Medium                      1.5
                Loose                       See Sec. 27-679.
  11-65     Nominally Unsatisfactory Bear-
              ing Materials                 See Sec. 27-679.
  ========================================================================
 
  Notes:
    (1) Classification.  The  soil classifications indicated in this table
  are those described in section 27-675 of this article.  Where  there  is
  doubt  as  to  the  applicable  classification  of  a  soil stratum, the
  allowable bearing pressure applicable to the lower class of material  to
  which the given stratum might conform shall apply unless the conformance
  to  the  higher  class  of material can be proven by laboratory or field
  test procedures.
    (2) Allowable bearing pressure on rock. The tabulated values of  basic
  allowable  bearing  pressures  apply  only  for  massive  rocks  or, for
  sedimentary or foliated rocks, where the strata are level or nearly  so,
  and,  then only if the area has ample lateral support. Tilted strata and
  their relation to nearby slopes or  excavations  shall  receive  special
  consideration.
    (3) Allowable  bearing  pressure on hardpan. For hardpan consisting of
  well cemented material composed of a predominantly granular  matrix  and
  free of lenses of fine grained material and inclusions of soft rock, the
  basic  allowable  bearing pressure shall be twelve tons per square foot.
  For hardpan consisting of poorly cemented material or containing  lenses
  of  fine  grained  material,  inclusions of soft rock, or a fine grained
  matrix, the basic allowable bearing pressure shall  be  eight  tons  per
  square foot.
    (4) Allowable  bearing  pressure on gravel and gravel soils. Values of
  basic allowable bearing pressure shall be as follows:
      (a) For soils of Soil Groups GW, GP, GM, and GC:
        Compact, well graded material--ten tons per square foot.
        Loose, poorly graded material--six tons per square foot.
        Intermediate   conditions--Estimate   by   interpolation   between
        indicated extremes.

      (b) For  soils  of  Soil Groups SW, SP, and SM, containing more than
    ten percent of material retained on a No. 4 sieve:
        Compact, well graded material--eight tons per square foot.
        Loose, poorly graded material--four tons per square foot.
        Intermediate   conditions--Estimate   by   interpolation   between
        indicated extremes.
    (5) Allowable bearing pressure on sands. The basic  allowable  bearing
  pressure  shall  be determined from the resistance to penetration of the
  standard sampling spoon. The basic allowable bearing  pressure  in  tons
  per  square  foot shall equal 0.10 times N but not greater than six tons
  per square  foot,  nor  less  than  three  tons  per  square  foot.  The
  appropriate value for the penetration resistance at various areas of the
  site  shall  be made by averaging the measured resistance within a depth
  of soil below the proposed footing level  equal  to  the  width  of  the
  footing.  Where  the average values so obtained do not vary by more than
  twenty-five percent of the minimum of the average values over  the  site
  of the proposed building, the lowest average value shall be used for the
  design  of  the entire building. Where the variation exceeds twenty-five
  percent, the allowable bearing  pressure  shall  be  predicated  on  the
  lowest  average  value  unless  appropriate  measures are taken to avoid
  detrimental amounts of differential settlements of the  footings.  Where
  the  design  bearing  pressure on soils of class 7-65 exceeds three tons
  per square foot, the embedment of the loaded  area  below  the  adjacent
  grade  shall not be less than four feet and the width of the loaded area
  not less than three feet, unless analysis shall demonstrate the proposed
  construction to have a minimum factor of safety  of  2.0  against  shear
  failure of the soil.
    (6) Allowable  bearing  pressure  on  fine  sand.  The basic allowable
  bearing pressure shall be determined from the resistance to  penetration
  of  the standard sampling spoon. The basic allowable bearing pressure in
  tons per square foot shall equal 0.10 times N but not greater than  four
  tons  per  square  foot  nor  less than two tons per square foot, except
  that, for loose materials (resistance to  penetration  of  the  standard
  sampling  spoon  ten  blows  per  foot or less), where the foundation is
  subjected to vibratory  loads  from  machinery  or  similar  cause,  the
  indicated  basic  values shall not apply. The allowable bearing pressure
  shall be established by analysis applying accepted  principles  of  soil
  mechanics and a report of such analysis satisfactory to the commissioner
  shall  be  submitted  as a part of the application for the acceptance of
  the plans.
    (7) Allowable bearing pressure on clays and clay  soils.  The  bearing
  capacity of medium and hard clays and clay soils shall be established on
  the  basis  of  the  strength  of  such  soils as determined by field or
  laboratory tests and shall provide a factor of safety against failure of
  the soil of not less than 2.0 computed on  the  basis  of  a  recognized
  procedure  of soils analysis, shall consider probable settlements of the
  building, and shall not exceed the tabulated maximum values.
    (8) Increases in allowable bearing pressure due to  embedment  of  the
  foundation.  (a)  The basic allowable bearing values for rock of classes
  1-65, 2-65 and 3-65 shall apply where the loaded area is on the  surface
  of  sound rock. Where the loaded area is below the adjacent rock surface
  and is fully confined by the adjacent rock mass and  provided  that  the
  rock mass has not been shattered by blasting or otherwise is or has been
  rendered  unsound, these values may be increased ten percent of the base
  value for each foot of embedment below the surface of the adjacent  rock
  surface  in  excess  of  one  foot, but shall not exceed twice the basic
  values. (b) The basic allowable bearing values for soils of classes 5-65
  through 8-65 determined in accordance with notes three,  four  and  five

  above,  shall  apply where the loaded area is embedded four feet or less
  in the bearing stratum. Where the loaded area is embedded more than four
  feet below the adjacent surface of the bearing  stratum,  and  is  fully
  confined  by  the  weight  of  the  adjacent  soil,  these values may be
  increased five percent of the base value for  each  foot  of  additional
  embedment,  but  shall  not  exceed twice the basic values. Increases in
  allowable bearing pressure due to embedment shall not apply to soils  of
  classes 4-65, 9-65, 10-65, or 11-65.
    (9) Increase  in  allowable  bearing  pressure  for  limited  depth of
  bearing stratum: The allowable bearing values for soils of classes 6-65,
  7-65 and 8-65 determined in accordance with this  table  and  the  notes
  thereto  (including  note eight), may be increased up to one-third where
  the density of the bearing stratum below the bottom of the  footings  or
  the  tips  of  the  piles  increases  with  depth provided that: (a) The
  bearing stratum is not underlain by materials of a lower class.  (b) The
  allowable bearing value of the soil material underlying  the  bottom  of
  the  footings  or the tips of the piles increases at least fifty percent
  within a depth below the footing or the tips of the piles which  is  not
  greater  than  the  width  of  the  footing  or the width of the polygon
  circumscribing the pile group.   (c)  It  shall  be  demonstrated  by  a
  recognized  means  of  analysis  that  the  probable  settlement  of the
  foundation due  to  compression,  and/or  consolidation  do  not  exceed
  acceptable limits for the proposed building.
    (10) Combination of loads. The provisions of section 27-594 of article
  two of subchapter ten of this chapter shall apply.
    (11) Correction  for  foundations  bearing  on  materials  of  varying
  bearing capacities. Where it is  shown  by  borings  or  otherwise  that
  materials  of  varying  bearing  value  will  be  used  for support of a
  building: (1) Where the weakest material does not rank below class 5-65,
  no modifications shall be required.   (2) Where  the  weakest  materials
  rank  as classes 6-65 through 8-65, if the difference in basic allowable
  bearing values for the several materials does not exceed thirty  percent
  of  the allowable value for the poorest material which is to support the
  foundation, the foundations may be proportioned  in  direct  conformance
  with  the  allowable  bearing  pressures.  Where the difference in basic
  allowable bearing values exceeds thirty  percent,  appropriate  measures
  shall  be  taken  to limit the differential settlements of the different
  portions of the structure to tolerable  values.  (3)  For  materials  of
  classes  9-65  and  10-65,  in  all cases, appropriate measures shall be
  taken to equalize the settlements  of  the  different  portions  of  the
  structure.
    (12) Inundated  footings.  The  provisions of this section relating to
  materials  of  classes  1-65  through  7-65,  shall  be  deemed  equally
  applicable  both  to  the dry and to the inundated condition of the soil
  provided: (a) That the  subgrade  is  undisturbed  by  the  construction
  operations,  (b)  The  bearing area is in a confined condition. For such
  cases, no reduction in allowable bearing value will  be  required  where
  the  soil  supporting  the  foundation  is  subject to a rising level of
  inundation. However, the effects on settlement of  a  falling  level  of
  inundation  occurring  after  construction  of  the  foundation shall be
  considered in the design.